wrote:
> While looking through a modern tube substitution book, I noticed
> something peculiar. They list the KT-88 as a direct sub for an
> EL-34/6CA7. You've got to be careful with those tube substitution books..
sometimes the subs they list aren't really direct substitutions,
some are kinda iffy, other cases they fail to point out
(as in this case) differences that could be a problem.
However, most companies that sell KT-88 list them as a 6550
> sub. Can anyone comment on the technical/audible merits of this swap?
There certainly are amplifiers that could take either KT88/6550 or
EL34's (this is a pretty common swap in Marshall guitar amps),
but AFAIK, there's very few cases that wouldn't require
readjusting the bias or changing the cathode-biasing resistor,
since all things being equal, KT88/6550 will draw more plate current.
In some cases you'd have to make changes in the bias supply
to get KT88 to bias properly.
KT88 and 6550 also draw more filament current...in a lot of cases
this isn't a problem, but sometimes it can be.
KT88's also generally want to see less DC grid circuit
resistance, especially as a fixed bias amplifier.
Some amps (like a Mk2 Dynaco) already have grid resistors
of low enough value (in a Mk2, 100K ohms), others don't.
Most amplifiers have enough headroom in the driver circuit
where reducing the value of the grid resistor won't be a
problem, but one can't count on this always being true.
How much difference you'd notice in actual power output would
depend partly on the power supply & output transformers,
could range from very little to quite a bit.
So IMHO, you'd have to take this substitution on a case by case basis.
Gold plated RCA jacks, binding posts, banana plugs etc, reasonable prices..
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