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Old 5th January 2007, 02:23 PM   #11
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Quote:
Originally posted by SY
The two cathodes are tied together at the bottom of their 1k bias resistors through the 11k tail. How about "diff-ish" amp?

That brings up another fatal flaw, the way the feedback is applied.
Yikes, I didn't notice this ...
Those bias resistors should be combined, and that feedback might be best connnected to the cathode of an input tube. Of course, if that input tube were direct coupled, those bias resistors could be dumped altogether.
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Old 5th January 2007, 02:38 PM   #12
SY is offline SY  United States
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You've still got the problem of needing 20 volts of drive from the preamp if you want to have a useful amount of feedback. EC's suggestion is an excellent one- start with a Williamson, then customize and update the circuit as you go (if you do nothing other than fix the LF time constant problem, you've got a terrific amp right there).

If you want to do something beyond that, a fixed-bias 6L6GC or equivalent can give you 40-50 watts in triode if you run it AB2, which it is very happy doing.
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Old 5th January 2007, 03:02 PM   #13
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Quote:
Originally posted by SY
You've still got the problem of needing 20 volts of drive from the preamp if you want to have a useful amount of feedback. EC's suggestion is an excellent one- start with a Williamson, then customize and update the circuit as you go (if you do nothing other than fix the LF time constant problem, you've got a terrific amp right there).
I was suggesting that if an input tube were added, it could make biasing of the diff amp easier and feedback application more logical, but it would easily fix the gain problem -- basically a Mullard circuit.

I also support a Williamson... I know we discussed that possibility a few months ago here, and still think it would be worth a go.
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Old 5th January 2007, 03:32 PM   #14
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Ah, gotcha. Yeah, a Mullard circuit (or a cousin) would work fine, but I'd want to use a tube in the diff amp that I know would swing a lot of volts cleanly.
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Old 5th January 2007, 07:53 PM   #15
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That NFB loop is all wrong. The NFB resistor should go to the other side of the 0.1uF cap and there should then be a resistor from the cap to ground. The ratio of that extra resistor to the NFB resistor determines the amount of NFB applied.

As already stated by SY, your gain with that schematic will be inadequate, especially considering that you could need up to 20dB of negative FB, depending on the damping required for your speakers. I don't think there's anything wrong with using the 6N1P as you intend, it just needs a voltage amplifier on the front end to give it some more gain. This should be a high gain stage, for which a high-mu triode would probably do.

If you do that, then you might as well use global NFB instead of the NFB loop you have there now. This will give more gain inside the loop, to make it more effective. Direct coupling of the first stage to the splitter, Mullard style, will be important to maintain LF stability with global NFB. (Avoid the Leak design, for that reason.)

I wouldn't try a Williamson approach if I were you, because you need to be careful to avoid LF instability with that design. Williamson used triode-strapped beam tretrodes, which didn't need much NFB to get decent damping, and even then his amp was close to being unstable. With UL, you might find you need more NFB and that will be a challenge. Not to say it couldn't be made to work, but why start at a disadvantage?
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Old 5th January 2007, 08:15 PM   #16
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There's nothing wrong with the Williamson circuit, just the way Williamson did it. Re-jig the coupling capacitor values and the instability problem will go away.
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Old 5th January 2007, 10:56 PM   #17
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If I built a Williamson, I would stagger the time-constants of the coupling (as SY has advised before now). I think I would also like to try to get a decent amount of additional NFB inside the global loop, with FB resistors from the OP tube plates to the driver cathodes. I believe it's possible to do this without using series capacitors, if you adjust the bias on the driver tubes.
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Old 6th January 2007, 02:27 AM   #18
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Well, I guess I've come full circle then. Thanks again for all the constructive criticism, I guess that schematic was too simple to be true (disregarding the really bad mistakes ) I remember discussing using .047 and .47uF caps, or in the opposite order. I'll dust off that schematic and start figuring out changing resistor sizes. I'm looking at possibly around 475V on the B+ with some Antek toroids in choke input... the problem is, I'll have a YUGE industrial control PT and 193M choke... maybe if I buy another of the PT and choke, I can make a ridiculous pair of monoblocks or something...
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Old 6th January 2007, 05:00 AM   #19
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What about modifying my 6L6GC amp in UL by using some other output transformer? Maybe its going to be ok for you. Plays fine as it is in triode but you probably need more power.
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Old 6th January 2007, 05:15 AM   #20
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Salas,

Just as a matter of interest, have you ever tried that amp with a 12AT7 in both splitter and first stage positions?
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