cluster 6/18/2017 3:34 PM
Marshall Major Blowing Power Fuse
Hello everyone,

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.

joseph
 
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).

[ATTACH=CONFIG]43820[/ATTACH]
[ATTACH=CONFIG]43821[/ATTACH]
[ATTACH=CONFIG]43822[/ATTACH]
 
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.

Justin

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
Quote Originally Posted by Justin Thomas View Post
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.
This is true for an 'open' cap, but not for one that is breaking down and shorting causing the fuse to blow. You will need to disconnect the suspect cap.
 
Justin Thomas 6/18/2017 6:42 PM
Thanks, nobody ever mentioned that nugget before...

Justin
 
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
[ATTACH=CONFIG]43823[/ATTACH]
[ATTACH=CONFIG]43824[/ATTACH]
 
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
Quote Originally Posted by cluster View Post
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.
The fundamental frequency of rectified 60 cycle power IS 120 Hz (100 in 50 Hz power areas) so not at all unusual to be hearing it. And if there's a short as I described, thru a screen grid resistor, that's what you would be hearing. Could be another fault too. We'll pick up the search after some shuteye.
 
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
Quote Originally Posted by cluster View Post
something else
It's looking bad for the OT but still worth looking for a short to ground thru a tube socket, OR thru one of those screen grid hockey puck resistors. The resistors are held to chassis with a nut & bolt, just loosen them, pull up the resistor, remeasure your primary-to-ground. If no joy there, remove the primary leads to pins 3 of output tube sockets & re measure. If after all that you still show resistance to ground, the OT has an internal short to ground,
 
cluster 6/19/2017 6:24 AM
Quote Originally Posted by Leo_Gnardo View Post
It's looking bad for the OT but still worth looking for a short to ground thru a tube socket, OR thru one of those screen grid hockey puck resistors. The resistors are held to chassis with a nut & bolt, just loosen them, pull up the resistor, remeasure your primary-to-ground. If no joy there, remove the primary leads to pins 3 of output tube sockets & re measure. If after all that you still show resistance to ground, the OT has an internal short to ground,
Thanks for that tip. Just to confirm what you said: lift the hockey pucks off the chassis and measure resistance to ground from each pin 3 ??? what should I be looking for as far as resistance? If I read 0 from pin 3 to ground, then there is a short in the OT?? Is that what you mean? thanks
 
Leo_Gnardo 6/19/2017 6:42 AM
Quote Originally Posted by cluster View Post
Thanks for that tip. Just to confirm what you said: lift the hockey pucks off the chassis and measure resistance to ground from each pin 3 ??? what should I be looking for as far as resistance? If I read 0 from pin 3 to ground, then there is a short in the OT?? Is that what you mean? thanks
There are taps on your OT primary, each goes to a pair of those hockey puck resistors. So - if a short developed thru that resistor to chassis, you would see the resistance to ground readings you are getting. It's an easy exercise to unbolt the suspect resistors & pop them off the chassis. If your short or near-short reading changes to open then you know where the problem is.

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
Quote Originally Posted by Leo_Gnardo View Post
There are taps on your OT primary, each goes to a pair of those hockey puck resistors. So - if a short developed thru that resistor to chassis, you would see the resistance to ground readings you are getting. It's an easy exercise to unbolt the suspect resistors & pop them off the chassis. If your short or near-short reading changes to open then you know where the problem is.

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.
I see what you mean. Looking at the schematic it seems that the OT primary also feeds the screens via those hockey puck resistors. I'll preform this and report back. So once I lift these hockey pucks off the chassis and read the resistance to ground at pin 3 and see "OPEN" then that means that there is a short to ground at one of these pucks? Is that correct? Thanks again for your generous help :-)
 
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
Quote Originally Posted by cluster View Post
So once I lift these hockey pucks off the chassis and read the resistance to ground at pin 3 and see "OPEN" then that means that there is a short to ground at one of these pucks? Is that correct? Thanks again for your generous help :-)
Yes. Let's hope that's the case, OR possibly the short is at one of the tube sockets.

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
Quote Originally Posted by cluster View Post
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?
There are two places that would probalby do it cheaper or same price as banzai. Magnetic Components (chicago) and Heyboer (Michigain). They will make whatever you want. Heyboer has no product line and only does custom transformers. Magnetic Components product line is Classic Tone but they will also make whatever you want.

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?

[ATTACH=CONFIG]43834[/ATTACH]
[ATTACH=CONFIG]43835[/ATTACH]
 
cluster 6/19/2017 2:57 PM
Quote Originally Posted by nsubulysses View Post
There are two places that would probalby do it cheaper or same price as banzai. Magnetic Components (chicago) and Heyboer (Michigain). They will make whatever you want. Heyboer has no product line and only does custom transformers. Magnetic Components product line is Classic Tone but they will also make whatever you want.

Mercury is a ripoff IMO and banzai is far away so no need to have it shipped overseas.
Sent classic tone an email but no luck. They don't make those. Will check with Heyboer. MM is a the biggest ripoff around but for resale the sheep would probably pay more for it than a no-name. I hope Heyboer can come through. Maybe, if i'm lucky, like Enzo said, it might be a simple fix. Lets see...
 
cluster 6/19/2017 2:58 PM
Quote Originally Posted by Enzo View Post
take notes take notes take notes... never rely on memory or "I'll figure it out" to get wires back where they belong.
Thanks Enzo. Yes, I am using my iPhone to photo document the work and I'm labeling each lead with different colour tape and writing down what it is.
 
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,

cjenrick@gmail.com
 
Leo_Gnardo 6/19/2017 3:34 PM
Quote Originally Posted by cluster View Post
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??
I'm guessing they bring the winding wires out from each coil, instead of solder splicing flexy leads on and keeping those connections inside the transformer shell. Real old fashioned build style there, with hi voltage leads covered with cambric cloth tubing.

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...

Justin
 
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
Quote Originally Posted by cjenrick View Post
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?
Now there is a generous offer that will be tough to beat!
 
The Dude 6/19/2017 7:44 PM
Quote Originally Posted by g1 View Post
Now there is a generous offer that will be tough to beat!
AMEN!
 
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
Quote Originally Posted by cjenrick View Post
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,

cjenrick@gmail.com
wow!! thanks so much. I would love to take you up on your offer. I live in Toronto, Canada but would be willing to ship to the US for a rewind. First I need to pull it off the chassis and do that test you mentioned. Will keep u posted. Thanks again.
 
cluster 6/19/2017 8:13 PM
Quote Originally Posted by Justin Thomas View Post
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...

Justin

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?

Thx

Joseph
 
Leo_Gnardo 6/20/2017 6:44 AM
Quote Originally Posted by cluster View Post
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? Thx

Joseph
Could be that your short is to one of the bellcaps. If you can find it and insulate the transformer wire adequately, you could save yourself a couple hundred $$$. So REMOVE the bellcaps and have a look around. Heatshrink tubing and GE RTV cement make dandy insulators. Good luck!
 
cluster 6/20/2017 6:53 AM
Quote Originally Posted by Leo_Gnardo View Post
Could be that your short is to one of the bellcaps. If you can find it and insulate the transformer wire adequately, you could save yourself a couple hundred $$$. So REMOVE the bellcaps and have a look around. Heatshrink tubing and GE RTV cement make dandy insulators. Good luck!

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?

Thx again,

Joseph
 
cluster 6/20/2017 7:11 AM
[ATTACH=CONFIG]43836[/ATTACH]

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.

Joseph
 
cluster 6/20/2017 7:22 AM
[ATTACH=CONFIG]43837[/ATTACH]

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

Joseph
 
cluster 6/20/2017 7:37 AM
[ATTACH=CONFIG]43838[/ATTACH]

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
Quote Originally Posted by cluster View Post
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?
Resistance of secondary (speaker) windings, a fraction of an ohm normally so don't worry if you read nearly short circuit.

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:

http://www.mouser.com/3M/Wire-Cable/...z0zp4dZ1z0vrwt

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
Quote Originally Posted by Leo_Gnardo View Post
Resistance of secondary (speaker) windings, a fraction of an ohm normally so don't worry if you read nearly short circuit.

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:

http://www.mouser.com/3M/Wire-Cable/...z0zp4dZ1z0vrwt

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.

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
Quote Originally Posted by cluster View Post
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
First get those wires spliced. Easiest way is scrape the lacquer off, twist the ends & solder. You could wrap that with electrical tape or use a wire nut, but my way would be to cover that splice with heat shrink, heat & shrink that first layer. Then add a second & maybe a third layer. It's probably overkill at this point but I'd coat the splice with RTV, let it harden for a day, then reassemble. Here comes some smart guys to tell me that's going too far . . . just wait they'll show up any time now. So the RTV is optional. Use the remainder to seal any leaks in your bath or shower or roof or gutters, that's what it's really good for.
 
cluster 6/20/2017 10:22 AM
Quote Originally Posted by Leo_Gnardo View Post
First get those wires spliced. Easiest way is scrape the lacquer off, twist the ends & solder. You could wrap that with electrical tape or use a wire nut, but my way would be to cover that splice with heat shrink, heat & shrink that first layer. Then add a second & maybe a third layer. It's probably overkill at this point but I'd coat the splice with RTV, let it harden for a day, then reassemble. Here comes some smart guys to tell me that's going too far . . . just wait they'll show up any time now. So the RTV is optional. Use the remainder to seal any leaks in your bath or shower or roof or gutters, that's what it's really good for.
[ATTACH=CONFIG]43840[/ATTACH]

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
Quote Originally Posted by cluster View Post
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. . . . .
I'm a fan of Ken Fisher & I used to add this to practically every amp I worked on. Now, only if I find there's a recurring problem that may benefit from this technique. In an amp with extra hi voltage like yours I'd series 4 x 1N4007 instead of just 3. On rare occasions I've seen the rectifier string short then you have to hope a fuse blows before the skinny wire in an OT primary burns or the power transformer fails. Not a good thing when your "safety circuit" causes unexpected wreckage.
 
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:

Primary winding:

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?

Thx,

Joseph
 
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
Quote Originally Posted by cluster View Post
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?
I haven't done it on a Marshall Major but in several repair/restorations I have been able to either clean out and stuff the old can with smaller, modern caps or mount new ones under the can with the can cap disconnected. As far as old caps are concerned... there's a lot of debate. The consensus would say replace anything to do with your bias. Filter caps? I have a 1963 Temolux that is bone stock except the output tubes. Works great even with modern line voltage. No hum at all and has a strong tremelo channel. I haven't even changed the power cord and death cap. BUT.. it's not a Marshall Major.
 
Leo_Gnardo 6/20/2017 5:13 PM
Quote Originally Posted by cluster View Post
Also, I'm considering replacing the can caps in this major but the 200uF cancaps are oversized. - - - - - Is it worth replacing these caps?
The one I worked on had replacement main filter caps, same diameter as the old ones. Replaced I guess 10-15 years ago. I'm not sure what the diameter was. FWIW JJ has some caps in 1 3/4 inch diameter. Other than that you can pore thru online catalogs, see what the big supply houses like Mouser and DigiKey have to offer. Downside - prices bound to be high. Good side - they have warehouses in Canada, should be able to ship without getting hit with customs & cross-border shipping charges.
 
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

Joseph
 
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
Quote Originally Posted by The Dude View Post
If you're going to test it without tubes installed, you don't need a load at all.
Thanks. Yes, the no tubes test is just to see if no fuses blow and to measure voltages at pin 3 (plates), the screens and bias. Hopefully if all goes well i'll put in the tubes and bring the amp up slowly with a variac.
 
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,

cj
 
catalin gramada 6/20/2017 7:19 PM
...
 
catalin gramada 6/20/2017 7:21 PM
Quote Originally Posted by cluster View Post
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?
Put some paper arround. And be aware in this type of series connection you have the first can cap shell at half potential. It is compulsory to have extra care for isolation between fixing clamp and capacitor shell. Don.t do it and have a permanent risk to shock you or put into a short. Isolate the top of the capacitor as well if metalic shell is exposed
If you clamps are extra large and have room enough you can use polyprop. motor RUN capacitors against electrolythics instead
Nice amp
 
cluster 6/21/2017 3:57 AM
[ATTACH=CONFIG]43856[/ATTACH]
[ATTACH=CONFIG]43857[/ATTACH]
[ATTACH=CONFIG]43858[/ATTACH]

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
[ATTACH=CONFIG]43863[/ATTACH]

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.

dave
 
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
Quote Originally Posted by Leo_Gnardo View Post
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.
That.s a good advice. Have to add: First, without any voltages turn it very,very gently from a end to another. Do it twice, that will broken the oxide film due to wearing. Than put it into the most negative position (eg. if works between -50 to -80v put it into -80v position) That will ensure power tubes will draw minimal current at first. Than you can start normaly, let it warm for a while and adust the bias to proper value. Recheck over half an hour or so for final adjustment
 
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.

Good luck!
 
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 Originally Posted by cluster View Post
Here is a close crop of the bias circuit. Where should I move the pot?
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 ( http://music-electronics-forum.com/t...36/#post457839 ), 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 Originally Posted by cjenrick View Post
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!

Tubes JJ KT88's. Here is the data sheet for them: http://www.thetubestore.com/lib/thet...re/JJ-KT88.pdf

Question; How does dropping the screen voltage effect tone and feel of the amp??
 
cluster 6/22/2017 4:34 AM
Quote Originally Posted by catalin gramada View Post
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
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 Originally Posted by Leo_Gnardo View Post
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 ( http://music-electronics-forum.com/t...36/#post457839 ), you managed to dodge the bullet with this one, finding & repairing the fault in the OT. Good work indeed!
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 Originally Posted by cjenrick View Post
maybe drop the screen voltage with some additional resistance
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 Originally Posted by 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.
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
[ATTACH=CONFIG]43875[/ATTACH]

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
Quote Originally Posted by cluster View Post
[ATTACH=CONFIG]43875[/ATTACH]

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??
No, no fusing into bias supply. This circuit should be as robust as possible. Take in account bias load pot/resistors are directly in series with grid leaks and is better to keep this impedance path as low as possible. You don.t want to break this connection in any circumstances. The pot/trim should be as hefty in respect with power dissipated by itself