switching between UL and Pentode

[loudthud]

Drifting back to the original point of this thread, for those who don't know the Sunn Model T runs in UL mode and there are 50 ohm resistors in series with each plate and 1.5K resistors in series with each screen.

Since it's arcing from the plate, the amp must be geting driven pretty hard. This will create spikes from the OT and an inductive load as the output stage gets a crossover dead zone. A little hotter bias will help as will conjunctive filters or just a resistor across the speaker. But if it's blowing fuses there must be a problem where the arc carbonizes a path and the B+ keeps it going. Are you using ceramic tube sockets?

[crisisat22]

Loudthud-
hey there. that's the setup Ra=50, Rg2=1.5k. I am using micalex sockets.

thanks!

[jreese2001]

Hey guys this is a great discussion... I am just now getting into the theory and am also designing a SE KT88 amp... so bear with the newbie questions.

I am trying to understand the calculations above... with 300V screen voltage and 75mA screen current, wouldn't that put you way above the 6W screen dissipation rating?

On a separate but related note, does anyone know how to calculate screen current as a function of anode voltage (so you can display it like how it shows on the tube characteristics as Ig2 at Vg1 = 0) ?

Thanks !

[Merlinb]

I am trying to understand the calculations above... with 300V screen voltage and 75mA screen current, wouldn't that put you way above the 6W screen dissipation rating?

Yes, but that value is for instantaneous dissipation, not average dissipation (which should be much less).

On a separate but related note, does anyone know how to calculate screen current as a function of anode voltage (so you can display it like how it shows on the tube characteristics as Ig2 at Vg1 = 0) ?

Here is an old article you may find useful. If you find it alittle dense to begin with, put it on the back burner and come back to it as you progress:
http://www.freewebs.com/valvewizard2...cteristics.pdf

[jreese2001]

Thanks Merlin, that is a great article. I bought your preamp book and am looking forward to digging into that as well.

[GainFreak]

Hi,

I'm not very much into theory so please elaborate a little bit more on the case where KT88s are used in 300-400 Watt bass amps with anode voltages in the range of 600-700V and screen voltages 300-400V, along with small value screen resistors?
Thanks.