[mike s]

On Tuesday, April 5, 2011 10:12:01 AM UTC+1, Big Bad Bob wrote:
On 04/04/11 14:49, mike s so wittily quipped:
Starving filaments is interesting. It's another of those things I
think about from time to time, but there are always other more
pressing things to do.

I would think that 'use the right component for the job' might be a
better way to go, rather than forcibly de-rating the device by starving
the filament. It's also more likely to give you unpredictable results,
or require tweaking each unit. Tube characteristics change over time,
after all, though I suppose lower heater voltage might slow that process
down a bit.

Now, something else comes to mind with reduced heater potential, and
that is a cooler cathode, which for a triode might reduce the noise
level. If you can squeeze out enough current to operate properly, the
reduced cathode temp might produce less entropy in the electron cloud.
That might result in lower noise, so for a 1st stage in a preamp it
might be a good idea on that basis. It's worth an experiment. But then
again, 'right component for the job' is probably the better choice.

Which is exactly what was needed for an electrometer. What is at first surprising is that Mullard did this with the ME1400 not by designing a new tube but by selecting from EF37A. But then selecting for low noise was established, I have some NOS ex BBC AC/SP3 selected for low noise, and RCA 1620 similarly selected from 6J7. What I hadn't appreciated is that running the heater of these valves much cooler would give a reliable, but quite different, characteristic. Though I'm still unsure if it's actually useful for an audio amplifier.


But reducing heater current on the power output stage? I think I'd shop
for a better tube instead. Maybe one of the high end tube makers would
spec one out specifically for low distortion power amps.

I have some RCA 1622 which claim to be a high reliability 6L6. That can't have been achieved by selection, so perhaps the design is slightly different. I bought these and a few 1620 to restore an old RCA Monitor amplifier.

[Big Bad Bob]

On 04/05/11 12:29, mike s so wittily quipped:
But reducing heater current on the power output stage? I think I'd shop
for a better tube instead. Maybe one of the high end tube makers would
spec one out specifically for low distortion power amps.

I have some RCA 1622 which claim to be a high reliability 6L6. That can't
have been achieved by selection, so perhaps the design is slightly different.

Most likely that's the case. Interesting that they gave it a different
number. Uusually it's something like 6L6[alphabet soup]

'6L6' really represents a pin layout and a set of performance curves
that correspond to the original 6L6 design, which could be improved for
better SNR or linearity or reliability and you could still call it a 6L6
(usually with alphabet soup after it so you know it's "different").

Anyway, Ruby Tubes has an interesting comment about their 12AX7's on
THIS page:

http://www.magicparts.com/tubetalk.htm

(quotes in case the text is updated later)

"The new RUBY 12AX7AC7 is the latest tube developed at our Factory. It
is the quietest of all our 12AX7 type tubes, both in terms of
microphonics and mechanical noise."

"To eliminate the mechanical rattles, we started with new, tighter
tolerance micas, redesigned the plate structure and added new assembly
controls to make the RUBY 12AX7AC7 free of mechanical rattles caused by
the vibrations inherent in a high gain combo amp. "


Anyway, without being a platform for advertising their products, they're
claiming improvements to the 12AX7 design, so you can expect their tube
to act like a 12AX7 even though the actual design differs from RCA's
original, and it has the alphabet soup at the end of the '12AX7' to
designate it.