OT protection diodes SR-2873?

[gui_tarzan]

Grounded? The plate?

[Enzo]

No bias pot? Isn't there a small black pot shaft sticking up through the chassis near the power transformer? -43 is right for EL34, flip the switch for 6L6 and it should go to about -55. The adjustment is of limited range, the idea is you can adjust it by ear and still not get in trouble. The test points and bias control are not designed for current measurement nor are they set up for the 70% thing. They are there so you can set it to factory spec, which is -42.5 for EL34 and -55 for 6L6.


The original diodes were probably OK from those readings.


So you have operating voltages on the power tubes. Do the heaters all come on in them? Is there ANY sound in the speakers? Even a background little hum or hiss when the amp is running, or is it dead silent?

Don't assume anything, check the speaker. Plug the speaker into some other amp, or touch a 9v battery to the plug and listen for a pop sound.

If the speaker works, then you have two speaker jacks on the amp, try both. Also there is an impedance switch, try all three positions, don;t worry about "mismatch" for this. All we want to find is will it make sound on other settings. Impedance comes later.


Make sure pin 3 is grounded? The plate of the power tubes?


Plug the guitar into the FX return. Sound?

[gui_tarzan]

No bias pot, no shaft anywhere. All heaters work, no sound at all, even with a guitar plugged into the effects return. No impedance switch, just a damping switch and a single 16 ohm jack. Tried two different speaker cabs with it. Same cord and guitar plays normally in other amps.

This is a XXX Super 40EFX, two output tubes, three pre-, one PI.

No bias pot? Isn't there a small black pot shaft sticking up through the chassis near the power transformer? -43 is right for EL34, flip the switch for 6L6 and it should go to about -55. The adjustment is of limited range, the idea is you can adjust it by ear and still not get in trouble. The test points and bias control are not designed for current measurement nor are they set up for the 70% thing. They are there so you can set it to factory spec, which is -42.5 for EL34 and -55 for 6L6.


The original diodes were probably OK from those readings.


So you have operating voltages on the power tubes. Do the heaters all come on in them? Is there ANY sound in the speakers? Even a background little hum or hiss when the amp is running, or is it dead silent?

Don't assume anything, check the speaker. Plug the speaker into some other amp, or touch a 9v battery to the plug and listen for a pop sound.

If the speaker works, then you have two speaker jacks on the amp, try both. Also there is an impedance switch, try all three positions, don;t worry about "mismatch" for this. All we want to find is will it make sound on other settings. Impedance comes later.


Make sure pin 3 is grounded? The plate of the power tubes?


Plug the guitar into the FX return. Sound?

[Jazz P Bass]

Grounded? The plate?

Heh, heh. Pin 8.

[gui_tarzan]

Just for kicks I pulled the speaker connector (P54) and measured ohms coming out of the OT - the 4 ohm reads .5 ohms, the 8 ohm reads .5 ohms, the 16 ohm reads .7 ohms. The damping switch must be the impedance selector but it just says "tight, mid, loose" on it. The values don't change at all when I move the switch.

[gui_tarzan]

Heh, heh. Pin 8.

LMAO! See? I do pay attention sometimes.

[Enzo]

Oh... well... A could have sworn all you said was XXX. yes the XXX40 is a totally different amp, there is no bias adjustment other than teh switch. No the damping switch is not an impedance selector, this amp doesn;t have one.

OK, so set the bias switch as appropriate.

So we are now dealing with a combo amp. The internal speaker runs through a contact on the external speaker jack, so again, test the internal speaker. Then get some other speaker and plug its cord into the extension jack. Does THAT make sound?

You still have an FX loop so does guitar into the FX return make sound?


We are trying to isolate teh problem. A no sound condition is either the speaker, the speaker wiring or the amp. And in the amp, there are jacks and preamps and power amps and we are trying to find which parts if any still work.

[Enzo]

You did already answer some of those, I know.

Plug into the regular input, and run a guitar cord from FX send to some other amp and speaker. That will be the preamp signal. Does that make sound out the other amp?

[gui_tarzan]

I plugged in an 8 ohm speaker to the internal speaker leads that come out of the chassis and have some sound. It's very distorted though and not real loud (sort of like being on 1) with the volume and gain dimed. I plugged my guitar into the effects "return" and get nothing. If I plug my guitar into the amp, then take the "send" into another amp I get sound. Really bad sound, but sound nonetheless.

So there is an issue in the power amp section just as I anticipated. It's not the tubes, I tried new ones. It's not the speaker, that's a known good one. It's not the diodes or cement resistors or solder joints on the tube sockets (which are soldered to the circuit board so the wiring is not suspect either) so I guess it's time to start measuring component values.

[g1]

If the preamp out sounds bad, then there is either the same problem in the preamp, or 2 separate faults.
Check all power supply voltages.

[Jazz P Bass]

Power amp issue: check the PI plate resistors for one or both having an open condition.

[gui_tarzan]

From the PT:

Orange - 22-0-22
Red - 323
Yellow - 6.6

Pre-amp tube plates

V1

1 - 148
6 - 174

V2

1 - 145
6 - 207

V3

1 - 217
6 - 216

V4

1 - 283
6 - 159

V4 looks way off for a PI.

[gui_tarzan]

Power amp issue: check the PI plate resistors for one or both having an open condition.

All resistors on the output tube board test correctly.

[Jazz P Bass]

Run a signal & check the output tube grid Vac.
Post #15.

[g1]

^^^^^ I've been hanging on since post #15 .

Can someone post this XXX40 schematic, I've been looking at XXX which is not correct?

Your PI plate voltages are bad, you said the PI plate resistors are good, but how about heaters and all connections to the socket (and resistors)?