The AM Forum

I have another question along the lines of my previous topic on exciter output needed to drive typical linears.

If one were to HB an exciter, and target an output of say 50 - 75 Watts, what would be the best way to be able to vary the output down into the 15 - 25W range needed for driving a linear?

Could you do something as simple as put a rheostat in the final screen grid circuit to lower screen voltage? (You would have to turn the audio gain down also, of course).

Here I'm assuming you design the tx from scratch rather than trying to reduce output of a Johnson Viking or some other boat anchor.

I think it would be best to have switchable plate HV values on the exciter's final in order to maintain a constant plate Z across the various power levels you're considering. Then design your final's tank for that Z.

Also, if your exciter is a single 6146, you could consider changing to a 2e26 when you want less output.

peter

If you're planning on a G-G amplifier, then the signal from that stage is not able to exceed 100% modulation. Am I correct on this? Then it need not be a plate modulated exciter stage... why not controlled carrier screen modulation or even simply cathode modulated? The power levels would then be more easily adjusted for the proper drive levels to the linear amplifier.

73, Phil

Yup
Your first thought about controlling the screen of the output stage (6146??) is the best way to control drive. I did that using an Elmac to drive a small amplifier using 3 ea 811A's.
Modulation will be bigger pos peaks. The modulators B+ stay the same.

Fred

Nope. If it's truly linear it will pass what it sees.

My Viking 1 has a negative peak clipper. It runs 90% negative and 115% positive peaks. That's what I see out of the amp! Confirmed on many a qso and on my scope and everything else i can turn on! ;D

The variable screen voltage is what was used in several commercial rigs and works well as those oldies would handle close to any plate or or output load impedance. If you use a roller coil such as the Viking I and II you can always be perfect.

OTOH you could always put in a Hi-Low power switch (or Variac for that matter) and maintain the same plate impedance using the roughly 72% ratio of the high B+ and low B+. Then use the screen control for fine adjustment. That way the pi net is less critical.

Carl

Me thinks that was a statement from some text where your plate modulating a GG final stage amplifier,not a lineeeiear. You could get closer to 100 percent if the driver was modulated in that configuration.

Ah-Ha, I see! And now that I think of it, I recall seeing a recent thread that basically said the very same thing.  Thanks for steering me straight on that.

73, Phil

I like the plate voltage idea. If you are going to HB this thing you can get a transformer that has multi taps.  THen you can lower the volts but load it up higher to keep the plate load on the modulation transformer correct.  That would be the best way as already stated.

That being said, I have a valiant that uses a small finger tube, I think a 6bq5 from memory, alot with some a rear mounted POT. This controls the screen volts.  I can vary the the power of the valiant up and down this way.  You need to make very sure you lower the audio down to match the lower carrier. Otherwise, You could arc out and damage the mod transformer.

C