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

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  • nevetslab
    replied
    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.

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

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  • stoneattic
    replied
    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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  • stoneattic
    replied
    Here are a bunch of pics of the inside of the T50:

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

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  • nevetslab
    replied
    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.
    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.

    Leave a comment:


  • stoneattic
    replied
    Originally posted by g1 View Post
    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.

    Leave a comment:


  • stoneattic
    replied
    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.
    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.

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  • g1
    replied
    Possibly due to the infinium circuit?

    Leave a comment:


  • nevetslab
    replied
    Originally posted by stoneattic View Post
    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.

    Leave a comment:


  • stoneattic
    replied
    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.

    Leave a comment:


  • Jon Snell
    replied
    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.

    Leave a comment:


  • stoneattic
    replied
    Originally posted by Jon Snell View Post
    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. Click image for larger version

Name:	Screenshot 2020-06-23 at 11.02.18.png
Views:	1269
Size:	74.0 KB
ID:	908013
    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!

    Leave a comment:


  • Jon Snell
    replied
    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. Click image for larger version

Name:	Screenshot 2020-06-23 at 11.02.18.png
Views:	1269
Size:	74.0 KB
ID:	908013

    Leave a comment:


  • glebert
    replied
    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.

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  • stoneattic
    replied
    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!


    Leave a comment:

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