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Old 25th January 2008, 06:11 PM   #1
bigwill is offline bigwill  United Kingdom
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Question Single stage preamp

Quick question - I just ordered the bits required to make a Gary Pimm battery biased CCS, and I was wondering if it's possible to simply use a CCS loaded 6SN7 as a single stage preamp - each half as a channel. Would this arrangement work and be able to drive cables / input stages?
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Old 25th January 2008, 06:34 PM   #2
SY is offline SY  United States
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It depends on the cables and the load. Assuming there's no cathode degeneration in the 6SN7 circuit, the source impedance will equal plate resistance. For usual operating conditions, that will be between 7-10k. That's pretty high. If your power amp has a high input impedance (more than 100k) and the cables are short, you can probably get away with it.

If there is cathode degeneration, that makes things worse; the effective plate resistance will be increased by a factor of 21 times the unbypassed cathode resistance.
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Old 25th January 2008, 06:42 PM   #3
bigwill is offline bigwill  United Kingdom
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I see. I sort of got it into my head it would be able to swing roughly the bias current into the load if needed - if only things were that simple. I guess I'll be using a cathode follower!
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Old 25th January 2008, 07:14 PM   #4
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Or you could just choose a more appropriate tube for the job. Same topology, but with a 5687, will give you roughly the same gain and a Z out of about 2 to 2K5. A 12B4 will give you less gain (which might be a good thing) and an even lower Z out.
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Old 25th January 2008, 07:19 PM   #5
bigwill is offline bigwill  United Kingdom
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Oh neat, it's possible - are there any suitable octals that would also work?
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Old 25th January 2008, 07:31 PM   #6
DougL is offline DougL  United States
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6H30 Octal

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Old 25th January 2008, 07:43 PM   #7
bigwill is offline bigwill  United Kingdom
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Hmm, the 6H30 does look awesome but it seems expensive!
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Old 25th January 2008, 07:49 PM   #8
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Maybe a mu-follower would be better for your needs? It would use both halves of the 6SN7 (or other double triode), would give practically the same linear performace and gain as a CCS load and would have the low OP impedance of a cathode follower.

The biggest advantage, though, is that you wouldn't need to use the Gary Pimm CCS, so you could save it for another project.
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Old 25th January 2008, 07:59 PM   #9
bigwill is offline bigwill  United Kingdom
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Quote:
Originally posted by ray_moth
Maybe a mu-follower would be better for your needs? It would use both halves of the 6SN7 (or other double triode), would give practically the same linear performace and gain as a CCS load and would have the low OP impedance of a cathode follower.

The biggest advantage, though, is that you wouldn't need to use the Gary Pimm CCS, so you could save it for another project.
Sounds like a good idea to me!
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Old 25th January 2008, 08:09 PM   #10
bigwill is offline bigwill  United Kingdom
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Hold on - If I use the mu-output of Gary Pimm's battery CCS won't that basically achieve the same thing?
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