Super Basic 813 Amp

[W9ZSL]

So I was digging around through various 813 amp projects and came across this. Says I, "That is REALLY BASIC."   But the output circuit is scary.  Could this be modified with a Pi Net output?  If nothing else, >the input tuned circuit looks like a keeper.<  A few 1" pill bottles and home free.  I have MANY pill bottles. Bad ticker. Comments?

[DMOD]

It looks like a basic single 813 tube amp and should work.

The output network is basically a transformer-coupled system with the Pi net as the primary of the transformer. The distance and orientation of the coils with respect to each other determines the amount of coupling and hence power output.  In most cases, the fewer-turn secondary was rotated wrt primary.

C8 acts as RF ground for both tuning capacitors.

This was a common output circuit back in the day.

You can convert it to a conventional PI net circuit by determining the output resistance of tube and using a pi net online calculator to determine range of cap values and coil inductance.

Phil - AC0OB

[ab1mn]

Hi Phil,

You could change the output to a pi-network. Since this is a grid-driven design, you would probably have to change the input circuit. Here is a link that discusses the Bruene neutralization scheme for single-ended amplifiers in section 5.3.

http://www.cpii.com/docs/related/22/C&F5Web.pdf

The 1996 ARRL handbook has a writeup of a series of 1kw single-band amps using a pair of 813s. They do not neutralize the amp on 80 or 40, but do on 20, 15, and 10. If I recall, some have mentioned that it is challenging to have a bandswitched 813 amp that is satisfactory on the higher bands.

Another other thing to keep in mind is whether you want to operate Class-C only or be able to set the bias for linear operation in Class AB1 or AB2.

I'm sure there are some that are more experienced in this area than I and hopefully, they will offer additional advise.

Bob,  AB1MN

[w1vtp]

Worked for me back in the 70's.  Couple of pics - incompleted stage bottom view showing pill bottle used for input tank. To be added was a regulator tube to make sure the fix biased stayed where it was supposed to.  

Top view showing 813's in place - incomplete. Next two pics in various stages of completion.  The link was functionally fixed - I did the loading using the built in tuner.  The hole in the third pic was for an ART 13 variable inductor that made up for a Transmatch tuner.  Worked well.  

Your tank was exactly what I used in my amp.  A PI net tank would work well for your purpose.  I've never been a fan of using a "goose tube" to control plate current when drive is lost.  I like a combo of fixed to bring the bias to cutoff and then a grid leak to put the amp into class C when correct drive is achieved

Go for it!!  Al