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

[stoneattic]

What are cathode (pin 8) DC voltages?

Pin 8 to ground on both EL34s are reading 0 VDC

Measure heater voltage (AC) between pins 2 and 7.

5.86 VAC & 5.82 VAC
I believe I want 6.3, so a little low, correct? Is that low enough to be a problem?

[stoneattic]

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.

I tried repeating the VAC readings that I took with the Craftsman DMM with the capacitor using the HF DMM with the capacitor and still read 0 VAC with that DMM.

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.

[stoneattic]

Is 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.

It's not intermittent anymore. The section to the FX out is still good, but the FX return/power section is slient.

[stoneattic]

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.

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. Up until this particular amp the Craftsman has done what I needed. The HF ones are free and are great to have in the house, in the car, in the tool box, etc. for quick checks, but not something I would do real work with.

[Helmholtz]

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.

I'm not familiar with today's DMM market. My professional DMMs are around 25 years old and no longer available. But I'm sure Fluke makes great ones.

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.

[glebert]

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?

[stoneattic]

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?

The tubes were replaced when the intermittent sound issue started, but the new tubes made no difference.

[nickb]

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.

[stoneattic]

To hazard a guess, the bias control PCB has decided there is a fault and set the bias where to a "safe" value.

If that were the case do you that the Infinium LEDs should light?

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.

I tried with a .1 µF/400V cap in series, which was the highest voltage cap I had and got the ~10 to 1 reduction. I'm not saying that is accurate because I no longer trust any of my DMMs for anything but the most basic functions. I'm researching new meters based on Helmholtz criteria now but I tend to over-analyze and over-research everything so it will probably take me a few days to pick one.

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.

The Infinium circuit is part of the main PCB. There are only two PCBs, (1) with all of the rear panel connections, switches, Infinium LEDs and (2) with everything else. I posted some pics here, but they are not great:

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.

[nickb]

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.

[stoneattic]

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?

[Helmholtz]

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?

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 Fluke 175.

[g1]

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?

For a hobbyist today, under $100 can buy you stuff a professional used to have to pay a fortune for.
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.

[Enzo]

Fluke is a quality brand for sure, but there are many brands of good meters.

[stoneattic]

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.

It seems like most reviews, particularly from that site I posted the link to, is from an electrician or home owner doing house wiring standpoint, Even the most of the manufacture's buying guides don't address electronics specifically. What features would make a DMM more for electronics (in my case specifically music/guitar gear like tube amps, SS amp and pedals)?