[Amps] The maximum amount of 811As and 572Bs in parallel

[Tom W8JI]

>I have an article using 6 of the 811A's in parallel for 160 
>meters from 73
> Magazine, early 1980's. I would imagine the output 
> capacitance of more than
> 4 would be too much to tune the plate circuit on say, 15 
> and 10 meters.

The real issue is feedback capacitance, not output 
capacitance. Amplifiers using two tubes are barely stable on 
upper HF because feedthrough capacitance is far too high. 
Four tubes actually require neutralization to be stable at 
upper HF, so six would the well beyond the limit.

Output capacitance of the tubes is not an issue at all. 
Adding tubes increases capacitance in the same proportion as 
it reduces optimum load impedance, and tank operating Q 
isn't a critical parameter at all as long as it exceeds a 
certain amount.

73 Tom