Patrick Turner wrote:

Andre Jute wrote:

I gave up the idea of direct tube drive of Quad ESL63 years ago as too
hot and too dangerous. But my recent experiences with Stax
electrostatic earphones driven by small OTL tube amps makes me wonder
if the next breakthrough in tube amps will not be a combination set of
a tube amp directly driving the associated fullsize electrostatic
fullrange speaker.

A full range ESL does require a large voltage swing.

Quad ESL57 have a 300:1 step up ratio tranny and so if 25Vrms is needed
at the low voltage input coil, then without the tranny
the voltage required is 7,500Vrms.

Where does one get a linear 7,500Vrms from?

A few hundred 6BK4. They're good for 16000V before they turn into X-
ray machines. Unfortunately, they're only good for 1.6mA, so you'd
need to stack a *lot* of them. I seem to remember 344 tubes per PP
channel with over 85W of filament heating for stereo. (God, I love
understatement: not even an exclamation mark.) But, hey, Iain lives in
Finland where it freezes in winter; we *are* talking about a niche
market amp.

But parallel 450TH might have the oomph with 6000V at 75mA each, 150mA
the pair at around 6000V would probably give reasonable levels with
almost ESL63-sized panels. You could use stacked panels and stack up
the tubes another layer to boost the bass. (No idea what 450TH cost
these days...)

ESL63 and later derivatives are slightly easier to drive, but basically
the drive voltage needed isn't ever going to become lower because of the
laws of physics
with regard to charged plates, the distance between them, membrane
travel requirements,
and high voltage Vdc charge biasing.

***If there is demand, we could, as a theoretical exercise, work
through the math, see why in the end I and so many others bugged out
and decided a transformer is the way to go to drive ESL. Some might
find the theory interesting.

Its all OK in a pair of ESL headphones because the levels of Vac drive
and bias etc are
all far lower than the potentially lethal levels in a full size ESL
which has to fill a room with sound, not just an ear held close.

Yeah. But I've been there, done that, as you know. (Thanks again for
your help, Patrick, and all the others who looked over the designs of
my electrostatic headphone OTL amps.) I was just wondering semi-idly
if I didn't give up too quickly on direct drive.

Morgan Jones explains how he uses a pair of PP 845 to get a high drive
Vac.
Many other tubes capable come to mind, 813 operating off several kV
perhaps but they will never ever
be adopted by anyone except the few clever types who enjoy doing things
the hard way.

2200V from 813 and (from memory, a similar tube available back in
c1995) 8068 just isn't enough to play fullsize electrostats at
reasonable volumes.

Radio amateurs routinely ran their transmitters with several kV but it
takes
huge dedication to be a really capable radio amateur if one builds
each and every part of one's own radio station.
DIY with HV to get better audio than available by other means is also a
challenge.

Like I said, I do love understatement!

I don't mind using magnetism as the means to get from tubes to ESL
panels.

In the end it doesn't matter whether you mind the transformer or not,
the alternative of several (to hundreds in the ludicrous case) of
valves operating on many kV is too dangerous and too hot and too big
and too expensive to consider sanely. And, in truth, the ESL
transformer, at least in the Quad ESL I know, is blameless; you can't
ask more of iron than that. I wonder of Sander still lurks: what about
in your Maggies?

Patrick Turner

Andre Jute
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