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Old 17th May 2011, 03:24 PM   #11
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Quote:
Originally Posted by richwalters View Post
The circuit enclosed using an ECF80 performs the function brilliantly with near equal swing symmetry. Expect 25V+25V rms swing and no excuse for signal voltage discrepancies.

richy
Yes, Ive used this 'scheme' with different tubes, and it does work extremely well....

Particularly like the 'DC' coupling from the gain-stage, and getting rid of the auto-bias and the coupling-cap of the phase-splitter.

Ive adapted it further to replace the pentode by a strapped triode and have a MOSFET on top, its bias set to place the anode of the triode at 110V, and the phase-splitter grid DC coupled by 470K to the anode of the 'mos-Mu' stage

Absolutely great sound.
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Old 17th May 2011, 03:31 PM   #12
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Well, that's the classic voltage amp- cathodyne used in millions of amps. It does work great. For stability, I'd be inclined totie the two B+ together, then adjust the plate load of the voltage amp to get the right DC conditions. Or use an adjustable CCS- the cathodyne has a very low input capacitance, so doesn't load down the voltage amp.
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Old 17th May 2011, 03:36 PM   #13
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Looks like we cross-posted there, Sy....

Your suggestion of the CCS in the plate-load of the gain-stage is exactly how I ended up, trimming the bias to get the correct voltage for the plate to grid voltage....

The CCS/MOSFET in the gain-stage really does make it more flexible and the sound is great too....
--I use it as my 'generic' gain/phase-splitter now....
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Old 17th May 2011, 04:47 PM   #14
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Convergence, all ideas together. Morgan Jones Valve amps 3rd edit doesn't praise the simplest and most effective enough. Page 403.
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Old 18th May 2011, 01:11 AM   #15
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Not to change the subject much, has anyone ever used the 6GH8a? Not sure why, but at one time or another I had purchased 10. Looking at the data sheets, the triodes do not appear to be very linear.
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Old 18th May 2011, 05:58 AM   #16
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Nor is the ECC82.
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Old 20th May 2011, 09:36 AM   #17
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The output impedance from cathode is much lower than from anode.
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Not so. Please see my article in Linear Audio which examines this theoretically and experimentally.
I have to correct my earlier statement where I said that the output levels of different outputs are unequal if the load resistance is too low.

One reason to this confusion is caused by the fact that the output impedance from anode is much higher than from the cathode.
(other maybe is that I have done all my consructions and analysis with LTPs)

The Concertina phase splitter seems to be that sensitive to unequal loads that even a level measurement done with my HP333A distortion/audio voltage meter gives noticeable difference from anode and cathode.

I made few tests to fully demonstrate the effect of unequal load impedance and it's effects.

I built a Concertina with 6N1P and with 68 kohms anode- and cathode resistors. Then I used a 680 kohms load resistor to load both outputs separately to see what happens.

The results are as follows:

1. Voltage measured from cathode and load connected to cathode: no level change

2. Voltage measured from cathode and load connected to anode: no level change

3. Voltage measured from anode and load connected to anode: level decreases some 9 %

4. Voltage measured from anode and load connected to cathode: level increases some 9 %

From these results it can be seen that the output impedance from the anode is practically equal to Ra (68 kohms) and from cathode the output impedance is much lower. (the actual value can not be determined with 680 kohms load)

I also tried to determine the optimum grid voltage i.e. the voltage that gives lowest distortion with high output level.

With Uo = 25 Vrms, Ub = 300 V, the optimum grid voltage was 85 V which is some 28 % of Ub.
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Old 20th May 2011, 09:38 AM   #18
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If the outputs are loaded equally (and that is indeed the case with the cathodyne in situ), the source impedances at plate an cathode are identical and low (~1/gm).

Quote:
Then, if the load impedance is too low - but equal at both outputs - the output voltages can be different.
This is entirely incorrect. Please see my article (with measurements) in Linear Audio Vol 0.
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Old 20th May 2011, 10:17 AM   #19
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Originally Posted by artosalo View Post
The Concertina phase splitter seems to be that sensitive to unequal loads that even a level measurement done with my HP333A distortion/audio voltage meter gives noticeable difference from anode and cathode.
What frequency are you measuring at ? There is a slight cäpacitive difference on the concertina, esp noticeable on square wave... the HP döes have a sizeable input capacitance. Old trick; Use a 4mm to BNC adaptor and use a X10 probe and scale accordingly. Somewhere I recollect there is a RCA 7199 data sheet around which describes the capacitive issue, if I can find it.
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Old 20th May 2011, 10:24 AM   #20
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Rich, I dealt with that in my article as well. The test lead often has significant capacitance- if you attach identical test leads to each output, the HF imbalance doesn't occur.
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