I am sure you have seen this crazy ebay auction where it is mentioned:
I've seen his auction for a single unit many times, didn't know he had two of them. Not that a pair of anything are worth that kind of money in my opinion.
Is this really a microphone pre amp?
It's a utility amplifier, much the same as the V72 was but was used as a mic amp in desks, as well as other functions.
I used to own one myself that came from EMI's Pathé Studios and have a schematic that was drawn out from the unit by a friend after I sold it - I forgot to do it myself! I'd prefer to not post it without asking first as it's not good form to do so without.
somebody told me that this amp is based on Mullard EF37a tubes,
Yes it is. There are 3 of them in the amp. If I remember it right, the first is triode connected, the second is as a pentode and the third is again triode connected and also loaded with a choke like the V72/V76. It's a nice amplifier and was used on a lot of EMI recordings up until about 1959. "Cliff And The Shadows" comes to mind.
IMO it's a more sophisticated design in some ways than the REDD47 was (no gain adjust though other than a fine adjust 6dB trim) but it's also a bigger unit and more cumbersome so, using 30 of them in a desk would be a lot of rack space and heat.
According to the Beatles Recording Book, RS61's were still used @ Abbey Road Studios up until the 1970's in the echo racks. And it's also the amp that is overdriven for the "Revolution" guitar sound.
I do know that there were more than 30 made BTW so the Ebay guy is wrong on that.
I'll ask about the schem.