PT question

[JoeCon]

Guys

I got a couple of Edcor PTs and was wondering if they'll be useable in a couple of builds.

They are both 280-0-280 but one is 200ma and the other only 150ma.
Would the 200ma be able to push a KT66 Rockster?

Likewise the 150ma I was looking at doing a 2 EL84 Rocket, enuff current?

Thanks...

[M Fowler]

Joe,

Yes to both questions.

Mark

[JoeCon]

Mark

Sir, thank you!

[JoeCon]

Could a 150ma supply drive the KT66 pair?

Where on the tube data sheet do you find this info?
I've read here the preamp tubes only draw about 10ma total so the power tubes are where the current is important.

How do you figure the total required and how much "reserve" is needed, I'm guessing you don't want to be running at 100% all the time.

Thanks.

[xtian]

One of my favorite tools for quick-and-dirty PT rating calcs:

http://www.dreamtone.org/Calculate_Current_Form.htm

[martin manning]

Could a 150ma supply drive the KT66 pair?

Where on the tube data sheet do you find this info?
I've read here the preamp tubes only draw about 10ma total so the power tubes are where the current is important.

How do you figure the total required and how much "reserve" is needed, I'm guessing you don't want to be running at 100% all the time.

Thanks.

You won't find that info on the data sheet because there are too many other variables. For 280VAC, cathode biased, and assuming that the screen voltage is close to the anode voltage, I'd pick 3400 Zpri. With that you could estimate HT current for the power amp as 2*HT VAC/Zpri = 164mA. I'd consider this to be a lower limit, and you might want to increase it by ~4/3 to 220mA. With that I'd say the 150mA is a bit too small and the 200mA is good.

The calculator xtian linked suggests 210mA, which agrees closely, but I don't like using anything that is making assumptions that are unknown to me. Note also that it does not use Zpri as an input, and this is a strong driver on the HT current requirement.

[JoeCon]

Martin

Where did you get Zpri is 3400 from? Is this the OT primary your using?

Thanks.

[martin manning]

I figured you'd have about 340V Va-k (365 B+and 25V on the cathode), and that the screen voltage would be 5V less at 335. I scaled the Vg1=0 curve from the Genelex KT66 data sheet to that screen voltage and picked an "available" Zpri that would pass the load line roughly through the knee of the curve. I think a 4K Zpri would work well too, maybe even better, still with a 200mA PT.