"JIMMIE" wrote in
oups.com:

On Apr 9, 11:07 am, "Ralph Mowery" wrote:
"Jimmie D" wrote in message

...

I am trying to build a small class C RF amp for 160 AM. The amp is
to be plated modulated and I have about 12 watts of audio available.
I have built amps before(30 years ago) but I have never built one
around a modulator, previously I built the modulator to suit the
amp. Any help with tube and voltage selections would be greatly
appreciated.

Jimmie

You have about enough audio power to plate modulate a 25 watt input
amp. Maybe something using a 2E26 or a 6DQ6 tube.

As Cecil mentioned you may want to modulate the screen of a pair of
6146 tubes or similar to get enough power out to make it worthwhile.

Thanks, I have a couple each of 2E26s and 6DQ6s I think I can come
pretty close to the impedance values I need by driving one of my LS-55
transformers witth a solidstate amp using one of the low impedance
connections as the primary of a modulation transformer.


Some thoughts:

If you have 12W of audio and you want to use plate and screen modulation,
you have enough to cover 24W of DC input at 100% modulation, but it
wouldn't hurt to have a slightly higher powered final with 60% to 80%
modulation. That would suggest up to about 36W input, which at 66%
efficiency would give you a 24W carrier, 66% modulated. In that case, you
would need a PA tube with more than 12W of anode dissipation. The 2E26
has an ICAS plate dissipation of 12W.

One approach is to choose a tube that is likely to remain available due
to its widespread use and the consequent replacement demand. I don't
think the ones you nominate would be high on that list. Although a 6146B
might seem overkill, it is cheap, can run comfortably from a 500V supply
at good efficiency, and will idle along at about 60% of CCS ratings.

Important factors will be the supply voltage and anode current
compatibility with your modulation transformer... otherwise your 12W of
audio might not go very far at all. The 2e26 has less room to move in
accomodating your mod tranny than the 6146B.

Owen