ping SY: Red Light LED question

[zobsky]

Not sure if I missed this in your Red Light article, .. but if I had non matched (or slightly unmatched) output tubes, wouldn't I need to put in a pot. to equalize the bias on both tubes?

For the record, I'm thinking of using this LED biasing scheme on a 2A3 PP project. Do you see any issues with this, apart from the large number of LEDs needed?

Thanks

[SY]

Well, there's the large number of LEDs needed...

Ah, anyway, regarding output stage balance, you can do one of two things. The easiest is to use matched output tubes. The vendor I buy JJ EL84 from, Triode Electronics, doesn't charge extra for matched pairs and quads. The harder, but more purist, way is to use separate screen regulators for each output tube. That's not as onerous as it sounds since each reg is pretty simple, small, and uses cheap parts.

Lazy guy that I am, I used the matched tubes.

[zobsky]

Thanks SY.

This project is for a PP 2A3. I did plan to buy matched 2A3 s, but as they obviously don't have screen grids, I wanted to know if building in a balance pot across the cathodes would work (may need it if unlucky, or when the tubes age), or would that negate the excellent overload recovery behaviour of the Constant Voltage LED array?

[SY]

If you keep the adjustment range moderately small, then you don't need to degenerate, thus no problem with blocking from cathode capacitors. The overload recovery problem then transfers to the grids. With a pentode, you can take care of that with a large resistor in the stopper position. That won't work well for triodes, so you're looking at a cathode (or source) follower to drive them, at least if you want to equal the overload recovery characteristics of the original RLD.

[zobsky]

Originally posted by SY
The overload recovery problem then transfers to the grids. With a pentode, you can take care of that with a large resistor in the stopper position. That won't work well for triodes, .......

The grid in this case will be loaded into a recently purchased electra-print phase splitting autoformer ( something like this http://www.magnequest.com/diy_lessard_circuit_2a3pp.htm )
which shouldn't have a high resistance (at least compared to the value of a typical grid resistor).

Is this idea still kosher?

[SY]

You're going to make me calculate that LC time constant, aren't you?

Assuming that the LC reaches down into the deep bass, my guess is that you're still compromising the grid recovery.