Constant current cap coupled reverb question

[octal]

I built a slightly tweaked version of Merlin's 12au7 constant current reverb driver circuit... the results were excellent IMO, but I have a question.

He (and others) state that the driver needs to have an output impedance of 10X the input impedance of the tank to approximate a constant current source. In his example, he has 2 X 12AU7 in parallel with a 22K anode load and a 22K series resistor driving a 1.5-1.9K reverb tank.

Wouldn't the output impedance of the amp stage also count towards the total constant current impedance? Could you use an amp with a higher output impedance and reduce the value of the series resistor?

Furthermore (general tube theory question) how is it possible for a gain stage to have a lower output impedance than the value of the plate/anode resistor? I mean, there's no way the circuit can source more current than whatever the maximum possible current deliverable by B+ in series with the plate resistor to ground. I understand that the Rp of the tube is usually far less than the value of the plate resistor, so I guess that can pull the AVERAGE output impedance down... but it seems like it's dramatically different for sinking versus sourcing current. Am I dead wrong here?


Nathan

[Merlinb]

Wouldn't the output impedance of the amp stage also count towards the total constant current impedance? Could you use an amp with a higher output impedance and reduce the value of the series resistor?

Yes you could do that. The 10x rule is not a mathematical exactitude, it's just a rule of thumb.

Furthermore (general tube theory question) how is it possible for a gain stage to have a lower output impedance than the value of the plate/anode resistor?

Total output impedance is ra in parallel with Ra. In other words, it's an average that applies for a whole audio cycle. However, at a given point in time the output might be anywhere between Ra and ra. When sourcing current the output impedance is Ra, and when sinking current it is ra. Most of the time we don#'t worry about it, but it can be important when driving capacitive loads, resulting in distortion (slew limiting) if the source impedances aren't equal in both directions.

[kg]

like merlin points out, don't confuse output impedance with current capacity. despite the fact that they seem directly related, they are actually two separate entities... coincidental, not causal.

it is possible to have a circuit which shows very low output impedance yet runs into slew issues constantly. consider a plate loaded 12ax7 with a local feedback loop run at 0.5mA. very low output Z, capable of extraordinary frequency response, but ask it to push/pull current and it'll slew constantly.

conversely, consider a power pentode (ie el34) run at high plate current (ie 100mA)... incredible current sink/source capacity--no slew issues here!--but also very high output impedance.

[octal]

Thanks to both of your for your responses.

Would the unequal sourcing/sinking abilities also be an issue driving inductive loads? (Such as reverb tank drive coils?)

Nathan

[Merlinb]

Would the unequal sourcing/sinking abilities also be an issue driving inductive loads? (Such as reverb tank drive coils?)

Potentially yes, but only if there's no significant resistance in series with the coil. Unlikely in any practical circuit.