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RE: red plates ... bob and weave ...

12BZ7A is a lot like a 12BH7A-- in construction.

The 12BZ7 series has more grid turns than the 12BH7--
it's amp-factor is 100.

Both tubes, due to their large plate, grid and
cathode areas, are capable of handling large
momentary current pulses-- which is what music is.

Some would ask, "why not use the 12AX7-- same
amp-factor, but lower noise?" Not enough plate area
for many driver apps-- it works well on efficient
speakers with d.c.-coupled amps-- it can drive the output
tube-- a 2A3-- but cannot go further-- it is smaller in
active area.

I run them all the time on d.c. couples-- it works like
gangbusters on my speaker systems.

The speakers are not super-efficient,
I'm running GPA 604's inside a large MLTL cabinet which is
heavily built-- no unwanted resonances which steal
power.

Lundahls I have some experience with-- Kevin likes to
see 4 ma. or more on most models that would be used for
low power coupling. 10-14 ma. is the accepted published
data on some of those. K & K Audio uses Lundahl interstages
which run 4 ma. on the input side in preamps.

The Lundahls which I did use had something extra
over some other brands-- a large tolerance for
differing operating levels-- in other words, they're
more efficient at low power levels...

An interesting comparison of iron is when we were looking
at various output transformers for the 2A3, and the KT-88
series (KT 120, 150, etc.).

I'm going to stay away from mentioning brands here-- people
can fight over them somewhere else.

We used-- over years-- most of the highest regarded
output trans from Europe, Italy, all the High-End
Japanese brands, etc. We also used several from ultra
expensive custom winders here in the USA and in
Gt. Britain.

Some of these were excellent, and were used in my
commercial amps. Price-- for those amps-- was not
considered due to it being a small part of the overall
cost of building the amp.

Most sounded more than OK at their recommended idle
current levels. a very FEW (all US made) could withstand
LOWER operating D.C. currents than recommended.

While most of them started sounding "Generic"- (not
super high-end) as soon as the output tube plate current
was reduced, a FEW sounded excellent at almost any
lower current level you want to name.

Yeah! A WHOLE lot better design & construction. One
builder, a commonly "generic" supplier, sold us some
of their "super premiums"-- which were designed years
ago by one of their best engineers ever. They had sold
very few of those-- ever.

Guess what? The 14 lb E-I core monster was rated at
over 300 ma. I ran it at 43 ma. on a 2A3. What happened?
It greatly outperformed all the other transformers, period.

In every possible way-- including transparency, speed
and extended highs. Bottom was also better-- expected
because of the large inductance.

Lundahls are like that. They handle ALL of the signal--
for the most part, regardless of idle current.

That's good engineering.

I'm not trying to tell anyone how to run Lundahls.
I have more to learn from them, and frankly don't
yet know how low one can go in ma.. and which
models will allow that. Just as in tubes, they're
going to differ in ways that the specs don't show-- we'll
have to find out-- as usual.

6SL7 can be run in the current range you may need if
you have the right Lundahl. I recommend calling someone who
knows their line better than I do, and getting an
Engineer's ideas-- from several engineers.

If you D.C. couple a 6SL7-- amp-factor is about 70--
you'll have a little extra gain into a 2A3, say over a
triode of 40-45 A.F. This may or may not suit the user--
depends on his system gain. My systems use an amp-factor of 100.

If you choose to run a KT-88, and wish for an industry-standard
amp gain structure, and are D.C. coupling 2 stages,
you'll want an amp-factor of about 33 on the driver.

Don't be afraid of running lower milliamperes on chosen
vacuum tubes. Some are very superior running half or less
of their continuous ratings. Some (transmitting tubes)
must run hotter-- pure Tungsten parts, etc. They bear
no resemblance to receiving tubes, and are used differently.

Running your own plate curves is easy. You need two
variable power supplies, and some milliampere meters,
and your graph paper. Set a plate voltage, set a load,
and insert different grid biasses and plate voltages and
plot all the points on the paper. You'll find out things
about the tube in question that aren't answered on tube
MFR data.

-Dennis-















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