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RE: make it work like DF ...

The 7B4 is a bit of trouble for me.

In all the years since I first used it,
myself and others have tried to find something
that would perform like it does-- get ALL of the
music right, and do it with a lot of verve,
dynamics and "just can't wait" personality.

By this I don't mean euphonic, I mean musically RIGHT.
Tonality is what everyone goes by. NOT ME! I want the
thing to follow music's dynamics, AND be tonally right.

Many triodes are easier to use (the pins on 7B4's
always have to be brushed and cleaned-- regularly).
Modern tubes don't need that!

One could do with more drive power. Never hurts-- as
long as you don't start bucking-out input detail with
excessive driver plate current. Fortunately, the 2A3 is
easy to drive as long as it is direct-coupled to its driver.

All sorts of triodes and pentodes have been tried as driver
for the veritable 2A3. AND with all sorts of driver milliamperes--
I do realize the problems of underdriving an output tube into
a heavy load. I also understand linearity problem theories.

None of that matters as much once you've heard what the 7B4
can do. And neither does any other tube matter in this app-
except the 12BZ7 when driving a 2A3 D.C. coupled.

That tube delivers more drive power, and has the same
amp-factor.

It sounds ALMOST as good. The extra drive power makes
no difference whatever, but it did cause me to bump-up
the plate current and plate voltage of the 7B4 A LITTLE BIT.
-- to see what happens. I like it, and do all of them
that way now, including any old amps I can get in here
to get the modification. The work and parts are FREE
all the way back to 1989 models.

Then, I finally found another match I really liked--
A KT-88 SE with a 6E6P tetrode, run in triode mode,
directly coupled to drive the KT-88.

At first, I ran it with conservative plate current
and voltage on the KT-88. It was obvious that this is
a great amplifier. Plate current was bumped-up on the
KT-88-- again-- to see what happens. VERY LITTLE
change-- both trials were excellent.

My next idea for this is to build a power supply
of about 600 VDC to run this affair, and get lots
of power WITHOUT excessive plate current. The filament
of the KT-88 will have to have its own B+ supply-- to elevate
the filament from ground so the tube cathode doesn't fail.
This voltage can be derived from the main B+ supply.

I intend to keep on running Direct-heated rectifiers,
so as to avoid any solid-state signatures. A solid-state
FWB would be obvious, but I won't do it.

The big bugaboo here is finding the right power transformer
that one can actually BUY anytime someone needs one. Off the
shelf beats-out unobtainium anytime! I would prefer a Toroidal.

The search goes on. In the meantime, this KT-88 SE is also,
sonically, a GREAT AMP!

-Dennis-

.



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