SE Power & OT Z calcs

[dennyg]

I'm using an OT with 2.5K & 4.0K primaries and trying to understand best one to use.  Also confirming my plate dissipation & max power calcs are correct:
Va           350v
Vk           33v
Va-Vk   317v
Ik   0.1
Ig2   0.004
Ia   0.096
Power   30.4
OT Z   4025.4

[dennyg]

SCRATCH PREVIOUS - hit the back button and sent incomplete post.  Here's correct one:
I'm using an OT with 2.5K & 4.0K primaries and trying to understand best one to use with a KT88 running the following values: 
Va           350v
Vk           33v
Va-Vk      317v (effective plate voltage)
Ik           100ma
Ig2          4ma
Ia           96ma

If my understanding is correct, plate dissipation is 317*.096= 30.4W and OT Z = 317^2/30.4 = 3302ohms which is half way between the two lead options. 
Please confirm if my thinking is right and if so, which primary lead I should use.
Thanks!

[PRR]

Try both. Won't take but a few minutes to swap. Be sure the unused lead is WELL insulated.

> sent incomplete post

BTW: there is a "Modify" button above your post where you can correct typos.

[kagliostro]

And, if you want, you can also delete an unwanted post

Franco

[dennyg]

Thanks for the posting edit tips guys!
PRR - I couldn't hear any difference between the two primaries - maybe if I could actually A/B there may be a subtle difference but I went with the 2.5K.
The biggest uplift I got from the more detailed calcs (i.e. offsetting Va by Vk and reducing Ik by Ig2, vs just using the measured Va and Ik to calc max power as I'd done previously) is I biased deeper into class A and picked up about 30% more watts which I could really hear. 

[PRR]

The lower Z tends to "suck" more power. You may see the cathode current, ideally steady, rise with high output in low Z. This is how audio pentodes are normally rated for SE, because the numbers are bigger. Also you get mostly 2nd harmonic up to a point (all amps go strong 3rd when clipped hard). The higher Z may be lower total distortion to a point, max power a dB lower than low-Z. I can accept that *you* can hear the difference. I doubt your bassist, manager, or the guy in the fourth row can tell.

So either can work, and not a huge difference, so I was egging you on to see what a skilled ear would say.

[dennyg]

Well I'll be darn - so the reason I back off treble when I push an amp hard is to reduce those somewhat harsh 3rd order harmonics? 
That right there was worth the post

[dennyg]

This may seem obvious but i've read conflicting info on calculating optimal Z.  In the above, i used calculated pdiss to determine Z based on operating values.  However i've also seen people use max pdiss per datasheet to calc Z regardless of how the tube is biased.  I've also read things like "KT88's like 2.5K" which would be true if you bias to max pdiss per datasheet, but if biased at say 90% of max I would think optimal Z would be higher. 
Clarification please? 

[PRR]

> read things like "KT88's like 2.5K"

Tube is a valve. It just passes-through (as best it can).

The optimum load depends first on what Voltage you have and what Power or Current you want to flow. Same as picking light-bulbs for 12V or 120V use: they will be very different resistances (though lightbulbs are not marked for resistance).

> "KT88's like 2.5K"

NOT if you are forced to use a 600V supply!  To make best power in 2.5K it needs ~~240mA, throwing 144 Watts!

OTOH 324V supply does lead to loads near 2.5K. For '88 or for ANY 42 Watt pentode (even two 21W 6L6).

[tubeswell]

... use with a KT88 running the following values: 
Va           350v

For centre-biased Class A operation, Zout= Va/(Pa/Va)

Pa for KT88 = 42W

350/(42/350) = 2,916R (3k)


This assumes you are idling a cathode-biased KT88 at about 100%

[dennyg]

This assumes you are idling a cathode-biased KT88 at about 100%

That last assumption is what I wanted to confirm because I've seen as-built schematics with posted voltages where they are running a KT88 at 30W with Va 350 but still using a 2.5K primary with a Z of 4K which I'd  think they'd go with 5K primary.  But i guess as PRR said, when Z is between available primary options, try both and see which one sounds best.

[tubeswell]

This assumes you are idling a cathode-biased KT88 at about 100%

That last assumption is what I wanted to confirm because I've seen as-built schematics with posted voltages where they are running a KT88 at 30W with Va 350 but still using a 2.5K primary with a Z of 4K which I'd  think they'd go with 5K primary.  But i guess as PRR said, when Z is between available primary options, try both and see which one sounds best.

Yep - If you bias the tube colder, you can go with a lower reflected load, but you begin to lose on the roundabouts what you gain on the swings