BTW, that parallel 100k resistor/diode Merlin shows to slow the discharge of the last cap doesn't buy much time. In one time constant, about 1 second, the cap loses 63% of its voltage. I'd make the resistor 470k at least.
When circuits go weird I like to divide and conquer. I would break the circuit at the red X and add a 100K to the cap just for a load. Then operate with just the circuit in the blue oval. Do you have voltage at the diode anode now?
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Holy cow!! What is going on with the Fixed/Cathode bias here?
Can someone explain the bias point that this amp should be set to?
How is it calculated?
Maybe that's why the bias reading (thinking fixed bias A/B) was way too high. Maybe the idle would be set higher with the Fixed/Cathode bias setup in this amp. Maybe I can just take my bias board out and let it eat as is. I mean, that is how it was, stock.
I am puzzled by this one.
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"Education is what you're left with after you have forgotten what you have learned" - Enzo
Measure the voltage on the cathode, divide by the resistance (R98 or R98 and R9 in parallel) and then divide by 2. That should give you the current (obviously not accounting for the fact that the valves could be mismatched).
It's a kind of hybrid fixed/cathode bias setup. I'm not sure of the benefits of this, but I think Merlin might mention it on his website.
Edit: Just checked, it's not. But I did a quick Google, apparently people use these setups to get the best of both worlds......I can't imagine what a mixed cathode and fixed bias amp would sound like....
Yes, mixed bias. The fixed bias voltage is the 28V, the switched cathode resistor network is how the different tube types are accommodated. Seems like a clever way to do that. You should calculate plate dissipation in the usual way, just use the correct Va-k.
My Marshall 4001 (Studio 15) has mixed bias, and there may be others.
