Need help on 811A partial feedback

[WT]

I would like to build a tube amp based on 811A with partial feedback. I got inspired by Gary Pimm's Tabor PP amp but I would like to do it in Single end.

I draw up a schematic and need opinion from experts around here if this will work. I have not design what input tube to use. Any suggestion?

[316a]

Originally Posted by WT

I would like to build a tube amp based on 811A with partial feedback. I got inspired by Gary Pimm's Tabor PP amp but I would like to do it in Single end.

I draw up a schematic and need opinion from experts around here if this will work. I have not design what input tube to use. Any suggestion?

With any valves running the grid positive , the driver needs to be run with plenty of current and have a low impedence such as a cathode follower or stepdown transformer . I don't think that circuit will work very well , especially as the grid has a zener shunting it 811A will be tricky to get good results from , may be a better idea to look at 826 3C24 or 809 . With 3C24 I used to use cathode feedback with the output transformer's 16 ohm tap in the cathode circuit , still found bass to be flabby and one-note , depends on the speakers used I suppose

cheers

316a

[Alastair E]

Ive used the 811A in both SE and in PP

I Really Like the power and clarity I had with the PP bread-board I made...

--It was quite amazing considering its a DHT, built I believe for R.F. work....!

The sheme you've drawn I doubt would work, TBH....

The 811A is a little bit different to the usual DHT tube, needing a Positive g1 volts to draw any current on the plate, below about 600V and it also gobbles up some considerable Current into g1 whilst doing so!...

You'll need about 22-25V positive on the G1 (I Forget the current) to make the tube draw about 60mA Plate current from a 450- 500V supply.

The 811A/572B needs a LOW Impedance drive to the grid to work at the voltages we 'normally' use in audio....

Best, (Easiest!) way I found to Drive the grid was via a MOSFET Follower from a 150-200V supply. This supply needs to be fairly stout and able to source say, 100mA if you're running 2 tubes from the same supply

--Ive seen implimentations of a big ole Pentode like a EL34 or KT66 driving the grid in C.F format but havent tried that myself, so cant comment on any issue from a Pentode C/F grid-drive

Drive the MOSFET Follower from summit like a 6SL7 strapped and you're halfway to what I built.

--The Output Tx can be anything from 2K to about 3.5K into your speaker-load, but about 2.5K load is good....

Issues I found were hum derived via the heater circuit, a hum-ballance pot in the heater supply helped but never completely eliminated the hum in my breadboard, but to be fair, the level was pretty low.

A nice DC heater could help, but at 6.3V @ 4A per 811A its a fair regulator in itself, and can make for a lot of heat!!

--Love the way they light up a dark room too!

[WT]

Hmm. Thanks for your help. Seem like I have to go back to drawing board.

[WT]

I make a mistake in circuit drawing. I means to make the R feedback from plate to grid not from B+ to grid.

[Wavebourn]

If you use 807 to drive 811 it will look cool!

[WT]

@ Wavebourn Any suggestion on bias schematic for 811A grid?

[Wavebourn]

Originally Posted by WT

@ Wavebourn Any suggestion on bias schematic for 811A grid?

I did not play with right-handed lamps practically. Need to think a little bit.

[Palustris]

Why not try a triode like a ECC99 as a choke loaded cathode follower to bias the grid positive, and to adequately sink the grid current? This seems like a much more practical approach.

[Michael Koster]

Originally Posted by Wavebourn

If you use 807 to drive 811 it will look cool!

That it would!

Here is how I would use the 811A in SE topology (surprised?) like a power grid tetrode but without the need for a g2 supply.

I would use a stable fixed bias supply and a MOSFET follower to drive the 811A grid (here a mu-follower/gyrator or mu-gyrator).

The feedback coupling cap is optional; you probably could drop the 475V across a feedback resistor but it will produce about 3.5W. The gyrator will fix the plate voltage and supply the needed make-up plate current.

A D3A is not needed but with a power triode you will want to keep the feedback impedance well below the Cgp reactance; Cgp is 5.6pF on the 811A. This will mean a reasonable driver gm to get good power sensitivity. Maybe an 807 would work after all. Anyway, the D3A is just an example.

This could drive the grid current in a linear way and you would get about 23 watts power out for 50W plate dissipation; pretty good for a class A SET on a balanced load line!

Hmm, if I didn't have a bunch of Eimac tubes already...