"Scott Dorsey" wrote in message

Arny Krueger wrote:

The sort of vacuum tube op amps I'm thinking of were more
on
the order of this one:

http://ed-thelen.org/dc_amp.gif

Man, that thing has a HUGE number of tubes in it!

There's really no excuse for vacuum tube amplifiers to be
terribly slow, after all Tektronics made some pretty fast
'scopes out of bottles....

There is indeed: remember that you can either have gain or
you
can have bandwidth, and the whole point of the op-amp is
that
you can trade one for the other. If you want wide
bandwidth
at high gains, you need a whole lot of open loop gain
inside
the box, and that means a big box with a lot of tubes in
it.
And it means some drift issues.

For most of the things op-amps got used for back then,
drift
was critical. If you didn't want response down to DC,
there
were plenty of other solutions that were easier or cheaper
(and often involved transformers).

Look at those 1.8M plate resistors! Yow! Gain at all
cost,
indeed. --scott

Look again - they 1.8 meggers are in series with the grids
and are bypassed. The plate resistors are 620K, 180K, and
240K. The 620K plate resistors for the input stage are a bit
misleading because there's 24K worth of local feedback.