Direct Coupling Modulated Bias (SET)

[serengetiplains]

Say, has anyone seen this circuit topology:

http://www.valvediy.com/simplexpg1.html

http://www.valvediy.com/simplexreduxpg1.html

http://www.plitron.com/PDF/polisois1957.pdf

I only just stumbled upon these articles yesterday. I find the circuit intriguing in the way it addresses the problem of coupling driver to output tube, and rids the output tube of the dreaded cathode resistor (!) ... very elegant. I do not know what type of output transformer the author, Ari Polisois, uses in the 6C33C version, but his output distortion specs look *linear* if a little high:

http://www.valvediy.com/simplexpg7.html

And this with unregulated supplies. Ari says he's built >10 amplifiers and that this amplifier circuit bests them all.

I suspect tying output cathode to driver power supply is the weak link of the circuit as any driver supply variations will modulate a very sensitive part of the circuit. Makes me wonder how the circuit would perform with rock-solid regulation.

My hat off to Ari Polisois for an interesting circuit innovation.

[serengetiplains]

Here's the schematic from the links above.

The SIMPLEX Single-ended Audio Amplifier
by Ari Polisois
©2002 A. Polisois

[serengetiplains]

Originally posted by serengetiplains
I suspect tying output cathode to driver power supply is the weak link of the circuit as any driver supply variations will modulate a very sensitive part of the circuit.

(continuing this conversation with myself) Actually, after further thought, driver supply variations always necessarily appear on the output grid. Coupling those variations to the cathode will therefore reduce their overall influence on the output signal ... the cathode coupling, if you will, providing a form of feedback relative to grid. Very elegant!

[EC8010]

The coupling and twin power supply idea is quite clever, and because a 6C33 needs a lower HT than its driver, makes a great deal of sense.

Unfortunately, your last post made me think about power supply noise, and I realised that taking the voltage between power supply and anode is not a good thing for a triode with low ra. The undecoupled cathode resistor raises ra, which helps, but without values, it's hard to see what the effect will be. Purely from the point of view of power supply noise, a pentode would be best in this particular application.

[gnomus]

I just started looking at this circuit too. And I am looking for parts. Some changes I am doing is a 10H choke input to replace the resistor in the driver power supply. A larger 10-20H choke input in the output tube power supply. By choke input I mean bridge rectifier then choke then caps. I have had good results with chokes in solid state. DC filiments for driver tubes. I also think its easier to build in dual mono. Mostly because you have more room to fit parts. Also separate channels are better. But, more costly as you now have two separate power supplies on each chassis. Finally passive input pot. Not sure if this will work well but it's easy to add.

[serengetiplains]

Originally posted by gnomus
I also think its easier to build in dual mono. Mostly because you have more room to fit parts. Also separate channels are better. But, more costly as you now have two separate power supplies on each chassis.

Hi Paul,

Separate power supplies pay big dividends soundwise. Also, as per my previous post, I suspect this circuit enhances power supply noise rejection of the driver supply by coupling similar amplitudes of that noise in phase to the output grid + cathode. I suspect adding a third power supply for the input tube would open the sound of this circuit even more than what the designer claims for it. My comments are conjecture because I have not built this amplifier.

The designer of this circuit told me very recently the circuit is covered by a French patent. Just so you know.

Tom

[Sch3mat1c]

Fine with me. Like an unchoked monkey but with a stepped supply instead of wasting incredible gobs of power through an absurd cathode resistor.

Power supply hum is an issue in any triode amplifier (with Rl typically 3xRp, 3/4 of the hum is transformed by the OPT, 1/4 of it is lost in the tube), the only differences between this and an 845 or 211 SE amp are the current and voltage levels, which necessitate an increase in PSU capacitance. With a choke, especially one in the 10-20H range as shown (which will be as large or larger than any other iron on the chassis, given the current a 6C33C needs; I'd go with 1H) this won't be necessary, at least in the stupifyingly brute force ways the SSatanic guys do it.

Tim

[Sch3mat1c]

Originally posted by serengetiplains
Actually, after further thought, driver supply variations always necessarily appear on the output grid. Coupling those variations to the cathode will therefore reduce their overall influence on the output signal ... the cathode coupling, if you will, providing a form of feedback relative to grid. Very elegant!

Actually, there is no feedback. Rearrange it as two power supplies, with the output supply on top of the driver supply. You can see the 6C33C's grid-cathode is across the driver plate resistor. Any driver variations will directly affect the output tube, making stable bias in such a high-Gm tube a problem.

Also, I never liked the topology because tubes never cut off. This means the output tube will never be fully utilized because for its grid to reach 0V (with respect to cathode), the driver plate resistor must have zero current through it. This takes an exponential(?) amount of additional negative driver grid voltage to obtain. As a result, distortion will be very high as you approach maximum output (taken as clipping of the output stage).

Tim

[serengetiplains]

Originally posted by Sch3mat1c
... absurd cathode resistor ...

... SSatanic guys ...

My compliments on your great wit.

[gnomus]

I have considered the 845, 211 or 805. I looked at a lot of designs and did not find one direct coupled. The other major problem is that NOS tubes sounds the best. But, they are rare and expensive. The 6C33B triode I have heard in many designs. From OTL to SET. It's performance was always good. I preferred it in a SET over OTL. And I think this has to do with a SET being class A. In fact I only like class A even in solid state. I Do not understand why everyone raves about class A pre amps and then mate it with a class AB amp. Yuck.