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Old 3rd August 2008, 07:26 PM   #1
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Default RJM with 12AY7

Has anyone ever tried to do the RJM passive phono with 12AY7 instead of the 6922? Mu is similar so it seems doable?

I have heard that the RCA circuit is not appropriate for AY7 due to lower mu (compared to AX7) but the topology is quite similar to RJM which uses a 40ish mu tube. As far as gain goes both RCA and RJM are two stage passive. So I would also be interested in hearing from anyone who has done the RCA phono with AY7s also.
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Old 4th August 2008, 09:01 AM   #2
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Hi, I'm not familiar with the RJM design you mention, but I have had good results using the 12AY7 as 2nd stage in RCA passive design, that 2nd socket has had all sorts of tubes tried, but I have kept going back to the 12AY7 in that slot, sufficient gain, and it seems to have a very even, uncoloured sound. In the first slot, I ran a 5755 for a long time, seemed to be free of what I heard as bass "mushiness" from a 12AX7, but changed back recently, just to find out if I was missing out on anything, not convinced yet.
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Old 4th August 2008, 07:48 PM   #3
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Default Re: RJM with 12AY7

Quote:
Originally posted by mashaffer
Has anyone ever tried to do the RJM passive phono with 12AY7 instead of the 6922? Mu is similar so it seems doable?

I have heard that the RCA circuit is not appropriate for AY7 due to lower mu (compared to AX7) but the topology is quite similar to RJM which uses a 40ish mu tube. As far as gain goes both RCA and RJM are two stage passive. So I would also be interested in hearing from anyone who has done the RCA phono with AY7s also.

Although the gain is similar, there is a large difference in plate resistance between the two tubes. I would think you would have to alter the values of the passive RIAA network if you used a 12AY7 to get the flattest response.
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Old 4th August 2008, 11:17 PM   #4
rman is offline rman  Canada
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No problem, just use this.riaa calculator

Cheers.
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Old 4th August 2008, 11:35 PM   #5
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So to use that tool would you include the Zout of the first tube in the value of R1?

BTW Here is RJM Phono
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Old 5th August 2008, 01:23 AM   #6
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Quote:
Originally posted by rman
No problem, just use this.riaa calculator

Cheers.

Thanks!!!
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Old 5th August 2008, 02:35 AM   #7
alexg is offline alexg  Philippines
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I don't think you need to re-calculate the RIAA if you are keeping the 6dj8 as the first stage.
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Old 5th August 2008, 03:04 AM   #8
rman is offline rman  Canada
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Hi guys.

Mike.
Right zout of first stage is part of r1, so just sutract that
amount from r1.

Alex. I belive you are right. If just changing second stage, I suppose you would just tweak the operating point to suit that tube. You said you used 6n1p's for yours? they have much less transconductance than 6dj8's. how do you like it so far?

Rolf.
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Old 5th August 2008, 07:06 AM   #9
alexg is offline alexg  Philippines
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Quote:
Originally posted by rman

Alex. I belive you are right. If just changing second stage, I suppose you would just tweak the operating point to suit that tube. You said you used 6n1p's for yours? they have much less transconductance than 6dj8's. how do you like it so far?

Rolf.
I prefer the sound of the 6n1p over the Sylvania 6922 that I have. I also changed the operating point of the 6n1p, higher B+ (270VDC), 22K Plate load resistors, 330ohm cathode resistor, and I replace the coupling capacitor from 0.1uf to 0.22uf. I am still working on the new RIAA filter, but I found that lowering the 82k5 resistor a bit is close.

I find it hard to compute the RIAA, I can't find reliable data on the plate resistance of the 6n1p, Svetlana says 4.4K, some say it is as high as 7K. RJM computed the RIAA using 6dj8 which has a plate resistance of around 2K. I am recomputing the RIAA using 4.4K on the 6n1p and 22K plate resistor.
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Old 5th August 2008, 07:18 AM   #10
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Quote:
Originally posted by alexg


I find it hard to compute the RIAA, I can't find reliable data on the plate resistance of the 6n1p,

It is always best to measure it anyway.
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