Push-Pull Cathode Bias Resistors

[Kashmire]

Question: Why do the KT88 datasheets emphasize that two cathode resistors are essential for push-pull operation with KT88 tubes?

Why not a single shared cathode bias resistor? (bypassed or not bypassed, depending on A/AB operation)

Also, if you feel like debating bypassing the cathode bias resistor, check out the old article just posed on Tube Cad Journal (TCJ):
“Effects of the Cathode Capacitor on P-P Output Stage.”
Excellent article.

[ray_moth]

Question: Why do the KT88 datasheets emphasize that two cathode resistors are essential for push-pull operation with KT88 tubes?

I can only presume that they must be talking about Class AB operation, which ideally requires fixed (grid) bias but can be achieved, to a limited extent, with separate cathode bias resistors, provided that they are bypassed. If you use a single, shared cathode bypass resistor, you are pretty well stuck with Class A, whether you bypass it or not.

I didn't have time to dig for the article in Tubcad but, as I understand it, bypassing a shared cathode resistor is advisable if there is any chance (which there always is) of imperfect matching between the OP tubes. Whether the cap should be between cathodes and ground or cathodes and B+ is also an interesting topic. I believe the latter has some nickname or other - 'Shining Path', perhaps?

[tubewade]

That is an interesting question for which I have no answer. In my datasheets the same recommendation is made for KT66, 6550, and KT90. For the most part these have commonly been run with fixed bias but there are some amplifiers that use them and use a single cathode resistor for both valves; Altec Lansing A340A, Leslie 147, and Leslie/Vibratone 22H, 122 and 145 to name a few.

[Jeb-D.]

I can only presume that they must be talking about Class AB operation, which ideally requires fixed (grid) bias but can be achieved, to a limited extent, with separate cathode bias resistors, provided that they are bypassed. If you use a single, shared cathode bypass resistor, you are pretty well stuck with Class A, whether you bypass it or not.

I'd have to second that assumption.

Related note:
If you have them Pentode wired and sharing a G2 resistor you will still have to add a G2 capacitor in class A. But you can get away without cathode bypass cap. I prefer no bypass cap on shared resistor push pull designs.

[tubewade]

Originally posted by Kashmire
...
Also, if you feel like debating bypassing the cathode bias resistor, check out the old article just posed on Tube Cad Journal (TCJ):
“Effects of the Cathode Capacitor on P-P Output Stage.”
Excellent article.

That is a really excellent article. I found it, along with some other great reading, here...

http://www.aikenamps.com/TI_Other.htm

[Kashmire]

tubewade, good articles. Thanks.


From http://www.aikenamps.com/TI_Other.htm:

For those of you who *still* think class AB amplifiers are "class A at lower volumes", read the difference in this next paper:

This is a nice summary paper. For the A/AB argument, the paper could have stated things a lot more clearly, but I don't think the intent of the paper was to do this.

I've pretty much talked myself into build a fixed negative bias supply, but I'm annoyed with myself for building an amp that will require potentiometer adjustments. DC servo loops would solve this problem, but that's just more circuity, making an otherwise simple amplifier into, well, a less simple amplifier.

Being from the Zen school of thought, it's hard to me to keep throwing components into an amp until I can't draw the schematic onto the back of a napkin easily. Is there a good way to have reliable auto-bias for class-AB amplifiers? It that the big hubub over the CCS threads on this forum? One CCS per valve?

Hmmmm ... maybe back to cathode resistors ... http://www.meta-gizmo.com/tri/sunschematic.html

The Japanese ego is very different, which makes possible such Zen circuits.

It's hard for westerners to understand the Japanese school of thought, but a worthwhile exercise to go through.

K.