Resistive plate load for 6V6

[lowell]

I want to use a 6V6 in my new design, but not coupled via an OT to a speaker. Instead to another following stage. I'm thinking this isn't doable with a typical 360v B+ and an Ra of 50k. (ignoring the input z of the following stage for now)

Using plate curves for the JJ 6V6 and in single ended operation:

Va=180v (B+/2)
Ra=50k
Quiescent current of 3ma !!!
I'd need a 6.7k Rk to bias here!!

THAT'S HALF A WATT!! Ha... I'm laughing at myself now writing this. But is this an issue if I'm not going for power and into another stage? FYI I'm doing this to get pentode overdrive.

Will the voltage on the plate will still move up and down as normal? The current can only go from 0-3ma.

Maybe I'm going about this all wrong and need to use an EL84 which has a 5.2k Ra? Damn, I wanna use a 6V6.

[tubeswell]

The plate resistance (ra) can vary enormously with the screen voltage. AFAICT the ra of a 6V6 can be between 50k to 100k under varying screen voltages. Going by OTM's rule of thumb for 1/6th- 1/8th of the plate resistance as a load for pentodes; Ra would be somewhere between 5k6 and 8k2 (for 50k ra) to between 12k to 16k (for a 100k ra). Take your pick. The output impedance would be high tho'. And you may need a hefty plate resistor methinks. No doubt somebody will chime in if I'm too far out with these guestimates.

[GregS]

I would think that the sound you would get from it operating under those conditions would not be what a 6V6 normally sounds like. Also, shouldn't the current go above 3mA on the positive part of the signal since 3mA is the current at quiescence?

[lowell]

Yeah I know the sound this will make might be unexpected. I think it will still sound cool and am gonna find out. Yes i think youre right about the 3ma. How did OTM come about those pentode load values of 1/6 or 1/8.

[tubeswell]

Its one of his rules-of-thumb. Why don't you ask him?

[lowell]

I sent him a pm.

[Steve Conner]

Just use a resistor of the same value as the recommended OT primary impedance for the tube. 4k, 5k or whatever. "Ra" for pentodes isn't really meaningful: it varies a lot with plate and screen voltages. It's not like a triode, where the load impedance has to be equal to Ra for maximum power output. For a pentode, the load impedance basically has to be whatever the manufacturer says. He's already done the hard work to figure it out.

Or, use a Champ OPT with an 8 ohm dummy load on the secondary, to get some of that transformer distortion too. A cheap SE OPT makes a very real difference to the sound, compared to taking the signal directly off the plate. To me at least, it makes it sound more "tubey".

[lowell]

Ok Steve thanks for the info. I'm gonna try it w/o the OT. But will eventually try the OT too to compare just for my own ears.