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S. W. Wortis wrote:
>
> Hello.
>
> In the Ultimate Tone, there is some discussion regarding tube replacement.
> He mentions three factors that might disuade one from switching to 6L6s in
> 6V6s amps. The first, heater current draw, seems pretty obvious and
> straightforward. The others, transconductance and mu, he doesn't explain.
> Could someone explain how transconductance and mu are factors in tube
> replacement?
>
> Thanks,
> ShaunHello Shaun
You have misunderstood something-- mu and transconductance differences
were not cited as reasons for not substituting 6L6s for 6V6s. I believe
you are mentally transposing a statement made about the complimentary
substitution, from an earlier part of the Power Amplifier chapter of "The
Ultimate Tone".
Maybe I can clarify this for you.
6L6s in 6V6 amps:
1) If the amp was designed for 4x6V6; you can plug in 2x6L6
2) If the amp was designed for 2x6V6; you CANNOT install ANY 6L6s
UNLESS-- you add an auxilliary heater transformer to supplement
the main winding-- 6V6 heaters draw 450mA each
6L6 heaters draw 900mA each
OR-- you have the manufacturer's nod that they say it is okay
6V6s in 6L6 amps:
1) If B+ is less than or equal to 450V; okay
2) If B+ > 450V; NOT okay
UNLESS-- screen voltage can be reduced to below 400V; preferably
below 350V
Regarding transconductance and mu: These parameters are a bit more than
half for a 6V6 relative to a 6L6 i.e. the 'V' has half the gm of an 'L'.
This fact means that for identical plate voltage, screen voltage and bias
voltage, the 'V' will settle to an idle current of about half the value
of a 6L6. This is borne out in actual use and greatly simplifies the
"bias option" circuit requirements for amps that are to accomodate both
types of tube.
More light will be shed on these issues in "The Ultimate Tone: Volume 2"
which is all-new information, and should be available in Feb/March.
I hope this helps!
Kevin O'Connor
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