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Single stage preamp

bigwill

Member
2004-12-25 8:36 pm
UK
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?
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
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.
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
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. :D
 

bigwill

Member
2004-12-25 8:36 pm
UK
ray_moth said:
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. :D

Sounds like a good idea to me!
 
Will,

If you use Gary Pimm CCSes to load 'SN7 sections and take advantage of the mu follower O/Ps, you will end up with too much gain and a "hair trigger" volume control. Expect a voltage gain of 20X, which is the full mu of the type.

You can convert that excess gain from a liability into an asset. Use Sowter model 8940 3:1 step down trafos at the O/Ps. ;) Talk about being able to drive a hellish load. :D
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
FWIW, I use a single 6SN7 for both channels in my preamp. Here's the separation curves. I think the limitation is the volume control, not the tube.

Don't sweat it. :D
 

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<snip>
Forgot that the load is a current source, cathode degeneration actually has little effect on the gain in this scenario. Choose a tube with lower gain. (Maybe a 56, 76 or its octal equivalent the 6P5 - all singles.) Feedback like you propose is possible, but the additional loading imposed defeats the purpose of the ccs in the first place, and it creates issues with the choice of volume pot as the input impedance will now be rather low if you want good ac performance.

Sitting out in the open like that it is likely that it is picking up stray fields, and it could also be oscillating if there is no grid stopper resistor close to the socket. Most of my bouts of oscillation occurred in bread boards that had a lot in common with yours.. Long leads randomly arranged, etc. :D
 

bigwill

Member
2004-12-25 8:36 pm
UK
Another odd thing I noticed

If I held the disconnected end of an rca cable connected to an amp and just held it near the tube and the leads carrying the output signal, I could hear the music and it seemed like all frequencies were present from what I could tell. Is this oscillation?