Ampeg Jet 12d- no sound

[g1]

Alright, but cathode and plate voltage are not consistent with a measured supply voltage of 274V and a 120k plate resistor, irrespective of rest of schematic.

Agree, but I think the 120K is an unconfirmed assumption based on incorrect version of schematic?
In case you have not come across them before, these 'Piazza' schems are non-factory and seem prone to errors. I never trust them unless I can compare them to the official schems.

[g1]

And the official Ampeg schem. does show the resistor as 120K. (see attached)
Still odd that the unit in question has 5Y3 rectifier. The only J12 schematic I can find with 5Y3 is the J12A which uses 6SL7 preamp tubes.

[Helmholtz]

Swapped v1&v2:

V2 plate pins 2,5,10=218, 170, 247 respectively
V2 cathode pins 3,4,6=2.17, 2.42, 1.69 respectively

V3 plate, pin 3=364
V3 cathode, pin 5=13.15
V4 plate=354
V4 cathode=13.15

Sorry, I missed that these values are for a different tube now. Do you get sound after exchanging V2 and V1?

Voltages are still confusing.
Let's concentrate on position V2a. Can you confirm that you have 247V at plate (pin 10) and 2.42V at cathode (pin 4)? Doesn't seem to make sense (see voltages in the schematic posted by g1).
I still suspect your pin numbers are wrong. 170V at pin 10 and 1.69V at pin 4 would make much more sense.

https://frank.pocnet.net/sheets/202/6/6BK11.pdf

[Perkinsman]

The voltages at pins 4&10 V2(a or 1/3) were as I’ve written. I’ve hunted for a schematic with a 5y3 too but can’t locate one with the correct tube configurations. My amp also has an add’l 22uf/500v cap on the turret board that doesn’t show in any schematic I’ve seen. It may just be a replacement cap.

[Helmholtz]

The 2 schematics we have are essentially identical. Rectifier type and filter caps don't matter much. It shouldn't be too big a problem to draw up the schematic of your amp. Start with the input stage.

Is V2 pin 9 actually the input grid?

[Perkinsman]

I’m not sure I know how to draw a schematic yet, however I noticed something interesting here. V2a Pin 9 wire actually goes to volume pot but pin 7 goes to the input jack junction of two 47k’s! Touching pin 7 with my meter probe makes a lot of noise thru the speaker.

You also requested the drop across the 120k, which is 91v but I see that the wire goes back to V2a pin 6! So neither of the pins are correct to the schematic.

[Enzo]

It is a Joe Piazza drawing, they are often wrong. Plus maddeningly, even in schematics that originally had them, he always leaves out the part numbers, so no C1 or R3.

[Helmholtz]

V2a Pin 9 wire actually goes to volume pot but pin 7 goes to the input jack junction of two 47k's! Touching pin 7 with my meter probe makes a lot of noise thru the speaker.

Either you are counting pin numbers wrong or the triodes within V2 are used in a different order. That would be an unusual wiring as the section 1 triode (pins 3,9,10) is different from the other two and typically identical triodes are used for the PI.
Is it possible that someone modified the circuit?

Assuming that section 2 (pins 5,6,7) is actually the input tube, plate and cathode voltages would make sense. As you get a noise when touching pin 7 with your meter probe, do you also get sound with a guitar now?

You also requested the drop across the 120k, which is 91v but I see that the wire goes back to V2a pin 6! So neither of the pins are correct to the schematic.

Is that 120k resistor connected to pin 10, measuring 247V? If so its other end should connect to a supply voltage of 247V+91V = 338V. That's close to the 341V yiu measured at the filter cap.
Pin 6 is a cathode and should not connect to any of the three 120k plate resistors.


Edit: Typo in my first line, it should read "section 1 triode (pins 4,9,10)" instead of (pins 3,9,10).

[Perkinsman]

Switching tubes didn’t make a difference, still very faint sound when turned up.

I thought we were calling it & looking at V2a 4,9,10, not “section 1, pins 3,9&10”...
this has been worked on before but the solder joints on V2 look original.
Pin 10 doesn’t connect to a 120k, it connects to a 22uf/50v, (I replaced the 20uf/30v) & a very small ceramic cap. Why would there be two caps?? Did I install something wrong there? It measures 247vdc.
I made an error regarding pin 6, it connects to a 2.2k & a 22uf/50v (uninstalled it too)

[g1]

I thought we were calling it & looking at V2a 4,9,10, not “section 1, pins 3,9&10”...

He meant pins 4,9&10, not 3,9,&10. Calling it section A or 1 means the same thing, B is section 2 and C is 3.

So the schematic is wrong in how V2 is wired up. Sections A and B are reversed. Where it shows V2 pins 10,9, and 4 should be pins 5,7, and 6. And vice versa.
Can you take some pictures of the insides and post them? What you said about pin 10 does not make sense. If there is a 50V cap there and 247V present, the cap would blow up.

[Helmholtz]

To crarify, the Compactron contains 3 independent triodes called section 1,2,3 (see datasheet). Section 1 has somewhat lower gain than section 2 and 3.

V2a, OTOH, indicates an amp function or a tube position in the schematic. In this case it means the input triode. In principle any of tube sections 1.2.3 could be used for V2a.

According to tube datasheet, pin10 is designated as 1P, which means plate of section 1.

Seems the amp uses section 2 (grid pin 7) in V2a position and section 1 (grid pin 9) in V2b position.

Agree with g1 that pin 10, being a plate, must not directly connect to low voltage ecap. Actually shouldn't connect to any ecap. Look for the plate resistor.

[Perkinsman]

Interestingly enough, after reading g1’s comment, I pulled the 22uf cap. I had replaced the original 25uf cap & must have reattached it incorrectly and I too wonder why it didn’t blow up with that much voltage, maybe u can direct me as to its correct location....the big surprise was after removing it I got sound! Much louder than with the cap in but not entirely sure it’s at it full potential. I also noticed that the that the 220 resistor is supposed to be attached to pins 3 & 6 but it is went from pin 3 to pin 4. When I attached 3 to 6, it lost sound again! Maybe you guys can look at these pics & tell me why.

[g1]

I also noticed that the that the 220 resistor is supposed to be attached to pins 3 & 6 but it is went from pin 3 to pin 4. When I attached 3 to 6, it lost sound again! Maybe you guys can look at these pics & tell me why.

It's supposed to go from pin 3 to 4, there is an error in the pin numbers on the schematic, remember from post #25 ?

So the schematic is wrong in how V2 is wired up. Sections A and B are reversed. Where it shows V2 pins 10,9, and 4 should be pins 5,7, and 6. And vice versa.

Do yourself a favour, and print out the schematic, then correct the pin numbers on it.

The 25V cap that didn't explode probably shorted, that's why it killed the sound.

[Perkinsman]

There are 4 25uf/25 or 12v caps, but I only have 2 installed in the amp currently, one runs to V2 pin 6, the other to V1 pin 4. Where do the others go?

[g1]

V2 pin4, power tubes pin5. They are all shown on schematic.