The problem with the adjustable W4ZT bias setup is..... come glitch time,? I can tell you exactly which components will fail.
?
A simple string of 6A10's....or? 10A10's is dead nuts simple.? ?Use the constant current graph to determine required number of diodes for a required idle current.? Then test, then tap more or less diodes for an exact idle current.?
Then install a? SPDT mini? toggle on the rear apron, to tap even more diodes in the circuit.? ?Then you can have a choice of 2 x idle current settings, one for SSB and one for CW.
On any GG high mu triode amp, you can't bias to cutoff on CW...... or you will get key clix.? ? For cw use, idle current is set to 10-40 ma.? ?On? SSB,? you can reduce the idle current by a bunch.? 180 ma on a SB-220? ( and a whopping 220-230 ma on my drake L4B amps is nuts.? ?I use? 10 x 6A10 diodes on the drake amps.? Then 100 ma idle on 2650 vdc position..and just 40 ma idle on 1900 vdc position.? IMD is superb with 1290 watts pep out in 2650 vdc position.? IMD is superb with 625 watts pep out in the 1900 vdc position.?
Come glitch time, you won't be blowing up a string of diodes..with their 400 amp surge rating.? You won't blow em up in normal operation with their? 6/10 amp? cccs rating either.?
On the drake amps,? I used a miniature DPDT-center OFF? toggle on rear apron.? In center OFF position, ALL 10 diodes are in the circuit...( or max number of diodes you want used in a given string).? The other 2 x positions provide for 2 x taps on the string..for a total of 3 x bias positions.? Easy peasy.? ?I wired both poles of the DPDT-center off mini toggle in parallel.
?
On that 3x6? HF amp, I had him use a 20 position HD rotary switch. I was a? double wafer unit, so wired both wafers in parallel.? ?Left the 1st 10 x diodes always in the circuit, then tapped off every 2nd or 3rd diode after that with the rotary switch.? The you can fully adjust the bias...on the fly, in mid sentence.? ?Cool, cuz then it can be biased to class B / C? for FM use. or a bit more idle for CW use, or more for SSB use.? ?He had the 20 position switch in the junk box..so used it.? Garden variety 12 position rotary switches are cheap, new.? ?You can also use 2 x? 12 position rotary switches in series....with position #12 of the 1st switch, feeding the input of the 2nd switch, to obtain a total of 23 positions of bias.
?
For extreme hardcore use, a spst mini toggle can also be used in conjunction with the rotary switch, to tap just the very last diode, for ultra fine adjustment. For 100%? bias regulation, wire a big value lytic across the entire string of diodes. When sucking more current, the V drop will increase by 10%.? IE: 7 vdc to 7.7 vdc.? With the lytic across em all,? you don't get the increase.? It does not budge.?
?
For really big tubes, diodes? can be wired in parallel for more current...and yes, the current will split 50-50....and also 33.33? - 33.33? - 33.33??
?
I tried EBS? in both dual state...and also tri-state.? None of em are fast enough to cut off the tubes? between syllables on ssb.? ?EBS only works if using PTT ssb.? ?EBS only works on VOX? SSB, between last word spoken...and whn VOX delay times out.?
For qsk CW use, just reduce the idle current to 10-25 ma. ..and ditto with semi break in CW.?
