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Pentode Power Tubes - Ultralinear vs Triode

While I’ve seen a bunch of posts on power tube rolling, I haven’t seen much mention of how preferences correlate to what mode you run if your amp has that option. I have a Primaluna DPHP that has the Ultralinear/Triode mode remote and can run a bunch of EL34 variants up to KT150.
So far I’ve only tried the stock EL34 and Tung-Sol 7581A and seem to prefer Triode mode with both. Before coughing up for another set of 8 to try I’m doing more research. I think based on my preferences so far that the bigger, punchier tubes may not be the way for me to go. I could be destined to find my way into SET amps eventually.

So which tubes do you prefer in what mode of operation?
cat_doorman
atmasphere's avatar
atmasphere
11,572 posts
 
Triodes are more linear and have a lower output impedance. If you can get them to make the same power and if you have two amps, one optimized for tetrodes or pentodes, and the other for triodes, the triode amp will sound better hands down. This is assuming that the build quality and topologies are otherwise identical.

'Better' in this case means lower distortion, so more neutral (since the ear interprets all forms of distortion as tonality), bandwidth will be the same, and output impedance will be lower, so again more neutral with more speakers.

wolf_garcia
6,595 posts
 
My Dennis Had Firebottle HO (it's a HO...says right there...) is a single ended pentode amp at about 12 to 17 watts (HO is "High Output") depending on tubes. Although Atmasphere's frequent comments regarding SET (Or SEP in my case) distortion make sense, after using various tube and SS amps over decades, my Firebottle never sounds harsh or distorted or anything besides beautiful through high efficiency speakers with all sorts of music at any level I can tolerate. That said, Gold Lion reissue KT77s seem to stick around in this amp longer than KT120s, 150s, 7581As, KT88s (GL or Sovtek), or anything else now living in my crowded tube drawer. 
russ69's avatar
russ69
3,360 posts
 
I only have two amps that are switchable AES Six-pacs and a Prima Luna. I prefer both in ultralinear mode. I have Gold Lyons KT-88s in the Prima Luna and it sounds great with better extension on the top end.  
atmasphere's avatar
atmasphere
11,572 posts
 
FWIW if ultra-linear is set up correctly, the result should be a pentode or tetrode power tube operating with the same linearity as a triode but with about 90% of the power it could make as a pentode or tetrode.
cat_doorman OP
236 posts
 
@atmasphere always nice to get answers from an expert. They never really covered vacuum tubes in the few EE courses required for an ME degree, and those are only a faint memory. From further reading I gathered that the ratio of the screen-grid and plate can vary quite a bit to find the sweet spot in ultralinear and would be dependent on the tube type. So with tube rolling on an amp like mine (Primaluna DPHP) would running in triode mode be more consistent when comparing different tube types?

What about tube life? Hypothetically, if you adjusted for the same output voltage, would one mode be significantly harder on the tube than the other?
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