[AMRadio] AMRadio Digest, Vol 93, Issue 30

[Jim Candela]

------------------------------ Message: 2 Date: Sat, 15 Oct 2011 
12:58:12 -0500 From: "John Coleman" <jc at pctechref.com> Subject: 
[AMRadio] New Project To: "'Discussion of AM Radio in the Amateur 
Service'" <amradio at mailman.qth.net> Message-ID: 
<000401cc8b64$0196eb00$04c4c100$@com> Content-Type: text/plain; 
charset="us-ascii" Well there will be many folks that will say "why 
bother", but I have a little project in mind that I have been discussing 
with Geoff, W5OMR. It is just a little fun and learning project. I want 
to build a push pull class C rig using a single 6SN7. Max Plate 
Dissipation is 7.5 watts for both sections or 5 watts for each So I plan 
to dissipate about 3 watts per section Peak voltage rating is very high 
on the 6SN7 as is the peak current - something like 1200 volts and 
300mils. I think I will run each section it at 400V on the plate and 
30mils of plate current. That's 12 watts input per section or 24 watts 
input total. This should yield about 18 watts output. The load 
reflection from the Tank is calculated Lz = Eb/2Ib = 400/.06 = 6666 ohms 
I'll figure the tank circuit from that later. At 400 Volts Eb, the 
charts show cutoff at about -30 to -40 volts grid bias Class C is 
usually biased about 3 X cutoff so I will apply -100 to -120 volts 
regulated to the grid RF choke and drive it till I get max output at 60 
mils plate current and then about 1.2 times more drive from my 
experience. And then measure the grid current. I'm expecting it to be 
about 5 mils of grid current, but that is just a guess. I'll calculate a 
grid leak from these findings for later use. Anyone got any other ideas 
or experience with these little triodes as class C devices. John, WA5BXO

Reply by Jim, WD5JKO:

    Go for it John!! I have heard of folks getting 10 watts from a 12AX7 
in grounded grid class B!!

Also look at the 6S4A as it is one beefy medium Mu triode like 1/2 of a 
6SN7 but with a 8.5 watt Pd rating.

73, and good luck!
Jim
JKO