6550 SE design, please comment.

[Miniwatt]

L.S.

What do you think?
Haven't done the power supply yet, too lazy now

[athos56]

Do you really need the feedback with the triode connected 6550? Or what about doing the Plate to Grid "RH" feedback.

[Wavebourn]

Looks fine, except for SE I would prefer self-bias. You could switch on/off feedback by switching a shunt cap from ground to the secondary.

[kenpeter]

6550 is 6L6 family, designed with square law (x^2) pentode curve for optimal AB crossing.
EL34 might be a somewhat closer to linear (x^1.5) pentode curve choice for triode strap?

Van Doorne Octode used EL34 for Class A triode strap, and 6L6 for Class AB dumper.
There was a reason he didn't use all EL34 , nor all 6L6. Each behavior having a purpose.

[kenpeter]

Is there higher power version of EL34, more or less similar to 6550?
Perhaps a beam power tube like 6550, but not curved for square law?
What octal pentode of compatible pinout gives the most linear triode?

Is transconductance law so overwhelmed by Mu feedback that the
underlying pentode curve (before triode strap) doesn't really matter?

[chrish]

I have heard that a common failure mode of potentiometers was for the wiper to lift from the track. With a bias setup as drawn, such a failure would leave zero bias on the output tube - very destructive. If you put a resistor from the bottom of the grid leak (220K) to V-. That resistor will normally be in parallel with the pot, but if the wiper lifts the grid will be pulled to the bias supply voltage - much safer...

Check tubelab site for Simple SE for example of switchable cathode feedback with cathode bias.

Cheers,

Chris

[Eli Duttman]

The HD spectrum of the 12AT7/ECC81 makes it (IMO) a poor choice for use in a SE amp. The sound will be very "wet". Check the 6GK5 out.

[ray_moth]

I notice that your ECC81 (12AT7) has a rather low value for its plate load. I'd rather see 33k or more. Also, this tube is not all that suitable as a single-ended voltage amp (although it does OK in PP or differential configuration). Maybe a 6DJ8 or 6N1P would be suitable?

If you put a resistor from the bottom of the grid leak (220K) to V-. That resistor will normally be in parallel with the pot, but if the wiper lifts the grid will be pulled to the bias supply voltage - much safer..

.

Yes, indeed. This is nearly always overlooked in commercial circuits.

[Miniwatt]

Thanks for the replies everyone!
I chose a ecc81 because I have one here. My intention was to make a prototype this weekend without having to order tubes and wait for them to arrive..
Other tubes I have are ecc83, 5751, 5755, 6072.
I'm still in the dark about what makes a good driver tube (in SE), I stole the ecc81 idea from a PP design.
But I have to buy some small parts, I might as well buy an extra tube, if it's not too exotic for my local shop..
I don't really like 6DJ8, but that's not based on actual experience..

How about an EL84 as a driver?

[Globulator]

Originally Posted by Miniwatt

L.S.

What do you think?
Haven't done the power supply yet, too lazy now

I think it would be better with a 6N2p or 6N1p SRPP on the input.
If you can make it parafeed too that would be nice, but then you need to worry about chokes, CCS or another SRPP