Ampeg Jet 12d- no sound

[The Dude]

They are all cathode bypass caps. See the schematic in post #17.

[Enzo]

Joe Piazza drawings cannot be trusted, I looked through mine, does ANYONE have the original Ampeg drawing?

[Jazz P Bass]

Here is the original schematic:

[Helmholtz]

He meant pins 4,9&10, not 3,9,&10.

Correct, sorry for typo.

[Perkinsman]

Here is the original schematic:

Not quite, mine has a 5y3 rectifier.
The best I can do is post a few pics of the torn schematic on the back of the amps back cover.

At the moment, I have sound again but with a bad hum/crackle. Its not the speaker, I ran to another speaker and still get the same. Could it be the OT?? I have 2 25uf caps that need to be installed today.

[Helmholtz]

The best I can do is post a few pics of the torn schematic on the back of the amps back cover.

Wow, why didn't you post these earlier? Would have saved a lot of time/guesswork.
Picture 2 clearly shows that the input triode uses pins 5,6,7 as opposed to other schematics. Even some essential circuit voltages can be seen.

[Perkinsman]

Ur right, I should have looked closer, I thought it was useless being torn. Now on to why I’m getting hum & placement of the 2 25uf caps

[Helmholtz]

..placement of the 2 25uf caps

As has been stated they must be cathode bypass caps.

One of them goes between input tube pin 6 and ground. Do you have a 100µ cap between pin 3 (PI) and ground (as shown in your schemaic)?
If not the other 25µ should go there.

[Perkinsman]

No 100uf on V2pin3. Why would I put a 25uf there if my schematic indicates 100uf? I installed the 100uf & it stopped the hum!

Also, on the schematic, there’s a 20uf cap from pins 5 cathode of the power tubes but on the amp there’s nothing on pin 5 except a common wire between the two. Should I just install a 22uf cap parallel to 140Ohm/10W & call it a day?

[Helmholtz]

No 100uf on V2pin3. Why would I put a 25uf there if my schematic indicates 100uf?

Your schemaic says 100µ. Other schematics say 25µ. Both will work. 100µ gives stronger bass response.

But also the tremolo LFO triode has a 25µ cathode cap.

[Perkinsman]

Yes, the trem cap is there but I was asking about the common power tubes 25uf cathode cap which wasn’t present. The amps works without it but I installed it across the 140 ohm/10w resistor anyway.

[Helmholtz]

Yes, the trem cap is there but I was asking about the common power tubes 25uf cathode cap which wasn’t present. The amps works without it but I installed it across the 140 ohm/10w resistor anyway.

As said in post #40.

It's kind of a worst case scenario if there is no valid schematic - and someone changed component positions.

[g1]

It's kind of a worst case scenario if there is no valid schematic

Well, the sheet glued in there says J12T (6C10/6BK11 & 7591 version). If that's what we had been looking for it's probably available somewhere.
Curious if the amp is actually marked as J12D or if that was a typo in the thread title.

[Helmholtz]

Well, the sheet glued in there says J12T (6C10/6BK11 & 7591 version). If that's what we had been looking for it's probably available somewhere.
Curious if the amp is actually marked as J12D or if that was a typo in the thread title.

Don't ignore the most important second part of my statement above. Even unknown circuits can be expected to be logical, but only if unchanged.

Apart from that I think we meanwhile know most of the circuit.

[Perkinsman]

Its working great so I’m going to thank you all again for your dogged assistance.