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Old 6th October 2010, 05:40 AM   #1
Bigun is offline Bigun  Canada
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Default CELLINI-2 a hybrid

Having built SS amps and recently my first SET (my CELLINI triode amp) I'm interested in bringing these families together to explore a hybrid that might offer some of the sonic benefits of the triode but with greater current drive and much less weight in iron. Well actually I'm just curious.

I've picked out what I feel are two proven building blocks. I'm not sure what kind of PSU I want yet, the circuit allows for different voltages for the tube and the buffer if desired.

The front end is a 6N23P triode that I believe to be flexible enough to operate at lower B+ and yet deliver good results. If I allow higher B+ I might also be interested in using something I already have on the shelf such as a 6SN7 or triode wired 6E5P.

The back end is the Krill diamond buffer invented by Steven Dunlap. It has been used in his all-SS amplifiers for a number of years with reports of good sonic results.

So the question is - what do I need to know and consider here having never made a hybrid before ?


p.s. I figure the tube/valve forum is a good place for this because you guys aren't afraid of glass or sand !
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Last edited by Bigun; 6th October 2010 at 05:42 AM.
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Old 6th October 2010, 08:18 AM   #2
Arnulf is offline Arnulf  Europe
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Keep the offset potentiometer value large enough to keep the tube load reasonable, add a grid stopper and consider adding some capacitance to compliment grid stopper and Cmiller. 6N23P (~= ECC88) is a VHF/UHF tube, you can never be too careful

Otherwise looks very straightforward, build it and report how it sounds !
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Old 6th October 2010, 02:48 PM   #3
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Arnulf View Post
Keep the offset potentiometer value large enough to keep the tube load reasonable, add a grid stopper and consider adding some capacitance to compliment grid stopper and Cmiller. 6N23P (~= ECC88) is a VHF/UHF tube, you can never be too careful
The offset potentiometer recommended for Krill is around 1Meg, although I suspect there is some tradeoff with noise ?

I've never worked with a VHF tube before, I'll follow your advice to avoid parasitic oscillations, I think I've read similar suggestions for use with high mu tubes.
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Old 6th October 2010, 03:22 PM   #4
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You could probably use a voltage doubler / tripler off of the SS PSU to provide for an ECC88 / 6dj8 type of tube for 100 or so volts. That would just leave you with a small filament transformer.
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Old 6th October 2010, 04:14 PM   #5
Arnulf is offline Arnulf  Europe
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Originally Posted by Bigun View Post
The offset potentiometer recommended for Krill is around 1Meg, although I suspect there is some tradeoff with noise ?
Yes but I don't think you went for CCS just to load it down with say 2.5K or so (two halves of 10K in parallel) You trade distortion for noise and vice versa and the general consensus established, uhh, well long time ago, seems to be that Rload should be at least 2-3x Rinternal for bearable amount of distortion, and more if possible.
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Old 6th October 2010, 04:39 PM   #6
Bigun is offline Bigun  Canada
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Originally Posted by Arnulf View Post
[...]seems to be that Rload should be at least 2-3x Rinternal for bearable amount of distortion, and more if possible.
...maybe the 'load' resistor gives me a means to tweak the sound of this thing ?

Anyhow, the datasheet I have for the 6N23P is one of those short-form bilingual jobs, it's not that clear to me what the internal resistance of this tube is - does anyone have a better datasheet perhaps ?

Pedroskova, a doubler's a good suggestion, because the CCS plate load should have good PSRR since a doubler will otherwise increase B+ noise I think ?
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Old 6th October 2010, 05:04 PM   #7
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Quote:
Originally Posted by Bigun View Post

Pedroskova, a doubler's a good suggestion, because the CCS plate load should have good PSRR since a doubler will otherwise increase B+ noise I think ?

Yeah, they can be noisy, but maybe some of the design solutions from Cavalli Audio might work.

Caveat emptor...I'm no EE
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Old 6th October 2010, 10:37 PM   #8
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Nice. That output buffer looks like one of those buffer chips that pinout like opamps but are only current boosters.
Why will a doubler be more noisy? U can make fullwave doublers too.
Will a unipolar positive voltage doubler create some asymmetry in the Vcc+/Vee- which inturn might give u a DC offset problem?
Then again, a doubler is probably more of a hastle than its worth. The input cap is a good idea anyways. I think ur schem is pretty good to go. But hey, we must discuss all options here!
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Old 6th October 2010, 10:42 PM   #9
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...but really. Shouldnt the ouput stage have its own separate supply anyways? I'm nitpicking maybe. Since u think hybrid u ought to try a mirror load for the tube and simply DC couple the whole way.

Last edited by SemperFi; 6th October 2010 at 10:44 PM. Reason: Spellin
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Old 7th October 2010, 12:55 AM   #10
Bigun is offline Bigun  Canada
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There are two reasons I thought a doubler could add to noise is simply that we have more diodes creating switching noise inside the box (I'm thinking of a single chasis). But it's probably not an issue to worry about. And a separate supply for the front end is also probably unnecessary with proper decoupling and the PSRR benefits of a CSS (or a choke).

With a thermionic valve plus a Krill buffer I should be calling this amplifier the "THRILL"
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