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Old 5th October 2004, 05:05 PM   #1
percy is offline percy  United States
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Default SE Pentode or PP Triode ?

Dont intend to start a war here but just wanted to get an idea if there are any sonic differences between a SE Pentode output stage versus PP triode output stage.

What kind of sound do the following combinations characterize ?

SE Triode
SE Pentode
PP Triode
PP Pentode

Thanks.
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Old 5th October 2004, 06:39 PM   #2
Wodgy is offline Wodgy  United States
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Comparing SE pentode versus PP triode, the latter makes a lot more sense. Pentodes are not particularly linear and typically have a weaker damping factor (i.e. poorer control over the bass) than triodes. In SE designs, which are often predisposed to weaker bass anyway, pentodes don't make a lot of sense to me. If you wanted more power, IMHO you'd be better off running two SE triodes in parallel. PP triode designs are probably the best sweet spot in amp design. Less power than PP pentode designs, but more inherently linear (less distortion) and a naturally better damping factor. Less need for a substantial amount of feedback.
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Old 5th October 2004, 07:03 PM   #3
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Quote:
Originally posted by Wodgy
Comparing SE pentode versus PP triode, the latter makes a lot more sense. Pentodes are not particularly linear
Um. Pentodes are linear. About 5 times more linear than triodes. And about as much more efficiency as well.

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Old 5th October 2004, 07:08 PM   #4
Wodgy is offline Wodgy  United States
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By "more linear" I mean triodes produce lower distortion in a similar circuit topology. I find it hard to believe that this is a controversial statement.
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Old 5th October 2004, 07:16 PM   #5
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If I was offered only two choices, SE Pentode, or P-P Triode, I would not even have to think about it. P-P triode would be my choice!
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Old 5th October 2004, 08:47 PM   #6
SHiFTY is offline SHiFTY  New Zealand
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The other interesting thing about PP triodes is this:

The distortion of a triode is usually almost all 2nd harmonic with a scant few higher harmonics.

In a PP amp, the even harmonics are cancelled out. Therefore, a PP triode amp will have very low distortion from the outset! You may want to experiment with a little NFB as well.

So yeah, my votes fior the PP triodes... 6B4G would be a good choice...
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Old 5th October 2004, 09:41 PM   #7
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Quote:
Originally posted by rcavictim
If I was offered only two choices, SE Pentode, or P-P Triode, I would not even have to think about it. P-P triode would be my choice!
I would have said the same thing, until I tried Gary Pimm's PP47 pentode design. Best amp I've ever heard (in my system), tied with Allen Wright's PP2C.

Right now I'm interested in building a higher power version of the PP47, using different tubes, for maybe 30W.

SE pentode can make a great guitar amp.
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Old 5th October 2004, 10:21 PM   #8
percy is offline percy  United States
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Quote:
SHiFTY:

The distortion of a triode is usually almost all 2nd harmonic with a scant few higher harmonics. In a PP amp, the even harmonics are cancelled out.
Speaking of even harmonics and how they are treated differently in a SE and a PP amp, how would one describe the sound of a SET and PPT ? Is it different ?
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Old 5th October 2004, 10:37 PM   #9
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Even harmonics cancel (..class AB SE has immense 2nd H but PP has near none...), odd cannot. Note you can't make a three-phase amplifier and cancel 3rd, or something...

As for distortion, take a two stage amplifier with pentode output and triode-connect it. No global NFB. Measure. Now remove the screen feedback and connect it up grid wise (an easy way being global NFB) so it has the same gain. Measure.

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Old 5th October 2004, 11:04 PM   #10
Wodgy is offline Wodgy  United States
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Quote:
Originally posted by Sch3mat1c
As for distortion, take a two stage amplifier with pentode output and triode-connect it. No global NFB. Measure. Now remove the screen feedback and connect it up grid wise (an easy way being global NFB) so it has the same gain. Measure.
It's not really surprising that an amp with global NFB would measure better than an amp with no global NFB.

Most people consider a triode operated at 5% second harmonic distortion as maximum output. This distortion profile is largely second harmonic, which cancels in a PP design. In comparison, pentodes run close to their maximum output typically have between 7% to 13% total distortion, largely third harmonic with appreciable higher order harmonics. Much of this distortion does not cancel in a PP design, and without NFB you end up with an amplifier whose distortion profile contains more higher order harmonics than an equivalent triode PP design. The psychoacoustic literature tells us that these are more objectionable to the ear, particularly the further away they are from the fundamental.

That said, total harmonic distortion numbers into a simple resistive load (especially measured at 1kHz) are not the whole distortion story anyway. Power pentodes run into a real loudspeaker load where impedance varies with frequency display a greater increase in distortion as the load impedance varies than do triodes (see the Radiotron Designer's Handbook, 3rd. ed., pp. 14-15). The greatest increases in loudspeaker load impedance are typically at the frequency extremes, and so with PP pentodes you end up with more distortion in the bass and treble.

Every engineering design involves compromises, but generally unless you need the extra power, a PP triode amp involves fewer compromises than a PP pentode amp. Or, as the Radiotron Designer's Handbook points out, "push-pull class A1 triode operation is regarded as providing the highest standard of fidelity."
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