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In Reply to: RE: 6SN7 circuit posted by Chip647 on March 25, 2017 at 11:52:51
What about Eric babour Brute Force? However, it has 6BL7 othee than 6SN7.
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I like line amps that don't invert. The Blue Velvet topology is one of the few that provides this functionality without either an IT or an overabundance of gain. I've worked out a slightly modified version on paper that will probably be my next line amp.
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Buy Chinese. Bury freedom.
TK,
I'd be interested in your BV 6SN7 preamp since it is non-inverting.
Can you post the schematic here?
dt 667
I worked out two schematics in SPICE. Both feature significantly less distortion and more typical tube loads than the original. Output Z of the first is about 4K Ohms. The second version adds a cathode follower to provide an output Z around 450 0hms. The follower is biased such that an additional coupling cap is not required. It will easily swing 50V p-p.
Now that I'm thinking about using 6W6s in triode mode instead of 6SN7s, the follower probably isn't needed. Wish I had a SPICE model for the 6W6!
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Buy Chinese. Bury freedom.
The SRPP as gain stage is not too bad, but it inverts and has a higher output Z.
These look like a better option for a preamp.
Thanks,
dt667
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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Buy Chinese. Bury freedom.
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.
Edits: 03/27/17
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