Post by Fabio BeruttiAFAIK the KT88 is a lot bigger than the EL34 (>40W anode dissipation instead
of about 25, depending on manufacturer and year of manufacture); moreover,
it has a much lower internal resistance and is therefore used customarily
with a much lower load than the EL34, say 3.5K for a KT88 instead of 6.5-7K
for a EL34.
If You just swap tubes, You'll have +/- the same output power, the same THD
% and so on.
Well hang on, we don't know what circuit the guy has.
If he goes to KT88, and uses pentode/tetrode mode,
and uses the same FB, and same load, and same operating B+ voltages,
and the same plate current, he should get only slightly more power.
Ro of the amp will rise because the Ra of the KT88 is 50% higher than the EL34,
but the NFB will leave the Ro at the output only marginally higher.
THD as you say could be anywhere up or down, but most likely down.
Just because the maximum Pd of the KT88 is 42 watts, with the EL34 at 27 watts,
it don't mean you should run KT 88 in an amp anywhere near 42 watts
since the design was for lower operating Pd of the EL34,
and this may only have been 20 watts.
Put it this way, I have seen guys run their KT88 at up to 45 watts,
but it shortens the tube life,
and extra power taxes the power suppy unnecessarily, and if the circuit
already produces nearly all class A power with EL34,
then having a higher bias current in the KT88 won't
increase the class A much unless the load value is lowered.
If I wanted to change to KT88, it might be because I wanted to
use say 6 ohm speakers connected to the 8 ohm outlet points.
The KT88 are capable of driving a lower load.
But KT88 offer the opportunity for triode operation.
If the amp has say a 3.5 k a-a to 4,8,or 16 ohm speaker,
then using the 4 ohm outlet connected to an 8 ohm speaker
means the the load seen by the KT88 in triode would be 7 k a-a,
which would give a very nice 15 watts+ of triode AB power,
wheras the EL34 in triode may only make 11 watts.
I have even placed KT88 in triode mode into Quad II amps with fixed bias
applied,
and got 20 watts AB1, which seemed to sound better tan the original KT66 + EF86
set up. But then I did some other drastic mods to the Quad II, which
I won't expand on now, although the archives would have what I did say before
now.
Post by Fabio Berutti"Sound" should change a bit, because the KT88 is a beam
tetrode and has therefore a bit more of the mellow "triode" sound (more 2nd
harmonic and less 3rd),
The 2H of either type of outpur tubes cancells, and the main difference would
be
a change to the 3H, probably be less though.
Post by Fabio Beruttibut id depends largely from the circuit. Some
people say that tetrodes (KTxx, 6L6, 6550, 5881,..) are better than pentodes
when used in triode or UL mode because they do not have that extra grid
which causes some unwanted perturbation in the electrons flow.
If You want to try and swap tubes, I'd say that the closer equivalent to the
EL34 is the KT66. Sovtek sells a good replica. Even easier to find is the
6L6GC, and there are still many JAN 6L6-WGB/5881 around at reasonable price
(the ones with the small bottle, really great tubes), but these last ones
may be a bit "weak" for Your amp (the max. all. V on grid 2 is about 300V
for the 6L6WGB and 450 for the EL34).
KT66 and 6L6GC require (+/-) the same output impedance of the KT34 and are
beam tetrodes like the KT88.
In PP amps, I have tried changing between EL34, 6550, KT88, and KT90.
They all sound a little different, but mostly they were not objectionable,
so sure there is a difference, but the "wines were all very drinkable".
Occasionally, you get tubes which obviously don't sound as well as others.
You know what to do.....
But just what preamp you have, and what tubes it contains,
and what source, speakers, and room also colours the sound
just as much as tube rolling in the output stage, and the mix of tubes interact,
and driver tubes in the power amp are also important.
Its a never-ending quest.
Post by Fabio BeruttiCheck bias and circuit requirements before swapping power tubes.
Yup
Patrick Turner.
Post by Fabio BeruttiCiao
Fabio
Post by ArmandAccording to Cary Audio, I can swap KT88's for the El34's. The amp's
self-biasing. I'm not familiar with KT type tubes. Do they sound better
than
Post by ArmandEL34's? Thanks.