Plate current and red plate problem
I have a home build amp, similar to the Fender Twin, 4 6L6GC tubes but with a switchable rectifier, bias and high/low option. See my drawing. It's only the power amp, the preamp didn't fit in this chassis and I'm making that separate, but I can still use it like this. And it sounds quite good too.
New Electro Harmonix 6L6GC tubes
Plate voltages: 404v
Plate current: 90mA per side of two tubes
Screen voltages: 400v
Screen current: 6-7mA per tube
Bias voltage at this current setting: -26v
Calculating the plate dissipation I get 18.2W.
But the plates have a small red spot on them and I can't figure out why at that level of dissipation.
I have set it at 74mA per side with the bias voltage at -35v, plate voltage about 412v and still have the redness. That's the extreme of the bias pots I have. I can go down to -22v but that would clearly be to much current.
What could be causing this red plate issue??
I increased the bias voltage to -41 and the red plating went away. But it measures only 56mA of current for two tubes at 408v on the plates for 11.4W dissipation. Quite low for a 6L6GC.
The pics are at -26v bias, 95mA and 389v respectively. And that calculates to 18.5W dissipation...
The flash pic doesn't show the red at all.
oscillation somewhere? is it producing RF?
It's as quiet as a church mouse. I don't have a scope to use either.
What source are you feeding it with?
The red plate is due to too much current.
Or a parasitic oscillation.
You need a scope to see that.
The 6L6 wants to be run in class AB, not class A at this plate voltage... I'd adjust the values of the resistors that go to ground from the bias pots *up* in ohms so you can get more negative voltage on the grids.
The other thing is to drop the screen voltage and current a bit.
As far as I can tell you are pretty close to proper operating voltages and currents, but these are import tubes, and the best thing to do is to adjust them empirically and not rely upon any mfrs specs.
You *do have negative voltage* there? Just checking...
Also, I don't see why you switch bias points AND switch to a cathode bias as well... I guess it does change the tone a bit, but why not just keep the same negative bias and switch in the cathode resistor of appropriate value??
Have you tried it with the NFB disconnected? What happens then,same thing?
Problem solved. I was using the OT shunt method and tried the cathode resistor method for measuring current. I measured 54.8mV on the cathode. That's my method from now on. I'm not sure if it's my meter or the method but it explains the redness.
"What source are you feeding it with?"
It's at idle. Or I plug my guitar directly into it.
"Also, I don't see why you switch bias points AND switch to a cathode bias as well... I guess it does change the tone a bit, but why not just keep the same negative bias and switch in the cathode resistor of appropriate value??"
It's the only way I know to go from fixed bias to cathode bias. It drops the output power a bit and makes the response spongier and loose-more for blues and rock than for jazz or metal playing-as far as I read...
I have not tried it without the NFB, but the circuit I have lets me get to about 0.8%. The Blackface Twin of the 60's has about 12%. Much cleaner and using the pot I can go from 20% to 0.8%-clean to mean...
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