Pentode PP amp for comments, please

[ray_moth]

This is a pentode mode PP amp. I don't have a UL OPT, so it's either pentode or triode mode for me. I already have a triode mode amp and it works just fine. However, I'm itching to try pentode!

I used this particular design because I wanted to apply fairly heavy local feedback, from the OP Tube plates to the driver cathodes. I found this worked better if the drivers were separate from the phase splitter, which ruled out the Mullard approach and left me with the concertina splitter (as in the Williamson). To minimise LF instability, I put in step-network coupling between the drivers and OP tubes.

The PS uses a hybrid SS and vacuum tube Graetz bridge rectifier. There is a C-L-C filter with 5H choke in the negative line, which feeds the OPT center-taps. The screen supply uses active regulation with a couple of MOSFETs. The negative B- supply uses passive regulation with a MOSFET.

I haven't built this amp yet but it looks OK modeled with LTSpice. One channel of the amp is shown in attachment below and the PS schematic, to serve both channels, is attached to the next post.

I would be grateful for your criticism of this amp design, for or against, before I build it. Thanks in advance!

[ray_moth]

This is the PS.

Voltages are approx:
B+ 1 = 460
B+ 2 = 370 (stabilized)
B+ 3 = 170
B+ 4 = 360
B+ 5 = 355
B+ 6 = 350
B- 1 = -128

[SY]

The CFs are there to prevent blocking? As long as you've got them, you might think about using 6L6 types instead of EL34 and run AB2.

I haven't used 6AU6 triode-connected. How linear are they?

[poobah]

Wondering the same thing... is there a better phase splitter?

[Giaime]

Originally posted by SY
As long as you've got them, you might think about using 6L6 types instead of EL34 and run AB2.

Why? I haven't used 6L6 much. Are they better for grid current than EL34? More linear, less current, what?

[SY]

Much better in AB2 than EL34.

[thevoice]

Originally posted by SY
I haven't used 6AU6 triode-connected. How linear are they?

An article from Radiotronics about doing just that, with curves.

Scroll down a bit, I can create PDF's, but not edit them...

link

[SY]

Thanks for that. The split load has something less than unity gain (and lots of degeneration), so the burden is placed on that first tube. I'd be interested in any actual distortion measurements on trioded 6AU6- I've got some, but can't seem to get the time to jig them up and measure it myself.

[ray_moth]

Thanks for the comments!

The first 6AU6 is a voltage amplifier in pentode mode. I thought of using EF86 (and in the end I still might do so) but, since I have some 6AU6s and they seem to have found favour in a few audio designs (Gary Pimm likes them, for example), I thought I may as well try them first. I'll use a well-behaved RCA 6AU6 for this voltage amp stage and a rather more microphonic GE 6AU6 that I have for the next stage.

The second 6AU6, in triode mode, is a convenient way for me to provide a single triode, with a small footprint, to act as the concertina splitter. The 6AU6 is quite linear used like this, it has lowish plate resistance of ~7.5k and likes a reasonably high current, so it seems a good candidate as a concertina splitter.

I'm trying to create an amp here that shows pentodes up in their best possible light. The cathode followers are in there, as SY said, to avoid blocking distortion and to give a bit more headroom, with more polite behaviour in the event of overload. I could omit the cfs, of course, without making any major changes elsewhere in the circuit but I've got the tubes to do it, so why not use them?

I'll take your word for it that EL34s don't give much extra power in class AB2 (pentodes usually don't, as I understand it). However, EL34 is the type of tubes I have and I didn't intend to use Class AB2, anyway, except for where it may occur through transients. I accept that beam tetrodes such as 6L6 can give worthwhile extra power in AB2 but the problem is that I don't own any 6L6s. My OPTs (3.5k p-p) wouldn't suit them anyway.

[Petralias Nik]

They are (6L6) much better tubes. Adorable sound!