Marshall 6100 30th Anniversary trouble

[mtlbasslad]

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]

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]

Thanks, now to get the amp on my bench tomorrow...

[mtlbasslad]

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]

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]

OK, opening beer now - proceeding to shed to engage rubber hammer with chassis. Good thing owner is not here LOL

[The Dude]

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]

I think the 50/42 bias thing is the 50 is for 5881/6L6/6550 and the 42 is for EL34. MAybe?

[mtlbasslad]

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]

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]

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]

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]

...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.

...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]

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]

Whichever connector that was crackling, double check it's solder connections. If they are fine you may have solved it by cleaning the connection.