Joule Electra preamps and amps. What is your impression?

^^Any amplifier using 6C33 power tubes will need the power tube sockets replaced on a regular basis. When the tube was used by the Soviet military, the recommended service life included replacement of the socket along with the tube.

The socket was originally designed in the US for a Sylvania tube called the 3C33 which was an RF transmitting dual triode. In that service it ran fine for decades. The Soviet design doubled the filament current and the result is that the connections fail. The best sockets we have seen were made by EF Johnson in the US, and there are some vintage Russian sockets that hold up fairly well too.

My older Sound Labs are very difficult to drive, but newer ones are easier, so I am not best to opine on that.

Just a FWIW, sbank, there are some simple mods to the Sound Labs that make them a lot easier to drive and also better sounding regardless of the amplifier. Or you can just get the new backpanels for them.

Did I hear right that OTL amps require a very careful match with speakers ? In what sense? I read that some use them with big Sound Labs, Classic Audio's modern vintage speakers are apparently excellent match too. But that's big unusual speakers. How about common dynamic speakers ?

Imagine a powerful SET and you have the idea of what an OTL needs in a loudspeaker (impedance of 8 ohms or more, particularly in the bass, no crazy phase angles). One of the most popular dynamic speakers with the Joule Electra and our amplifiers was the Merlin VSM. Audiokinesis is another, Coincident, Audio Note, Utopia, Spendors, etc.

If you have a question about a speaker, just let me know what it is and I can let you know. The requirements for our amplifiers are similar to the Joules.

It is Michael Green Audio Revolution 80i free resonance speakers.

I've not been able to find any information on this speaker, but in general it appears that the speakers on Michael Green's site are pretty tube-friendly in terms of impedance and phase angles. They do need a bit more power but that is usually not a problem for OTLs.