832 ultra-linear push pull?

[torrence]

OK, I will admit I have a thing for goofy looking tubes, so I have been working on an idea for an 832 PP amp for a while now. I was wondering if anybody has tried to use the 832 dual tetrode in UL mode?

The screen (G2) in the 832 is shared between the two units, so UL with a single tube is not possible, but if I use two tubes per channel, I can make this work and get a nice power increase at the same time.

One problem is the fairly anemic 250V rating for G2. I would much rather run the plate voltage above 300V. Does anyone have any experience with higher screen voltages on these tubes?

The second question is whether this is even worth it. For a non-audio tube like the 832, will UL actually help, or is it better to just provide a nice regulated 250V supply for the screen and run the plate around 350V?

I have all the equipment to actually test the various possibilities, but I am away from my lab for several months, so at this point all I can do is ask questions to the forum...

[aardvarkash10]

...and the internal capacitance to cathode on the G2... I'd be looking into the second suggestion rather than u/l, but if you have time and tubes to burn, you could always do a "tubelab" on it and find those limits...

[SpreadSpectrum]

Why not run the screens as intended and use some plate to grid feedback? Turn those pentode curves into triode-like curves without loosing efficiency.

Curves in the image are:

Blue - KT88 triode connected, mu = 8
Yellow - 300B, mu = 3.8
Pink - KT88 run as pentode with 20% plate-to-grid feedback, mu = 5

Note: KT88 pentode curves don't contain any positive control grid voltages. Pentode efficiency is maintained.

[Gordy]

Originally Posted by SpreadSpectrum

Why not run the screens as intended and use some plate to grid feedback? Turn those pentode curves into triode-like curves without loosing efficiency.

Curves in the image are:

Blue - KT88 triode connected, mu = 8
Yellow - 300B, mu = 3.8
Pink - KT88 run as pentode with 20% plate-to-grid feedback, mu = 5

Note: KT88 pentode curves don't contain any positive control grid voltages. Pentode efficiency is maintained.

That pink trace looks very interesting and useful. I have always tinkered with triodes so I do not know how to hook up a "pentode with 20% plate-to-grid feedback". Is it just normal pentode connection + standard feedback resistors (using a resistor in line to the grid then a second from grid to plate, along with a dc blocking capacitor)? If not is there a tutorial or diagram you could point me at a so I can learn? Thanks.

[revintage]

The only problem with plate-plate feedback is that Zin will be in the ballpark of a few kohms, Zin=Rfb/(Gm*Rload+1).

It can be a little problematic but but you can try finding the right hiGm pentode driver at high current, do not use triodes.

You find it all in RDH4 pages 332-333, fig 7.31. You can easely adopt it to PP.

10-20% FB is a good idea to get triode characteristics as Spreadspectrum indicates. Pete Milletts latest PP is an example. Read his text where he also mentions the important distortion-cancellation taking place in this type of amp.

[Gordy]

I will dive into RDH4 and read Mr. Millett today. Thanks for information, I appreciate that.

[pmillett]

Originally Posted by revintage

Pete Milletts latest PP is an example.

I did some breadboard testing with a single 832 tube in this circuit (well, same topology, different operating pounts and such) and the 832 worked very, very well this way. Amazingly low distortion as I recall. I think I was using 7W7 tubes as drivers.

There was somebody at ETF last hear who had this type of amp as well (832, single-tube, P-G feedback). It sounded very good, one of the better amps there IMHO. Not sure who it was... it'd be nice to see his schematic.

Here's a picture of it (Thanks Raymond):



Pete

[torrence]

Originally Posted by pmillett

There was somebody at ETF last hear who had this type of amp as well (832, single-tube, P-G feedback). It sounded very good, one of the better amps there IMHO. Not sure who it was... it'd be nice to see his schematic.
Pete

It is great to get some real-world reports on this. I am going to have to try playing around with some of these topologies. The RDH4 didn't seem too positive about plate to grid feedback (as opposed to plate to plate) but it wasn't clear to me why. I guess I may need to find a lower output impedance driver, though.

[revintage]

Hey Torrence,

The RDH4 didn't seem too positive about plate to grid feedback (as opposed to plate to plate)

Now I got curious, I might have missed something important.

Plate-plate or plate-grid is the same in my world if we by grid mean G1. If it means G2 we are talking triode-strapping. Havent found any negative words there except that they for obvious reasons didnt recommend using triodes as drivers.

[torrence]

RDH4 shows fig. 7.30, where there is direct feedback from plate-to-grid, then discusses the 'improved' circuit in 7.32, which looks to me like plate-to-plate feedback, although I guess it depends on your point of view since the plate is also AC coupled to the output pentode grid.