PP-Triode , PP-UL , PP-Pentode-strapped-as-Triode

[Bandersnatch]

Hey-Hey!!!,
There are advantages to all the topologies you mention. Attempting to lumpthem into three groups without any description of how their advantages will be played to, or how their faults dealt with leaves one and only one conclusion:

There can be no conclusion.

On the output TX design, the primary L interaction of interest is with the load it is in parallel with( relfected from the secondary by the turns ratio squared ). The output Z of the devices is of no consequence at power. Assuming otherwise would require a pentode amp to have more primary L than is possible, while a low mu triode would be able to get away with two orders of magnitude less.

The load then interacts with the plate curves to determine things like distortion. At very small signal levels, the output Z does play a little part, but only at the levels used to quote output bandwidth when no power is delivered...

I like building with pentodes. The care involved in picking an OP and load is so much more complex than it is for triodes. It is easy to get it wrong. When it is wrong, it can be very wrong indeed. Judging from a lot of the quoted load/OP I see, I believe that it goes wrong far more often than it goes right. I know it has given me difficulties.
cheers,
Douglas

[Wavebourn]

Originally posted by gingertube


I confirm that Morgan Jones does'nt agree with Preisman and advocates the use of a "build out" resistor in the cathode output to equalise the source impedances. I built his little EL84 Amp which uses the split load phase inverter. In the end I connected a dual trace oscilloscope to the anode and cathode (to keep the loads equal) and then adjusted the "build out" resistor in the cathode output empirically to give exact balance at 120kHz. The resistor value I ended up with was about 1/5th the value that my calculations of the source impedances suggested would be required.

I guess that puts me in "CONFUSED" camp.

1. Did you measure outputs SIMULTANEOUSLY using probes with equal impedances, especially capacitances?
2. Why do you need a balance on such high frequency?

Balance in push-pull amps is needed for efficiency and only for efficiency. A first, on DC to compensate magnetizing of a transformer core by constant current. Ans second, on lower frequencies that have most of the power in the signal.

No other means for push-pull amps even exist. Such advantages as cancelation of even order harmonics of distortions is not the advantage, though it was said that it reduces distortions. Probably this reduction is significant for marketing purposes about better specs tnan competing production has...

However, the better is the amp balanced over the entire spector the more efficient it is, and it is true... If you are afraid of disbalance of split load amps on high frequencies use a differential cascade after it, no big deal, especially when you need to drive tubes that require big voltage swing on first grids... But using one more cascade with additional RC network we should remember of stability on lower end, in such case I prefer directly coupled by DC stages like in Altec Lansing 1568-1569 amps.

[gingertube]

Retraction,
My comments regards Morgan Jones and Priesman related to 1st edition of "Valve Amplifiers". In 3rd edition Morgan gives an expanded analysis of the split load phase inverter which confirms Priesmans analysis.
Cheers,
Ian

[serengetiplains]

You might wander over to Lynn Olson's Nutshell HiFi website. He prefers PP triode, DHT triode in particular. Some good discussion on his site.