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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 27 Aug 2020, 21:34 
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a little off thread but does anyone see anything wrong with this cct. or can some one point me to a better designed 6SN7 cathode follower on this forum. I'm sure Suncalc put up one but I cant find it?
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6SN7_CF.png


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PostPosted: 27 Aug 2020, 21:42 
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I found the link. no need to reply unless you have some tips for construction or a change in values. I'm going to make a hybrid HP amp with this cct. for frontend and a CMoy driver backend. The CMoy has gain so I just wanted a tube sound up front with no gain.

Ill post my progress.

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PostPosted: 13 Dec 2020, 13:31 
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This looks like a fun and useful project. I’m putting together a parts list. On the filter capacitors (33uf and 100uf) - how far can I stray without changing the existing design? I have most of the parts except the 100uf cap. I do however have two filter cans with two 40uf sections each. Can I use one 40uf section in place of the 33 and combine the remaining 3 sections for 120uf in place of the 100uf?then I can mount those on top away from everything else.

I’ll try to read the power supply article again tonight. I started and got bogged down.


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PostPosted: 13 Dec 2020, 15:39 
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christian_jennings wrote:
On the filter capacitors (33uf and 100uf) - how far can I stray without changing the existing design?
At these current levels the capacitor values are very forgiving. Your 40µf cap for the first filter is fine. I think I would start with just using one section of your can in each position (i.e. 40µf after the rectifier and 40µf after the 2H choke).This should be more than an adequate level of filtering for this preamp.

Using the 40µf cap with the 2H choke yields a filter factor of ≈ -33dB (verses ≈ -41dB with the 100µf cap). Given the small draw current, the ripple out of the rectifier cap should be very small (smaller in fact than with the 33µf cap). Combining the two will likely still put the ripple well below the audible level.

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PostPosted: 14 Dec 2020, 15:30 
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Thanks Matt. I’ve got pretty much everything thing I didn’t have on order now. I’ve been reading the power supply article. Really well written. Theoretically if I wanted to design this preamp with a socket for each 12A?7 (6) and switch between them how would that change my power supply?

For example: I have 1 set of rca input (L&R) connected to a switch with 1-6 positions that directs the input to one of the 6 tubes. Because only 1 tube is under load at any given time, is the power supply unchanged from the original design?

Not going to add 6 tubes but might be interesting to have 2 side by side to compare by flipping a switch. And more than anything just curious how that works.


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PostPosted: 14 Dec 2020, 20:51 
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christian_jennings wrote:
... but might be interesting to have 2 side by side to compare by flipping a switch.
You could do this but beware that if they were different tube types, the volume would jump rather dramatically. You could do this to A/B test two different tubes of the same type however. Actually, I'm surprised that no one has thought about trying that until now.

As for the power supply, the driver currents are rather small. Doubling up on a few milliamps is not really going to affect the power supply.

Hope this helps.

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PostPosted: 15 Dec 2020, 16:18 
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Hmm. I always turn the volume down all the way before changing input so it wouldn’t be a big deal for me. It would be nice to flip back and forth though and hear the difference side by side like that. Although turning it down flipping and turning it back up is way better than turning it completely off swapping hot tubes and turning it back on. As a product it might have challenges.

I guess it would jump regardless where in the circuit you added the volume pot. But seems like you’d want the volume at the output in case the switch introduces noise. Or at least put the switch on the input side of the potentiometer so any unwanted noise in output is throttled with all the noise you want.

I call it the tube rolling amp.

And you could just leave any extra sockets empty if you’re not rolling and don’t want extra hours on tubes you aren’t planning to listen to.

I also thought of putting a gain switch on the bypass. Could all the tubes use the same bypass cap or would I need one per tube? So they would all connect to the same 1K2 resistor. Or does it not work that way?

Thanks!


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PostPosted: 16 Dec 2020, 20:01 
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christian_jennings wrote:
Could all the tubes use the same bypass cap or would I need one per tube? So they would all connect to the same 1K2 resistor. Or does it not work that way?
No each tube would need its own grid resistors, load resistors, bias resistors, cathode bypass capacitor, and output capacitor. All the output caps could be tied to the 250kΩ volume pot. Although if you had more than two tubes, I would recommend increasing the volume pot to 500kΩ.

This is possible because you are only driving one tube at a time. With nothing on the input, the unused tube is in a quiescent DC state and nothing goes through the output capacitor on the undriven tube. I would also recommend grounding the inputs of the undriven tube to limit noise.

Such a preamp would make A/B testing various tube really simple.

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PostPosted: 18 Dec 2020, 00:03 
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Ok so like this:
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PostPosted: 18 Dec 2020, 13:19 
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christian_jennings wrote:
Ok so like this:
Exactly. :thumbsup:

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