Using tube amp with electrostatic speakers.

Lewm,

Congrats to you and Will on the work you have done in bringing forth a significant improvement to the Soundlab speakers.  I can attest that the transformer upgrade was a significant improvement over the prior toroid l transformer.  It is not very often that a user pioneers improvement that results in commercial changes.  Kudos!

Yes the inductor was in parallel with the resistor.  I only have the resistor now (inductor removed) per Dr. West (another thing you probably had your hand in).  I do not know what they are using now for a resistor value but I believe it is the same (36uf and 10 ohms (4-2.5ohm potted resistors)).  As mentioned, Dr. West stated that the resistor value could be increased with the Toroid ll but not by a significant amount (I have 12 or 14 ohms).

What are the consequences of completely removing the brilliance control that is in series with the transformer?  Does it make the speaker much brighter?  Does it have some effect on the transformer at high frequencies or is it reliability issue? 

I would also like your final thoughts on the topic of the thread which is using tube amps with electrostats and more specifically Soundlab electrostats.   The Soundlabs obviously have wild impedance swings from HF to LF.  From the thread I assume that a tube amp should be selected to be able to handle the HF low impedance (with the zero transformer being a nice option to aid the tube amp).  Are there any issues you know of on why a tube amp cannot drive a high impedance load such as 30-200 ohms?

Once again, thank you for all your help.  Your knowledge has aided me tremendously.  I really like the fact that even old Soundlab speakers (like mine) can be upgraded and made to sound great.

Regards,
Gary

4 of those 2.5-ohm "coffin"-type resistors?  They are probably wired in series/parallel, but do you know the net resistance of the bundle?  Could be 10 ohms or could be 2.5 ohms.  Neither value is gonna make your OTL any happier than mine was before I chucked the crossover. (Really, give all credit to Will; all I did was follow his lead because I was so unhappy with the sound I was getting from my 845PXs.)  You are definitely better off with 12-14 ohms than 10 ohms.  However, I am a little surprised at the values; they are identical to what I was using with my "old" toroid (10 ohms/36uF), before the big modification (removing the crossover entirely).  The -3db point of the high pass filter is inversely related to the product of R times C, where C is in Farads and R in ohms.  For a 10-ohm resistance with 36uF, I calculate a crossover point of 440 Hz.  I would have thought they could use a much lower crossover point with the new toroid, but I bear in mind that Dr. West is very conservative in his design so as to favor reliability.  From my experience making measurements, the intrinsic impedance of the speaker is much higher than 10 ohms at all frequencies up to somewhere between 2kHz and 5kHz. At 5kHz, it's about 8 ohms for my 845PXs.  (Bear in mind, measurements were made with NO crossover, driving the SL factory bass transformer and the full-range Australian transformer in parallel with each other, directly with an audio frequency generator.) So, as you can imagine, when you interpose the RC network, at low and mid frequencies, much more of the energy is going through the resistor than through the speaker (R is smaller in value than speaker Z at those frequencies, so R dominates). This may be why some guys on SLOG discovered long ago that very high wattage resistors seemed to sound better.  There is almost no limit to the wattage ratings they tried, more than 100W, even.  Before my epiphany, I was using an enormous 10-ohm resistor hand-made by one of the British contributors to SLOG.  It could tolerate at least 500W. (One benefit of bi-amplification is that only the treble amplifier then "see"s that R in parallel with its output; the bass amplifier drives the bass transformer only through the inductor.  Thus paradoxically, your SS amplifier might work best on the treble (because it can handle the power demand and the low impedance created by the R, with your tube amp on the bass).

If you go up to 20 ohms, the crossover point would be ~220 Hz. (The beauty of the equation is that once you know the values for one set of parameters, you can calculate in your head what will happen if you change R or C.  If 10 ohms and 36uF give you 440Hz, then 20 ohms and 36uF give you 220Hz, etc.)  I would have thought that the new toroid would be quite happy at around 300 Hz, not too far from where you are at if you are using 14 ohms now.

There is no difference between bypassing or removing the brilliance control and setting it to zero. But I preferred to remove it; it's a cheap part that muddies up the signal path. You will hear an improvement. No, the speaker is not too bright without it, in my opinion.  The easiest thing to do is to re-solder the input and output wires to the brilliance control together on one binding post of the control.

As far as impedance variations of the speaker per se, the ESL can be thought of as a giant capacitor.  It has very high impedance at very low frequencies and then gradually falls off in a fairly linear manner to very low impedance at very high frequencies.  I wrote here earlier that in my opinion, very low impedance at 10kHz or 20kHz is not a real problem, because the energy requirements are very low at those frequencies.  What makes for problems is the circuitry that comes before the panel, e.g., that 2-ohm impedance dip at about 1kHz that came with the old toroid due to its crossover.  Nearly every company does something to make ESLs more friendly to SS amplifiers, which in turn makes it hard on tube amplifiers. 

Lewm,

     The resistors are in series (10 ohms).  That is what I had in my circuit when I changed to the Toroid ll.  I do not know what they are using in the new Toroid ll backplate circuit.  I did exactly the same thing (you described) when I replaced the resistors. I have large ceramic non-inductive wire wound resistors (hi wattage).  I will definitely look into increasing the resistance to around 20 ohms.  I am not sure what transformer saturation is but that is what concerned Dr. West when increasing the resistor value (and lowering the croosover point for the Toroid).
     From your suggestion I might just stick with my SS amps.  They have ample power (Innersound 800 ESL monoblocks (800W@8 ohms)).  My journey to this thread was based on understanding if a lower power tube amp could be used on a speaker such as Soundlabs.  I believe I understand the tradeoffs you have outlined.
    As usual, thanks for all the information, it is much appreciated.  I try completely removing the brilliance control from the circuit.  I will also try lowering the crossover point of the Toroid ll.  I will then try crossing over to my B1S somewhere between 350hz to 80hz and see what works best for me (when I get the time of course).  Merry Christmas and Happy Holidays Lewm.  I have learned a lot.

Regards
Gary

My wonderful Quad ESLs(57) on 16ohm taps of rebuilt 60's EICO HF35 monos work great. 

Yep. Quad 57s, KLH 9s, early Acoustats, Janszen tweeters.  They were all made with tube amplifiers in mind. For a real treat you might look into the Quad mods made by Dave Slagle, for driving them with tubes. Fantastic results.

Gary, Most of what I have done is to make the speakers sound optimal with my OTL tube amplifiers, which I genuinely believe are superior, and I did once borrow a pair of Innersound amplifiers for comparison. Since you already own a tube amplifier as well as your Innersounds, you might want to try bi-amplification, with the tube amps running the bass transformer and the Innersounds running the treble.  You would have to make allowances for balancing the output, by attenuating either the Innersounds or the tube amp, depending upon what you hear.

On the other hand, with the 20 ohm resistance and the same 36uF capacitance, you may like your tube amplifier, full range, best.