[ARC5] adventures in battery ops - 6K6 vs 12A6

[Kenneth G. Gordon]

On 7 Apr 2015 at 7:53, Ian Wilson wrote:

> I think it's the 12A6 for which there is no direct 6-volt replacement.

Correct. There is a 6K8. There is no 6A6...well, there is, but it is a completely 
different tube than the 12A6. The 6A6 is a 7-(large) pin dual-triode.

> I've seen the 6K6 used in 6v mods. This is not quite correct in terms of
> heater current (0.4A) but it's reasonably close.

Well, yes, but IMHO, using it in a series-parallel filament setup would really 
require a balancing resistor across the "other" tube.

Yes. Under the same conditions of applied voltage and class of operation, 
the output power of the 6K6 is identical - 3.4 watts.

The interelectrode capacitances are slightly different, but not enough to 
worry about, especially in audio service, and the transconductance is sligthly 
lower for the 6K6, but, again, in audio service that wouldn't matter.

The 6K6 metal tube is somewhat uncommon though, so the 6K6G or later is 
OK.

One other factor which MAY be important, but probably isn't, is that the 6K6 
is a power pentode while the 12A6 is a beam-pentode.

One could also use the 6V6, (base connections are identical) which is a 
beam-pentode, although the filament current is even higher (450 mA) than 
that of the 6K6 (400 mA), so using either tube in a series-parallel filament 
setup would, again, require the addition of a balancing resistor somewhere.

Ken W7EKB