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Old 3rd February 2007, 10:05 PM   #1
engels is offline engels  Israel
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Default 6C8G phase inverter

I've got a couple of nice RCA 6C8G's and I've read they were primarily intended for phase inversion. Surprisingly I couldn't find any schemes featuring 6C8G as a PI.
Any links?
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Old 3rd February 2007, 11:22 PM   #2
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Take a look at the data sheet. Gm is low at 1.6 mA./V. and Rp. is high at 22.5 KOhms. IMO, those characteristics suggest mediocre performance in either a "concertina" or LTP circuit.
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Old 4th February 2007, 03:25 AM   #3
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Default Re: 6C8G phase inverter

Quote:
Originally posted by engels
I've got a couple of nice RCA 6C8G's and I've read they were primarily intended for phase inversion. Surprisingly I couldn't find any schemes featuring 6C8G as a PI.
Any links?
Try Googling up "Long Tail Pair" or "Schmidt Phase Inverter", and you should come up with plenty of schematics.

Quote:
Gm is low at 1.6 mA./V. and Rp. is high at 22.5 KOhms. IMO, those characteristics suggest mediocre performance in either a "concertina" or LTP circuit.
Sure, as a cathodyne it would be "TEH SUXXORZ", but it would make a fine LTP, especially with a CCS in the tail. Those specs are similar to that of the 6SL7, and I did use one of those as a LTP splitter. Worked great, especially in conjunction with cathode follower drivers.
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Old 4th February 2007, 11:38 AM   #4
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Quote:
Those specs are similar to that of the 6SL7, and I did use one of those as a LTP splitter.

OK, but the 6C8's mu is 1/2 that of the 'SL7. My concerns lie with low net gain and slew limiting. A high gm in a differential splitter is protection against slew limiting. Slew limiting is an issue, when loop NFB is employed.
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Old 4th February 2007, 07:01 PM   #5
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Quote:
Originally posted by Eli Duttman
My concerns lie with low net gain and slew limiting. A high gm in a differential splitter is protection against slew limiting. Slew limiting is an issue, when loop NFB is employed.
Slew rate is why you should use a cathode follower driver if you're gonna use VTs like the 6SL7 or 6C8: these are voltage amps that are suitable for driving high resistance, low capacitance loads, which doesn't describe the grid circuit of many a power final, but does describe the input of a cathode follower where you don't get Miller effect, and that bootstraps the Cgk to a very small value. VTs like these are quite capable of driving such a load, and do so while preserving slew rate.

I sure didn't connect that 6SL7 LTP splitter directly to the 807 grids for just that reason. Of course, there are occasions where it doesn't matter, like guitar amps where you don't have such a high end anyway.
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