Ampeg Jet 12d- no sound

[Perkinsman]

Powers up but no sound unless volume all the way & then still only faint. Slight hum that doesn’t fluctuate with volume. Tubes tested strong. Lifted legs on coupling caps & test in spec. Replaced 2 25/30v Ducati with 2 22/50 Panasonic Audio caps. Measured .02uf coupling caps, both in spec. Voltages on PT plates are 365vdc. No other voltages are listed. Anyone with similar Ampeg experiences?

Ampeg-Jet-J12D-Amp-Schematic.pdf

[Helmholtz]

Please post V2 and V3 plate and cathode voltages and check if pulling V1 restores volume.

[Perkinsman]

• Do you mean v2&v3? V1&v2 are the preamp tubes. V3&v4 are the power tubes. Also, the schematic shows a solid state rectifier but this has amp has a 5y3.

[Helmholtz]

According to schema, all 3 systems of V1 are used for tremolo. Amp should work without it.
V3&4 should have same cathode and plate voltages.

[The Dude]

Since you already stated your output tube plate voltage was at 365V, I think Helmholtz means plate and cathode voltages on all 3 triodes of V2. We are trying to find out if any of those are not conducting. I believe that's what he's after. If not, it's what I'd be looking at.

Edit: Helmholtz got in while I was typing.

[Perkinsman]

Ok, I think I understand now. I’ve never dealt with a tube with 3 plates & cathodes!

Pulled V1, dead silent (except for low hum). Even the faint sound at full volume was lost.

V1 plates= 0, 0, 350vdc
V1 cathodes= .09, .09, 0vdc

V2 plates= 220, 177, 250
V2 cathodes= .03, 2.1, 1.5

How do I know what the voltages should be?

[Helmholtz]

Seems I wasn't clear enough.

I asked for V2 and V3 voltages. V3 plate voltage you already posted but I need cathode voltage as well. V1 (tremolo) doesn't matter for now.

Please specify tube pins with voltages.

I also need supply voltages as measured at the 3 filter caps.

From the voltages I can calculate tube currents and see if there are "normal" operating conditions. Even a bad plate or cathode resistor can be identified.

[Enzo]

From my own perspective, I want to see plate and cathode voltages for the triodes as well. I am not so worried they are wrong as I am some are missing. Your issue is NO sound, not crappy sound. SO if we find a 350v plate with a zero volt cathode, that tells us that triode is not conducting. Not conducting means cannot amplify. If we get a volt or two on the cathode and substantially lower than B+ on the plate, that means the triode IS conducting, and even if it is "wrong", it still should make some sound.

Lets use your meter as a hum generator. Disconnect the black probe from the chassis ground, leave it sitting unattached to anything. Now use the red probe as a signal injector. The meter and probe wires are acting as an antenna to pick up hum in the area. Touch the red probe to pin 7 of V2, the phase inverter input. Do you hear hum out the speaker? Turn the volume up, and touch pin 9 of V2, does that hum?

[Perkinsman]

V2 plate pins 2,5,10=220, 177, 250 respectively
V2 cathode pins 3,4,6=.03,2.1,1.5 respectively

V3 plate, pin 3=360
V3 cathode, pin 5=13.09
V4 plate=370
V4 cathode=12.8

378, 341, 274vdc on the cap can
There is an add’l 22uf cap on the turret board=375vdc

i get hissing noise from 7 & 9 on V2.

[Helmholtz]

Let's agree to call the input triode V2a (pins 4,9,10).

There is something wrong with this stage or the measurements. The 120k plate resistor drops only 24V, meaning a very low plate current of only 0.2mA. Expected current would be about 3 times larger.

Also you can't have 250V at pin 10 and 2.1V at pin 4. You probably mixed up pin numbers. (A cathode voltage of only 0.03V is suspect as well.)

V3 and V4 cathodes are directly connected and must have identical voltages.

Please check pin numbers and voltages again.


Next step would be swapping V2 with V1 and measuring again.

[Perkinsman]

V2a pin 10 is 250. Pin 4 is 2.1, however pin 3 is 1.94, not .03. V3 & V4 cathode both 12.3.

Ill swap v2 & v1 & send a list of those voltages as well next message.

[Perkinsman]

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

[Helmholtz]

V2a plate and cathode voltages are not consistent with schematic. While 2.1V at the cathode (corresponding to a plate current of 2.1/3200 = 0.66mA) looks good, the plate voltage seems too high.
Please also measure voltage drop directly across the 120k plate resistor. And measure the plate and cathode resistors.

[g1]

V2a plate and cathode voltages are not consistent with schematic.

He mentioned in post #3 that it is not the correct schematic.
Maybe finding the right one would help.

[Helmholtz]

He mentioned in post #3 that it is not the correct schematic.

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.