Using tube amp with electrostatic speakers.

That's brilliant - thank you Ralph! I'll try this weekend.

Regards,

Andy

Ralph, Are you referring to the resistor "R" in the RC network?  I totally agree that R is a big problem in the original design, but if you remove it entirely, that would make a dramatic change in the crossover point.  For the example given above where 10 ohms and 36uF values of R and C give you a X-over point of ~440 Hz, removing the R entirely would give you a high pass -3db point at ~4400 Hz.  This would leave a big gaping hole in the midrange, because the bass transformer does not work much above 2kHz, based on my earlier measurements of my speaker.  I am sure you are aware of all this, which is why I think I might be misunderstanding the nature of your recommendation.  You could remove R and preserve the x-over point, if you add more C, in this example.  I would suggest increasing the value of R so that less current flows through it and the crossover point is thereby lowered to a frequency more in the bass range.  This needs to be done with caution because of the limitations of the toroid itself, which we don't really know for sure (because SL is a bit secretive in this regard).  But I bet you could go to 20 ohms, giving a hi-pass at 220 Hz (assuming 36uF of capacitance).  That's actually what I thought SL was doing with their new toroid, lowering the hi-pass X-over point.

Jazzdude, Plitron do (or did) make a "full-range" ESL step-up toroid with a 1:75 ratio, and in theory this would work.  However, bear in mind that I cannot personally say I have tried it.  There are also other companies that make EI type transformers with a step-up in the needed range (1:75 up to 1:90).  The Plitron looks to be pretty robust, and it would be my choice, if I was starting from scratch, is all I meant to say.  If you go to some other brand and to an EI type, just be sure to pick a "big" substantial transformer that is rated for full-range use.  Some of the aftermarket ESL transformers look like toys and would not be up to the job.

So, if you try the Plitron, I wondered whether it could mount on the very same bracket used to mount the existing toroid.  If the diameter is too large for it to fit in that bracket, probably the bracket can be modified to accommodate a larger diameter toroid.  This is what I was thinking.

Thanks for the info Lewm!

Andy

To summarize what Lewm said above (that he and Will have tried in their speakers) :
1. You can remove the brilliance control (or short the terminals) which is in series with the Toroid transformer (I removed the brilliance control and put in discrete resistors but will remove it entirely per Lewm's recommendation).
2. You can increase the shunt resistor value (mine had 10 ohms 4 - 2.5 ohm resistors in series) to lower the frequency of the pole (with the 36uf capacitor) from 440hz to a lower value.
3. Dr. West said it would not be an issue to increase the value from 10 to about 12-14 ohms with the toroid ll (I'm sure this is very conservative) to move the frequency pole lower.  Lewm suggested 20 ohms.  From my recollection, the concern that Dr. West had about increasing the shunt resistance was saturating the toroid transformer.  The toroid ll transformer offered by Soundlab is much larger than the toroid l.  I believe Dr. West said that it can handle an increased resistance without saturating.
4. You can use a better quality shunt resistor and it may add some minor benefits.

Regards,
Gary

Gary, That's a pretty accurate summary.  The notion to remove or bypass the brilliance control is actually older than the ideas to modify the backplate that came from Will and me.  

I remain puzzled that Sound Lab would upgrade to a larger toroid with presumably a better low frequency response and then continue to use the same values of R and C, thereby maintaining the same old crossover point (at round 440Hz for the values 10 ohms and 36uF).  But I don't actually recall what values SL used as OEM standard with the "old" original toroid, so it could be that the old values were actually something less than 10 ohms with something less than 36uF capacitance, which together or separately would give a higher crossover point.  Therefore, the values of 10 and 36 are "new" to Sound Lab.  As I now recall, the old values might have been 6-8 ohms (depending upon year of manufacture) and maybe 32 to 34uF.