Possibly due to the infinium circuit?
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Bugera T50 - Intermitent (mostly no) Sound at the Speaker Out, but signal at the FX Send
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Originally posted by nevetslab View Post
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.
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.
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Originally posted by g1 View PostPossibly due to the infinium circuit?
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Originally posted by stoneattic View Post
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.Logic is an organized way of going wrong with confidence
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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.
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Originally posted by nevetslab View Post
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.
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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.Logic is an organized way of going wrong with confidence
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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.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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Originally posted by nevetslab View PostI'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.
"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?
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Originally posted by g1 View PostYou 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.
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.
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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.htmlLast edited by Helmholtz; 07-01-2020, 02:30 PM.- Own Opinions Only -
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Originally posted by Helmholtz View PostI 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
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!
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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.
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