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M Gregg 8th July 2013 08:28 PM

SS phase splitters?
Does anyone have,

Any circuits for SS phase splitters to drive tube sections? (I'm thinking discreet FET based)i.e. Van Scoyoc cross-coupled splitter.
And what if any problems were encountered?

M Gregg

ingenieus 8th July 2013 09:05 PM

If you have a tube circuit, it can probably be adapted. Here is an article on how triode tubes can be replaced directly by high voltage MOSFETs.

MOSFET Follies - letting the Solid State Demon into your tube amp

Eli Duttman 9th July 2013 12:57 AM

It's no secret that I like MOSFET voltage followers. As a "concertina", AKA split load, phase splitter is a tweaked voltage follower, a MOSFET doing that job rates to be quite satisfactory. Look here, where Jeff Yourison is working (with a little help from yours truly) on a PP 6Y6 amp that employs a MOSFET "concertina" phase splitter.

While FET current amplifiers are highly satisfactory, I'm very dubious about FETs as voltage amplifiers. The cross coupled phase splitter is a somewhat complex bit of circuitry. Voltage amplification is occurring, along with the generation of the 180o out of phase signals. I can see depletion mode (for self biasing ease) MOSFETs source followers at the I/P of the transformerless version, but linear triodes being employed in the voltage gain positions. Perhaps the DN2540N3-G would be suitable in the voltage follower "slots".

ingenieus 9th July 2013 09:02 PM

2 Attachment(s)
Here is am improved version of the Van Scoyoc cross-coupled splitter published in 1957:

I ran it through LTspice, and it works perfectly. The only thing is, I don't get the bit about "cathode voltage of power tubes must be increased by 20 volts." My limited knowledge of tubes does not help, especially since the simulation works without this. Why is that necessary? To improve the voltage swing range? In practice, would this be implemented with voltage divider resistors off B+?

Eli Duttman 10th July 2013 12:37 AM


The only thing is, I don't get the bit about "cathode voltage of power tubes must be increased by 20 volts."
The phase splitter's O/P cathode followers and the O/P tubes' control grids that are DC coupled to them "sit" at +20 VDC. Correct operation of a tube, with few exceptions, requires the control grid be negative with respect to the cathode. Implicit in the remark is a cathode (self) biased set up. Since the control grids of the O/P tube pair are at +20 V., instead of 0 V., the cathode has to be raised above ground 20 extra volts to achieve the desired operating point. :yes:

If the cap. coupled O/P cathode followers are replaced by DC coupled ZVN0545A source followers, cap. coupling to the O/P tubes' grids can be employed and any valid biasing method will work. :)

Wavebourn 10th July 2013 01:23 AM

1 Attachment(s)
Here you go, just a concept:

Cookj 10th July 2013 10:16 AM

Novotone - Projects 35 and 37

Published in Electroniqe Pratique



tinitus 10th July 2013 10:30 AM

1 Attachment(s)
I think this one is from a Japanese site

MiiB 10th July 2013 11:23 AM

'The last circuit is a LTP, to me one of the best ways to make a phase splitter. I would make it with two lsk170's or something like that, then cascode it with transistors that can withstand the high(ish) voltage.

Or maybe looking to Calvin's fine buffer-thread make it with a CFP pair as input + a shielding cascode. Then you can run the output tubes without any coupling caps.

ingenieus 10th July 2013 09:10 PM


Originally Posted by Cookj (
Novotone - Projects 35 and 37

Published in Electroniqe Pratique



I can read Dutch and Flemish, but my French is non-existent. It's a shame because it seems like the site contains a lot of useful information.

I eventually managed to figure out that one needs to click on "Schéma de l’amplificateur" to see the schematic of the amplifier. Here are the actual links. ;)

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