Pentod PP - too much NFB needed to sound nice?

[ray_moth]

I am have 4 x EL34 and a pair of PP OPTs with no UL taps on the primary winding and only one 0-4-8 ohm secondary winding. This effectively rules out UL/distributed load configuration. The impedance is 3.5k P-P and power handling is nominally 100w but is probably nearer 50w. This gives me the options of either pentode mode or triode mode, if I use an 8 ohm speaker on the 4 ohm secondary.

Up to now, I've been using PP triode-mode but the ratio I get with my 8 ohm speaker on the 4 ohm tap gives me a rather sub-optimal load of 7k P-P instead of the more ideal 5k. I'd like to try pentode-mode PP, to get more output power, but I realise I'd need a lot more voltage gain and NFB to get acceptable damping and distortion.

In fact, the amount of NFB called for with pentodes (at least 26dB) would seem to be much more than some people would advocate these days for an open, detailed sound. I'm confused, because some high-NFB amps (e.g. Citation II with 36dB of NFB) are spoken of with reverence. Or are these different people who are commenting?

I'd try it for myself, except that I think I'd need a quite different topology to change from triode to pentode mode, because of the extra gain required for high NFB. So I'd rather hear what others have to say before going to any great effort to try pentodes.

[analog_sa]

It all depends on your musical priorities and taste. And your speakers. No matter how much i like open loop triode amplifiers they fall short on at least 50% of the music i listen to. NFB pentodes may not be able to compete in directness, soundstaging and unrestricted dynamics but complex music is better controlled and more listenable. And most speakers expect a low driving impedance.

Pay particular attention to a stable and clean G2 source - it makes a huge difference subjectively.

[audiousername]

Originally posted by ray_moth
I'd try it for myself, except that I think I'd need a quite different topology to change from triode to pentode mode, because of the extra gain required for high NFB. So I'd rather hear what others have to say before going to any great effort to try pentodes.

High NFB need not necesarily mean that you have to realise extra high gain and then use loop feedback from the secondary of the tranformer. Read John Broskie's article on "Partial Feedback": http://www.tubecad.com/march2001/2001_03.pdf , then read it again. And then read it tomorrow. It's quite complicated and takes some time to sink in. He uses a 300B as an example, but the concept is arguably of more use to pentodes as they have little input capacitance in comparison to triodes and have more need for feedback due to high output impedance and higher distortion, but the calculations in the triode example are equally applicable. The reduced output impedance of the output stage also makes life easier for the output transformer - it need not have as much primary inductance for a given cutoff frequency.

In short, you can reduce the both output impedance and the distortion produced by the output stage by approximately a factor of its gain + 1 (thus realising 100% feedback) without enormous drive voltages. A humble small signal pentode will suffice.

[Bandersnatch]

hey-Hey!!!,
Look at the early RCA pentode amp design. No loop FB requird.
http://mysite.du.edu/~etuttle/electron/elect35.htm

That one I've heard( and even in its PPP version the SP20); best off-the-rack amp yet. I'm contemplating it for use with sweep pentode finals.

I've got a Citation II and it is a good amp. I can't get over the feeling that its VA/PI circuitry is a bit much. My E-Linear W6m sound much better and have nearly the same Iron. Before I give up on the Citation, I'll put its OPT in place of the stock ones and see how we do.
cheers,
Douglas

[Eli Duttman]

Ray,

As the happy owner of a Cit. 2, I have considerable respect for the doings of Harman and Hegeman, in that time frame. You don't have to get involved with the complex circuitry of the "Duece", when EL34 O/P tubes are being used. Instead, model the circuitry of the simpler Cit. 5, which uses 6L6 family O/P types. Look here.

Hegeman relied on bandwidth to make lots of NFB work very well. Don't use his circuits in combination with mediocre O/P "iron".

High gm to provide slew limiting resistance is very important, when substantial HF error correction signals are present. With that in mind, replacement of the 6CG7 by an ECC99 is something to consider. Obviously, gm is not an issue with the 12BY7 and its close cousins.

BTW, 1 of the changes McShane makes to the Cit. 5 is the inclusion of a choke in the g2 B+ supply. Surprise (NOT!), better regulation there improves things.

[Bandersnatch]

Originally posted by Eli Duttman


Hegeman relied on bandwidth to make lots of NFB work very well. Don't use his circuits in combination with mediocre O/P "iron".

That circuit has relatively little FB that requires high BW from its Iron. There's plate-to-grid around the LTP stage, and cross-coupled plate FB from the power tubes to the LTP grids. The loop from the secondary is minimal compared to most( like the marginally stable Heathkit W5 ).
cheers,
Douglas

[JoshK]

I find myself fascinated with these local feedback techniques in lieu of global FB (or to reduce amount). Makes me wonder why they aren't used more often. Does it require just that much more thought to implement correctly?

[JoshK]

P.S. Doug, what is the W6m? I've seen you mention it a couple times. I am not familiar.

nm....I found it here

[Bandersnatch]

Originally posted by JoshK
P.S. Doug, what is the W6m? I've seen you mention it a couple times. I am not familiar.

It is Heathkit's TotL 70W monoblock amp. Came with a Peerless 16431 opt very similar in spec to the Citation II. 3k2 to 4, 9, 16 and 64 Ohms. It had simiilar PS too; same HV doubler, but w/o a choke, larger negative rail for a cathode follower stage and a user-adjustable mix of current and voltage NFB. Alledgedly very low production.
cheers,
Douglas

[Shoog]

Plate to Plate feedback or Partial feedback does deliver the goods. I have two amps at the moment which use the technique. The sound is clear detailed and punchy with plenty of air. No hint of the usual GNFB ills of which people report.
However the design is very exacting as it is a compromise between enough anode load on the drivers verses enough current through the drivers. You don't get both together. The choice of suitable drivers can be very limited. One of the best drivers is a 6AU6 in pentode mode as this is capable of high output impedance (essential) and good current.
It took me months to work out a suitable set of compromises and in the end it looked just like Gary Pimms Tabor amp. Dc coupled and no Global feedback, it kicks some tight bass alright.

Shoog