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

[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!

[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]

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]

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.

[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.

[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!

[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.

[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 pulled the power tubes and measured the following:

Pin 3: 382 VDC
Pin 4: 381 VDC
Pin 5: -51.7 VDC

Both sockets read the same values. So the bias fits right into Enzo's expected range, but the B+ is on the low side. I did some Googling and it seems like the B+ is in the acceptable range for EL34s, but pretty close to the minimum. Without having the schematic I don't know if that is what it should have. But if the first step is checking to missing voltages, it appears to have passed.

I also measured the anode to V4, which I assume is the PI since it is the last 12AX7. In this case I measured:

Pin 1: 256 VDC
Pin 6: 252 VDC

The tube was in place when I took those measurements.

I'm not sure how to the test for signal safely at the anodes there. I have an audio probe that I built for working on pedals. Of course these are 9 volt pedals. Would that be safe to use with such high voltages?

Thanks again!

[glebert]

The signal should be pretty large on the PI anode, if you put a test tone into the amp input (I use my phone with a function generator app) you should be able to use a DMM to measure AC voltage there.

[Jon Snell]

It is not suitable to try and measure a high voltage signal on the high voltage supply rail = output valve anodes. Don't do it!
Use a known signal source, from your phone is OK or a signal generator and follow it from the FX loop socket into the 12AX7 phase splitter. The voltages on pins 1 and 6 are what we needed to know and they seem OK.
Have you good signal on pin 2?
Have you good signal on pin 5 of both output valves?
I enclose a schematic for referral. It is not the exact model but should serve to help you find the issue.

[stoneattic]

It is not suitable to try and measure a high voltage signal on the high voltage supply rail = output valve anodes. Don't do it!
Use a known signal source, from your phone is OK or a signal generator and follow it from the FX loop socket into the 12AX7 phase splitter. The voltages on pins 1 and 6 are what we needed to know and they seem OK.
Have you good signal on pin 2?
Have you good signal on pin 5 of both output valves?
I enclose a schematic for referral. It is not the exact model but should serve to help you find the issue.

What would be the best way to follow the signal from the FX loop? The audio probe I mentioned before or using the DMM set to VAC? I don't have a scope, or more accurately I have an old scope that I don't know how to use.

Thanks!

[Jon Snell]

Very difficult to measure signal with a DVM.
If you insert a 1kHZ sine wave into the input, you will be able to see with the AC selected all the signal until the grids of the output valves. The voltage may be too high on the output anodes but with a DVM that can handle 750volts, you should get a useable reading. If you have good signal on the grids of the EL34s and nothing on the anodes, then there is a problem with the valves, biasing or the transformer/speaker/wiring itself.

[stoneattic]

I input a 1KHz sine wave in using an app on my tablet and measured V4, which I am assuming is the PI since it is the 12AX7 farthest from the input. I'm not sure about that though as the printing on the chassis labels them as:

12AX7A
12AX7B
12AX7C
12AX7C (again)

I am measuring the second 12AX7C.
On pin 1 I read 13.13 VAC and pin 6 I read 10.60 VAC.
Not knowing any better I would have assumed they should be roughly the same.

I do have a cheap DMM that will handle 750VAC (or so it claims) but I'm a little nervous to try it.
Do the measurements on what I believe is the PI make sense?

I measured the grids on pins 4 and 5 on the EL34s and got 0 VAC on pin 5 and the meter jumping all around on pin 4, which I assume means it's reading nothing. Both tubes gave the same result.

[nevetslab]

I input a 1KHz sine wave in using an app on my tablet and measured V4, which I am assuming is the PI since it is the 12AX7 farthest from the input. I'm not sure about that though as the printing on the chassis labels them as:

12AX7A
12AX7B
12AX7C
12AX7C (again)

I am measuring the second 12AX7C.
On pin 1 I read 13.13 VAC and pin 6 I read 10.60 VAC.
Not knowing any better I would have assumed they should be roughly the same.

I do have a cheap DMM that will handle 750VAC (or so it claims) but I'm a little nervous to try it.
Do the measurements on what I believe is the PI make sense?

I measured the grids on pins 4 and 5 on the EL34s and got 0 VAC on pin 5 and the meter jumping all around on pin 4, which I assume means it's reading nothing. Both tubes gave the same result.

As you're getting driver voltage on the PI tube, 13.1VAC on one side, and 10.6VAC on the other, but, NOT seeing those voltages at the Grid Inputs on the EL34 Power Tubes (pin 5). that's where your problem appears to be. You should be seeing those same voltages, or very close to it. Plus, you should also be seeing the bias voltage of -51.7VDC there, as you found earlier, while the power tubes were removed. Odd that you don't see either driver signal voltage at the power tube input grids. Verify you see the signal voltages on both sides of the coupling caps between the PI anodes and the input grids of the two power tubes.