Very high idle current WEM Dominator Bass 25

[Stratfordade]

I'm frankly unfamiliar with cathode biased amps, which is what the WEM appears to be from the schematic, but I'm struggling to make sense of the idle currents I'm seeing and the implied dissipation.

Since it's cathode biased I'm not using my bias probe (which only switches one valve into the circuit at a time) but just measuring voltage across the 100 ohm cathode resistor for the EL84 output pair with no signal and controls at zero.

There's 312V on the anodes, and 9.3V across the cathode bias resistor (100 ohm on schematic and measures 99 ohm). This implies 46.5mA idle current per EL84, but that would be 14.5W which is close to maximum power for an EL84!

The amp actually sounds pretty good, and if I put a scope across a dummy load it will run 12.2W RMS into an 8 ohm load (the speaker in the cabinet is a 15 ohm, and I believe the amp is nominally 15W so the impedance mismatch may account for the slightly reduced output power).

What am I doing or thinking wrong to calculate such high idle power? If I do use my bias probe the the EL84s run well matched with 77 and 74mA idle current when switched in one at a time, and they test well-matched on an AVO so Idon't think either has a problem accounting for the high idle current. If I look at a data sheet then an EL84 with -10V bias (less bias than my -9.3V) has anode current of about 25mA at 300V, not the 46.5mA I'm seeing.

Thanks for any advice.
Dominator_mk3.pdf

[Jazz P Bass]

Personally, I wouldn't worry about that idle current.
Designers tend to run the heck out of EL84 tubes.
It could be that set of tubes runs hot with that 100-ohm resistor.
If you feel uncomfortable with it, change the cathode resistor with a higher value.

[DrGonz78]

I find every set of tubes runs hot in a cathode biased circuit that uses a 100-150 ohm resistor. I usually pick 200-300 ohm value for the cathode resistor because of this issue. I usually get it around 95% dissipation or so.

[Stratfordade]

Thanks both. I didn’t realise that cathode bias runs such a high idle power (am very used to the 60-70% of fixed bias). Agree that whatever the amp, EL84s always run very hot! I’ll probably keep the 100 ohm as is since the two channel Dominator has no master volume, so needs to be turned right up to get any kind of grind. With the 100 ohm I expect a suitable pedal would help get a wider range of sounds from it. It’s a lovely thing to work on anyway

[glebert]

I don't know if this is the "right" way to think about it, but to me cathode biasing is a form of negative feedback. More current through the tube pushes the cathode voltage up, reducing the grid to cathode voltage (making it more negative) which reduces the current from anode to cathode. When you have a fixed bias voltage on the grid you don't get this sort of self regulation, so you need to have the 30% or so headroom.

[Stratfordade]

I don't know if this is the "right" way to think about it, but to me cathode biasing is a form of negative feedback. More current through the tube pushes the cathode voltage up, reducing the grid to cathode voltage (making it more negative) which reduces the current from anode to cathode. When you have a fixed bias voltage on the grid you don't get this sort of self regulation, so you need to have the 30% or so headroom.

That’s useful, thank you

[J M Fahey]

Thanks both. I didn’t realise that cathode bias runs such a high idle power (am very used to the 60-70% of fixed bias). Agree that whatever the amp, EL84s always run very hot! I’ll probably keep the 100 ohm as is since the two channel Dominator has no master volume, so needs to be turned right up to get any kind of grind. With the 100 ohm I expect a suitable pedal would help get a wider range of sounds from it. It’s a lovely thing to work on anyway

We deserve a gutshot or two

[Stratfordade]

Here you go

[Mars Amp Repair]

Hello Stratfordade,
I don't know if you've considered this, but since the two tubes share one cathode resistor, the current/Idle wattage calcuated would be 1/2 of that per tube. It seems that it would make the idle current/wattage considerably less and they would be pretty cold. Doesn't seem as if that's the case here. Just a thought.

[g1]

Is that dirt or soot on the board? Seems odd that it would build up such a layer like that unless the chassis was left out in the open for a long time. I would worry about stray resistance where the dirt remains.

[Stratfordade]

Hello Stratfordade,
I don't know if you've considered this, but since the two tubes share one cathode resistor, the current/Idle wattage calcuated would be 1/2 of that per tube. It seems that it would make the idle current/wattage considerably less and they would be pretty cold. Doesn't seem as if that's the case here. Just a thought.

I think I’m considering that. 93mA through common resistor so around 46mA per EL84. That’s high but I understand adjustable bias does run very high idle.

[Stratfordade]

This photo in daylight shows more clearly that it’s just dusty areas and nothing dramatic.

[g1]

Looks better in that pic but still a lot more dust than I've seen in chassis that are closed up with no fans. I'd give it a blast with an air compressor if you can, arcing loves dust.

[The Dude]

If that doesn't work, I'd get out the component cleaner and a toothbrush.

[Stratfordade]

Both good suggestions (toothbrush and air) so will try both! I’ve only given it a cursory clean so far and was focused on cleaning pots and changing power supply caps. All now performing ok so can focus on cleaning and other tasks including replacing speaker cloth which is very threadbare in parts. Anyway I have a good compressor so will try air first on that dust.

I’d not seen dust like this before but the cabinet is not actually sealed. There is a small gap between front panel and top of cabinet and it’s apparently designed that way which was a bit sloppy of WEM! That’s using the original holes in the cabinet for the chassis screws. They may have been put in wrong position originally so that’s something else I can investigate now the electronic side is done.