[Amps] 4CX1500 class AB1 vs. AB2

[Tom W8JI]

> Negative feedback has little to do with being better at 
> reducing the
> distortion produced by running into grid current than 
> having no feedback.
> Sure an amp with feedback is going to lower the overall 
> distortion X number
> of Db but on a relative scale overshoot problems are going 
> to create the
> same amount of difference in distortion products whether 
> there is feedback
> or not.

A  sharp transient is quickly gone and has little average 
power. Until we clip it. The worse thing we can do is turn 
that sharp transient into a long period of non-linerarity as 
the grid voltage pumps way up and then recovers.  Making the 
already clipped waveform even flatter for a much longer time 
doesn't help bandwidth either.

> The question still remains, how do you protect the grid of 
> a 4CX type tube
> from excess grid current with a stiff bias supply?

We could just shut the amplifier off until the person fixes 
the drive problem, and/or do fast attack ALC with a long 
hold time. We don't need to turn the PA class C for multiple 
RF cycles over and over again just so the operator can keep 
on transmitting without popping the grid.

I had a few rigs that were terrible. One was an IC775DSP. 
When set for 50-60 watts it would spike to around 350-300 
watts. My AL1500, when loaded normally at 2000w carrier, 
would just dump when I used it. If I had a loose goose grid 
I'd never have known it was a problem.

A very fast grid trip circuit not only can save the tube.... 
it can let the operator know something is wrong.

73 Tom