Making a very low power consumption tube amp.

[ELS]

I want to make a very efficient, no need for a lot of volume, tube amp. For background listening.
But I feel some ideas might also be relevant in guitar amps.
I was thinking of using a single 6N6P double triode for the output tube. The heater is 600mA @ 6.3V so it's still a bit much.
The biggest dilemma is to find a tube with low heater power draw, but high Gm so I can use an 8k OPT. So around 11.3mA/V for gm requirement.

I also want it to sound very airy. like an easy-listening amp you could say. From what I read SET's most often sound like that.
I have 2 tube types that seem promising.
6FC7/6N24P: 12.5mA/V 300mA heater (basically the same as 6DJ8, but cheaper)
6N6P: 11mA/V 600mA heater

But now I'm questioning myself... it IS gm that changes the ideal transformer load right?
Just seems a bit unlikely that a 12AX7-sized tube can drive an 8k OPT

 

[ELS]

Well messing around in LTSpice, simulating the output section of the amp, it seems that I really can't use the 6N24P, the plate dissipation is too small and it just doesn't mix with my OPT.
however the 6N6P's max dissipation is 4.8W instead of 1.1W.
This allowed me to get around 1.4watts output from the amp, with reasonable THD. while with the 6N24P it'd let me just get around 50mW. pretty much nothing
it probably wouldn't have even been able to drive the transformer core with that low power.
But would raise the power draw of around 15w total. Maybe I can do something to dynamically adjust the bias voltage. I think there was a solid state amp that did this, the class was called something like Class XD (yes I know)...

 

[ELS]

Well it delivers around .8watts at 0db input.
doing the math it comes to around 20watts of power draw. now to build it.


there's the response, not impressive but maybe it sounds good anyway

 

[tschrama]

Single Ended Triode is the worst configuration for output efficientcy… and due to the DC current needs a SE output transformer…. Triodes are lower efficient that pentodes. So a PP pentode with be much more efficient that a SET triode.

But given that you are only looking for 1 watt or so, efficiency might not be so important. IIRC there are few parallel 12AT75 SE triodes in parallel designs on the web that might fit the bill.

Ultimately its the plate current capacity that determines the OT impedance, not the gm.

 

[ELS]

Single Ended Triode is the worst configuration for output efficientcy… and due to the DC current needs a SE output transformer…. Triodes are lower efficient that pentodes. So a PP pentode with be much more efficient that a SET triode.

But given that you are only looking for 1 watt or so, efficiency might not be so important. IIRC there are few parallel 12AT75 SE triodes in parallel designs on the web that might fit the bill.

Ultimately its the plate current capacity that determines the OT impedance, not the gm.

Well I was more worried about the heater power, the 6N6P has a 750mA heater (not 600), while with EL84's it would be twice that. maybe with something like a 6AK6 would work too, but that has low gm and my load impedance is only 8k.

 

[WiderGates]

Hi, I throw a 6360 in the round. Class AB at page 5.