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

[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 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.

I used the Craftsman DMM. It claims to be good at 400Hz. I will try with the HF tonight.

[stoneattic]

Bias is the cathode/grid voltage, current through the tube is what the bias adjusts. So current is what we measure so we can set the bias to achieve the desired flow.

I guess I need to remember that it is FLOW that matters. So what does it mean when I'm measuring -51VDC bias but measuring no current with the bias probe? This is really stumping me.

[Helmholtz]

It looks like the DMM doesn't like frequencies much above 150Hz as it appears to struggle even at 300Hz. I have no idea what sort of VAC values I should be reading there.

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.

[Enzo]

Bias is the cathode/grid voltage, current through the tube is what the bias adjusts. So current is what we measure so we can set the bias to achieve the desired flow.

[stoneattic]

If R71/R61 are the PI plate resistors, they should have signal only at one end (plate side) as the other ends should be tied to B+ (DC).
Please try to measure signals directly at the power tube socket pins 5. You can use a 400V series cap as long as circuit point voltages are below 400 VDC.

Also measure power tube screens DCV (pins 4).

I don't know what the voltage should be at pin 5 on the power tubes since I don't have schematic, but my previous measurements were below 400 VDC so I soldered a .1µF/400V cap on the tip of a probe and measured the following on the bad amp:

EL34 #1
Pin 5
@150Hz sine = 1.095 VAC
Pin 4 = 374 VDC
@300Hz sine = 1.45-1.51 VAC (bouncing around in this range)
Pin 4 = 375 VDC

EL34 #2
Pin 5
@150Hz sine = 1.203 VAC
Pin 4 = 374 VDC
@300Hz sine = 1.58-1.66 VAC (bouncing around in this range)
Pin 4 = 375 VDC

It looks like the DMM doesn't like frequencies much above 150Hz as it appears to struggle even at 300Hz. I have no idea what sort of VAC values I should be reading there.

[Helmholtz]

I did get a chance to try and measure AC signal last night. I set the function generator to 150Hz and measured the following using the Craftsman DMM:
Note the first reading is on the lead of the resistor closest to the EL34s and the second reading the other lead.

"Good" amp
R71: 29.56/9.01
R61: 29.59/13.60
R68: 18.30/3.10
R67: 18.39/2.91
PI pin 1: 13.49
PI pin 6: 9.14

"Bad" amp
R71: */11.23
R61: */12.18
R68: 18.59/2.96
R67: 18.47/2.81
PI pin 1: 12.25
PI pin 6: 11.30

The "*" indicates I could not get a reading. However I'm not sure that means there is not a signal there. I had a tough time getting a reading on pretty much all of the measurements. The meter would jump all round including "OL" pretty much every time I took a reading. If I would lift one of the leads and try again sometime it would give a steady reading. Sometimes it took multiple tries before I would get a steady reading. It seems like the DMM was "confused" easily. I did try MANY times to get the 2 readings indicated by the "*" but eventually gave up. It doesn't make sense to me to have a reading on one side of the resistor and not something on the other side.

If R71/R61 are the PI plate resistors, they should have signal only at one end (plate side) as the other ends should be tied to B+ (DC).
Please try to measure signals directly at the power tube socket pins 5. You can use a 400V series cap as long as circuit point voltages are below 400 VDC.

Also measure power tube screens DCV (pins 4).

[stoneattic]

I checked the idle current again on the bad amp, making sure I didn't anything stupid and still read 0. I measured the cathodes to ground as glebert suggested and read 5.1 ohms on the bad amp and 5.2 ohms on the good amp. I check the bad amp with and without the EL34s in place and got the same reading. I'm not sure what to make of that.

I don't have a cap with a voltage rating above 400 so I can't use that as a DC blocker to read AC with my DMMs or to use to put a signal tracer together.

[stoneattic]

I did get a chance to try and measure AC signal last night. I set the function generator to 150Hz and measured the following using the Craftsman DMM:
Note the first reading is on the lead of the resistor closest to the EL34s and the second reading the other lead.

"Good" amp
R71: 29.56/9.01
R61: 29.59/13.60
R68: 18.30/3.10
R67: 18.39/2.91
PI pin 1: 13.49
PI pin 6: 9.14

"Bad" amp
R71: */11.23
R61: */12.18
R68: 18.59/2.96
R67: 18.47/2.81
PI pin 1: 12.25
PI pin 6: 11.30

The "*" indicates I could not get a reading. However I'm not sure that means there is not a signal there. I had a tough time getting a reading on pretty much all of the measurements. The meter would jump all round including "OL" pretty much every time I took a reading. If I would lift one of the leads and try again sometime it would give a steady reading. Sometimes it took multiple tries before I would get a steady reading. It seems like the DMM was "confused" easily. I did try MANY times to get the 2 readings indicated by the "*" but eventually gave up. It doesn't make sense to me to have a reading on one side of the resistor and not something on the other side.

[stoneattic]

On some websites it is labelled a class A, but this is how Bugera describes it. "Switchable Class-A/AB operation for ultimate power amp voicing: Class-A for classic warmth and Class-AB for raw, high-speed power" Maybe the mode switch is a place to look for a problem? I would try to do a resistance measurement from cathode (pin 8) to ground on a cold amp.

Yes, there is an A/AB switch. I'm not sure what it really does since I don't have schematic. But I was set to AB in both amps. I like the way it sounds there better.
I will measure the switch and cathode to ground on both amps when I get home tonight.

[stoneattic]

Bias is a voltage between cathode and grid, negative in this field.
The 'Ik' reading will probably be cathode current at idle, which is controlled by the bias (voltage) but it is not, per se, bias. Ik might be referred to as, idle / static / quiescent (cathode) current

Previously, about -50Vdc was measured on the 'bad amp' EL34 control grids; that's the bias, it's probably excessive, hence this is not a 'no bias' scenario.

Sorry for being a pedant but using terminology incorrectly won't be helping communication or analysis here.


The 350V triode plot on p8 of https://tubedata.altanatubes.com.br/...129/e/EL34.pdf indicates about 10mA.


If the 'good amp' is only drawing about 30mA idle cathode current, I don't think it's likely to be intended to be class A.

Thanks for the clarification on the terminology. I guess since the probe is sold as a bias probe I assumed that the current reading is the "bias".

[stoneattic]

Maybe I'm missing something, but it seems like with -51V bias you should get some current.

Wait a minute, this is a class A amp? I can't think of a EL34 based class A amp to even get a frame of reference, much less a 50W amp. Wikipedia says the bias needs to be much higher than push pull, so maybe -51V would give zero current. Could it be a blown cathode resistor?

It doesn't make sense to me either. I'm wondering if I screwed up the current reading doing something stupid like not taking it off standby, but I would think if that were the case I would not have read plate voltage. Doesn't standby typically mean only the heaters are on? I'm going to measure again when I get home tonight.

[glebert]

If the 'good amp' is only drawing about 30mA idle cathode current, I don't think it's likely to be intended to be class A.

On some websites it is labelled a class A, but this is how Bugera describes it. "Switchable Class-A/AB operation for ultimate power amp voicing: Class-A for classic warmth and Class-AB for raw, high-speed power" Maybe the mode switch is a place to look for a problem? I would try to do a resistance measurement from cathode (pin 8) to ground on a cold amp.

[pdf64]

...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

(using the same tubes in both amps)

So it looks like this confirms that there is no bias on the EL34s...

Bias is a voltage between cathode and grid, negative in this field.
The 'Ik' reading will probably be cathode current at idle, which is controlled by the bias (voltage) but it is not, per se, bias. Ik might be referred to as, idle / static / quiescent (cathode) current

Previously, about -50Vdc was measured on the 'bad amp' EL34 control grids; that's the bias, it's probably excessive, hence this is not a 'no bias' scenario.

Sorry for being a pedant but using terminology incorrectly won't be helping communication or analysis here.

Maybe I'm missing something, but it seems like with -51V bias you should get some current...

The 350V triode plot on p8 of https://tubedata.altanatubes.com.br/...129/e/EL34.pdf indicates about 10mA.

...Wait a minute, this is a class A amp?...

If the 'good amp' is only drawing about 30mA idle cathode current, I don't think it's likely to be intended to be class A.

[glebert]

So it looks like this confirms that there is no bias on the EL34s.

Maybe I'm missing something, but it seems like with -51V bias you should get some current.

Wait a minute, this is a class A amp? I can't think of a EL34 based class A amp to even get a frame of reference, much less a 50W amp. Wikipedia says the bias needs to be much higher than push pull, so maybe -51V would give zero current. Could it be a blown cathode resistor?

[stoneattic]

I still need to trace the signal, but just received my socket savers that I needed to connect my bias probes. The sockets are PCB mounted and too deep into the chassis for my probes to fit. Anyway, here are the results:

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

(using the same tubes in both amps)

So it looks like this confirms that there is no bias on the EL34s. Which I assume means that the Infinium circuit is not functioning. I will go ahead and try to trace the signal at a lower frequency just for completeness, but it looks like the problem is going to beyond what I will be able to do with the Infinium circuit. I'm thinking now my best bet is to pull the board out and look for obvious issues on the back side (cold solder, etc.) and if I don't find anything maybe look towards building a fixed bias circuit for it.