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.
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...
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...
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.rcavictim said:
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.
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.
Tim
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.
Tim
Sch3mat1c said:
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."
Found a very interesting note on Aikenamps website about Push-Pull output stages and even order harmonics -
..."This does not mean that the (PP) amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages will pass right through to the output. Only those generated in the output stage itself are canceled."
http://www.aikenamps.com/ClassA.htm
So then do the even-order harmonics generated in the preamp stages get amplified in the output stage ? or just "pass right through" ? If they do get amplified (I am guessing they are) then I'd imagine an "All Triode PP amp" would give the best of both worlds.
Also see http://www.vac-amps.com/page0004.html for nice reading on the subject.
..."This does not mean that the (PP) amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages will pass right through to the output. Only those generated in the output stage itself are canceled."
http://www.aikenamps.com/ClassA.htm
So then do the even-order harmonics generated in the preamp stages get amplified in the output stage ? or just "pass right through" ? If they do get amplified (I am guessing they are) then I'd imagine an "All Triode PP amp" would give the best of both worlds.
Also see http://www.vac-amps.com/page0004.html for nice reading on the subject.
Wodgy said:
Even local NFB, say shunt or CFB, will produce lower figures than a straight triode-mode pentode. You acnowledge this, so why the long post? The point was made, pentodes have lower distortion... that says nothing of psychoacoustics (which I personally would argue are inaudible anyway, being <0.1% *total* at even top listening levels (i.e., music with peaks to maximum amplifier output). Or for a real good one, 0.2% or so at continuous *maximum* output.) Of course, low distortion doesn't color the sound (2nd H I presume), which I'm sure is what many people look for in triodes.
Tim
Brett said:
Brett,
I have a SE 6BQ5 amp from a Magnavox stereo console. Uses half of a 12AX7 to drive each SE output tube. 5U4 makes DC. This amp is quite impressive on the right speakers but if you want room filling bass before you hit the distortion wall then very efficient speakers are needed. Armed with this knowledge I have ordered a pair of 25 watt SE output xfmers 10K with 40% screen tap, from our good friend at Handwound. If Mr. Lucas ever gets his act together on my order which is now almost a year old, my plan was to make a stereo amp using a SE 813 bottle in UL in each channel. If that doesn't sound good then my xfmers will suit 845's or some similar triode.
I've built this amp in EL84 and 12V6 and it sounds quite good, but it's not a patch on the PP47.rcavictim said:
Even though I have efficient speakers (>100dB/W), I have a cavernous room, and I like to crank it on occaision, well, a lot....so I need more power and I don't want to use up my rare 47's on background listening.
I'm also planning another set of speakers (still drawing board - new thread on this next week) that will fit in the room better than the big horns. A highish output Z would also be an advantage.
Based on lots of other reports, I hope you're not holding your breath....
I love the 813, and ran a low voltage PP813 triode for a while. My house gets really hot in summer and the 813 just put out too much heat, it weighed more then me when finished and was the size of a small bar fridge, so I dismantled it. I've read reports of others going SE UL with 813's and look forward to seeing your implementation and reading your experiences. I might follow with a 'home' bass guitar amp in a similar format.
Brett said:
Talking abiut heat, I have a 300B driven 805 SET stereo amp (40 watts/ch) taking the dampness out of my basement home theater and best lstereo listening setup. It is all I need in a SET amp and I love it. In class-A SET the lower the percentage of full power capability used the lower the distortion. At that rate I'm in the very, very low distortion area all the time.
Upstairs though (picture in my avatar) I am using any of several P-P tube amps 6BQ5's or 6L6's, or P-P triode wired 6CA7's on my new and quite efficient speakers. I have a parallelled 6080 (three bottles per channel) SET amp under construction that will probably (I hope) be the last word upstairs.
Fourteen watts makes all the slam on these 14 driver behemoths that I need so twenty will be plenty!
If I never get the transformers from Handwound I will be upset but at least not without my top quality tube sound. I am more interested in building the 813 amp as a learning experience than from actual need. For me this audio interest is all about the journey, the learning experience and the chance to be creative.
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