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

[Helmholtz]

The 115 there says 'true RMS' right on it. I don't think you will find a true RMS meter that is not AC coupled on the AC range..

Maybe by convention. But real RMS should include both AC and DC, as RMS is a power equivalent and both AC and DC contribute to power (the RMS value of 1 VDC is not zero but 1 Vrms). I would love to have a true RMS meter that allows measurements with and without DC components.

I read the 115 manual and it wasn't clear about that point. I tend to think that the normal ACV range is AC coupled, while the AC mV range might be DC coupled.

Edit: Read the 115 manual again and found the all important note:

"AC volts is ac-coupled. Auto-V LoZ, AC mV, and AC amps are dc-coupled. "
https://dam-assets.fluke.com/s3fs-pu...bVGUu8WPBGUjJu

So the 115 would be fine.

[g1]

The 115 there says 'true RMS' right on it. I don't think you will find a true RMS meter that is not AC coupled on the AC range.
I think all but the cheapest meters will have proper AC coupling when on AC range.

[stoneattic]

It should state "AC coupled" for ACV ranges. The ACV symbol should look like this ~ without any additional lines (indication DC coupling).
I aleays thought this would be normal until I found out that many of the cheaper meters don't have this essential feature.
I'm not even sure if the more expensice Fluke 115 is AC coupled (deleted it from my post above).
Without AC coupling you will need the additional cap for measuring AC on DC.

The fluke 115 has a ~ over the V on the AC voltage setting.

So does that mean it is AC coupled?
Ugh, I still have not been able to figure out which multimeter to get.

[Helmholtz]

Maybe I'm just missing it, but I have not seen specs or features listed on a meter that explicitly says it will read AC on DC. Would it be described in another way?

It should state "AC coupled" for ACV ranges. The ACV symbol should look like this ~ without any additional lines (indication DC coupling).
I always thought this would be normal until I found out that many of the cheaper meters don't have this essential feature.
I'm not even sure if the more expensice Fluke 115 is AC coupled (deleted it from my post above).
Without AC coupling you will need the additional cap for measuring AC on DC.

[stoneattic]

See my post #67. Electricians probably don't need the most important first three features of my list. They hardly have to measure AC on DC, DC mVs, frequencies above 400Hz and many meters have too low input resistance/impedance to correctly measure plate voltages or voltage drop across large resistors.

I wouldn't buy a meter without knowing about meter input impedance with DCV and ACV.

Maybe I'm just missing it, but I have not seen specs or features listed on a meter that explicitly says it will read AC on DC. Would it be described in another way?

[Helmholtz]

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)?

See my post #67. Electricians probably don't need the most important first three features of my list. They hardly have to measure AC on DC, DC mVs, frequencies above 400Hz and many meters have too low input resistance/impedance to correctly measure plate voltages or voltage drop across large resistors.

I wouldn't buy a meter without knowing about meter input impedance with DCV and ACV.

[stoneattic]

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

What would some of those good brands be? Are there some to avoid? There are so many brands out there. Some sound good when reading their feature list and specs, but the reviews seem to be by home owners who are just looking for something to check the wall outlet or their car battery, not trying to read an AC audio signal on their tube amp PI anode.

[stoneattic]

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.

I read both of those threads doing my initial research. There's always the guys who have been using Flukes forever and then a few brands I've never heard of, which may be great but I don't seem to find a lot of folks touting them.

[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)?

[Enzo]

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

[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.

[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.

[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?

[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]

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.