Removing Cathode Bypass Cap on Power Tubes

[Mr. Lime]

I'm currently planing an EL84 PP amp with switchable PI / Self Split.
Because SS doesn't work with a bypass cap (turns the amp to single ended), I'm wondering if the bypass cap can be left out for the classical push pull with cathodyne mode and how it will sound?

Haven't found a lot of reading regarding this. Some write the bass response will suffer, some write it will cost gain and output.
There are postings where people write they like the unbypassed PA better and get more sag because the tubes draw more current.
I understand that the bypass cap blocks feedback, so it's a kind of feedback going on in the power valves?

Has someone experience with removing the cap and tweak the rest of the power amp circuit?
Any good results?

Sure a switch can be added later with a cap to have 4 modes:
1) PI + cap
2) PI without cap
3) Self Split
4) SE

[pdf64]

For valid responses, a schematic sketch would helpful.

[Stevem]

Even the cheapest Cathode biased Hi Fi 10 watt mono tube amp back in the day had a 10 uf bypass cap for bass responce atleast down to 100 HZ across the outputs cathode resistor.

And a bypass cap across any type of tubes cathode will always add gain, this is a fact.

[pdf64]

Even the cheapest Cathode biased Hi Fi 10 watt mono tube amp back in the day had a 10 uf bypass cap for bass responce atleast down to 100 HZ across the outputs cathode resistor..

Would need to know the tube type, cathode resistor value etc before being able to work out the corner frequency.
http://bmamps.com/CapCal.html

...And a bypass cap across any type of tubes cathode will always add gain, this is a fact.

Have you considered the case of a push pull pair with a shared cathode resistor operating in class A? Given well balanced input signals, then due to the opposing signals cancelling each other out at the cathodes, there will be negligible Vac on the cathode, hence no gain increase from a bypass cap.

[Mr. Lime]

Thanks for the response.

For valid responses, a schematic sketch would helpful.

Meanwhile I found a solution for a SS / PI switching that incorporates a bypass cap when the PI is engaged.

Would need to know the tube type, cathode resistor value etc before being able to work out the corner frequency.
http://bmamps.com/CapCal.html

I'd like to use EL84s so they will distort pretty quick - Class A won't last long I guess..

In the schematic the bypass cap is removed in self split mode.
Not sure if R14 paralleled to R16 is really necessary against popping. Otherwise the switch could be placed in front of C2. Shouldn't AC be avoided on a switch?

EL84 PA mit PI SS.PNG

[bmx]

That's a cool design. Self Split needs to be Class A, so both options are classA? Which means no sag as current is constant or possibly decreases with increasing signal depending (the opposite of sag).

[Mr. Lime]

Depends on the B+ I would say.
But it's not my design goal for the PI mode so I would rather have class AB biased..
I think SS is class A as long the power tubes don't distort?

I would use a 3PDT On-Off-On switch as power or standby switch so the amp is turned off between switching the modes.
If the switch is placed hard to reach someone could use an On-On-On to get 3 modes. Like single coil mode or bypass cap removed, but remember to turn the amp off before switching..

[Stevem]

I can't recall any of the 1000s of cathode biased push pull HI FI amps that I worked on in the 60s and early 70s that where biased in class A, but yes your example is true but that's not what I posted about.