Bugera T50 - Intermitent (mostly no) Sound at the Speaker Out, but signal at the FX Send

[Jon Snell]

If you check for anode voltage and signal passing through the phase splitter, you will soon find the issue. Look for about 230 volts on the 12AX7 anodes and signal of course.

[stoneattic]

I believe, from what you have written, there is pre-amp signal on the effects socket, (FX send) when you send a signal to the main power amplifier, (FX return), there is no sound.
Have you heaters on V3? If not check R28 = 47R 5W.

Correct, that is the symptoms.
It's kind of hard to see on the 12AX7s because they fit pretty deeply in the chassis (PCB mounted sockets) but it appears that filaments are lit on all the tubes.

[stoneattic]

SO is this a not normal condition of the lights? I mean are they normally dark? or do they normally light to tell you OK condition? If missing lights, check for missing low voltage power supply.

And in general, pull the power tubes and measure voltages right at the sockets. Pins 3 and 4 should have good B+ and pins 5 a good bias amount. I have no idea what voltage, B+ likely 450-500v. Bias something in the -45 to -60v area. I am worried about MISSING voltages, not WRONG voltages.

The lights being off are a good thing. When lit it is supposed to indicate that the tubes should be changed. Of course if the problem is related to that circuit them all bets are off.
I will measure the pins you suggested and report my numbers.
Thanks!

[Jon Snell]

I believe, from what you have written, there is pre-amp signal on the effects socket, (FX send) when you send a signal to the main power amplifier, (FX return), there is no sound.
Have you heaters on V3? If not check R28 = 47R 5W.

[Enzo]

SO is this a not normal condition of the lights? I mean are they normally dark? or do they normally light to tell you OK condition? If missing lights, check for missing low voltage power supply.

And in general, pull the power tubes and measure voltages right at the sockets. Pins 3 and 4 should have good B+ and pins 5 a good bias amount. I have no idea what voltage, B+ likely 450-500v. Bias something in the -45 to -60v area. I am worried about MISSING voltages, not WRONG voltages.

[stoneattic]

You did half the test. PLug the guitar into the FX return, now what happens? We have bypassed teh whole preamp. And THE first thing to ever do in these cases: Plug a spare cord from FX send to FX return. The FX jacks can easily cause this problem.

Sorry, I should have mentioned that I did run the FX send of the other amp to the FX return of the T50 and got no sound. But just to be sure, after seeing your response, I used a pedal jumper cable to bridge the FX send and return with the same result of no sound.

I'm not sure how much to read into this, but the Infinium lights that was supposed to indicate a tube needed replacement are not light. Of course if the issue is with the Infinim circuit then that doesn't really help I guess.

[Enzo]

You did half the test. PLug the guitar into the FX return, now what happens? We have bypassed teh whole preamp. And THE first thing to ever do in these cases: Plug a spare cord from FX send to FX return. The FX jacks can easily cause this problem.

[stoneattic]

So I'm not holding out too much hope repairing this Bugera T50 as it's mostly surface mount and I haven't had any luck finding a schematic but I'm going to give it a go.

It will occasionally pass sound to the speaker cab, but mostly not. I have not been able to figure out what conditions get sound. It just randomly outputs sound for a few seconds and goes silent again. Sometimes this happens when it's first turned on and other times and a couple of minutes. It always goes silent very quickly, like 3 seconds. It has a fresh complete set of JJs that did not affect the problem. If I take a line from the FX send to the return on another amp I get good sound. All of the channel switching and channel knobs work. So I believe it to likely be an intermittent connection somewhere in the PI or power amp section. Usually in this case I would just reflow all of the solder joints in those areas, but with surface mount...not so much.

My plan now is to start measuring voltage on the PI 12AX7 and the two EL34s and compare the results to other EL34 based amps just to see if there is a pin that is not getting or sending what it should. Does this seem like a reasonable approach to an amp without a schematic? The PCB is black and tracing the circuit is going to be difficult to impossible.

Thanks for any suggestions!