2xKT88 in UL Twin?

[jazbo8]

Fender Twin UL 130W Schematic

This is the schematic I am working from, hopefully it is the one you have.

[potatofarmer]

Awesome, thank you! And that schematic does match the amp I have. I should've posted that a while ago.
Just a couple points/questions though:

Are those plate curves for a 40-ish% ultralinear connection or the "distributed load" 12.5% taps in this amp?

And that screen dissipation calculation - that resembles the pentode connection equation...? It's nice to see confirmation that just making a new LC node off the OT CT for the screens is a bad idea. It doesn't look like that equation factors in the screen-protecting abilities of distributed load operation, minor as they appear to be in this amp. Actually, the equation that I used doesn't take into account the additional screen voltage reduction you get from using a screen stopper, let alone a large one.

In case I didn't link to that paper: Determining Screen Grid Dissipation in "Ultra-Linear" Amplifiers

So, now thoroughly out of my depth: it looks like "P = EI - ei" only accounts for the dissipation-reducing contribution of the reactive load. Could we factor in the resulting drop in DC and AC voltages (from the DC and AC current passing through the screen stopper) into the P = EI - ei calculation? A quick 'back of the napkin' calculation would show the screens being very happy in that case, but I feel there's a reason I shouldn't be making that assumption, though I don't know what it is...

[jazbo8]

Awesome, thank you! And that schematic does match the amp I have. I should've posted that a while ago.
Just a couple points/questions though:

Are those plate curves for a 40-ish% ultralinear connection or the "distributed load" 12.5% taps in this amp?

You are welcome, sorry I forgot to mentioned the UL tap used, it is 12.5% - you can see the curves are really close to the pentode curves.

And that screen dissipation calculation - that resembles the pentode connection equation...? It's nice to see confirmation that just making a new LC node off the OT CT for the screens is a bad idea. It doesn't look like that equation factors in the screen-protecting abilities of distributed load operation, minor as they appear to be in this amp. Actually, the equation that I used doesn't take into account the additional screen voltage reduction you get from using a screen stopper, let alone a large one.

So, now thoroughly out of my depth: it looks like "P = EI - ei" only accounts for the dissipation-reducing contribution of the reactive load. Could we factor in the resulting drop in DC and AC voltages (from the DC and AC current passing through the screen stopper) into the P = EI - ei calculation? A quick 'back of the napkin' calculation would show the screens being very happy in that case, but I feel there's a reason I shouldn't be making that assumption, though I don't know what it is...

You are right, neither RDH4 nor Kaplan's equations factor in the screen stopper, they both assume that all of the dissipation occur in the screen only, which I think is understandable, since you seldom see UL with screen stoppers. Also bear in mind that, unless the amp is used at its full power continuously, the actual dissipation (both for the plate and the screen) is usually lower.

BTW, I ran the calculations for the KT88 and came up with almost identical results, except of course the KT88 has higher Pda and Pdg2 ratings, and it is not stressed at all for the same output power, this of course will prolong the longevity of the tubes, so you had the right idea about subbing them for the 6L6s.