Hi,
In my particular considered implementation, of course without signal.
Essentially all this circuit would do is to take a "bog standard" fixed bias system (well, almost, electronic pots do not like 100V, so we need some DC Amplifiers to give us usable bias levels).
Then simply use a PIC (with build in A2D and MUX to sample the tube current) and via cereal control (pun intended) adjust our electronic bias pots. I would also drive some "mute" relays off that MCU, the rest is actually really quite simple.
We can invoke the automation based on a number of conditions. Obviously one check would be every turn on, with enough delay to allow tubes to warm up and supplies to settle.
Program in a set of maximum tolerances for both pair bias balance and absolute values, if the actual tube currents are outside tolerances, adjust them, otherwise leave well enough alone.
Basically the same one would do with a bias meter and a set of pots etc. on the Amp. The concept was considered for an OTL Tube Amp, BTB...
Ciao T
So at which moment in time should one press the button
In my particular considered implementation, of course without signal.
Essentially all this circuit would do is to take a "bog standard" fixed bias system (well, almost, electronic pots do not like 100V, so we need some DC Amplifiers to give us usable bias levels).
Then simply use a PIC (with build in A2D and MUX to sample the tube current) and via cereal control (pun intended) adjust our electronic bias pots. I would also drive some "mute" relays off that MCU, the rest is actually really quite simple.
We can invoke the automation based on a number of conditions. Obviously one check would be every turn on, with enough delay to allow tubes to warm up and supplies to settle.
Program in a set of maximum tolerances for both pair bias balance and absolute values, if the actual tube currents are outside tolerances, adjust them, otherwise leave well enough alone.
Basically the same one would do with a bias meter and a set of pots etc. on the Amp. The concept was considered for an OTL Tube Amp, BTB...
Ciao T
Thanks Stuart. I concede that Morgan's is an elegant solution. Sadly for me, my 3rd edition already had a bookmark between P420 & P421!What, you don't have a copy of Valve Amplifiers????
Morgan's circuit took the usual analog comparater and combined a clipper at twice the idle current (well, twice the sense voltage off a cathode resistor that represents the idle current). That way, the circuit won't cause an AB-biased amp to go overbiased when the amp is run past class A. When I get home tonight, I'll give you a page number where you can find the circuit. The Audionics (Berning) circuit was a pile of logic chips which sampled the voltage on the sense resistor when the AC output of the amp dropped below 0.7V, then ratcheted the bias supply regulator (also digital) accordingly to match the reference voltage. I think the schematic is still in the Support section of Berning's website.
Basic details please
Not wishing to be spoon-fed
Well, I did something sorta similar, by using a simple mosfet referenced by zener across the cathode-resistor to act as a Clamp.....
The operating point/voltage of the clamp is set to a little above the normal self-biassing potential of the valve.
At the point where the stage would start to go into blocking-distortion by its bias volts charging the cathode-by-pass cap, the clamp then holds the voltage from going any further into incorrect bias...
Upshot is, It does work, but you'll still not get the full power as you're still losing volts across the cathode-resistor....
This can be compensated for in the design by increasing the supply volts accordingly--It aint perfect, But it does have merit....
MCU's, Op-Amps, and all that complicated stuff--Who needs it! Doing it this way--You Dont Need an additional Negative Bias supply as is most often needed....
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