tube amps and electrostatics

Play yes Atmasphere, but really you and I both know they were not able to give a flat 20hz to 20000hz into those hard loads.
They acted more like they had tone controls in them and made the FR look like a dogs hind leg.
I have a old friend that has two pairs of Stax F81's and not even his mighty Rogue Magnum 150 monoblocks drives them as well and with flat FR with a vice like grip as a good stable BJT amp can, like the big Krells and such.

Cheers George

I copied this from a post from Kent of ESS. He's the expert on F81s. Has them. Sells them. Rebuilds them.

The Stax ELS F-81 is one of the most difficult loads one can find. From 40Hz-800Hz the impedance is well above 100 ohms. Around 180Hz the impedance peaks around 512 ohms! Amazing.

Solid state does not like this sort of load. A Zoebel network is highly recommended if using solid state. OTL tube amps are ideal but can be pricey. Transformer coupled tube amps will handle this kind of load.

Power output will depend on required SPL in the listening room. A smallish room with a nearfield set-up will yield SPL in the 90-92 range with a mere 60 watts. I use an ARC D-70, 65 watts, with good results except for clipping if I want to drive things. Or I have an ARC Ref300, 280 watts, that will drive them louder than I like without clipping issues. Well I can clip the Ref300 but then I am pushing the speaker close to failure, probably the transformer.

Any power amp will need to have a large, robust and stable power supply. An ARC VT130, 110 watts, would be ideal with these speakers and is within your budget. The AtmaSphere M-60 would be a great match if maybe just a little under powered.

You forgot to mention Dhcod that they also dive down to below 2ohms as well. And don't go by the Stax published impedance graph, it's fudged quite a bit, we measured it and it's far from what they pictured. Add to that it's capacitive loads and you have a speaker that is easy to drive at some frequencies and a down right pig at other frequencies.
And a quad esl 57 is even worse. And other ESL's are just as bad if not worse again, look at the Martin Logan Montise crazy amp impedance load and add to that the nasty phase angle, amps with current need only to apply.

http://www.avsforum.com/t/1454033/amp-for-electrostatic-speakers post number 12

http://www.stereophile.com/content/martinlogan-montis-loudspeaker-measurements

For over 40 years I've only used ESL's
Quads, RTR's, ML, Acustat, Stax. Sure some of my big tube amps will sound good into them, but nothing drives them like a strong current bjt output stable solid state can. Direct drive esl's can also sound good but that's getting into the deadly territory

Cheers George

Mr G- You may want to realign your blanket statement, re:
MOS-FETs
and
current. MOS-FETs are innately self-protecting, regarding
thermal
runaway, unlike bi-polar transistors
(http://www.onsemi.com/pub_link/Collateral/AND8199-D.PDF).
I sold(mostly modded)
David
Hafler products, for years, which all used MOS-FETs as
output
devices. As a sales point: I used to demo the DH-500,
measuring the amperage into a 2 ohm dummy load. I had read
as
high as 48 amps, without damage or overheating(and thermal
protection activation). You could practically arc weld with
the suckers! I, as well as a couple other customers, owned
Acoustat Model III's, and had excellent results with the
Hafler amps, at realistic SPLs.

Sorry but Mosfets can't do current like BJT's can. They can do watts no problem though.
Show me published specs on a Mosfet amp that can almost double it's "tested" watts output all the way from 8 to 4 to 2ohms and I'll show you a case for legal action.

Cheers George