[AMRadio] Another 813 TX question

Larry Szendrei ne1s at securespeed.us
Sun Mar 29 09:37:13 EDT 2015

On 3/27/15 2:04 AM, CL in NC via AMRadio wrote:
> A while back, a friend gave me a homebrew transmitter with and 813 final to check out.  It had a bad plate xfmr, gave it back to him to let him see if he could find a replacement for it.  It was intended for CW only for 80-20, but had a novel AM section added at a later date apparently.  The 'modulator' was nothing more than a transformer put in series with the 813 grid bias, and the primary was fed with a carbon mic, with low voltage fed through it, the primary and ground.  I never did get the TX back, but I did plug a carbon mic in and looked at the grid with a scope and there was quit a bit of audio there. There was no gain control other than talking loud or louder I guess.  Anybody ever run across this circuit or used it and how did it sound and what kind of percent mod did it give?
> Charlie, W4MEC in NC
I'm not very familiar with that particular circuit, Charlie, but it's an 
implementation of control-grid modulation, which is one of the 
"efficiency modulation" techniques similar to screen or suppressor grid 
modulation. The bias on the 813 control grid would need to be increased 
(made more negative) to the point where the 813 RF carrier power output 
is no more than 25% of the maximum CW output power, preferably less to 
allow headroom for positive peaks exceeding 100%.

It can sound very good with good audio quality applied to the PA control 
grid, which of course you aren't going to get from a carbon microphone. 
I don't believe there are any limitations on modulation percentage other 
than the positive peak won't exceed the maximum CW output.


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