Could the transformer be eliminated?

It is desirable to eliminate as many things as possible from the signal path. Output transformerless vacuum tube amplifiers eliminate the transformer, but at a cost of many complications to match to a 4 or 8 Ohm speaker. Gangs of tubes in parallel are certain to corrupt the signal.

But the electrostatic speaker has a much higher impedence, after the audio signal is stepped up by its transformer. Also, no crossover, electronic or passive, would be necessary if it has its own low frequency rolloff around 200 to 400 Hz, where a pair of active subwoofers could take over. With the right amplifier, this would be possible.

Why not build two 45 SET amplifiers that share the same power supply and are fed by a balanced signal. Each 45 anode would be parallel fed through a prodigious choke and between both plates would appear a push pull but altogether class A signal with a peak audio voltage of as much as 200 and an impedence of 10,000 ohms. Then both the anodes of the 45's could connect directly to the stators of an electrostatic speaker and for safety the stators would be covered with plastic.

This would have all the virtues of the 45 SET with common mode distortion and noise removed and the transformer eliminated without the feedback, parallel gangs of tubes, or the prodigious power of OTL amplifiers. The speed of the electrostatic speaker along with the loss of the limitations of the transformer should offer undreamed of refinement.

Do you think it might work?

Barney Vincelette
Not enough voltage for most electrostats.

There are direct drive electrostats that are sort of like what you are describing. But not using the 45 triode. They are using very high voltage transmitter tubes. I've heard of some Beveridge that were done that way. There is a guy named Alan Kimel that is one of the most experienced at this sort of thing, and he probably is at the VSAC show right now. David Berning did a ZH270 for, I believe, Alan Baghan, to directly drive his Soundlabs. It has been done, but it is still kind of experimental, and somewhat dangerous to work with this high voltage.

Interesting concept.
The big problem is the surface area and the thickness of the ESL drivers - the bigger they are, the higher the voltage. A typical ESL has voltages in the thousands. This presents the problem of having to supply a very very low amperage; even the current output from the 45's is too much to be supplied directly. In theory, what you propose may work (although extremely inefficiently). But probably only for a very thin, small surface area ESL driver. The "no-free-lunch-in-audio" axiom is hard to shake.
Weren't the original Acoustat's powered by their own OTL tube amps ? Sean
Butler Audio 3150/5150 utilizes a transformerless output in their 6SL7GC tube hybrid design that`ll handle anything from 2-16ohms at 225 watts@4ohms with <0.15%THD to produce stunning dynamics with a tube sound nirvana. This might be what your looking for? Regards, Robin
The Acoustat amps used a high-voltage pentode tube, I forget the exact number... I used to own a pair of highly modified Acoustat Servo-Charge amps that used 6LB6 high-voltage transmitter tubes, and they worked really well. As I recall, those amps could drive up to 8 Acoustat panels per channel -- quite a feat!

Roger Sanders of InnerSound has been wanting to come out with a direct-drive tube OTL to power his Eros system, but I don't know if he will ever get that going now that InnerSound has a new partner. But the day he comes out with that setup is the day I'll return to audio reviewing. Hey Rog, you listening!??? :)
A 6LB6 would typically be considered a 25 - 30 watt tube in audio circles, possibly less if looking for low distortion figures. If you can remember, how many of these tubes did the amps run per channel ?

For the record, i think that a 6LB6 was primarily used as a horizontal output tube in early TV's. It could also have been used as an output tube in an RF transmitter, but they wouldn't hold up that long in that type of a situation. That is, unless the circuit was very conservatively tuned. Sean
You may want to see this link about the Acoustat direct drive amps. I owned them for years and loved them.

They used the 6HB5 tube. I think it was also a horizontal TV tube.
Hi Gang. Sean,you are correct regarding the original acoustat servo amps. They were in fact, OTL designed servo-amps and employed the 6HB5 tube. They would supply the 5,000 volts the panels required.A brilliant design in their day, but somewhat Quirky and unreliable in stock form.There are a number of companies updating the design with modern parts and circuit mods.Once revised: there is not a commercial amp on the market that could come remotely close to their performance on the acoustat panel.Still use mine[highly modded] on a pair of 2+2's in second system.