Seeking troubleshooting tips on my Marshall JTM30

[cassoulet]

At first glance, it seems that previous owner or someone else has removed the DC filtering part for preamp heaters and chose to power all heaters (preamp and output) with AC right from the PT secondary. That's why the diode bridge and both filtering caps C122 and C123 have been removed. Now, why did they do that, and is it the reason for this failure, I don't know.

BR101 is indeed the DB1 on the schematic. And you can see that it has been replaced with two straps to connect all AC heater lines together.

[maddtapper]

Yeah, this started out as simply a burnt resistor and tube replacement...

Should putting those parts back in cause issues?


I don't mind throwing some parts at it... But at this point almost all that's left is the power transformer...

[Stevem]

If you have at least 6.3 volts AC between paired pins 4 and 5 then pin 9 on each preamp tube, plus 6.3 volts across pins 2 and 7 of the output tubes then I would leave as is for now and just get the thing back up and running!

[cassoulet]

I quickly re-read this topic and, maybe I have missed something, but it seems that you have put back two new caps without removing both straps where the diode bridge used to be, and without installing a new bridge. If so, you are filtering 6.3 VAC without first rectifying it, and that's not good at all. Either you leave it as is, without caps nor diode bridge, or you put two new caps and a new diode bridge in order to restore the DC filtering for preamp tubes.

D108 and D109 are in the ground lift circuit (typical to Marshall), but have no purpose other than protection.

R128 and R108 which you replaced are in the output tube screen circuit and were probably blown when the tube failed.

[maddtapper]

Thank you, cassoulet that's what I was looking for! And that makes perfect sense! I was just worried about the changes done in the past...

I'm going back over all the joints now to make sure there aren't any broken traces/eyelets... I'm sure it's gonna be something small now...

And again, THANK YOU GUYS!!!

[maddtapper]

Got the diode bridge and caps reinstalled, and touched up a few solder joints....
And...
We have lift off!!

Again, THANK YOU!


Maybe next I'll try to tackle the adjustable bias mod... This thing is pulling some juice! 470 plate volts and around 50mA per tube! Gotta get that tamed down!

[xtian]

Maybe next I'll try to tackle the adjustable bias mod... This thing is pulling some juice! 470 plate volts and around 50mA per tube! Gotta get that tamed down!

Wait, how are you checking bias? Is that 50mA for the PAIR of tubes? Because you're right, for ONE tube, that would be near red-plating. For TWO, that would be cold, but working.

[maddtapper]

Sorry, xtian... Just saw your reply... I checked the bias with a weber bias probe... I have since done the bias mod and brought it WAY down! It was actually closer to 60mA per tube! Now it's down in the lower 30's...

Though there is one other interesting thing of note...

First off, I have a box of "takeout tubes" that have come from amps I've bought over the years... They are all still working, but I don't really know the history of them... So I change tubes whenever I get a new ( to me) amp... Then I know where I'm starting from... But I have a box full of older tubes that I don't mind using for a short time... As is the case with this amp... I have an older quartet of Sovtek 5881's that I put the best matched pair in this little Marshall...
So what's interesting is the plate voltage isn't the same on each power tube socket... One is around 470 while the other is around 440... I know that's definitely not kosher... So that'll be my next puzzle to solve...
Now that being said, I've also noticed that the amp will make a bit of a popping sound if I play on it too long... And fwiw my Bogner Shiva is doing the same thing too... I'm thinking they need the power tubes replaced and that may iron that part out... Hopefully... If I smack the top of the amp, sometimes I can get it to do it... But usually only when they are hot... And I've already cleaned all of the tube sockets... Thoughts?

Also, any ideas as to why the plate voltage may not match are definitely welcome!! Maybe what and where to test...

And again, thank you guys! I NEVER would've gotten this up and running without your help!

[cassoulet]

Different plate voltages just mean output tubes are not matched perfectly, but that should not prevent the amp from working.

As for the popping noise when you smack your amp, may be preamp tubes become more microphonic when they heat up?

[maddtapper]

Oh ok... Well The plate voltage was off period... No matter what tubes went where... One socket was lower regardless.

And maybe so on the preamp tubes... But I'll let it get hot and give it a try...

[cassoulet]

Oh I see now. Sorry about the mismatched tubes. I was wrong. If you always get the same unbalanced voltages always lower on the same side, that probably means your OT is part of the problem, if any.

What I mean by "if any" is that it may be due to a difference in DC resistance between both primary windings. This difference may be due to the OT construction: one winding is around the other one, so the outer one is much longer in wire length than the inner one, therefore more resistive.

Some better quality OT have interleaved primaries. Windings are of the same length and do not show such unbalanced DC resistances.

May be I'm wrong. Just thoughts.

[Stevem]

That is a BIG difference in plate voltage to sum up to being due to OT winding length and I have never seen so much in my life time doing this stuff and still say that the OT is good!
If the voltage you read on pin 3 and 4 with both outouts removed is the same then your OT is toast and the sometimes popping you hear is the OT arcing over at times or one or both of the tube sockets is doing such!

[cassoulet]

Yes, you are right. I knew I was wrong at the minute I posted this. A 30V drop with about 30 mA per leg would make a 1k difference in DC resistance. That's definitely too high.

Too bad for the iron.