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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 Helmholtz View Post
Which DMM did you use? I recommended the cap probe for use with the Harbour Freight meter, because this seemed to have a useful frequency response and high enough input resistance.
If that doesn't work either it's time to get a decent DMM.
If everythig is ok (including meter), the AC signals at the power tube grids (pin 5) should be only slightly less than the PI plate (pins1,6) signals.
Screen DC looks ok.
From all we've seen I think that your problem (is it still intermittent?) is restricted to the power tube grid circuit. The symptoms being missing/low grid signal at both tubes and large negative bias voltage are unusual (the fact that you don't see idle currents may be due to your meters not being able to display low mVs).
It seems that something is muting the grid drive and it's very likely that it's caused by a defective Infinium circuit as being said earlier.
The AC signal is significantly lower at the power tubes (pin 5) than the PI plates (pins 1 & 6) by a factor of pretty much 10.
The problem has pretty much stopped being intermittent it's now always silent.
I agree it is likely the Infinitum circuit.
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Originally posted by glebert View PostIs the amp still giving some sound intermittently? Wondering if you can "catch" a current reading when it is actually making sound. Not really the current value, just to see if there is measureable current during that time.
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Originally posted by Helmholtz View Post
And you probably connected the probe to one of your cheap meters.
At a bias of -50V a typical EL34 should have a cathode current below 5mA.
I wonder if your meter can measure voltages below 5mV.
Anyway, idle currents that low don't allow proper class AB amp operation. But that's not the reason for the low grid signals.
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Originally posted by stoneattic View Post
All I have at the moment are cheap meters. What meter would you recommend? I've been wanting to pick up a better meter but don't know what features would be most useful.
Especially cheap meters require a good understanding of the meter's limitations and their consequences on results.
I like to have:
- High input impedance (AC and DC), preferably 10M to minimize the loading of high impedance circuits (e.g. tube grids), absolute minimum 1M.
- Good mV resolution (especially DC)
- Good frequency bandwidth, at least up to 1kHz.
Nice to have:
- True RMS
- Auto-ranging with units displayed.
The ability to measure ACV floating on DCV is a must.Last edited by Helmholtz; 07-14-2020, 05:23 PM.- Own Opinions Only -
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I wonder if the AC voltages are a wild goose chase. They may be low, but they aren't zero, and there is zero sound coming out, which supports the evidence that the tubes are not conducting. If it were just one tube I would wonder if there is a burned or broken pin but can't see that happening with both sockets. The bad amp had the JJ tubes, right? Was there a failure with the original tubes or just an upgrade?
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Originally posted by glebert View PostI wonder if the AC voltages are a wild goose chase. They may be low, but they aren't zero, and there is zero sound coming out, which supports the evidence that the tubes are not conducting. If it were just one tube I would wonder if there is a burned or broken pin but can't see that happening with both sockets. The bad amp had the JJ tubes, right? Was there a failure with the original tubes or just an upgrade?
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To hazard a guess, the bias control PCB has decided there is a fault and set the bias where to a "safe" value.
The large reduction in signal between the PI and the EL34 grids seems key to solving this. I lost track of the meter situation but if you put a 10nf 630V cap in series with your meter lead, as has been suggested, then you can be sure the readings are meaningful in the presence of DC. If after doing that there is still ten to one reduction find out why. Likely you'll have to pull the main PCB and trace some of it out. While you have it out look at the connections the bias control PCB ad try to figure out what they do as it might be useful info later. It's possible, but I don't know why, the bias control board kills the drive to the EL34's but I don't recall seeing anything like that on the 6262 I looked at once. How about a clear (hi res and well illuminated) image of the guts? We might see just something suspicious between the PI and the EL34 with a bit of luck. Also intermittents are often associated with bad connections so you can inspect solder joints and so forth while at it. Might be worth chop sticking so find anything loose before you start.
If the bias board is anything like to one I've examined, it seems possible to simply disconnect it and add a resistor from each EL34 grid one to ground to set the bias voltage, somewhere in the 100's of k ohm range. If you do that it will eliminate the bias voltage uncertainty until you get to grips with the real issue.
PS: A pic of the bias control PCB will help as I can see if it is the same as the one I have here.
Experience is something you get, just after you really needed it.
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Originally posted by nickb View PostTo hazard a guess, the bias control PCB has decided there is a fault and set the bias where to a "safe" value.
Originally posted by nickb View PostThe large reduction in signal between the PI and the EL34 grids seems key to solving this. I lost track of the meter situation but if you put a 10nf 630V cap in series with your meter lead, as has been suggested, then you can be sure the readings are meaningful in the presence of DC. If after doing that there is still ten to one reduction find out why.
Originally posted by nickb View PostLikely you'll have to pull the main PCB and trace some of it out. While you have it out look at the connections the bias control PCB ad try to figure out what they do as it might be useful info later. It's possible, but I don't know why, the bias control board kills the drive to the EL34's but I don't recall seeing anything like that on the 6262 I looked at once. How about a clear (hi res and well illuminated) image of the guts? We might see just something suspicious between the PI and the EL34 with a bit of luck. Also intermittents are often associated with bad connections so you can inspect solder joints and so forth while at it. Might be worth chop sticking so find anything loose before you start.
If the bias board is anything like to one I've examined, it seems possible to simply disconnect it and add a resistor from each EL34 grid one to ground to set the bias voltage, somewhere in the 100's of k ohm range. If you do that it will eliminate the bias voltage uncertainty until you get to grips with the real issue.
PS: A pic of the bias control PCB will help as I can see if it is the same as the one I have here.
http://www.stoneattic.com/t50/t50.htm
I tried to take some more pics this evening, focusing on the area that I believe is the Infinium circuit but they are not great. I will sort through them and post any good ones and/or take new ones tomorrow.
I'm likely going to pull the PCBs and check underneath for any obvious problems and see if I can trace the bias path and try to figure out if I can bypass the Infinium. Although I may wait until I get a new DMM and retest the AC signal.
At possible new twist that may mean nothing, is when I was poking around with a chop stick I had plugged my tablet in to play music and when I unplugged it and touched the input cable there was a the typical pop/static that you usually get out of the speaker. I tried another cable (also known good) and it did not make the noise. Putting the other cable back in and it made the noise again. I don't know what, if anything that might mean. Other than that I have not heard a sound out of the speaker for a few days.
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Thx for the piccies and sorry for missing them earlier. A pity the bias control is integrated quite unlike the 6262 I looked at. That just makes it so much harder as does the black resist.
But... the volume control is dual pot and it kinda sorta looks like the one of the traces from a PI output coupling cap head off in that direction making me think it might be a PPIMV type of control. Maybe that is where the PI signal reduction is happening? Fire it full up and see what affect it has on the signal level on the EL34 grids relative to the PI output, assuming that the pot itself isn't the actual problem.Last edited by nickb; 07-15-2020, 12:55 PM.Experience is something you get, just after you really needed it.
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Sorry I didn't get a chance to post the pics yet, I will within the next couple of days (band practice tonight).
I've been spending (way too much) time researching different DMMs. I found what seems to be a pretty good site that has reviews, comparisons, etc. You pros may already be aware of it, but just in case:
http://testmeterpro.com/
While I know Fluke is the industry standard, I am only a hobbyist, and after doing a lot of reading at the above site and and elsewhere I think I'm leaning towards an Amprobe AM570 or AM530:
https://www.amprobe.com/product/am-570/
There are a few differences in features/capabilities and well as a big difference in price between the two. While I have no problem paying for the 570 I also don't want to waste money on features or capabilities that I don't need The big thing jumping out at me is the max DCV voltage (600 for the 530 and 1000 for the 570).
Does anyone have any thought or opinions on the Amprobe and/or difference in models, or other recommendations? Am I wasting my money on anything besides a Fluke?
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Originally posted by stoneattic View PostSorry I didn't get a chance to post the pics yet, I will within the next couple of days (band practice tonight).
I've been spending (way too much) time researching different DMMs. I found what seems to be a pretty good site that has reviews, comparisons, etc. You pros may already be aware of it, but just in case:
http://testmeterpro.com/
While I know Fluke is the industry standard, I am only a hobbyist, and after doing a lot of reading at the above site and and elsewhere I think I'm leaning towards an Amprobe AM570 or AM530:
https://www.amprobe.com/product/am-570/
There are a few differences in features/capabilities and well as a big difference in price between the two. While I have no problem paying for the 570 I also don't want to waste money on features or capabilities that I don't need The big thing jumping out at me is the max DCV voltage (600 for the 530 and 1000 for the 570).
Does anyone have any thought or opinions on the Amprobe and/or difference in models, or other recommendations? Am I wasting my money on anything besides a Fluke?Last edited by Helmholtz; 07-17-2020, 04:00 PM.- Own Opinions Only -
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Originally posted by stoneattic View PostDoes anyone have any thought or opinions on the Amprobe and/or difference in models, or other recommendations? Am I wasting my money on anything besides a Fluke?
Here are a couple previous threads that might help your meter search.
https://music-electronics-forum.com/...m-for-under-50
https://music-electronics-forum.com/...-to-multimeter
Fluke makes nice reliable meters. But anybody that tells you that you are "wasting money on anything besides a Fluke" is not very credible.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 Helmholtz View Post
I dunno. These meters seem to be more suitable for electricians than for electronics. Can't find info on input resistance. A lot of features I don't need. My reference would be something like a Fluke115 or 175.
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