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

[g1]

Possibly due to the infinium circuit?

[stoneattic]

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.

I am seeing the same bias reading on pin 5 of the EL34s with them installed

Without the schematic I don't know what the coupling caps are for sure. It's a black PCB that appears to have most of the traces on the "other" side. There's a lot of disassembly to get there and it's going to be very difficult to connect everything to the PCB to test turn it on and test after pulling it. Of course that may be my only option. From what I do see I believe the coupling caps are C26 and C48 which are the little box (polyester?) style caps and I don't have access to test those as described above.

I'm very confused as to what I'm seeing here. Should I see a change of voltage on pins 1 & 6 of the PI with a change in input volume (same 1khz sine wave input)? I read the same VAC regardless of the preamp gain setting on the amp or the output of the function generator.

My "good" DMM max VAC is 600 so I used one of my many free Harbor Freight DMMs set to 750 VAC and measured 856 VAC at both pin 3s. I don't have a ton of faith in the reading from the DMM especially if its max is 750 but I am reading a steady 856 on both tubes.

[stoneattic]

Possibly due to the infinium circuit?

I have not been able to find any real info on the Infinium circuit. So far I have not found a schematic for this amp or any Infinium amp for that matter. Maybe I'm just not looking in the right places. But since I'm reading a bias of -51.7 VDC, which is in the range Enzo suggested I was assuming that the Infinium circuit was doing it's thing.

[nevetslab]

I have not been able to find any real info on the Infinium circuit. So far I have not found a schematic for this amp or any Infinium amp for that matter. Maybe I'm just not looking in the right places. But since I'm reading a bias of -51.7 VDC, which is in the range Enzo suggested I was assuming that the Infinium circuit was doing it's thing.

While I haven't been inside one of these, the film caps between the PI plates and the power tube input grids, you'll see probably 0.1uF/630V, or maybe 0.047uF/630V. Sounds like they are radial box caps from your description. As you ARE seeing the bias voltage, you're NOT seeing that AC voltage swing from the PI plates? With box radial caps, you can only trace it from the foil side.

[stoneattic]

Here are a bunch of pics of the inside of the T50:

http://www.stoneattic.com/t50/t50.htm

[stoneattic]

I should have done this earlier, but I have a functioning T50, so I feed it the same 1Khz sine wave and took measurements. Here is what I got compared to the one that is not sending signal past the FX send, with problem T50 labeled as "Bad" and the fully functioning one as "Good".

PI

Pin 1
"Bad" 256 VDC, 13.3 VAC
"Good" 260 VDC, 14.1 VAC

Pin 6
"Bad" 252 VDC, 10.6 VAC
"Good" 235 VDC, 9.5 VAC

EL34

Pin 3
"Bad" 382 VDC
"Good" 368 VDC

Pin 4
"Bad" 381 VDC
"Good" 365 VDC

Pin 5
"Bad" -51.7 VDC
"Good" -29.6 VDC

It should be noted that the "Bad" has a set of new JJs and the "Good", the factory unknown (Chinese) tubes. This many explain the differences, especially in the bias on EL34 pin 5.

The weird thing is I still get no AC on EL34 pin 4 on the "Good" amp. I'm very confused.

[stoneattic]

While I haven't been inside one of these, the film caps between the PI plates and the power tube input grids, you'll see probably 0.1uF/630V, or maybe 0.047uF/630V. Sounds like they are radial box caps from your description. As you ARE seeing the bias voltage, you're NOT seeing that AC voltage swing from the PI plates? With box radial caps, you can only trace it from the foil side.

It looks like the coupling caps are 0.047uF, 400V perhaps as they are label 473K400.

[nevetslab]

I'm guessing in reviewing your photos that V4 is the driver tube. The Yellow 47nF/400V caps (C48, C26) look like they're the AC coupling caps between the driver plates @.resistors R71 100k 1/2W & R61 82k 1/2W and the power tube input grids. .R68 & R67 look to be the 220k bias resistors feeding the power tube input grids. See if you're getting AC signal on both the plate resistors and the bias resistors (the sides that are common to Pin 5 of each power tube. Pin 4 on the power tubes are the screen grids. You won't see the driver signal on those pins.

[g1]

You didn't give your AC signal voltages at pin5 of the power tubes. I think you will find similar voltages on good and bad amp.
The power tubes appear to be cut-off. (big difference between -30 and -50V bias). Which is also why the power tube plate voltages are high on the bad amp, the tubes are not conducting.
I believe they are being turned off by the infinium circuit.

[stoneattic]

I'm guessing in reviewing your photos that V4 is the driver tube. The Yellow 47nF/400V caps (C48, C26) look like they're the AC coupling caps between the driver plates @.resistors R71 100k 1/2W & R61 82k 1/2W and the power tube input grids. .R68 & R67 look to be the 220k bias resistors feeding the power tube input grids. See if you're getting AC signal on both the plate resistors and the bias resistors (the sides that are common to Pin 5 of each power tube. Pin 4 on the power tubes are the screen grids. You won't see the driver signal on those pins.

So I ATTEMPTED to measure AC signal at the locations I believe you suggested. The reason I say ATTEMPTED is that I believe that either I am measuring incorrectly or there is an issue with some or all of my DMMs. I measured by putting the negative probe on the chassis and the positive probe on the wire coming out of the resistor and tried touching the copper eyelet if I was not getting a stable reading on the resistor wire just in case there was some of the resistor coating on the wire that was insulating it. Is it possible that there is a clear insulative coating on the copper eyelets? For what it's worth all three of my DMMs read 120VAC coming out of the wall. I measured both sides of the resistors, first the end physically closest to the EL34s, figuring that would be the side common to pin 5 of the EL34s. Anyway, here are my results using the DMM I would most trust.

"Good" amp:
Plate resistors
R71: 29.5 VAC (closest to EL34s), 8.7 VAC (farthest from EL34s)
R61: 29.5 VAC (closest to EL34s) , 13.4 VAC (farthest from EL34s)

Bias resistors
R68: no reading VAC (closest to EL34s), 3.1 VAC (farthest from EL34s)
R67: no reading VAC (closest to EL34s), 2.9 VAC (farthest from EL34s)

Pin 5 on EL34s: 0 VAC


"Bad" amp:
Plate resistors
R71: no reading VAC (closest to EL34s), no reading VAC (farthest from EL34s)
R61: no reading VAC (closest to EL34s), no readingVAC (farthest from EL34s)

Bias resistors
R68: no reading VAC (closest to EL34s), 3.0 VAC (farthest from EL34s)
R67: no reading VAC (closest to EL34s), 2.9 VAC (farthest from EL34s)

Pin 5 on EL34s: 0 VAC

"no reading" means my DMM jumped all over
"0" means the DMM actually read 0

I really don't think I'm getting legitimate AC signal readings. I am going to attempt to get these reading again with more DMMs. Any hints on what I might be doing wrong or other ways to measure AC signal? I have an OLD Techtronix 453, but I have no idea how to use it. Would an analog voltmeter be better? Can I use my homemade audio probe? (http://diy-fever.com/misc/audio-probe/) That works great when working on pedals, but I don't know how well it would handle the amp voltages/levels and if it would be safe to use. No fear with a 9V pedal, but with the voltages in a tube amp?

[stoneattic]

You didn't give your AC signal voltages at pin5 of the power tubes. I think you will find similar voltages on good and bad amp.
The power tubes appear to be cut-off. (big difference between -30 and -50V bias). Which is also why the power tube plate voltages are high on the bad amp, the tubes are not conducting.
I believe they are being turned off by the infinium circuit.

I'm not reading any AC voltage on the power tubes even on the "good" amp when I can hear the 1khz sine (man, that's annoying). The DMM reads 0VAC. See my previous post regarding my issues with measuring AC.
Since i can't find a schematic of this amp I'm not sure what the Infinium circuit actually is, but it has to be more complex than just a steady negative voltage like I usually see. The bias I did measure seemed in the range that Enzo thought it should be on the "bad" amp.

[Helmholtz]

I just looked up the specs of some Harbor Freight DMMs and found that bandwidth is limited to 450Hz (or even only 150Hz?). So you won't get decent AC readings with a 1kHz signal. Try lowering frequency to 400Hz.
Another problem might be a low (AC) meter input impedance (not specified) which would load down grid voltages.

https://www.harborfreight.com/7-Func...ter-63759.html

[stoneattic]

I just looked up the specs of some Harbor Freight DMMs and found that bandwidth is limited to 450Hz. So you won't get decent AC readings with a 1kHz signal. Try lowering frequency to 400Hz.

https://www.harborfreight.com/7-Func...ter-63759.html

I think you nailed it!
The DMM I was using is this one: https://www.manualslib.com/manual/48...man-82139.html
On page 6 of the manual it appear to list the AC Voltage as (40-400Hz)
One of the others I tried was a HF like the one you linked. The other I tried was an old Heathkit that I can't find specs for but I'm guessing is topping out at 400-450Hz as well.
I will try again at lower frequencies.
Thanks!

[stoneattic]

A follow up regarding my question about using an audio probe before Helmholtz figured out that my DMMs won't read 1KHz properly:

I search around and found this: https://el34world.com/Hoffman/tools.htm
Check out the bottom of the page. He made an audio probe that is good for amp by adding a 1M pot to my setup. Seems like a useful tool.

[Enzo]

Also called a signal tracer.