Bugera V5 Infinium Low Wattage

[tdlunsfo]

Customer complaint is it appears to be losing wattage. I thought for sure this would be one of those "perception of loudness" issues and my watt meter would prove them wrong. Well.... with the amp cranked I am getting 1 watt of what should be 5 watts. Double checking my numbers I measure 1.9V max AC into the 4 ohm speaker and that calculates out to 0.9 watts. I disconnected the power attenuator and got the same result. All my voltages are there, DC filaments, 15V, 5V, and B+ on my plates. Scratching my head on this one and I am sure it is right in front of my face. I want to blame this auto bias Infinium circuit but it doesn't appear to be the cause that I can tell. I threw my bias probe on the EL84 and got 37mA at idle at 300plate volts. Another strange one is if I dime all the knobs (minus the reverb) the power light starts blinking within about 10 seconds. I assume this is some kind of "alarm" to the user although the manual doesn't mention anything about it. Oh and the tube life circuit led is off throughout all these tests. Tubes were both verified to be good on my hickock.

ACV through out at 400Hz:
12AX7 Pin 2 - 80mv
12AX7 Pin 1 - 5.3V
12AX7 Pin 7 - 1.8V
12AX7 Pin 6 - 59V
EL84 Pin 2 - 21V
EL84 Pin 7 - 47V
Speaker - 1.9V


Thoughts? What am I missing?

Attached is part of the V5 schematic, I couldn't find an Infinium version. No voltages listed that I can see so no help really.

[Helmholtz]

Check EL84 screen voltage at pin9.

[Enzo]

And always check things in the speaker line, like the headphones jack cutout contact and the power selector switch. Also, monitor signal level at someplace like the top of the volume control and top of the gain control. This thing is little more than a Champ.

[Helmholtz]

Are your AC voltages peak-to-peak or RMS?
21Vrms at the grid should be far more than enough to drive the EL84 into full saturation/clipping.
Actually 6Vrms would be sufficient for full output power.

[tdlunsfo]

Screen voltage is 280v. I have checked the headphone jack but will check again. Agreed it is simple but for the life of me I can't find the smoking gun.

[tdlunsfo]

Measured with my Fluke 87V so true RMS and yes it is clipping as expected as it is dimed. I guess I would just expect more than 1.9v rms at the speaker. Could I just be getting confused in my calculations? Wouldn't be the first time.

[Helmholtz]

Could I just be getting confused in my calculations?

Calculation is correct. 5W would correspond to 4.5Vrms into 8R. Did you try a new EL84?

BTW, what is EL84 cathode voltage?

[tdlunsfo]

About 15v cathode to chassis and yes I tried a used old stock EL84. No difference.

[Helmholtz]

About 15v cathode to chassis and yes I tried a used old stock EL84. No difference.

THat looks extremely high, would correspond to a tube current of 63mA. Does R9 measure 240R?

[tdlunsfo]

238R with it off but I also read that these have the autobias circuits which I don't have a schematic for (proprietary?) so I wouldn't begin to understand what is going on with the bias. I left my bias probe on the EL84 and cranked it. Bias jumped up to 60mA but immediately started to fall down to 45mA. Removed the audio it drops down to 10mA and slowly climbs back to 37mA idle.

[Helmholtz]

238R with it off but I also read that these have the autobias circuits which I don't have a schematic for (proprietary?) so I wouldn't begin to understand what is going on with the bias. I left my bias probe on the EL84 and cranked it. Bias jumped up to 60mA but immediately started to fall down to 45mA. Removed the audio it drops down to 10mA and slowly climbs back to 37mA idle.

Independent of any additional autobias circuit 15V across 238R means 63mA idle current. So we have some contradiction here. I assume all (idle) measurements were done with zero signal as large signal increases cathode voltage.

[tdlunsfo]

My apologies, at idle cathode voltage is 9.7v.

[Helmholtz]

Now that makes more sense. 9.7V across 238R means a cathode current of 41mA. That includes a screen current of around 4mA so that plate current is 37mA. Fine.

I would do the following: Remove both OT secondary wires from circuit, directly connect 4R load to the wires and measure output again.

[tdlunsfo]

Ok. 1.6vAC rms into 4ohms measured at the OT secondary connector (disconnected) with 80mv input signal applied and tone, gain, and volume all the way up.

[Helmholtz]

Is that just at the onset of clipping? It's even lower than you reported earlier. What is B+ at full output?
I'm beginning to suspect the OT, especially as plate signal should be more like 150Vrms at full output.