|cluster||6/18/2017 3:34 PM|
|Marshall Major Blowing Power Fuse|
Got me a Marshall Major. I was playing it loud (9 or so) the other day and all of a sudden it blew a power fuse. Replaced the fuse, turned on Ok but as soon as I took it off standby, the power fuse blew right away. Thought it was the tubes, so I pull the tubes to see but it blew the power fuse again immediately as I took it off standby. Pulled the chassis out, looked around. No smoke or burn marks. Unsoldered the PT from the standby switch and hooked up a variac. Turned on ok. Tuned up the variac to 117 VAC with the standby off no problem. PT seemed fine. Wired back the standby switch to the PT and brought the variac from 0 very slowly (still no tubes). As soon as I engage the 1 volt on the variac I get a low hum on the speakers. Turn the variac to 5 volts and the hum is loud. Its a loud girthy hum and gets louder as I turn up the variac. Can't get the variac over 30 volts AC. The hum is super loud. Tuned it all off and scratched my head. Could it be the diodes, filter caps, OT?? Put the fluke on the diodes in place and they seem to read fine. Used my blue ESR meter on the filter caps and all read around .68, which seems in range for the ESR. Don't know what else to try. Hope I didn't blow my OT. I'm sure if I didn't use the variac and just took the amp off standby I would blow the power fuse again, so I didn't try. There is a power surge coming from somewhere. Any suggestions? Thanks in advance.
|Leo_Gnardo||6/18/2017 3:47 PM|
|Next troubleshooting step I'd take, is to pull each output tube from its socket, test the tubes individually, also inspect the tube bases & sockets for signs of arcing. Have you used this amp successfully in the past, ran it up to 9 & let 'er rip?|
There are several types of Majors, which is yours?
|cluster||6/18/2017 4:37 PM|
|Fuse blows when tubes are OUT, so it's not the tubes. Marshall is a 1970 standard model (not the bass or the pig).|
|cluster||6/18/2017 5:39 PM|
|I'm thinking either bad diode or bad filter cap. loud hum points to filter cap issues. can anyone tell me how to test for a bad filter cap? Thx.|
|Justin Thomas||6/18/2017 5:57 PM|
|I'm sorry, I have to ask - when you said Volume on 9 or so, did you mean BOTH or just one or the other? I mean, 200W isn't <THAT> loud... most people think 20W on 9 is too loud.|
Edit: okay, I should probably contribute something... you can check a suspect filter cap by placing a known good one across the One you think is bad - in parallel. If the hum gets better, the initial cap is bad.
DON'T FORGET TO CHECK THE BIAS CAPS!!!
|g1||6/18/2017 6:42 PM|
|Justin Thomas||6/18/2017 6:42 PM|
|Thanks, nobody ever mentioned that nugget before... |
|Enzo||6/18/2017 6:43 PM|
|Not for fuse blowing. If a cap is shorted, a parallel cap won't cure that.|
Rectifiers are just diodes, a few seconds with an ohm meter tells you if they are shorted. You can always measure across a cap to see if it tests shorted. Of course your meter runs on a volt or two, and some caps are real leaky at working voltages, but not down at 2v.
Pin 3 of the power tubes is the plate, measure resistance to ground at each. If low resistance, you have an issue on the B+. Then disconnect the output transformer center tap wire. Does the short go away?
Think about this, if your speakers make a hum with no tubes installed, SOMETHING is drawing current through the output transformer. And sorry to say that may be a short to frame or to secondary.
I don't recall these having flyback diodes. A schematic would sure help.
|Leo_Gnardo||6/18/2017 7:05 PM|
|I worked on one like this recently. Although the following won't lead you to what's wrong, the bias adjustment pot was intermittent in the one I worked on. Additionally it's in series with the supply. You may want to alter your bias supply so that won't be a potential problem before you finally button yours up.|
Something worth checking, there's four 250 ohm screen grid resistors, hockey puck shape & bolted to the chassis. If you developed a leak to ground thru one of these you could be having the symptoms you describe. I eightysixed all of 'em, and replaced them with 1000 ohm 10W wirewounds suspended on tie strips. There was a small loss of power, about 10%, but I reckoned this change would make the output tubes a tad more comfortable & expect longer life out of them. KT88's with about 600V on the plates, and nearly that much on the screen grids with the output transformer primary having ultralinear taps, for those unacquainted with this model. It is a beast!
|cluster||6/18/2017 7:21 PM|
|This all good stuff. I'll have another look in the morning. In the meantime here are some schematics|
|cluster||6/18/2017 7:57 PM|
|BTW, the hum I'm hearing is clearly more like 120 cycle vs the more common 60 cycle. Let me ask this: Could a bad filter cap (or diode) cause 120 cycle hum through the speaker even though tubes are OUT?? Looking at the schematics the filter caps do connect to the OT primary. Thx again.|
|Leo_Gnardo||6/18/2017 8:19 PM|
|Enzo||6/18/2017 9:08 PM|
|If it hums with no power tubes, then as I said, something else is pulling current through the OT. There are no flyback diodes, so that leaves us mostly with a transformer failure. SO please do read resistance to ground from pin 3 of each socket/|
120Hz hum is from power supply ripple currents. It may be lack of filter cap, but also quite common if the supply is loaded down even with good caps.
|cluster||6/19/2017 5:49 AM|
|Thanks for the help everybody :-) |
I took resistance readings at the primary of the OT via pin 3 and the centre tap and here is what I got:
pin 3 (left side) to pin 3 (right side) = ~28.7 ohms.
pin 3 (left side) to CT = 14.9 ohms.
pin 3 (right side) to CT = 13.6 ohms.
pin 3 (left side) to ground = 0
pin 3 (right side) to ground = 30 ohms
it seems the left side winding has a short to ground. now could this be a fail or something else, since all the other readings seem normal??
|Leo_Gnardo||6/19/2017 6:01 AM|
|cluster||6/19/2017 6:24 AM|
|Leo_Gnardo||6/19/2017 6:42 AM|
If that doesn't show any difference there's still the possibility of a short thru a tube socket. To test this you'll have to unsolder the OT primary leads from the suspect sockets, just a little more work than unbolting those resistors.
If you still have a short to ground after having done these, there's no choice left, the short must be internal to the OT, and that will be a costly pain in the neck to replace. It would be a double disaster if you replaced the transformer only to find the short was thru a socket or SG resistor.
|cluster||6/19/2017 7:05 AM|
|Enzo||6/19/2017 7:46 AM|
|Why lift anything? You have about 30 ohms through the winding. One end is grounded, and it makes sense the other end then shows 30 ohms. You have screen taps? FIne, measure resistance to pins 4 of each socket. I bet you find the value of the resistor plus the transformer resistance. Lift the zero ohms end wire from the socket. Still shorted?|
Also a good possibility instead of shorting directly to ground, your primary may be shorted to secondary. Secondary resistance is usually VERY low normally.
|Leo_Gnardo||6/19/2017 7:49 AM|
FWIW in the Major I worked on a couple weeks ago, there was already one output tube socket replaced. I retubed it, sent it out - with a 6 amp fuse that ought to be enough. Sure thing the fuse blew when the owner started pushing his amp hard. One of the new output tubes shorted and its socket was covered with arc debris so I replaced that too. Was it the socket or the tube? Chicken & egg question. I wasn't about to risk another tube so replaced its crusty burnt socket too.
|cluster||6/19/2017 8:46 AM|
|Raised the hockey pucks off the chassis and measured the resistance on the leads of the ultra-linear taps. Left side 9.2 ohms to ground. Right side 20.7 ohms to ground. Left pin 3 is still 0 ohms and the right pin 3 is 29.5 ohms to ground. Not getting a good feeling here. Next is to disconnect the the primaries at pin 3 to see if there is a short in the tube socket.|
On a side note, I'm sourcing an OT just in case. Here are 3 choices;
TAD at 279 euros ( $311) plus shipping: Output transf. for Marshall Major 200 W Ultralinear - Output Transf.
Banzai Music at 154 euros ($171) plus: https://www.banzaimusic.com/Transformer-T-OP-M200A.html
Or Mercury at a wapping $446: http://www.mercurymagnetics.com/imag...TC-MMAJ-OA.pdf
Any experience with TAD or Banzai?
|Enzo||6/19/2017 9:10 AM|
|Those readings simply verify the zero ohm end is shorted to ground or secondary. the taps are mid-resistances that fit right into the scenario.|
|Enzo||6/19/2017 9:14 AM|
|Since it looks like the transformer might be bad, we are at the what is there to lose stage. Note carefully where they all go, and disconnect all the secondary wires. Is pin 3 still shorted to ground? If so, the primary is shorted to frame (most likely). If the short went away, then the short was to secondary.|
So pull the transformer, and CAREFULLY cut into the fish paper or whatever surrounds the windings. Look for the points where the winding wires are bonded to the insulated wires that leave the transformer. Is it possible somehow that joint is touching and could be corrected?
|cluster||6/19/2017 2:09 PM|
|Bad news. De-soldered the primaries at pin 3 (left and right) as well as the CT. Tubes sockets are fine (no short to ground at pin 3). One side of the primary is still 0 ohms to ground, so it seems like there is a short from one side of the OT to the frame/chassis. Next step is to do what Enzo said. This will be a pain since I will need to de-solder the secondaries at the Christmas tree stand-off array at the top of the chassis. Keep you posted. This amp was made in 1970 and it took 47 years to blow this OT. Hopefully, i will able to fix it and keep it original otherwise a new OT will be put in. Either way, it was sad prying apart those primaries that were stuck together with some kind of wax.|
|Enzo||6/19/2017 2:27 PM|
|take notes take notes take notes... never rely on memory or "I'll figure it out" to get wires back where they belong.|
|nsubulysses||6/19/2017 2:28 PM|
Mercury is a ripoff IMO and banzai is far away so no need to have it shipped overseas.
|cluster||6/19/2017 2:52 PM|
|so here are a couple of photos from the christmas tree array on the secondary OT. For sake of what Enzo said to do would it be better to unsolder from the top (white wires - shorter time) or the bottom (more work but if the OT is shot then it's already to pull out of the chassis). Thoughts?|
|cluster||6/19/2017 2:57 PM|
|cluster||6/19/2017 2:58 PM|
|cluster||6/19/2017 3:04 PM|
|Question: Why does each tap coming off the secondary have two wires: 2 reds wired together, 2 yellows wired together, 2 greens wired together, 2 blacks wired together and a single green to ground??|
|cjenrick||6/19/2017 3:18 PM|
|what kind of output transformer is it? Drake? Partridge?|
been dying to get the data for this beast, would be willing to rewind for the price of copper (about 20 bucks)
that way you get a cheap OPT and i can start winding those for people,
do you live in the US?
if you have a signal generator or a variac you can try putting ac volts into the secondary and see what you get out of the primaries, be careful to use low volts, like 5 volts on the sec so you do not get hurt with the step up ratio volts which might be 20 to 30 times higher,
Mercury is probably using a layered paper approach as opposed to a nylon bobbin, claiming sonic improvements, as long as the clearance from the first layer to the core is the same with the nylon approach, there will be no difference in sound, if you use the same thickness and type of paper between layers and sections, plus you get a tougher transformer as there is less creepage of HV around the margins due to the nylon flange,
|Leo_Gnardo||6/19/2017 3:34 PM|
I'm pretty sure the Major's iron is Partridge.
I'm not impressed with Mercury's claims of sonic anything, since I ran across one of their replacement OT's in a blackie Fender Bandmaster. It sounded shattering bright, not at all what you expect from a Bandmaster. The rest of the amp was stock and working correctly.
|Justin Thomas||6/19/2017 5:30 PM|
|Cluster, where are you located? It might make it easier to find someone to help. Maybe (just MAYBE) someone has something suitable locally... |
And, is it me, or does this seem like something Hammond might actually make regularly? Or at least something that would work...
|nsubulysses||6/19/2017 6:52 PM|
|Hammond 1650W might be overkill but should work fine. |
Does the major do 2 ohms? 1650W has 4, 8, and 16 ohm taps
|g1||6/19/2017 7:37 PM|
|The Dude||6/19/2017 7:44 PM|
|The Dude||6/19/2017 8:00 PM|
|If you have a scope, do check the output for ultrasonic parasitic oscillation after replacing the transformer. I recall having one of these myself and have read about others with this problem. The oscillation riding on the audio causes the voltage to exceed transformer ratings and can cause them to fry. I'd hate to see someone go through all of the expense and time replacing an OT only to cook another one.|
|cluster||6/19/2017 8:05 PM|
|cluster||6/19/2017 8:13 PM|
Thanks for that info. I'll check with hammond for sure. I've also heard that the MM stuff is very bright and not vintage sounding. Not to mention very over priced.
|cluster||6/19/2017 8:17 PM|
|unfortunately, i don't have a scope. I should get one and learn how to use it though. They sound very useful. I do need to figure things out though since I don't want the repeat situation.|
|cluster||6/20/2017 6:29 AM|
|I was taking some resistance readings on the OT this morning and discovered that if I loosened the bellcaps, I would get an "Open" reading on all primaries and if I tightened the bellcaps, I would get my previous readings of 0 ohms on the left and about 30 ohm on the right (to ground). Could anyone please explain this? How do the bellcaps effect the ohms to ground on the primaries? |
Also, I was taking resistance measurements across the secondaries at the impedance selector and all my readings gave me 0 ohms. Is this normal? I would expect some resistance across some of these windings. Could this mean that there is a short in the secondary as well?
|Leo_Gnardo||6/20/2017 6:44 AM|
|cluster||6/20/2017 6:53 AM|
Thanks so much Leo. Could you please post links to the insulation you are referring to. It would be a great help.
Also, I was taking resistance measurements across the secondaries at the impedance selector and all my readings gave me 0 ohms. Is this normal? I would expect some resistance across some of these windings. Could this mean that there is a short in the secondary as well?
|cluster||6/20/2017 7:11 AM|
Well, well, well... what do we see here?? Lifted the bell covers and notice some fool at the factory forgot to tuck in the wires properly before fighting the bellcovers and smushed up a primary. Notice the broken insulation. Maybe this baby doesn't need a rewind after all. Now what is the best way to insulate this without de-sodering wires. Suggestion. Thanks again to everyones help.
|cluster||6/20/2017 7:22 AM|
Not sure if you could see this but not only was this primary shorted to ground but it was also cut in the process. Any clue what i need to do? Should i splice a new wire or try to solder the 2 back together? Thoughts? Would want to use the least intrusive method possible. Thx
|cluster||6/20/2017 7:37 AM|
Ok, I managed to peel away some of that dried out cloth insulation and expose the break. can anyone tell me what that red coating is on the wire? Is it a type of insulation? How would I strip it? Sanding paper? Would it be ok to solder them together and shrink wrap them? That seems to be the simplest way to go about this. thanks
|Leo_Gnardo||6/20/2017 9:02 AM|
GE RTV silicone cement sold in nearly every hardware & home-center store. Handy 3.5 oz tube about the size of a large tube of toothpaste I expect will run you about CD$5. I used to use it in a hi-vacuum hi-voltage lab and it insulated well up to 25000 volts.
Heat shrink tubing, if you still have any open Radio Shack stores, they sell it. If any other electronic supply stores, they should have it too, in several diameters. Worst case you have to send away for it, Mouser has a nice selection, I prefer 3M polyolefin which they provide in a wide selection of diameters & colors in handy 4-foot or 6-inch cuts:
275 choices of diameters & colors, I'm sure you'll find something to suit your needs.
Red coating on your transformer wire is insulating lacquer. You can scrape away near the ends you need to splice, twist & solder then cover with a layer or two of heat shrink. For extra insulation you may want to coat that with a layer of RTV, let it dry then reassemble your transformer. Hopefully you'll have no further trouble from it. Whew, what an adventure.
|cluster||6/20/2017 9:15 AM|
Thanks so much. Couldn't have done it without everyone's help. Please tell me how and where I should apply this RVT cement? Thx
|Leo_Gnardo||6/20/2017 9:30 AM|
|cluster||6/20/2017 10:22 AM|
Thanks so much. Could you comment on this and would it be useful to put in my major? :
OUTPUT SPIKE PROTECTION
This modification involves taking three 1N4007 silicon diodes in series with the banded end connecting to pin three of your output tubes and the other end to ground. You use one on each side of a push-pull output. So for example, on a 100 watt Marshall the two tubes on the left, either pin three of either one of those tubes can connect to this diode to ground. And on the other side either pin three can connect to the other set of diodes to ground, as the tubes on either side are in parallel.
We use 1N4007 silicon power diodes which are effective in suppressing spikes at certain frequencies. These will not necessarily prevent spiking on all amps! There is a fast recovery or high speed diode which will suppress higher frequency arcing however these diodes have a very distinct disadvantage as they alter the tone of the amp! They make the amp sound muddy as they bleed off high frequencies from the output transformer! Trainwreck does not use these kinds of diodes because of their effect on the tone.
While the Trainwreck method does not eliminate spiking in every amp, it does not effect the tone at all. Every other method which will totally eliminate spiking will have a negative effect on the tone of your amp.
Another point to remember concerning spiking is that hot biased amps tend to generate more high voltage spiking than properly biased amps.
|Leo_Gnardo||6/20/2017 11:32 AM|
|cluster||6/20/2017 12:31 PM|
|So I spliced the primary wire and double wrapped it in shrink wrap. Put the OT back together and took some readings:|
end to end = 29.1 ohms
CT to left = 14.8 ohms
CT to right = 16.4 ohms
UL leads end to end = 11.5 ohms
CT to left UL = 7.4 ohms
CT to right UL = 6.7 ohms
All leads to ground = open (no resistance)
Does this sound right? Are there any other tests I should do before testing it with the PT. Would it be worth testing the OT with the battery method on the secondary?
|cluster||6/20/2017 1:24 PM|
|Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. Modern cans are smaller. Does anyone know of way to modify the large clamps to accommodate the smaller cans? BTW, I know these cans are 47 years old. ESR meter was giving me a reading of .67 which should be good? Is it worth replacing these caps?|
|cluster||6/20/2017 3:53 PM|
|Just any update. I used a small transformer to deliver 7.4 VAC to the secondaries to test the windings and I believe it to be working well. Please check my work. 7.4VAC on the 16 ohm tap of the secondary resulted in 90.6 VAC across the primaries with an even 45.3/45.3 VAC from CT to each end. Seems like it's working. Might be time to wire it all up and put the amp on the Variac.|
|cluster||6/20/2017 4:15 PM|
|Just follow up... I did the math to calculate the OT's impedance:|
90.6VAC/7.4VAC = 12.24 turns ratio
12.24 squared (impedance ratio) x 16 ohms = 2,398.4 ohms or 2.4 KOhms, which is dead on spec.
Next step wire it all back up and put it on the variac (no tubes) and hope she purrs....
|olddawg||6/20/2017 4:50 PM|
|Leo_Gnardo||6/20/2017 5:13 PM|
|cluster||6/20/2017 5:50 PM|
|Question: I don't have a speaker cab here to use as a load but I do have a 16 ohm THD hotplate with a load option on it. I don't intend to test it with tubes in. Would the hotplate be a good enough load to test the OT w/o tubes or should I wait to have the proper cab? If all goes well and I want to test/bias the amp with tubes in (no input signal - just idle) would I need a cab or could I use the 16 ohm THD hotplate load? Thx|
|The Dude||6/20/2017 5:54 PM|
|If you're going to test it without tubes installed, you don't need a load at all.|
|cluster||6/20/2017 6:05 PM|
|cjenrick||6/20/2017 6:18 PM|
|go ahead and stick the tubes in there, you are ready to go, yes secondary resistance is usually .25 ohms for the 4 ohm wind and increases slightly with each tap so you need a 4 wire meter to measure the DCR.|
vinyl glass sleaving is real handy for covering splices, you can buy it on evilbay, you could also used glass cloth tape from Scotch 3M , Permacel, etc
.scrape the enamel off with a razor blade, tin the wires, stick them side by side and hit them with the iron and they stick nicely. a few turns of black tape and them some sleaving and you are good to go.
that might be a Radio Spares xfmr, the Partridge was the one we were hoping to rewind, they sometimes use a tape wound core which is very expensive compared to EI lams but has less leakage.
and as luck would have it, we had a Sovtek Mig 50 come in today with a blown OPT so we get to find out what makes this thing tick,
|catalin gramada||6/20/2017 7:19 PM|
|catalin gramada||6/20/2017 7:21 PM|
If you clamps are extra large and have room enough you can use polyprop. motor RUN capacitors against electrolythics instead
|cluster||6/21/2017 3:57 AM|
I notice that the big 200uF cans have 3 terminals with two tied together, so I'm guessing they are actually 100+100uF? Is that right? The schematic says 2 cans of 200uF are in series but doesn't say that they are internally stacked. I would expect the can to read 100+100 instead of 200uF or else why have the tree terminals.
Also, could some explain the power filtering section in this amp. The 2 375uF cans seem to feed the bridge rec? The 2 x 200uF to the plates via the OT? But it also feeds the plates of V2 preamp and PI.
|catalin gramada||6/21/2017 6:42 AM|
|Just supose: big ones could be 200+200 meant 400uF in parallel, pretty close to 375 schematic shows. Also I saw an error in power supply drawing. 56k/2w should be tied over each cap to ensure balance. There are corectly tied in you amp but schematic didn.t shows. You dont.t need balance resistors over the big ones as time HT center tap was tied in between|
|cluster||6/21/2017 7:34 AM|
with 118VAC via the variac I can see 631 VDC on the plates (no tubes). Hooked up those diodes from pin 3 to ground for extra precaution. Next step is bias the amp with the tubes in. Take measurements. Listen for hum and/or strange noises. Bang the OT and PT a couple of times to see if I get any noise. Feel for any excessive heat on the OT. Let it sit for 15 minutes or so then pug in my lester for it's final test.
|dstrat||6/21/2017 7:41 AM|
|Hi, those caps usually are marked outside the can but may need to turn them to see it. might be a good idea to check that.|
also know that there are errors on most of the marshall schematics.
|Leo_Gnardo||6/21/2017 8:03 AM|
|Don't forget, it would be a good idea to alter that bias supply before you put tubes back in. The adjustment pot is between the rectifier & filter network. If it fails open, as pots do, the bias voltage will fall & you'll be popping fuses or worse. The adjustment pot on mine was intermittent. You may choose to trust yours but if it does become intermittent you'll wish you had changed it. I simply bypassed the factory pot with a 10K fixed resistor, and put a 10K trim pot plus fixed resistor in series with the 10K to ground. Also I changed the value of the 820 ohm resistor so I had a useful voltage range. The bias voltage is well up there, somewhere around -70V.|
|catalin gramada||6/21/2017 8:32 AM|
|Familyortiz||6/21/2017 9:20 AM|
|This is something I do when troubleshooting anything.|
1) Look for the obvious. It sounds like you did.
2) If you can power the amp, you can measure the output voltages for all of the supply outputs. All of them. If not, measure the resistance to ground for all supply outputs. The shorted guy will standout.
3) When the shorted supply is found, at that point you can try to isolate which connection to that shorted supply is the culprit. There are only a number of connections to each supply so it's not a huge task.
|cluster||6/21/2017 3:54 PM|
|Good news at last. Put the tubes in and variac'd the amp up to 118VAC. Bias the KT88's to 35ma/tube and got the following measurements: V4 plate: 590VDC screen: 585VDC V5 plate: 587VDC screen: 586VDC V6 plate: 591VDC screen: 587VDC V7 plate: 590VDC screen: 585VDC V1a Plate: 177.7VDC V1b Plate: 233.0VDC V2a Plate: 205.0VDC V2b Plate: 305.5VDC V3a Plate: 292.0VDC (PI) V3b Plate: 281.0VDC (PI) Bias feed = -66.7 VDC and -69 VDC at each grid pin I would have thought the screens to be lower that what I measured but those were the readings. Leo, could you explain a little more how I should mod my bias pot. I do want to do this before putting the amp back together. I also have a PPIMV for Metro that I've been thinking of putting in but part off me wants to leave it original. Still haven't decided. Thanks everyone for the great help. Could have done it with out you guys :-) Joseph|
|cluster||6/21/2017 3:54 PM|
|Good news at last. Put the tubes in and variac'd the up to 118VAC. Bias the KT88's to 35ma/tube and got the following measurements: V4 plate: 590VDC screen: 585VDC V5 plate: 587VDC screen: 586VDC V6 plate: 591VDC screen: 587VDC V7 plate: 590VDC screen: 585VDC V1a Plate: 177.7VDC V1b Plate: 233.0VDC V2a Plate: 205.0VDC V2b Plate: 305.5VDC V3a Plate: 292.0VDC (PI) V3b Plate: 281.0VDC (PI) Bias feed = -66.7 VDC and -69 VDC at each grid pin I would have thought the screens to be lower that what I measured but those were the readings. Leo, could you explain a little more how I should mod my bias pot. I do want to do this before putting the amp back together. I also have a PPIMV for Metro that I've been thinking of putting in but part off me wants to leave it original. Still haven't decided. Thanks everyone for the great help. Could have done it with out you guys :-) Joseph|
|cluster||6/21/2017 4:10 PM|
|[ATTACH=CONFIG]43869[/ATTACH] Here is a close crop of the bias circuit. Where should I move the pot? Also, if there are any other mods I could do to make this amp more stable and less likely to blow up, I would appreciate it. Thanks.|
|Leo_Gnardo||6/21/2017 8:27 PM|
|[QUOTE=cluster;457826]Here is a close crop of the bias circuit. Where should I move the pot?[/QUOTE] What I did was snip the connections to Marshall's pot, bridge over it with a fixed resistor. Don't remember value exactly it may have been 2K4 or 3K3. Left the pot physically in place, it's a bear to remove. Next resistor (820R) swapped out for 10K or 15K. Finally the existing 10K snipped out & replaced with a 10K or 20K trimpot in series with a fixed resistor, value chosen so that bias voltage could be swung say -60 to -80V. The fixed R may have been 56K - 68K, somewhere around there. These higher value resistors allow better filtering of the bias supply, and if there's a failure of the bias pot, bias voltage rises to a point the output tubes cut off. Marshall's way, if that pot opens up you lose the bias supply and output tubes go red plate. I use a 6 amp slow blow mains fuse in this amp. Marshall's 10 amp slow blow imho isn't protective enough. Not a bad idea to replace any original electrolytic caps there may still be in your Major. Someone already had replaced the caps that pass signal to the output tube control grids. If you suspect leakage here or think it's a good idea to replace them as preventive maintenance, now's the time. Unlike that da'esh chappie mentioned by G1 ( [url]http://music-electronics-forum.com/t41470-36/#post457839[/url] ), you managed to dodge the bullet with this one, finding & repairing the fault in the OT. Good work indeed!|
|cjenrick||6/21/2017 10:52 PM|
|screen voltage is usually pretty close to plate voltage in a four tube amp as they usually use a choke or UL transformer which has lower DCR than a 1 K voltage dropping resistor like you might see in a Princeton Reverb, chokes are usually around 100 ohms DCR, so figure a few ma across 100 ohms plus maybe a 470 ohms screen resistor , .002 * 570 = about 1 volt, 4 ma = 2 volts, when you start cranking that screen voltage will come down due to the AC current, what kind of KT88's are those? 585 plate volts in kind of scary, maybe drop the screen voltage with some additional resistance or switch to 7027/6550 , Music Man runs 750 on the EL34 plates but the screens sit at about 350, or use the Jerome Brown trick (PV, Kustom) and insert cathode resistors in the pwr tubes, you get a mix of self bias and cathode bias for a bit more compression and you offset the plate voltage by whatever cathode voltage you have, this also helps prevent thermal runaway (red plating) and can also balance the tubes a bit, power comes down a tad depending on the cathode resistor value, but that can be a good thing on a brute like this, good to hear you are up and running!|
|catalin gramada||6/21/2017 11:46 PM|
|Be aware with this types of ppimv. Usually the pots replace grid leaks resistors of power tubes.(and usually this was designed for EL34 tube type going to 2x250k-2x500k value). You don.t want to exceed nominal value of 68k which you KT88 Marshall did (it is allready high), so be sure you PPIMV didn.t and check first. If you don.t have to remove grid leaks and you ppimv is decoupled from power stage (eg. Orange ppimv type) it is not a concern|
|cluster||6/22/2017 4:28 AM|
|[QUOTE=cjenrick;457867]screen voltage is usually pretty close to plate voltage in a four tube amp as they usually use a choke or UL transformer which has lower DCR than a 1 K voltage dropping resistor like you might see in a Princeton Reverb, chokes are usually around 100 ohms DCR, so figure a few ma across 100 ohms plus maybe a 470 ohms screen resistor , .002 * 570 = about 1 volt, 4 ma = 2 volts, when you start cranking that screen voltage will come down due to the AC current, what kind of KT88's are those? 585 plate volts in kind of scary, maybe drop the screen voltage with some additional resistance or switch to 7027/6550 , Music Man runs 750 on the EL34 plates but the screens sit at about 350, or use the Jerome Brown trick (PV, Kustom) and insert cathode resistors in the pwr tubes, you get a mix of self bias and cathode bias for a bit more compression and you offset the plate voltage by whatever cathode voltage you have, this also helps prevent thermal runaway (red plating) and can also balance the tubes a bit, power comes down a tad depending on the cathode resistor value, but that can be a good thing on a brute like this, good to hear you are up and running![/QUOTE] Tubes JJ KT88's. Here is the data sheet for them: [URL="http://www.thetubestore.com/lib/thetubestore/JJ-KT88.pdf"]http://www.thetubestore.com/lib/thetubestore/JJ-KT88.pdf[/URL] Question; How does dropping the screen voltage effect tone and feel of the amp??|
|cluster||6/22/2017 4:34 AM|
|[QUOTE=catalin gramada;457870]Be aware with this types of ppimv. Usually the pots replace grid leaks resistors of power tubes.(and usually this was designed for EL34 tube type going to 2x250k-2x500k value). You don.t want to exceed nominal value of 68k which you KT88 Marshall did (it is allready high), so be sure you PPIMV didn.t and check first. If you don.t have to remove grid leaks and you ppimv is decoupled from power stage (eg. Orange ppimv type) it is not a concern[/QUOTE] Thanks for that info. I'll skip the MV at this stage. Try to keep the amp original. I've heard some people say that Marshall Majors don't take overdrives well (especially Fuzz) and some blow up on a count of square wave signals hitting the OT (something to do with it's ultra linear character). I have used a transparent overdrive (a TIM) on this amp and it really makes this amp come alive at volume 4-6 or so. Any thoughts, pros/cons on using OD pedals with a Major.|
|cluster||6/22/2017 4:37 AM|
|[QUOTE=Leo_Gnardo;457856]What I did was snip the connections to Marshall's pot, bridge over it with a fixed resistor. Don't remember value exactly it may have been 2K4 or 3K3. Left the pot physically in place, it's a bear to remove. Next resistor (820R) swapped out for 10K or 15K. Finally the existing 10K snipped out & replaced with a 10K or 20K trimpot in series with a fixed resistor, value chosen so that bias voltage could be swung say -60 to -80V. The fixed R may have been 56K - 68K, somewhere around there. These higher value resistors allow better filtering of the bias supply, and if there's a failure of the bias pot, bias voltage rises to a point the output tubes cut off. Marshall's way, if that pot opens up you lose the bias supply and output tubes go red plate. I use a 6 amp slow blow mains fuse in this amp. Marshall's 10 amp slow blow imho isn't protective enough. Not a bad idea to replace any original electrolytic caps there may still be in your Major. Someone already had replaced the caps that pass signal to the output tube control grids. If you suspect leakage here or think it's a good idea to replace them as preventive maintenance, now's the time. Unlike that da'esh chappie mentioned by G1 ( [url]http://music-electronics-forum.com/t41470-36/#post457839[/url] ), you managed to dodge the bullet with this one, finding & repairing the fault in the OT. Good work indeed![/QUOTE] Thanks for this. I'll try to draw out a little schematic and check back with you. Amps that red-plate are scary.|
|catalin gramada||6/22/2017 5:15 AM|
|Is hard to predict as time are tied in ultralinear connection and its effect is related by OT but I saw from you pics you screen resistors are allready oversized from original. Seems someone tied a series 750 ohm with original 250 going to 1K. This mean more voltage drop over when dimmed. The screens draw very litle current at idle and in consequence at idle the voltage will be as same as the plate,more or less. When dimmed they draw more, so more voltage drop over screen resistors. Bigger screen resistors meant less voltage to the screens on "dynamic" mode.|
|Leo_Gnardo||6/22/2017 5:29 AM|
|[QUOTE=cjenrick;457867] maybe drop the screen voltage with some additional resistance [/QUOTE] That's what I did with the Major I was working on, 1K 10W instead of those 250 ohm hockey puck resistors. Power figure dropped to 168W at clip instead of 186, still plenty enough to rock the house. [QUOTE=cluster]I've heard some people say that Marshall Majors don't take overdrives well (especially Fuzz) and some blow up on a count of square wave signals hitting the OT (something to do with it's ultra linear character). I have used a transparent overdrive (a TIM) on this amp and it really makes this amp come alive at volume 4-6 or so. Any thoughts, pros/cons on using OD pedals with a Major. [/QUOTE] As long as you're not overstressing the power amp, I don't see why not use OD pedals to get your fuzzy tone. A Major, being driven well into square wave zone by dialing it up & wailing on it, that's what wrecks them. I consider them "friendly giants", they make good bass amps, or clean guitar to earsplitting volume. Treat 'em gently and you'll get good results. If you like the tone of a real overdriven Marshall, get or build a late 60's - early 70's 50W with a B+ in the 360-400V range. Real power will top out around 30W at clip with EL34 or 35W with KT88 or 6550, you'll get plenty of power amp clip tone & save a lot of headaches. Luv those amps!|
|catalin gramada||6/22/2017 5:38 AM|
|...And it is very easy to recoverable mode if you feel is to loud. Just connect it into triode mode. It will sound slightly different but maybe is in you spot.|
You amp looks in pretty original condition. Don.t mode it. Play it like a Major damn... Love it.
|cluster||6/22/2017 6:53 AM|
Took a photo of the bias pot underneath the bias board. Looks like a heavy duty sucker of a pot. As an alternative to redesigning the bias circuit, would it be possible to put a fuse in the existing one so if there is a fail, the fuse blow instead of red plating the tubes??
|catalin gramada||6/22/2017 7:04 AM|
|g1||6/22/2017 12:12 PM|
|Enzo||6/22/2017 12:22 PM|
|Fuse in the bias? Fuse blows you lose bias completely, making whatever else was wrong even worse.|
|cluster||6/22/2017 12:30 PM|
|Enzo||6/22/2017 1:14 PM|
|Well, safe guard it from what? There is no current stress on a bias circuit. SO mostly I concern myself with the failure of the pot wiper. If that loses contact, our circuit reverts to some condition. We want that condition to be the maximum bias voltage.|
Most bias supplies rectify something to form a base negative supply. The a voltage divider adjusts the portion of it you want. I never make a bias control that takes the bias off the wiper. I always wire the pot as a variable resistor. If that pot is used as the top resistance in a divider, the loss of the wiper means the resistance maxes, which throws the bias voltage to the least - the opposite of the desired goal. SO if I make the pot the lower element, then an open wiper reverts the supply to the maximum negative voltage.
So your circuit in post #72 for example I would make the 10k resistor the variable and the 3k just a resistor. I would not use a 10k pot, I would chose a pot value plus a resistor, so maybe a 5k pot with 5k resistor to make 10k. That way if the wiper opens, we have 10k resistance for the max voltage. And the resistor means if someone turns the bias control all the way down, it won;t go to zero, it will only drop to whatever 5k represents.
My numbers are arbitrary, you might find a 20k resistor with a 3k pot works well. The exact values would be determined by the base supply voltage and make up a divider that gives you the adjustment range of bias voltages, say 40-60v. Whatever.
That is how I would protect it.
|cjenrick||6/22/2017 11:15 PM|
|doh on me, i thought those were 66 tubes instead of 88's, so you are alright with that B+ voltages and screens too., |
dropping screen volts can be bad for tone if you take it too far,
UL transformers might be a little too clean for some people, if so, simply safe off the taps and put a screen dropping resistor to the pwr supply, wire up some 470's or 1 K five watt resistors and rock on,
ever play the Fender Super Twin? (UL transformer) Great for steel guitar but mega clean with guitar, and they are real back breakers,
|cluster||6/23/2017 4:23 AM|
|pdf64||6/23/2017 6:54 AM|
I think it's better to refer to them as distributed load.
I think that they seem super clean as they have a high stiff HT, and very good mitigation for bias shift under overdrive.
|Justin Thomas||6/23/2017 7:38 AM|
|They crunch without using the "Distortion." Eventually. And they're not <THAT> loud. >|
I'm sure this Major can do the same. Actually, gimme a Pig. No Master.
|catalin gramada||6/23/2017 8:08 AM|
|cluster||6/23/2017 8:56 AM|
Could someone here explain the two black secondaries off the PT going to the 200uF can? I don't see this anywhere on the schematic. As far as I can tell the 2 red secondaries off the PT go to the standby switch -> rectifier -> HT fuse -> 200uF -> 200uF. Here I see these 2 mysteries black wires connected to the positive lug of the 2nd 200uF to secondary of PT. The other strange thing about this PT is that the heater wires seem to be connected on the primary side. I thought heater wires are always off the secondary. Maybe the heater secondary was fed to the back of the PT to make it closer to the last power tube at the end? Still this mysterious black wires.
|dstrat||6/23/2017 9:04 AM|
|hard to say without measuring but i would think that is the HT secondary center tap, those two wires are connecting the two half's of the secondary.|
that is what it looks like to me, its a voltage doubler circuit.
oh yeah nice amp btw.
|catalin gramada||6/23/2017 9:31 AM|
late edit- please, take very serious my advice: be very,very sure the top caps aluminium shell from series bateries have not any chances to touch the fixing clamp and/or the chassis ! You can put some extra paper arround. Sometime the cap isolation is so thin as can be very easy punched by a burr!
|g1||6/23/2017 11:56 AM|
|As far as the heater wires, you can't tell what is primary or secondary just from the outward appearance. Some transformers may have all wires coming out of the same area, both primary and secondary. Normally you would look at the wiring diagram to figure out what colour wires are primary or secondary. The physical location does not really mean anything.|
|pdf64||6/23/2017 12:12 PM|
Your increased value of the screen grid current limiting resistors will give them an even easier life.
Yes there will be a bit more current in the OT primary with UL but I think it is inconsequential and that retention of UL will give best reliability / tube life, all else being equal.
Yes, dropping UL and properly changing to regular pentode/tetrode mode would require an additional HT node with 2 more stacked filter caps.
|catalin gramada||6/23/2017 12:31 PM|
|catalin gramada||6/23/2017 11:38 PM|
|Still blowing fuses ?|
|cluster||6/24/2017 5:41 PM|
I've read that the original Dagnalls were crap (Partridges were very expensive). That the insulating material that was used was really thin and cheap. Because of this, arcing would occur between windings and the OT would smoke. Later OTs were better made and didn't blow.
|catalin gramada||6/24/2017 7:44 PM|
|I know there are 3 models of Majors which comes after original "pig". 1966,1967 and 1978 maybe something fit with you amp|
From you pics I can imagine the PS looks more like this:
Horrible drawing have any sense for you ?
late -No need to redraw the schematic. I checked for you. Except 100k which not looks factory instaled NFB resistor instead 47k and extra screens resistors everything in you amp shows original 1967 schematic values probably made in "68-69.What seems to be suspect for you ?
Changing electrolythics is a good movement. Take your time and buy the best you can afford.
...and well...yes, there are few little modern design improvements you can do it against 20th century standard industry but didn.t worth it, are not completely justified... just if you amp shows signs of instability or is excessive noisy
|cluster||6/25/2017 4:42 AM|
So far that is what i've found.
Also, I've read that in some modes the bias secondaries has a centre tap. Not in mine.
The red plastic cover on my indicator light has broken off, so I'll need to get a new one for that.
Question: would it be worth just replacing the 2 x 375uf cans (i can get these) and the 50+50uF can (preamp) but leaving the massive 200uF cans? No-one sells those massive cans and I have no idea as to how to safely install a set of 100+100uF cans there instead. Any ideas??
|catalin gramada||6/25/2017 5:20 AM|
|what diameter have those fixing clamp? Take a look at F&T . There are large cans at 40-45mm dia. not necessary double or 400uF as they are in amp. Search for 105*C series 680 or 820uF, 450-500V Are pretty large. It.s not a big deal to put a little bigger value. Usually electrolythics have extralarge tolerances. Two 680 or 820uF series meant 300-400uF average. Good quality caps, really German made stuff|
680uF/450V have 40mm = 1.575"
820uF/450V 45mm = 1.77"
|cluster||6/25/2017 2:01 PM|
So I pulled out some old caps from another Major I used to own. Surprise, the big caps are actually 200uF + 150uF. I able to read the values on the ones in my amp but now that I have these, I can clearly read the values.
Would these work?? -> Found these JJs at the tube store 390uF @ 450VDC www.thetubestore.com - JJ Can Capacitor 390uF / 500V
|catalin gramada||6/25/2017 2:25 PM|
|JJ sound very nice in my replacements and projects. I cannot tell you nothing about their reliability, but this can be a concern when are exposed to excesive heat and it is not you case.(btw are rated at 85*C). But are so cheap and sound so good it is hard to beat, so I can consider a good option. Change all elcap. in you amp , included bias and bypass cathodes. Be aware I found standard 35mm dia. can is a little bit large, I think 36-37 mm. It fit in standard 35 mm clamp but need to be mounted it in clamp first and then plant to chassis. Clean very well from oxide the ground lugs and chassis ,You don.t need to have residual contact resistance here.|
|catalin gramada||6/25/2017 4:25 PM|
|Cause you work at you power supply, I suggest to do a simple useful change. Broke the wire between minus rectifier pole and bias ground reference. Let the bias ground reference to run to chassis through those black wire. Run a separately wire (should be a relative thick one) from minus pole rectifier straight to minus pole first series batery cap, directly on the minus cap lug not chassis lug . like blue line in my sketch. That.s a heavy dirty noisy supply return, is better to keep it separately and not run ,contaminate ,the chassis|
|cluster||6/25/2017 4:59 PM|
|catalin gramada||6/25/2017 5:25 PM|
And the bias goes as it is to original ground conection ( you have a black wire there) . Just disconect the bridge and run a separate wire to the cap. You will keep noise supply return out of chassis. That is very important from noise consideration cause you run through chassis signal returns. It is in whole beneffit to give no chance to not contaminate it an keep as clean as posible, Those return is very noisy return containing AC ripple and have a lot of energy, Run a AWG 18 there if you can and try to keep it as short
|cluster||6/25/2017 5:37 PM|
Thanks for that. I noticed that the amp had a noticeable hum at idle today. this cure some of the hum.
|catalin gramada||6/25/2017 5:47 PM|
|No worries. clean the ground lugs when you screw in back and instal properly|
We talking about 100 hz ripple noise, or 120 ...sorry
|catalin gramada||6/25/2017 8:22 PM|
|What are those two wires tied together at first ground lug? It is a green one, suppose it is CT from bias winding and a yelow one-suppose to be a shield or what ? can you enlight me ,please ? Which one is bias center tap ? and what a heck is the other ? Cluster the heater wires, including heater CT are on the other side you said ?... |
|catalin gramada||6/26/2017 4:40 AM|
|Cluster From you pics looks like bias secondary is also made from two series windings. Green and yellow are tied together to form a center tap. Am I right ? (I.m not sure hundred percent so need to check it)|
If is so,in this case you can lift green and yellow from ground lug, Keep it tied together and run a wire from that connection to the turret where bias circuit is referenced to the ground( black wire connection on the small board). You can run this wire in chassis bundled with red bias wires. Keep black wire bias reference very safe , just lift yelow/green and run it to that spot on bias board. You will separate in this way bias supply by signal path in bias circuit
|catalin gramada||6/26/2017 9:40 AM|
|Trying to do more understandable|
|cluster||6/28/2017 4:21 AM|
|catalin gramada||6/28/2017 4:36 AM|
|I know what Larry did and he is right. There are a lot of schemes functionaly talking but be aware no schemes worth nothing if you don't understand what you did. And that means one thing, one principle : keep as much izolate signal path by supply path. Separate as much low currents path by large currents path. Stick on this principle and can develop any schemes you want in respect of complexity of your amp.|
Of course in many situations inevitable mixing but good practice mean to have care to minimise the effects as much as possible
|Tom Phillips||6/28/2017 10:37 AM|
|Caution for readers of the ground scheme illustrated in post #114 claimed to be "the Larry ground scheme." Note 6 of the posted illustration, Power Cord Ground, contains a major mistake. It indicates the black hot lead is the power cord ground. It should be the green wire which is correctly shown in the photo bolted to the chassis. However, the overlay note "6" is in the wrong location. These kind of typos are a problem for novices who build from layouts because they haven't learned to read schematics and don't yet understand the circuit principles.|
|DrGonz78||6/28/2017 1:27 PM|
|Justin Thomas||6/28/2017 2:31 PM|
|My question is, I can't see where that black wire actually IS going... But it sure doesn't look like it's going to the center lug of the fuse, does it? Which is where it SHOULD be going FIRST, right? Do I see the Neutral fused but not the Hot? |
Just asking for my own security... if I've been doing it bass-ackwards all these years, I have a few amps to fix!
|catalin gramada||6/28/2017 3:13 PM|
|Guys take it easy. This pics is a drawing made from a user who dīnd.t understand nothing by basic circuits at the moment trying to understand what Larry Grohmann aka Novosibir on metro forum trying to explain. It is a whole valuable topic in Metro about with very valuable infos.and Thanks to Larry Grohmann for that. But Larry never did a drawing. There are hundred of almost identical pics on internet made from different novice users wich didn.t understood what they dīd. If you are really interested by subiect go to Metro and folow the related topics under Larry - novosibir- directions|
For this reason I mentioned above- no schemes worth nothing if you don.t understand what you did
|DrGonz78||6/28/2017 3:47 PM|
Edit: Also make note that the two prong cable I mentioned had marker on it to show polarity when plugging it in. If you looked in the amp they used white wire everywhere you would normally expect to see black wire too. Of course this was the 70's...
|Justin Thomas||6/28/2017 3:54 PM|
|I don't think anyone is beating up on Larry or his ground scheme. I've actually used something like it that I came up with out of practical necessity. But to anyone who is following along HERE, we should police that... |
Generally, it's not a big deal around here when someone makes a wiring error. But when it comes to Mains, it's a little different. It's the capability for a wall outlet to deliver a practically unlimited amount of current until something burns down, possibly your house, possibly yourself.
There's no way we can police the entire internet for errors, but we CAN police what goes up here. I'm also open to the possibility that the photographed amp is from a different country. But given the Black/White/Green, and that the Neutral seems to be fused instead of hot, then it's definitely a possible issue that goes beyond melted tubes or transformers.
There <ARE> a lot of people who are cloning and building based on layouts. That doesn't mean that we shouldn't warn future readers about it. In some ways, I do think we should even feel a certain social responsibility to do so, and explain why. But nobody is beating up anybody, simply calling for anyone following along to pay attention. A few years back there were pics of "boutique" amp posted - the power transformer had the HT terminals exposed, right beside the power tubes. Reach back there while that amp is in operation, ZAP. While no-one WANTED to beat up on that builder, that is a fairly egregious error for a commercial product... I think this is in the same class...
No disrespect to anyone involved here is meant by me.
|Tom Phillips||6/28/2017 3:59 PM|
|catalin gramada||6/28/2017 4:00 PM|
|You right Justin but posting here a drawing with massive error claimed as Larry grounding scheme is a little bit to much.for me at least.This guy is a precise german mind and did for community a lot. cannot assign him this kind of errors such as cause he didn.t. Don.t you think ?|
|cluster||6/28/2017 4:08 PM|
sorry... is this better??
|catalin gramada||6/28/2017 4:13 PM|
It looks allright to me...but have to do a mention.still. It is a long red wire going through lengh of chassis. I think it mean the heater center tap. I said I think cause I remember he said he found the quiest spot for this CT is the first at least noisy spot. And he is right. I tried it. But from my practice I found it is not absolutely necessary to run there. You can found a quiet spot allover the chasis. Just be sure you don.t choose a noisy one like tranny bolts or power stage grounding lugs. I always walk a wire inside the chassis to find a quiet spot for a shortest path
|Justin Thomas||6/28/2017 6:04 PM|
|I don't knock Larry's ground scheme at all. And Larry did not actually build the amp used in the picture, correct? But whoever built that particular amp did it wrong, at least as far as USA laws are concerned. I think what we're trying to get across to any new users here is, "this is largely correct or safe to use for an overall ground scheme, but please do not just blindly copy THIS PARTICULAR amp as far as mains wiring goes." Larry is certainly not responsible for anyone who uses his grounding but makes mistakes... |
then again, in the USA, there will always be SOMEone who will look for a way to make a few million in easy lawsuits... so by hammering this point home, we may actually be SAVING Larry from a LOT of hassle. Bottom line: no disrespect toward Larry intended. But, that amp's mains is wired wrong and should not be copied.
|catalin gramada||6/28/2017 6:16 PM|
|Hundred percent agree|
|cluster||6/28/2017 7:55 PM|
|does anyone have a proper Larry Ground layout on hand they could post??|
|catalin gramada||6/29/2017 3:29 AM|
|Wich version? You have to follow the directions for US users if you live in States as I understand due to the main safety conditions. I live in Europe and here is equal as time we can turn the main plug in wall socket means live and neutral wire revert|
|cluster||6/29/2017 5:00 AM|
I live in Canada, so it would be a USA version of the scheme.
|Justin Thomas||6/29/2017 12:16 PM|
|The actual layout would be different, because your Major is different. Basically, the Larry Ground system seems to be what is called a "Galactic" one - some grounds are brought together and met at one point, a few others at another point... so using the photograph of the amp posted, just bring the noted and listed points together and ground them somewhere logical for the chassis design of your major. |
The "problem" with the amp pictured is the mans wiring and the fact that the Earth Safety Ground is mislabeled. But the actual grounding scheme is correct for the Larry Ground scheme. The most important thing is that your green wire from your mains/power cable is the ONLY thing grounded at that point, and has to be bonded to the chassis. I will use a bolt with a keps nut, and then solder the whole thing straight to the chassis (if it's steel). Also a good idea to leave that green wire a bit longer than the other two, so that if the amp falls and the cord is ripped out or damaged, that green wire is the last one to break/tear free.
As Merlin points out, it's the single most important connection in your amp... otherwise, just go down the notes in the pictured amp and do the local "star grounds."
Edit: the other option would be if someone could mark up a schematic of the Major and show which ground point to with which... and a full-size internal chassis shot of EVERYthing may help for finding a good point. Like somebody did with the Spitfire a few years back...
|catalin gramada||6/30/2017 2:17 AM|
|Don.t run high pulse ripple through chassis. Do the mods wiring as I allready showed you and see if you.re happy with that. There are just two wires to run|
This is compulsory for any good practice to do from my point and have nothing in common with International safety reglementations
Use black wires to be in accord with wiring code if you want...
|cluster||6/30/2017 7:40 AM|
|catalin gramada||6/30/2017 7:47 AM|
|The red wire I sugest to run is not bias ground but center tap from bias secondary winding .Do you see a black wire on small turret board ? those wire is bias reference to the ground. Just check and ensure is firmly tight to chassis|
First check if secondary bias is made from two sections and green wire and yelow are related as center tap. I think so but not sure hundred percent. Please check first. One of this wire (green or yelow) is bias center tap for sure. I.m not sure for the other. but my thought is your bias winding is made from two sections as HV secondary is.
|catalin gramada||6/30/2017 8:23 AM|
|You bias looks like that at the moment (it should - please check green and yellow wires). charging current goes through chassis. |
Not red and yellow but green and yellow. But check first . It is possible to be a split winding and both wires to form a center tap as I showed you in drawing , but also is possible one wire to be bias center tap and the other to be related to another winding (eg. heater CT). In this case you will lift just the wire represent bias center tap and solder this wire to the red one I showed you.
You need to run just the bias center tap directly to the bias cap instead the chassis ground lug. But if you bias center tap is made by those two wires (green and yellow) you have to connect all of them
|dstrat||6/30/2017 10:27 AM|
|according to the older 1968 schematic the yellow is bias CT and green is heater CT.|
but its always best to verify with meter IMO.
|catalin gramada||6/30/2017 10:46 AM|
There are allready some errors like blue-yelow-blue for bias winding. There are pink as I saw
The green wire is indicated as be heater CT or electrostatic shield. Let it in place