EF80s in push-pull: Is there a role for a bias cap?

[King Fan]

A recent discussion got me wondering about this on my planned Bassman Micro. Let me try to loop in @robrob , who designed this cool cricuit, and maybe @dougsta, who made many us aware of EF80s for PP micro output use.

Here's the Bassman Micro around the EF80s. No bypass cap on the cathode resistor, right? Of course this works; lots of these have been built. OTOH, I see Rob's EF80 version of his 5E3 Deluxe Micro has a traditional 25uF bypass cap on the 300Ω bias resistor. Optional? Pros? Cons?

FWIW, my only possible clue is this from Merlin discussing cathode bias output: "A small capacitor (less than 100uF say) also increases non-linear distortion, which may be significant in hifi. Using a very large capacitor (greater than 470uF say), or using no capacitor at all, reduces this effect."

 

[bebopbrain]

You can bypass the power tube cathodes for more gain and, depending on the value, possibly less bass.

But probably nobody needs the extra gain. The amplification factor (mu) on the EF80 is likely more than the 6L6. B+ is only 300V for the micro, so a smaller output voltage still clips.

 

[King Fan]

You can bypass the power tube cathodes for more gain and, depending on the value, possibly less bass.

But probably nobody needs the extra gain. The amplification factor (mu) on the EF80 is likely more than the 6L6. B+ is only 300V for the micro, so a smaller output voltage still clips.

Ah, that makes a ton of sense. Thank you. And maybe the Bassman Micro needs the gain less than the Deluxe Micro, if I'm guessing right....

 

[robrob]

The EF80s in push-pull are pretty loud so I deleted the power tube bypass cap to cut gain and volume. You can alligator clip one in to see if you like it.

 

[King Fan]

The EF80s in push-pull are pretty loud so I deleted the power tube bypass cap to cut gain and volume. You can alligator clip one in to see if you like it.

Thanks, Rob, I did see your note that at 2.5W they get approximately as loud as many small 5W amps -- and though I don't need 1W bedroom volume, 5F6a sounds at roughly 5F2a volumes sound just right.

Thanks for this and for all you do.

 

[andrewRneumann]

FWIW, I read on another forum that in Class A (both tubes conducting), the AC current from one tube runs up the other tube. This is effectively reducing the cathode impedance to near zero. There will be some gain loss once one tube stops conducting, but posters reporting not being able to hear the difference. Makes sense to me.

If there was a difference, it would be in the high drive regions, not at low volumes.