813 homebrew amp-- go for three or four tubes?

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K1ZJH:
Hi Shane!

Great info! I was going to use a 0.4 uH inductor in series with the plates to C1, since I happen to be using coils from an Illumitronix tank--and it included a .4 uH coil made from silver plated strap!  I can tap that coil for .3 uH easily.

I'll check your charts for the remaining values. Basically you are using the tube capacity and .3 uH coil for the first leg of the Pi L network?  What is the intermediate impedance (in case it isn't in the charts, which I still have to print out.)

I take it you didn't need parasitic suppressors?  Also, did you bother to neutralize, I was going to copy the W8JI feedback system he used in the AL-572 and other amps, or use a winding on the filament choke for feedback.

Pete

K1JJ:
Yo Pete -


Fun-Factor:

Quad 813s will make a nice amp and a fun project to talk about on the air. I mean heck, half the fun of home brewing is showing off the project to your buddies, and quad 813s are way-cool! ... ;)    4-1000As followed by 813s are my favorite tubes.


Technical:

I didn't see anyone mention if this amplifier was grounded grid or grid driven, but I assume it is GG since a feedback winding off the fil choke was mentioned.

Yes, the issue on ten meters becomes the 813's plate to cathode capacitance that would probably make a 4-holer 813 linear scream for minimum C1 on 10M.  The L1-C1-L2-C2  L-Pi method is a great solution, especially since you have a nice input silver plated L1 inductor ready for the job..

The neutralization might not be needed, even on 10M, since it is GG, but it will add some additional negative feedback (in addition to the inherent GG configuration FB) which will make the amp cleaner for IMD, so definitely do it if you have the motivation. The NFB will make it slightly harder to drive, but no big issue.

I would use parasitic suppressors - but maybe try it without first to see if they are needed. They may be needed since it is a 4-holer on 10M. All depends on how  short and well-shielded your layout is and the input not seeing the output - as well as "random" parts anomalies that may make VHF feedback paths possible.

Frank/GFZ once gave me an idea to help stabilize the fil/input circuit:  Hang a 2K non-inductive resisitor in series with a .001 cap from each fil lead to ground. (at each fil tube pin) This will help lower the Q for high impedance VHF choke parasitics, if the choke happens to have them. I did it as a precaution on all of my linear amps. FWIW...

The 813 4-holer can use a small breeze with a Variac-controlled muffin fan to be quiet as a mouse.

I say do it!

T

KD6VXI:
Pete,

The impedance transformed is present, near the bottom of one of the pages.

If you Google G3SEK spreadsheet, you can get yourself a copy.  It's pretty much invaluable.  Let's you design for pi or pi L, let's you enter stray C and the L for the interconnects.

Keep in mind if you want to keep the tuning (Q) as designed.  You need to make that  .3 uH the entire assembly, from anode to Ctune.  Meaning, any strap or wire for the interconnects needs to be measured, with your inductor in circuit, for it to work as designed.  Not saying it won't work if you don't do it this way, but you may end up with a Q of 20 or so, which is what we are trying to get out of anyway.

I //believe// the correct terminology Is an L-Pi.

If you desire an L-Pi-L, I can run those numbers, too.  I believe it wasn't feasible with the image impedance, etc. I was looking at.

Yet another nicety about the L-Pi is that the CTune //should// see about 550 volts under properly loaded conditions.  This is on ten meters, so youll still need a vacuum cap for 80.....  But....  My last 3cx3000 deck worked 10 to 15 meters fine business.  So quad band hf amp isn't out of the question with a decently sized CTune.

--Shane
KD6VXI

K1ZJH:
Shane

I will be using a Pi-L for 80 and 160 meters, and I assume that L1 will be nearly invisible below 15 meters. Between 30 and 17 meters the design reverts back to a Pi Net.  I found those values yielded the most practical values for the plate and tune caps.  Perhaps that is why Ameritron used a similar tank design in many of their amps?

I was planning on having L1 go directly between the plate RFC and then 68 pF section of a dual plate tuning cap salvaged from a retired Ten Tec amp. I am almost thinking that I will abandon the ceramic plate caps, and use metal heat dissipating ones instead.  That will allow using flat copper strips to connect the short distance between plate caps to the RFC and plate blocking cap. You think that will gain enough to be worth the effort?

Pete

KD6VXI:
L1 does disappear as you go lower in frequency.  It doesn't really disappear, more absorbs some of C1.  But, it is negligible at 160.

Ive got metal caps on all my tubes except 572Bs.  Those I bought new ceramic ones for, as rf parts didn't have any metal ones.  I prefer metal, I think it adds a bit of heat sink, whereas the ceramic  tends to be an insulator:  keeps the heat in.  I've no proof, just makes sense.

I have also used a normal pi circuit and put a small amount of L between C1 and the pi tank coil, when I just needed a few pf less than the cap was willing to give.  On air variables, bend a plate, not so easy with vac variables.

As to the strap, flat and wide is superior.  And strap, not braid.  You can always bend the strap into a U to attach it to the ceramic caps.....  But again, I like the metal ones, too.

I can't comment on ameritron......  🤔☺️

--Shane
KD6VXI

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