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

[stoneattic]

Other than teh HArbor Freight, the other two would at least measure AC siting on DC.

Drop your audio frequency. At least under 400Hz. I used 100Hz most of the time, because unlike a piercing 1kHz, I can listen to 100Hz for extended time. Once the system is working, I can play full band audio and listen with my ears.

Test your meters again: turn on your audio generator to the highest level it makes, now measure its output as AC volts. Try each meter at 100Hz, 400Hz, 1000Hz (1kHz).. That will tell you if teh meter can handle audio signals. No meter will respond well to actual music signal.

I tested using the above method and on the Craftman read .033 VAC at 100Hz and 400Hz. I read .031 VAC at 1kHz. The other 2 meters did not read anything.
Does the .033 seem like a legitimate value?

I didn't get home until late and did not have time to try the 0.1µF/630V cap yet.

[Helmholtz]

That ^^^^. Or wire a 0.1µF/630V cap in series with the positive Harbour Freight meter lead to block DC while measuring ACV.

[Enzo]

Other than teh HArbor Freight, the other two would at least measure AC siting on DC.

Drop your audio frequency. At least under 400Hz. I used 100Hz most of the time, because unlike a piercing 1kHz, I can listen to 100Hz for extended time. Once the system is working, I can play full band audio and listen with my ears.

Test your meters again: turn on your audio generator to the highest level it makes, now measure its output as AC volts. Try each meter at 100Hz, 400Hz, 1000Hz (1kHz).. That will tell you if teh meter can handle audio signals. No meter will respond well to actual music signal.

[stoneattic]

Yay, the site's back up!

Let's test your meters. Set the meters to AC volts. Get a 9v battery, and measure the "AC VOLTS" coming from it. Yes, I know it is DC. A good meter would show a quick volt or two spike and settle down to zero volts AC. If your meter reads 9v battery as something like 12vAC, then the meter will be confused in circuits trying to read audio.

I tested my various DMMs as suggested and got the following results:

Craftsman 82139: 9v battery test shows ~120VAC for a split sec, rapidly drops to a couple of volts and eventually settles to 0. (doesn't appear to actually read audio though)
Harbor Freight freebie: 9v battery test shows a steady 20VAC (read high VAC when trying to read audio)
Heathkit ~1990 handheld DMM - 9v battery test shows nothing, just shows 0 (failed with audio as well)

All three read wall outlet VAC, so they work for standard VAC, just not the audio (frequencies) I need them for.

So it looks like none of my meters are capable of reading audio. The only one that actually APPEARED to read audio (the HF) read crazy high values and failed the 9v battery test.

It looks like I might need a better DMM. Any recommendations for a reasonably price model that will read audio? I know "reasonably priced" is a relative term, but I'm curious as to what would be recommended and what out there.

Thanks!

[Enzo]

Let's test your meters. Set the meters to AC volts. Get a 9v battery, and measure the "AC VOLTS" coming from it. Yes, I know it is DC. A good meter would show a quick volt or two spike and settle down to zero volts AC. If your meter reads 9v battery as something like 12vAC, then the meter will be confused in circuits trying to read audio.

[stoneattic]

As a general range, that's right. But he said that before you measured the -30V bias in the working unit.
The computer that is running the bias circuit probably also shuts down the drive to the grids. Is the infinium circuit on a separate board that you can swap between the 2 amps?
Even if not, swap the power tubes and see if the working amp now biases those tubes (from bad amp) at -52VDC.

I swapped power tubes as suggested and the bias on the bad amp measured -51 VDC. I guessing that does point to the magic Infinium circuit being the problem, or does that just mean something else is wrong and the Infinium circuit is reacting to that? My understand of the how the Infinium circuit is supposed to work is that the LEDs should light if it sees a problem.

Unfortunately everything except the panel with the back panel jacks is one PCB. It looks like the Infinium is part of the main, except for the LEDs themselves.

[stoneattic]

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!

I set the signal generator to 380Hz so that theoretically my "better" DMM should be able to read the AC signal. Still no luck with that meter, but the freebie HF one did read, although I have my doubts on it's accuracy since I read over 800 VAC on a meter that is only supposed to be good to 750 VAC. Here's what I got:

"Good" amp:
Plate resistors
R71: 814 VAC (closest to EL34s), 487 VAC (farthest from EL34s)
R61: 812 VAC (closest to EL34s), 542 VAC (farthest from EL34s)

Bias resistors
R68: 631 VAC (closest to EL34s), 314 VAC (farthest from EL34s)
R67: 632 VAC (closest to EL34s), 300 VAC (farthest from EL34s)

Pin 5 on EL34s: 0 VAC


"Bad" amp:
Plate resistors
R71: 821 VAC (closest to EL34s), 507 VAC (farthest from EL34s)
R61: 825 VAC (closest to EL34s), 522 VAC (farthest from EL34s)

Bias resistors
R68: 629 VAC (closest to EL34s), 302 VAC (farthest from EL34s)
R67: 630 VAC (closest to EL34s), 289 VAC (farthest from EL34s)

Pin 5 on EL34s: 0 VAC

"0" means the DMM actually read 0

So from the above I would say things look okay there....but...see my next post.

[g1]

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.

As a general range, that's right. But he said that before you measured the -30V bias in the working unit.
The computer that is running the bias circuit probably also shuts down the drive to the grids. Is the infinium circuit on a separate board that you can swap between the 2 amps?
Even if not, swap the power tubes and see if the working amp now biases those tubes (from bad amp) at -52VDC.

[Enzo]

Also called a signal tracer.

[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.

[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!

[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]

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.

[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?

[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.