What is the plate load impedance of a pair of 4cx800A tubes run at
2500VDC and 1500 watts out?
I have read varying opinions on the amp groups ranging from 800ohms to
2100 ohms. I thought it might be 1600 ohms.
Once the impedance is known I can try to determine the minimum tuning
capacitance that I need.
The pair of tubes are 22pf. What would be guesstimate of the stray
capacitance in the tank circuit?

I am undecided if I will use Pi or Pi-L network and whether it will be a
load of 50 ohm or 200ohms with a toroid to bring it down to 50 ohms. I
have to use real world parts....

I can use a vacuum cap to keep things simple.
since I am not an engineer, I want to be sure that my guesses are in the
ballpark...

any input would be appreciated.

Last time that I built an amp was over twenty years ago. It was a
4-1000a and I built the Hypersil plate transformer by hand winding it
This time I will have Peter Dahl earn his money.

Jim n2go

On Sat, 22 Oct 2005 19:15:35 GMT, James Skalski
wrote:

What is the plate load impedance of a pair of 4cx800A tubes run at
2500VDC and 1500 watts out?

Depends on what class of operation you intend and on the screen
voltage.

The best way to determine it is using the Eimac Tube Peformance
Calculator, which was a graphical way to implement Chaffee's method.

This is described in "Care and Feeding of Power Grid Tubes" section 3,
which can be found online at:

http://www.cpii.com/eimac/PDF/C&F3Web.pdf


I have read varying opinions on the amp groups ranging from 800ohms to
2100 ohms. I thought it might be 1600 ohms.
Once the impedance is known I can try to determine the minimum tuning
capacitance that I need.
The pair of tubes are 22pf. What would be guesstimate of the stray
capacitance in the tank circuit?

Probably figure 5-10 pF.


I am undecided if I will use Pi or Pi-L network and whether it will be a
load of 50 ohm or 200ohms with a toroid to bring it down to 50 ohms. I
have to use real world parts....

I would not use a transformer here.


I can use a vacuum cap to keep things simple.
since I am not an engineer, I want to be sure that my guesses are in the
ballpark...

With your plate voltage you might not need the vacuum cap but it won't
hurt and the minimum C will be an advantage.


any input would be appreciated.

Last time that I built an amp was over twenty years ago. It was a
4-1000a and I built the Hypersil plate transformer by hand winding it
This time I will have Peter Dahl earn his money.

Way to go.