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From: fehlinger_jim@jpmorgan.com (Jim Fehlinger)
Subject: Re: McIntosh Equipment
Date: 10 Jun 1994 18:24:40 GMT

I also have an amp which permits one to choose
among these output tubes (it's a Jadis JA-80).
I got this amp used, with KT-88s (on their last
legs), and replaced them with Sovtek EL-34s.

The differences?  Well, the folklorish, non-technical
answer is that 6550s or KT88s will give you a bit
more power than EL34s, and a bit more bass control.
However, EL34s are supposed to be sweeter, more refined
sounding than 6550s.

The KT88 is (was) a premium audio tube
designed (by Mullard, I believe) to give the best of
both worlds.  They also have a characteristically
shaped envelope which some people like a lot.
They're also rare and expensive.  Shuguang, the
Chinese manufacturer, may be a current source for
these, but I don't know whether they're supposed
to be any good or not.  NOS (New Old Stock) European
manufactured KT88s are highly sought after by the
people who can afford them.

6550s are easier to find and cheaper than KT88s,
but it is apparently somewhat difficult to find
a reliable and good sounding brand these days.

EL34s are still easily available, relatively
cheap, and available in good-sounding and reliable
versions.  The Sovtek brand is currently popular.
Your amp should run a bit cooler with EL34s, and
will also have more physical space between the tubes
for cooling.

No doubt there are more knowledgeable tube mavens
on the net who can fill in the gaps in this thumbnail
sketch, if necessary.

Jim Fehlinger