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

[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]

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.

[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?

[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.

[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.

[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]

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]

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?

[glebert]

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.

[Helmholtz]

I used the same probes, meter and EL34s on both amps:

Good Amp
Ik (bias current): 33.2/33.6
Vp (plate voltage): 355.6/355.6

Bad Amp
Ik (bias current): 0/0
Vp (plate voltage): 365.4/364.0

Since it is not impossible that I screwed something up taking the readings, I took them again the following night with the same results. This is the probe I have:
https://images.reverb.com/image/uplo...qk9osetdwh.jpg

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.

[stoneattic]

As he has reasonable plate and screen voltages and obviously 5R cathode to ground resistance, I strongly doubt that there is zero idle current. Rather I suspect that the meter doesn't correctly display mV readings.
Unreliable/unsuitable meters (and bad meter batteries) can waste a lot of time.

I used the same probes, meter and EL34s on both amps:

Good Amp
Ik (bias current): 33.2/33.6
Vp (plate voltage): 355.6/355.6

Bad Amp
Ik (bias current): 0/0
Vp (plate voltage): 365.4/364.0

Since it is not impossible that I screwed something up taking the readings, I took them again the following night with the same results. This is the probe I have:
https://images.reverb.com/image/uplo...qk9osetdwh.jpg

[Helmholtz]

On pin 4 I measure: "Bad amp" 381 VDC, "Good amp" 365 VDC
Is ~4% difference normal/acceptable? Is the bad amp being higher an indication of something? But either way there is B+ there.

I can see the heaters glowing on both EL34s. Is is possible that they are not heating enough?

On pin 3 I measure: "Bad amp" 382 VDC, "Good" 368 VDC
The same relative difference between the good and bad amp. This would seem to indicate that the heaters are working, correct?

I measured 5.1 ohms on the bad amp and 5.2 ohms on the good amp between pin 8 and ground. It's higher than the 0 to 1 you stated. Is that high enough to be a problem? Could the Infinium circuit throw off some measurements vs a normal fixed of cathode bias setup?
I found a schematic (only 2 out of 3 pages though) online that appears to be someone's attempt at tracing/reverse engineering an Infinium model. I'm not sure what model, but it's definitely not the T50 because it has 4 EL34s. Maybe the 1960 or 333XL? It shows 4.7 ohm resistors between the cathodes and ground. If the T50 has the same 4.7 that could be what I'm reading.

In all of the above measurements I got~same between both EL34s so I only listed the measurement once.

What are cathode (pin 8) DC voltages?

This would seem to indicate that the heaters are working, correct?

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

[Helmholtz]

It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

-51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.

As he has reasonable plate and screen voltages and obviously 5R cathode to ground resistance, I strongly doubt that there is zero idle current. Rather I suspect that the meter doesn't correctly display mV readings.
Unreliable/unsuitable meters (and bad meter batteries) can waste a lot of time.

[stoneattic]

It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

-51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.

On pin 4 I measure: "Bad amp" 381 VDC, "Good amp" 365 VDC
Is ~4% difference normal/acceptable? Is the bad amp being higher an indication of something? But either way there is B+ there.

I can see the heaters glowing on both EL34s. Is is possible that they are not heating enough?

On pin 3 I measure: "Bad amp" 382 VDC, "Good" 368 VDC
The same relative difference between the good and bad amp. This would seem to indicate that the heaters are working, correct?

I measured 5.1 ohms on the bad amp and 5.2 ohms on the good amp between pin 8 and ground. It's higher than the 0 to 1 you stated. Is that high enough to be a problem? Could the Infinium circuit throw off some measurements vs a normal fixed of cathode bias setup?
I found a schematic (only 2 out of 3 pages though) online that appears to be someone's attempt at tracing/reverse engineering an Infinium model. I'm not sure what model, but it's definitely not the T50 because it has 4 EL34s. Maybe the 1960 or 333XL? It shows 4.7 ohm resistors between the cathodes and ground. If the T50 has the same 4.7 that could be what I'm reading.

In all of the above measurements I got~same between both EL34s so I only listed the measurement once.

[Enzo]

So what does it mean when I'm measuring -51VDC bias but measuring no current with the bias probe? This is really stumping me.

It means the tube has some reasonable bias voltage, but is not conducting current. Why would a power tube not conduct current? First suspect is open screen grid resistor. Is there B+ on pin 4? Next maybe the heater is not running on that tube, look and see. Also if the B+ never reaches the plate, so is there B+ on pin 3? Or the cathode circuit is open, measure resistance to ground from pin 8, it should either be zero ohms or maybe 1 ohm when using a sense resistor.

-51v is higher than I would expect for an EL34, but not near enough to put it in cutoff.