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Old 1st May 2006, 01:40 PM   #1
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Default Pentode Push-Pull

Hi,

I decided to start a new thread on this subject because I've become intersted in meeting the challenge of achieving good quality sound with pentodes and I would like to discuss people's experience in this area. It rarely gets discussed, possibly because it's considered to be a relatively poor solution for "hi-fi" and better suited to guitar amps. However, Gary Pimm seems to have achieved success with it.

I guess most of us have heard about:

* the excessive amount of high odd-order distortion from pentodes;

* the sonic superiority of ultralinear and triode modes over pentode mode (I have an EL34 triode-strapped PP amp that sounds pretty good);

* the difficulty of getting a good damping factor, due to the pentode's high plate impedance;

* how the only way to achieve acceptable sound (good damping, low distortion) with pentodes is through the application of heavy NFB, which is not in favour these days;

* the difficulty of achieving stability with so much NFB;

* etc; etc; etc.

The fact remains that you can get more power from the same tubes in pentode mode than you can in the other modes. You are also able to experiment with a much wider range of operating conditions than with either UL (unless you use a separate tertiary winding for the screens) or triode modes.

The EL34 is my personal choice. It has been praised, in some circles, as the best audio power pentode ever made. And it was definitely intended (initially, at least) to be used as a pentode; if you read Mullard's advertising literature carefully, it's clear that they developed the EL34 before they were aware of the possibility of using it in an ultralinear circuit. The fact that the maximum plate voltage is 800v and the maximum screen voltage is around 400v tends to support this.

I have some ideas of my own about how to achieve success using EL34s in pentode mode. They don't involve my own personal favourite all-balanced design approach, which I have found to be too gain-limited to be able to support sufficient NFB for pentodes. I have tested them, as far as I can, with LTSpice and the results were very encouraging. I'd be the first to admit that it's a big step from Spce simulation to actual building and testing, but that's all the more alluring to me.

Anyway, I'd be really interested in what others have managed to achieve with pentode PP and what advice they may be able to offer.
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Old 1st May 2006, 03:00 PM   #2
Joel is offline Joel  United States
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I've built four pentode push-pull amps so far, all using 12-22dB of NFB, and I feel like they are even more "high fidelity" sounding than the triode amps I've built. There is a control, and an ease that triodes just don't seem to have; a lack of congestion.

To address some of your other points:
Pentodes do not always have more higher order distortion than triodes. That is a myth. It is totally dependant on loading, the circuit particulars, and the average drive to the output stage.

Also, stabilizing a feedback amp is not difficult. The techniques are all well documented. What is difficult is finding coherent sources for those techniques, and applying them correctly to your particular circuit/output transformer combination. But the reward is worth the time spent, IMO.

Joel
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Old 1st May 2006, 03:18 PM   #3
ttan98 is offline ttan98  Australia
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Joel,

My experience is the more feedback means, the greater the damping factor hence you mention better "control " I asume you mean control of the speaker cone. Most visitors here do not like this type of sound, too much control hence "harden the sound output". Some prefer a more airy and open sound where the damping factor can be a low as 2-3(ie less feedback). With correct partnership of speaker it can still sound quite good, while other type it may sound sloppy, esp those that dip in its impedance. an example, there is a much acclaimed amp.,based on a single-ended KT88 with output impedance of 3.8ohms, however it has a special feature ie an ability to adjust its output impedance to match the speaker impedance(I don't how it achieve this).

Just a thought.
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Old 1st May 2006, 07:24 PM   #4
Joel is offline Joel  United States
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It has little to do with damping factor, IMO. It is much more related to being able to deliver real power, at or below rated distortion, at the frequency extremes (and having a smooth response rolloff beyond those points as well).

This is, of course, not the sole providence of pentode w/ feedback amps - but they are usually much better performers in this regard. Yes, this is a generalization - the output transformer is vitally important. However, it's utterly astounding how poorly the typical triode no-feedback amp does at 20Hz and 20kHz, in terms of THD. This is what I mean by a "congested" sound.
I honestly can't hear damping factor. Maybe you can though.

Joel
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Old 1st May 2006, 10:38 PM   #5
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"The EL34 is my personal choice. It has been praised, in some circles, as the best audio power pentode ever made."

Not mine. I favor the 807 instead. This one has a specified Q-Point that gives a THD= 1.8%. I haven't seen any other type that gives such a low THD figure.

As for how I used them: stiff screen supply. For this, I used an active regulator. The active regulator not only nails down the screen voltage (0.8V drop with the DC rail going down some 40V) it also has a very low AC impedance. Both factors go a long way towards improving linearity.

I included two feedback loops. The inner one taking input from the 807 plates back to the grids of the cathode follower drivers. According to an app report, 6.9db(V) of feedback for this loop is recommended. They certainly knew their stuff since that worked just fine. Getting the output impedance of the finals below r(p) with feedback helps with both speaker damping, and low frequency performance since the xfmr's Xl(pri) becomes a larger percentage of output impedance. That local feedback also helped to tame the excessive "brightness" of the high end.

Next, added ~12.0db(V) of gNFB. This outer feedback loop from xfmr secondary to first preamp cleaned up the mids and highs, and also gave the bass plenty of "authority" without any of the under damped "sloppiness" that can occur with VT amps.

Use fixed bias (better linearity and more power since you aren't losing any across cathode bias resistors). Include a decent driver that can source enough current at the high end to charge up the stray and input capacitance to avoid the slew rate problem, and can drive into Class AB(2) transparantly. I favor a DC coupled cathode follower for a driver. That way, there is no coupling capacitor to accumulate excess bias voltage when the signal forward biases the control grid/cathode parasitic diode. In my 807 amp, the OPT actually saturates before the finals clip.

Sonically, beats hell out of a commercial MOSFET amp from the Big Box store.
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Old 1st May 2006, 10:40 PM   #6
ttan98 is offline ttan98  Australia
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you must know high damping factor means less power is being actually delivered to the speaker. some power is lost in the output impedance.

I have built and listened to triode, pentode, low and high damping factor amplifier.
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Old 2nd May 2006, 03:39 PM   #7
Giaime is offline Giaime  Italy
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I'm starting to think that most of the hype about triode mode is the possibility to drive cheap OTs and get good damping factor out of them... go and get some serious ones and you'll see how good a pentode amp can sound!!!

I'm designing a prototype of an EL36PP amp, I'll have the option of configuring it in triode or pentode mode, maybe I'll post some results... I like the approach of returning error signal from the anode of the drivers to the driver tube.

Miles, could you point me at your 807 design? I have some, I love their look and have heard great things about this beam tetrode.
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Old 2nd May 2006, 07:11 PM   #8
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Quote:
Originally posted by Giaime
I'm starting to think that most of the hype about triode mode is the possibility to drive cheap OTs and get good damping factor out of them... go and get some serious ones and you'll see how good a pentode amp can sound!!!
Hear, hear!

Quote:

Miles, could you point me at your 807 design? I have some, I love their look and have heard great things about this beam tetrode.
You can find out more about it Here (under a different handle) or see a pic in the Photo Gallery. They do have a nice "retro" look don't they? "...have heard great things about this beam tetrode." And from them.
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Old 2nd May 2006, 07:17 PM   #9
SY is offline SY  United States
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I'm using push-pull EL84 in pentode at the moment with terrific results. Distortion spectra look fine (at 1W out, 2nd is dominant at -75dBW, 3rd and 5th are below -85dBW, nothing higher above residual), and the advantages that pentodes bring to the table are considerable. But, yes, it means the output transformer has to be an excellent one.
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Old 2nd May 2006, 09:12 PM   #10
Giaime is offline Giaime  Italy
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Do you use feedback, Stuart? Schematics?

807 are cheap good for me
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