Jim,
FB on the rotary switch and grouped increments. I do the same here.
I have also had problems with zeners shorting in this service - as biasing in the CT leads, and they are always a short. I thought it was just me. Individual diodes in a string never crapped out. The BIG amp stress takes them out, whereas a docile 3-500Z amp may survive for years.
FB on adding the elec cap across the string. I have done that on some amps here but will check to be sure I did it on all. The stable key up is important.
From all the amps you tried, did you find a better efficiency using cut-off bias on AM as discussed?
T
I have not tested ur concept on AM..yet. Here's what I want to know, what happens to the anode dissipation, with just the Un modulated CXR... with the amp biased to class C, or at least cut off ??
In 'normal' AB-2 biasing, an UN-modulated cxr typ results in 25 % tank eff. (and 65-70-% at max pep output.... amp tuned for max pep output).
IE: 375 watts UN modulated cxr and 25% eff = 1.5 kw dc input. 1.5 kw - 375 watts = 1125 watts anode diss ! 1125/375 = 3
IE: anode diss is a whopping triple the UN modulated CXR.
Ok, what happens when the GG triode is now biased to cutoff..or just beyond cut off ? What happens to the anode diss, when UN modulated? How much current does the typ 2 x 3-500Z amp sit at when UN modulated with 'normal' bias, like 180 ma..... vs idling at 0 ma ??
The 11m folks may well have figured out the class C bias scheme eons ago, dunno. Some of em used an adjustable wire wound..with the slider on the side. Slider set the bias..and idle current. Problem is... when driven hard, and way more plate current, the V drop across the resistor increases, putting the amp more towards class C.... or well into class C. It amounts to a sliding bias scheme, except they are sliding in the wrong direction. Some will wire a cap across the adjustable resistor, so the V drop hangs up higher/longer on voice peaks.
Years ago, I tried. and failed, to cook up a sliding bias scheme for ssb use, where the idle current was really low..and increased, as more drive applied. In the end, I used the adjustable bias scheme, with the rotary switch and string of diodes..and just reduced the idle current to where IMD was still acceptable.... like 100 ma on my drake amp...( ssb position, 1290 watts pep out)...and just 40 ma ( cw position, 625 watts pep out).
Jim VE7RF
