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In Reply to: RE: 6SN7 circuit posted by Triode_Kingdom on March 25, 2017 at 21:59:58
That's a nice circuit. I had looked at the Blue Velvet some time back and considered building it, but alas my system gain is such that I don't need to run a preamp - well at least for now. The only thing that I'm a bit hesitant about, is that the input tube is running into a 11.2K impedance as presented by the next stage.
The effective load for the first stage is at 3K which is kind of low. The 15dB of NFB will cancel distortion at the second stage but the input tube has only the degenerative current feedback from the unbypassed Rk to do some HD reduction. This may, or may not be enough (I'm just thinking out loud here, not saying that this is actually the case). I'm wondering if a bit of cathode feedback at the input tube might be of some benefit.
I agree, the impedances in the circuit are too low. I've changed that (on paper), as well as modifying the feedback network to reduce output Z and gain. Lately I've also been thinking about using different tubes. The 6SN7 is an excellent choice, but the 6W6 in triode mode is extremely linear and offers an even lower output Z. The only downside is heater power, which would be almost 5A for four tubes.
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I like the 6W6 tube myself and think it'd be an excellent choice. As a matter of fact I was the one that suggested its use (along with the 6V6) in a preamp thread of yours a few months back, if I remember correctly.
When I was looking into the BV preamp, I thought that instead of using around 15dB of local NFB at the output section only, it would be a good idea to cascade two anode followers each having around 10-12 db of local feedback. I ran some numbers at the time but couldn't get the input impedance of the first stage high enough to my liking and didn't want to transfer the burden of driving the preamp to the source ahead of it.
I didn't spent too much time on that exercise, but now that I think about it, maybe a high mu tube such as the 6SL7 at the input position, configured as an anode follower with a gain of ~2 driving another AF, say a 6w6, wouldn't be a bad idea. But then you're moving completely away from the 6SN7 which you probably don't want to do.
Anyway, thanks for reminding me of this circuit, I now might take another look at it.
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