mtlbasslad 2/7/2018 11:09 PM
Marshall 6100 30th Anniversary trouble
Hello all, I'm back with the toughest problem I've had so far...
Friend who thinks i know what I'm doing brought me his Marshall 6100 - blowing fuses (I think there are 7) and making explosion/smoke...
This is no simple amp - 3 channels MIDI switching etc - there are 5 separate boards under the hood

Rickey told me he saw a black spot near where the smoke came from (he now knows how to check fuses) on the power supply board - Ha! methinks this will be easy, then I opened it up - arrgh there are eleventy-seven chips & ribbon cables everywhere. I feel like water rising to neck level

OK so R401 is fried, but wait - R401 seems to be 3 1/4 watt resistors ganged together in parallel - value obliterated by explosion/smoke - schematic says "10R 1/2W" ... I see the Brits have a slightly different symbol/language - is this 10 ohms?

I stare at the board(s) for some time accompanied by my best friend Stella Artois, looking for the 3 bridge rectifiers indicated on schematic... I see nothing that looks like a big black square... try a shot of Polar Ice (vodka) to see if that improves my vision. WTF? there are 3 little things marked DB1 DB2 DB401 that look like kernels of corn...??? these little things can handle the current for a 100 watt tube amp? Colour me confused...

I can see that someone has been in here before me, there are French sharpie (felt pen) markings here and here - 'NOIR, JAUNE' etc

Also, I see an additional 5.6K resistor on pin 5 of the output tube closest to the PT - OK I see them on the schematic but not on the board except this one ??

With my new Fluke 115 meter (congratulate me for spending $250) I measured those big square (very technical term) 5 watt 470 ohm (X4) & 1 4 watt 2.2K resistors surrounding the power tubes just because Enzo or another of you wizards would ask me about that (I think)

So friend Rickey has been dispatched to the nearest parts store an hour away (we got 15cm/6 inches of snow today) & the fun will continue tomorrow...

He has a show soon & really does not want to use his backup amp - a Line6

I am way over my head here, help!

P.S. Checked power tubes (Ruby 6L6) on my old Knight tester, they seem OK

Waiting on your wisdom, remember I'm an amateur with a good meter

Lorne & Joanie
 
Enzo 2/7/2018 11:38 PM
A schematic you can post or link?

10R instead of 10 ohms is not Brit, it is world wide. Not everyone uses it, but it gets more and more popular.

10R - 10 ohms
4R7 - 4.7 ohms
3K9 - 3900 ohms

No decimal point to miss.

The little 3/8" rectifiers are rated at an amp, just like a 1N4007.
 
mtlbasslad 2/8/2018 12:15 AM
Hi Enzo, thanks for the explanation - so 10R is 10 ohms

Not clever enough to post a link, but the schematic I'm looking at is from Dr Tube...
 
mtlbasslad 2/8/2018 12:25 AM
update - drtube.com/library/schematics
 
Jazz P Bass 2/8/2018 10:19 AM
Here is the power in schematic.

[ATTACH]47030[/ATTACH]

R401 is on the CT of the secondary 6 volt transformer.
 
g1 2/8/2018 1:24 PM
You said 6L6 so here's the 5881 version of the schematic. Power amp schematic is version 10 so there may be differences to what you have.
Which fuses are blowing?
 
mtlbasslad 2/8/2018 4:08 PM
Thanks Jazz P, I had the schematic, just not savvy enough to link to it...
 
mtlbasslad 2/8/2018 4:15 PM
Thanks G1, that's the same schematic I've been working from. As far as I know the 5881 is just a ruggedized 6l6. This amp was 5881 from the factory.
All the boards have different numbers - PS says 6100-65-00. Fuse was the 5 amp
 
The Dude 2/8/2018 4:34 PM
Do you have a light bulb limiter or other current limiting device?
 
mtlbasslad 2/8/2018 4:38 PM
Todays fun
So Rickey drove 2 hrs to pick up some 10 ohm 1/2 watt resistors (R401) to replace whatever was there before. The old 3 stacked 1/4 watt arrangement with 1 blackend measured 150 ohms - kinda far from the 10 spec.

Solderded in the new resistor & powered up with the light bulb limiter I built last year on the advice of you wise people
Everything OK on standby, bulb went bright for a few secs as caps charged up, then went dim = good
Flipped off standby, one of the 6L6's that tested weak but ok put on a light show !!
Replaced said 6L6 with a spare, repeated the process - no smoke, no light show, YAY
Yes, we had a speaker connected, but no git in the shop, big hurry so 'bye & thanks..

Back at home with a git & the proper cab connected... no smoke, everything lights up, but no sound except pop & crunch switch noises...
I'm thinking the famous grungy jack thing, but who knows...

Updates to come...
 
Justin Thomas 2/8/2018 4:42 PM
FYI: Don't pay attention to the vintage 5881 specs for the "5881s" in this amp. The 5881WXTs that came in this amp are comparable to a 6L6GC or even tougher, and nobody's entirely sure just how much abuse they can really take. That said, any "reissue" "Tung-Sol" 5881s won't put up with the same punishment as the ones that came in your amp will. Just be aware. I know, it's confusing how they reuse old tube designations, but it's just a fact of life in our world...

Justin
 
The Dude 2/8/2018 4:50 PM
If you had a shorted/arcing power tube, it very likely also took out HT fuse(s), which would account for no sound. Check that it's not open.
 
mtlbasslad 2/12/2018 3:12 PM
So, just before bringing the amp back to me Rickey decided on one more try - and all is golden!
I suspect the dreaded 'switching jack' (as in the sticky) but who knows...
I recommended a can of contact cleaner as his next purchase - but he's a guitarist, so faint hope for that...

Thanks to all who responded, no doubt I'll be back with more problems soon as my undeserved reputation locally as a 'fix it' guy grows LOL
 
mtlbasslad 2/12/2018 7:29 PM
Well, back sooner than I thought...

The amp worked great for a couple of hours - then sound faded away - 2 power tubes were redplating, so shut down.
The amp is 20 miles away, & I'm talking on the phone to a guitarist, not tech-minded...
Seems he has 2 6l6's & 2 5881's in there, never a good idea...

I suggested he get a new set of 5881's and we will go from there.

Here's my problem - no voltages on the schematic, so what am I looking for? Somewhere in the 400 - 500V range, but it would be nice to know something a little closer, eh?
My understanding of redplating is wrong biasing, but there is only one bias adjustment for all 4 tubes... so perhaps a resistor somewhere around those tubes?

Over my head again!

Waiting for wisdom,
Thanks, Lorne
 
The Dude 2/12/2018 7:40 PM
I'd start by pulling the power tubes and measuring bias voltage on all sockets. Monitor it for a while and make sure it stays put, particularly the red plating tubes. There could be a number of things wrong- leaky coupling cap from PI, connection problem, bad tube(s), etc. Let us know what you measure for bias voltage on the sockets as a step one.
 
mtlbasslad 2/12/2018 8:31 PM
OK, so that I'm clear - PI is the phase inverter tube?
Bias voltage is on pin ?
Schematic lists -50V but notes -40something 'for USA models' ? Bias changes crossing the Atlantic?

Thanks Dude, putting a fresh battery in my Fluke LOL
 
The Dude 2/12/2018 8:43 PM
Yep. PI is the phase inverter. There are 2 coupling caps from the PI to the output tube grids (C219 & C220). One of them could be leaky, but lets check first before jumping to conclusions.
Bias is on pin 5 of the output tube sockets and it should be in the neighborhood of -50V. You'll be looking for something way out of wack, so anywhere close to -50 is ok.
 
mtlbasslad 2/12/2018 8:53 PM
Thanks, now to get the amp on my bench tomorrow...
 
mtlbasslad 2/13/2018 1:01 PM
OK, amp is back on my bench...

Bias voltage is -50V +/- about .75V after a half hour on all tubes. On the schematic I have there is a further note "For USA only set bias to -42V no signal" ??
C219 & C220 are mounted so close to the board I cant get my smallest clip-on leads in - I'd have to remove about eleventy-seven jumpers & remove the entire board... ugh
Checked R235 & R235 OK at 150 ohms
R232,233,237,238 OK at 5.6K (or 5K6 in newspeak)
R251,252,253,254 OK at 470 ohms (470R)
R236 OK at 2.2K (2K2)

Only V8 & V9 redplated yesterday - Sovtek 5881"s the Ruby 6L6's (V10 & V11) did not.
Only obvious difference is the tubes, but I fail to see... anything.
Waiting for a new set of JJ 5881's, although all old ones 'test' OK the boat anchor Knight 600 (sometimes called 'footrest' by some techs LOL) HI Enzo

Comments & suggestions are welcomed - encouraged even...

Thanks, Lorne

P.S. I've loaned Rickey my Mesa Boogie DC-3 in the meantime, let's hope I never have to go into that one...
 
The Dude 2/13/2018 1:11 PM
Try holding the probe on pin 5 socket of V8 or 9 and give the amp a couple good whacks on top while monitoring voltage to try ruling out an intermittent/connection problem.
 
mtlbasslad 2/13/2018 1:16 PM
OK, opening beer now - proceeding to shed to engage rubber hammer with chassis. Good thing owner is not here LOL
 
The Dude 2/13/2018 1:19 PM
That'll work, but I usually just ball up my fist and give 'em a few bumps. Of course, don't get too crazy. We're not trying to break it!
 
Enzo 2/13/2018 3:49 PM
I think the 50/42 bias thing is the 50 is for 5881/6L6/6550 and the 42 is for EL34. MAybe?
 
mtlbasslad 2/13/2018 10:29 PM
Quote Originally Posted by The Dude View Post
That'll work, but I usually just ball up my fist and give 'em a few bumps. Of course, don't get too crazy. We're not trying to break it!
Yes but - Dude, I have very sensitive 'artiste' fists
 
mtlbasslad 2/13/2018 10:37 PM
Quote Originally Posted by Enzo View Post
I think the 50/42 bias thing is the 50 is for 5881/6L6/6550 and the 42 is for EL34. MAybe?
Hi Enzo, this amp is clearly marked 'Mashall 5881' - I may be mistaken (happens a lot) but I thought EL34's needed a whole different setup from 6L6/5881...

Rumor has it that Jim only switched to 6L6's at a certain time when there was a dearth of good EL34's - I dunno

I noticed while monitoring bias voltage that if i tweaked the trimpot it went from -45V to -62V... hmm
 
mtlbasslad 2/13/2018 11:10 PM
Latest update...

Not wanting to leave out any variables, I made Rickey schlep his 2X12 bottom, lead cord, speaker cord & anything else I could think of to my garage/shop/mancave - but not his Strat, 'cuz I figured I could ask the lovely Joanie (wife) to borrow her MIJ Strat - she said yes, yay

Plugged everything in, all tubes glowing nicely... but no sound! WTF? Changed tubes, cables... opened another beer - my local rep is dropping fast...
Tried Dude's recommendation - pound on it! That produced a loud hum & REDPLATING on at least one tube before I hit the power switch... ah shite

So sent very disappointed guitarist home & pondered... intermittent fault, oh joy. Then decided to use high-tech tool (HB pencil, no chopsticks handy) to poke around - & found a nice crackling sound when I nudged one of the (many) ribbon cables. Pull off many cables, jumpers etc spray with Deoxit & air - power up, wait, WE HAVE SOUND!

Wait 10 mins & pound with rubber hammer again - repeat 5 times - everything still OK - Could this be the solution? We sure hope so, 'cuz I'm almost finished the vodka he brought me...

Tomorrow Guitarist will return to play through the amp, I refuse to let him take it home until it can survive at least an hour...

Stiil wondering about bias - it's set at -50V now - which way makes it hot/cold - is -45 hottter & -60 colder or???

Thanks , Lorne (Joanie says HI to all my forum freinds)
 
Enzo 2/13/2018 11:35 PM
42 is a typical EL34 value, and 50-55 is typical 6L6 value. Some Marshall models were made using both. If it says 5881 on it, then set to the 50, or whatever gets you where you want to be.

In the life of Marshall amps, there have been tube supply issues, there have been reliability issues - like we want heftier tubes for the American market because USA is shipped farther, and there have been marketing issues. That note on the drawing covers more than one situation.
 
pdf64 2/14/2018 2:09 AM
Quote Originally Posted by mtlbasslad View Post
...decided to use high-tech tool (HB pencil, no chopsticks handy) to poke around - & found a nice crackling sound when I nudged one of the (many) ...
A pencil is unsuitable for use inside an amp, as the 'lead' (graphite) is conductive.
In a live amp, using one as a probe could result in electrocution, and even in a isolated, de-energised amp, conductive deposits might be inadvertently made in there.

Note that schematic notes to set bias to a specific voltage only apply to tubes selected by that manufacturer at that time.
Tube characteristics have always varied over a wide range from their bogie type standard.
See http://www.mif.pg.gda.pl/homepages/f...086/k/KT66.pdf 'bias must be variable over a +/- 25% range for each tube'.
Hence it's good practice, even when using tubes that conform to that manufacturer's selection criteria, to verify that the plate / cathode current resulting from the bias voltage is reasonable.

Quote Originally Posted by mtlbasslad View Post
...Stiil wondering about bias - it's set at -50V now - which way makes it hot/cold - is -45 hottter & -60 colder or?...
A bias voltage of -60 is 'colder' than -50, ie it will result in a lower plate current / dissipation at idle.
 
Enzo 2/14/2018 3:43 AM
Maybe we should stop obsessing over the bias voltage. Just set it to whatever it takes for good operation, like any other tube amp.
 
g1 2/14/2018 12:20 PM
Whichever connector that was crackling, double check it's solder connections. If they are fine you may have solved it by cleaning the connection.
 
mtlbasslad 2/15/2018 8:15 AM
Quote Originally Posted by pdf64 View Post
A pencil is unsuitable for use inside an amp, as the 'lead' (graphite) is conductive.
In a live amp, using one as a probe could result in electrocution, and even in a isolated, de-energised amp, conductive deposits might be inadvertently made in there.

Note that schematic notes to set bias to a specific voltage only apply to tubes selected by that manufacturer at that time.
Tube characteristics have always varied over a wide range from their bogie type standard.
See http://www.mif.pg.gda.pl/homepages/f...086/k/KT66.pdf 'bias must be variable over a +/- 25% range for each tube'.
Hence it's good practice, even when using tubes that conform to that manufacturer's selection criteria, to verify that the plate / cathode current resulting from the bias voltage is reasonable.


A bias voltage of -60 is 'colder' than -50, ie it will result in a lower plate current / dissipation at idle.
You are right of course about the pencil, using a dowel now
 
mtlbasslad 2/15/2018 8:17 AM
Quote Originally Posted by g1 View Post
Whichever connector that was crackling, double check it's solder connections. If they are fine you may have solved it by cleaning the connection.
Unable to get to solder connections without removing the board...
 
mtlbasslad 2/15/2018 8:30 AM
I think we got it
With all connections cleaned up, I ran the amp (with old tubes) for 8 hours while doing other shop chores, while applying special diagnostic tool (rubber hammer) every 1/2 hour - no noises, no redplating

Guitarist showed up with new JJ 5881's, so installed them & he played for a hour while I played with bias - he seems to like it at -48V so that's where it will stay.

How that bad connection ribbon cable (from the 'master output' board to 'power amp' board could cause redplating I don't understand, but living with women has taught me that I will never understand everything...

Thanks all for your advice, let's hope the thing holds up...

Lorne