Spud, Schade, PP, Anti-triode ECL86

[revintage]

Got this idea when discussing Shoogs EL86. MJK showed the embryo of this one with his 417A Spud.

I get indications of 5-6W with quite low THD, this with SE signature. The input triode does need 400V so its working point is not optimal, but this is as good as it gets. Distortion-cancellation helps a bit.

Balance is set with the "bias" voltage at the MOSFETs gate. There is a problem with current balance though, as screen current varies.

Any suggestions to make it even simpler?

I have the chassis, tubes and OPTs just have to order the PT.

Feel free to use the Spice-file at the bottom. Just change *.text to *.asc and open in LTSpice, the tube-models are included



http://www.eflatjump.se/SchadeECL86PPMOS.txt

[Yvesm]

Wonderfull depletion mode MosFets

[Michael Koster]

Originally Posted by revintage

Got this idea when discussing Shoogs EL86. MJK showed the embryo of this one with his 417A Spud.

I get indications of 5-6W with quite low THD, this with SE signature. The input triode does need 400V so its working point is not optimal, but this is as good as it gets. Distortion-cancellation helps a bit.

Balance is set with the "bias" voltage at the MOSFETs gate. There is a problem with current balance though, as screen current varies.

Any suggestions to make it even simpler?

Replace the tail CCS with a resistor?:

http://www.diyaudio.com/forums/showp...59&postcount=7

A balance servo is probably needed with the toroid. Maybe regulate the tube current using the screen voltage? Not very simple though. Probably easiest to make the MOSFET follow the tube DC.

Startup is strange with the MOSFET hogging all the current until the tube conducts, but nothing burned up.

It would be cool to make this into a complete amp. A little re-arrangement of feedback and coupling and this could drive class A2

Cheers,

Michael

[smoking-amp]

Maybe the DC xfmr balance could be tweaked by adjusting R29 (lower probably) so the increasing screen current roughly balances out (subtracts out) the non-linear increasing component of the U1 plate current at large signal excursion.

Or use a sense resistor in the U1 screen current to pull the M1 gate voltage slightly upward (another P type transistor needed) to control DC balance.

Don

[smoking-amp]

SE <---> P-P control idea:

If M1 were another identical pentode to U1, call it U2, a SE <---> P-P control can be implemented by controlling the effective gm of U2:

A differential pair or Op Amp would compare an adjustable DC reference voltage (set at the nominal common cathode voltage) with either a fast or slow reference ("A") derived from the common cathodes. It would control the grid of U2 to keep these matched.

A pot would span between the direct common cathode voltage (fast) to a LP filtered common cathode voltage (slow). The pot's wiper would then provide the reference "A" above.

When the pot is at the "slow" end, U2 grid 1 is controlled slowly to keep DC balance in the xfmr. So giving a P-P signature with essentially matched U1 U2 gms.

When the pot is at the "fast" end, U2 grid 1 is controlled quickly to keep the common cathode voltage rock stable. U2 looks like a very high gm tube in this case. So giving a SE signature from the very unmatched U1 U2 gms.

The adjustable DC reference could still have some DC balancing signal added to it for DC servo balancing.

Don

[smoking-amp]

Can use the pot resistance itself for the LP filtering. Connect one end of the pot to the common cathodes and the other end of the pot to a big cap going to ground, or to a B+ reference (PSRR).

[revintage]

Hey Don,
Good idea. For listening evaluation this must be great to be able to dial in the correct amount.

Solving the dc-balance is my main concern at the moment. Probably a servo is needed. It should, as I see it, take the pentodes anode as reference comparing, with the MOSFETs drain and with the correction voltage applied into MOSFETs gate. It is beyound my knowledge though to solve it.

[Shoog]

Most DC servos work to keep cathode current constant - which would be no use to you here.

If you used a conventional EI OT and an 807 as the output pentode the problem would be a lot less. 807's were designed to draw much less screen current than most pentodes. I believe that most pentodes draw about 20% of the anode current through the screen - but the 807 is less than 5%. Coupled with the lower plate current/higher plate voltage of a 807 this makes the screen imbalance much less of an issue.

Shoog

[revintage]

Hey Shoog,

I suggested a servo to compensate for current-differences in the OPT only. The servo has to be referenced to B+. Don&#180;t know how to design it though.

This one was ment to be a Spud(single tube amp). Have to breadboard this one to see what happens IRL with respect to current imbalance. Also have a pair of Nuovotem 6V-toroids waiting. Might try a simple currentmirror first.

As currentmirrors/CCS/LM317 use some of the necessary cathode-swing there will be a tiny problem with them together with low-Ug tubes like ECL86. Maybe one should add some positive gridbias to make more room and compensate? Or go for cathode-resistor and accept higher THD?

I have a 6L6(807 w/o top cap)+6E5P(trioded) on the drawingboard, but that one will be a Schaded-SE. Will use a rewired 125ESE at 3,4kohm for that one.

[Shoog]

I had a similar problem with designing a 6080 amp with the OT in the cathodes. My problem was that I needed perfect current balance which could be achieved with a CCS, but that would have created different cathode voltages - which would have created a DC current through the OT. I am still thinking about that one. I think Broskie has a DC servo which would address my issue. He revived the notion of the Garter Bias, and has recently produced a transistorized version. Interesting musing - but not a solution to your particular problem.

I am not certain that there is a solution to your particular problem. It seems to me that a DC servo on the anode side is likely to inject large amounts of distortion (I could be wrong though).

Have you considered going triode or pentode and supplying the DC to each winding side with a CCS (ie not CT'd). The cathodes would have to be none CCS which would make the self splitting very poor.

I really don't think there is a good solution to this issue.



Shoog