EMI RS61 schematic?

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rock soderstrom

Tour de France
Joined
Oct 14, 2009
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Hi,

I am searching for information about the EMI RS61 Electron Tube Microphone Preamplifier.
I am sure you have seen this crazy ebay auction where it is mentioned:

http://www.ebay.de/itm/EMI-RS61-Electron-Tube-Microphone-Preamplifier-Pair-with-Fairchild-PSU-V72-U47-/330719730922?pt=LH_DefaultDomain_0&hash=item4d006e9cea

;D

Is this really a microphone pre amp?

The reason I am interested is, somebody told me that this amp is based on Mullard EF37a tubes, I found some of them in NOS condition for small amount of money. This tube is pretty famous for low noise operation...so I want to build a mic pre with it.

Is there any schematic or further information about this EMI RS61 available for free?

best regards  8)

 
rock soderstrom said:
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.

rock soderstrom said:
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.

rock soderstrom said:
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.

Cheers.


 
Jean Clochet said:
 
According to the Beatles Recording Book,  RS61's were still used @ Abbey Road Studios up until the 1970's in the echo racks. 

Yes, this is true: I recognize them from the photo.  They were not at all special, but if you want to match them with an M49 that has been left in the damp for 10 years, you will probably get the same sound!

David
 
david-p said:
I recognize them from the photo.  They were not at all special

Actually, I quite liked the one I had and would have picked it over a few other bits of kit at the time.  I ended up selling it only because I was offered what I thought was a good price at the time.  Given how this stuff has rocketed in £ over the years though, I could have easily sold it today for 10 times the money and still been at about 10 times less £ than the Ebay guy is asking.  That's the way the cookie crumbles I guess!

Attached is a JPG of a schematic.  It's a bit faded but should still be readable.
I notice that the ground connection is missing but it's obvious where it's supposed to go so...

P.S. The blacked out bits are just where I was asked to remove some personal info before posting, no big deal.

Cheers.
 

Attachments

  • RS61 Amp.jpg
    RS61 Amp.jpg
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@mitsos - I guess you noted the ratio on the OT.  Another one for the files.

I'd call it a light drive program amplifier.
 
Anyone interested in a clone? I was thinking about doing a PCB layout and front panel design this weekend.

Where can we find the 72H choke on VR3?
 
dustbro said:
Anyone interested in a clone? I was thinking about doing a PCB layout and front panel design this weekend.

Where can we find the 72H choke on VR3?

Interested! Are you going to implement a tube substitute (6J7, 6SJ7, EF40, EF86 ) or stick to the original tubes? It´ll be hard to find these tubes or they quite expensive.
regards
Bernd
 
I was planning on using the original tubes...

http://www.ebay.com/itm/LOT-Philips-EF37A-tubes-made-in-England-by-Mullard-1965-NOS-NIB-59pcs-/290706570980?pt=Vintage_Electronics_R2&hash=item43af763ee4#ht_1877wt_1296

maybe if we get enough interest, we could buy this lot of 59 tubes for $1250.

 
I have to admit that these tubes are eyecatchers, but personally I would prefer the EF86 or one of its substitutes. Soundwise they wouldn´t be that much different if there is any difference at all. It would make this project less exotic. So my vote: noval sockets.
regards
Bernd
 
Hah!  :)
I can see the difference between these tubes, but I doubt that I could hear the difference. I find the topology of three pentodes, 1st and 3rd wired as triodes and the 2nd as pentode, very interesting. Another interesting aspect is the "dual feedback" thing. I´m far away from being a tube specialist, but these things caught my attention. Maybe some of the tube gurus can give us a lesson ....
regards
Bernd
 
Triode pentode triode.  Gates SA-20.  Gates SA-39 limiter designed in conjunction with the BBC.

The adjustable feedback loop is a trim; only 6 dB I think.  The Gates design makes more sense; only one FB loop (last 2 stages), and a gain pot between v1/v2. 

The 3 stage FB loop affects input Z, I believe.  Think of it as an amplified signal driving backwards through the input transformer, into the source.  This seems more purposefully designed for line signals.

This parallel feed output arrangement is like the RCA OP-6 design.  Do we know the dcr of the choke? 

Several eq tweaks certainly designed for the specific transformers, likely need modification in a clone. 
 
Anecdotal riffing a bit more, 1triode for lower stage gain and input noise, 2pentode immune to Miller effect I think (and plate/load R low enough to avoid loss of highs), 3triode for current drive to outside world. 

No reason not to build this with 6c5/6sj7/6c5.  Octal will have lower distortion profile on average.  Small bottle will typically need higher NFB to achieve same distortion, and harmonic spread will be broader.  But you could build it with (2) 6c4 and an ef86.  I'd put money on that seeming brighter/dirtier/cloudier/grainier.
 
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