Ultralinear vs. Triode vs. SET

Trelja,

I do agree with a lot of what you said above. The transmitter tube SETs (845, 211) do not have as much of the favorable qualities of the low-powered triode tubes--like almost everything else, it is a matter of picking tradeoffs--but I don't find them completely wanting.

My primary amp is a parallel SET (two 2a3s per channel). I like this amp a lot, particularly when the 2a3s are EML meshplates. A friend, who designs amplifiers, said that he personally could not get that topology to work; one tube in the pair inevitably ended up hogging the current and doing most of the work even when the tubes started out as a matched pair. I have also heard other claims that some "purity" is lost and the sound becomes a bit muddled when more than one output device is used (the same claim is made for pushpull and even transistor amps). Also, while output is higher in parallel (I would guess primary impedance is also lower) we are still talking about pretty low output. My amp is rated at something like 6 watts from the parallel 2a3s. Am I losing something because it is a parallel SET? I don't know, but I like the amp.

I have heard a number of nice sounding pushpull pentode amps that put out 40-60 watts using two tubes per channel. For the money, small pushpull amps running EL84s sound pretty good to me (punchy and fun sounding). What has been most disappointing to me are the really big, expensive pushpull amps with many output tubes, particularly if those output tubes are 6550s. These sound brittle and dry to me (MUCH more so than decent solid state amps). Those that can be operated in triode (screen tied to the plate) do tend to sound better that way, but, I still don't really like them that much.

What two-tube per channel P/P true pentode amps are there in current production?

Pubul57,

Off the top of my head, there is the Audiospace AS-6m (KT88, ultralinear/triode switchable), Primal Luna Prologue 5 integrated amp (KT88 ultralinear), Cayin A-88 integrated amp (KT88 ultralinear/triode switchable), at least one Ayon integrated amp(KT88?), Rogue Atlas (EL-34 ultralinear), Quicksilver Audio Mini-Mite and Mid Mono (EL 34, KT88, KT90, 6550, etc).

If the amp is ultra-linear then its not the kind of amp Trelja was referring to. An example of that is the Citation 2 made by Harmon Kardon.

Now its a simple fact that pentodes, left to their own devices, are not very linear. So you usually have to do *something* as they won't otherwise be hifi. So you either run an ultralinear circuit, loop feedback, cross-coupled cathodes or maybe even all three.

Cathode cross-couping has not been mentioned so far, but works very similar to ultralinear operation. This is a local feedback technique that involves the secondary of the transformer and the cathodes of the power tubes. The technique was first used in the Leslie speaker cabinets for Hammond organs in the 1950s, and has been used in most ARC amplifiers made. You can get enough linearity using this technique that you don't have to use loop feedback.

Thanks Ralph, that's what I meant to ask about "pentode"
operation. By the way, can someone explain tetrode [?]
operation - I often see that word used. Is this yet another
possible tube application?