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Old 27th June 2005, 01:14 PM   #1
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Default Phase Splitters

Usually, phase splitters (in a PP tube amplifier) are in the stage immediately before the output tubes.
Is there anybody that can argue on a different topology: that is a phase splitter (with gain, like a Long-Tail, Schmitt) as first stage, followed by the driver tubes, then by the output tubes ?
I plan for a low impedance driver circuit, like the one on can get with a 6C45P-E loaded by a solid-state constant current source (IRF830), I circuit I have alredy used (with success) in some other my realizations.
Output tubes are to be driven a little bit in class A2.
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Old 27th June 2005, 01:20 PM   #2
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Yes, looks at the williamson circuit.
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Old 27th June 2005, 01:47 PM   #3
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It depends on the phase splitter and overall gain setup for the amp. For example, the "Crystal Palace" amplifier shown in Morgan Jones's "Valve Amplifiers" has a long tail in the first stage. The Williamson that Tekko mentioned puts a split load in the second stage, between the input amp and the driver. My amp uses an input transformer to do the dirty work.
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Old 27th June 2005, 02:12 PM   #4
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If I've read into this correctly, and you're thinking of using 6S45s in a long-tailed pair to drive a PP output stage into class *2, it may well be best to do phase splitting prior to the driver stage as an LTP relies in part on equal load impedances (assuming a high Z tail) to split phase properly. This will not be the case when one output valve is into grid conduction when the other is nearing cutoff.
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Old 27th June 2005, 03:03 PM   #5
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Default To be more precise ...

... the first stage should be a Long-Tail phase splitter (12AU7, I'm still simulating), the second stage should be hybrid cathode follower (2 x 6C45P-E), the final stage should be a PP of 2A3 (drive a little bit in class A2).
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Old 27th June 2005, 03:46 PM   #6
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Default I just built ...

... a pair of monoblocks with

1) phase splitter by input transformer (cinemag)
2) push-pull of 6H30
3) interstage LL1660 PP
4) push pull of 2A3
5) OPT LL1623

simple enough for a newbie like me and, yes, it works nice.

A few days ago I modified the interstage connections and the amps are now working as SE to PP just to make some experience and hear the differences.

Ciao
Gianluca
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Old 28th June 2005, 06:57 AM   #7
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I prefer to start with the splitter and use PP throughout. It helps especially with minimizing hum. E.G., please see attached schematic of my 16W PP EL34 triode amp.
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File Type: gif rwm amp el34 triode p-p gfb.gif (16.1 KB, 1040 views)
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Old 28th June 2005, 11:29 AM   #8
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Default Interesting point ..

Good point Ray. This the kind of comments I need.
You say that this configuration helps in preventing hum. Good.
It is interesting also you usage of the pentode as Constant Current Source. Usually I do CCS in Solid State (mainly because I have more reliable -than pentode- SPICE models).
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Old 28th June 2005, 10:31 PM   #9
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I like to keep stages to a minimum. Preamp direct coupled to a cathodyne or LTP, coupled to output (buffered if necessary; you mentioned grid current, that'll do it). Cathode followers don't really count as stages because they contribute no poles, they're basically invisible (assuming you drive the OP grids directly from the CF cathodes).
If a lot of gain is necessary, I'd rather use a pentode front end (gain circa 400, do that with a 12AX7!) than a second gain stage (balanced or SE). Reason being, a mere two coupling caps in the circuit combined with the inductance of the OPT makes a large phase-shift oscillator.

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Old 29th June 2005, 08:40 AM   #10
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Agreed, 2 coupling caps and an OPT within a global NFB loop can easily cause oscillation - one of the main criticisms of the Williamson design. I have tried DC coupling between my splitter and driver stages, which would be the ideal arrangement, but the 6SL7 and 6SN7 tubes I have are not sufficiently well balanced and I got almost 20% difference between the 6SN7 driver plate voltages. However, I've found that, using a step network with a local loop around it, as shown in the schematic I posted, gives me sufficient stability margin for global NFB, plus excellent balance.

My choice of a pentode as a current sink is due to the fact that I had some 6AU6 tubes available. My power tranny has a 50v 0.1A winding, so, using a voltage doubler, I was able get 110v @25mA for a negative supply, sufficient for both the 6SL7 LTP/6AU6 CCS combination and the 6SN7 driver.
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