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RE: 5751 work in Precision Fid C-7A Rev.?

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Dman

In Reply to: RE: 5751 work in Precision Fid C-7A Rev.? posted by pix4work on October 21, 2011 at 10:59:03

NO! for two reasons-

1. The impedance of the 5751 is different than a 12 AX7 (you can download the data sheets off the 'net for both). This can cause RIAA errors. IIRC, the phono stage on both those preamps seemed REALLY sensitive to 12AX, AU changes, as well as 5751.

2. The 5751 draws a bit more current than a 12AX7 (350ma vs 300ma), and can cause the regulator (which is already running very near its limit) to oscillate. Due the gain of the phono stage, this will be audible as a faint whistling through the loudspeakers as the regulators go into overload protection. I had this problem with both the C-7a and C-8a (I actually gutted the innards of my C-8a and built a new preamp, using a similar 12AX circuit, and a 12AX7 line stage, with a 6N1P as a cathode follower!).

Not knocking the PF stuff, but it could have been better designed, IMHO. You can also look in the archives here, as I know I conversed at great length about this a few years ago...

"If the audio industry built gear that sounded as good as it did 50 years ago, there would NEVER be a need to re-issued anything!"

Follow Ups:

RE: 5751 work in Precision Fid C-7A Rev.? - pix4work 21:22:48 10/21/11 (2)
Thank you for your timely reply, as I was about to go with a pair of 5751's for the phono circuit. Which 12AX7's did you find to work well with your C-7A?...I know there are quite a few varieties. TIA.

pix4work
- RE: 5751 work in Precision Fid C-7A Rev.? - Dman 06:48:22 10/22/11 (1)
  I used EI (Yugo) for a time, until I managed to find a set of Telefunken smooth plate 12AX7. They seem to work well and have a nice, neutral voice in this pre. Sovteks (yes, regular basic ones) also were fine, but tended to highlight the top end a little (not grainy though)...
  
  As usual, YMMV, so try a few different ones. Good luck!
  
  "If the audio industry built gear that sounded as good as it did 50 years ago, there would NEVER be a need to re-issued anything!"
  - RE: 5751 work in Precision Fid C-7A Rev.? - pix4work 10:36:19 10/22/11 (0)
    Thanks for your thoughts and comments. I have used this unit with tube and solid state amps and I would like to keep it going as long as possible. Different tube brands will act as tone filters, I have discovered, so I will be trying a brand or two to come up with a reproduced sound that's pleasing to my ears, and ultimately, that's what it's all about in this hobby, isn't it?
    
    Thanks again for posting your reply.
    
    pix4work

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