Lou can chime in on this one.? On? the..... "big 3"? ? ameritron amps....... they don't use a resistor at all.? On RX it's wide open !? ?I got into an argument with W8JI about this years ago.? ?IMO, it's a stupid idea...... and 'saves' all of a few pennies at best.? ?While it ( a wide open across the relay contacts)? might? just work on the ameritron amps...it sure as hell doesn't work on that single and also dual GS35B? 6m amp I designed for a buddy in W7 land years ago.? ?I contend that with the B-? in the cathode open on RX........? the cathode will? try and assume partial to full B+.? ?The handbooks say the same.?
On those GS35B amps, the builder used red millens for the B+...and black millens for the B-.? ?One day, while putting it back together...( HV supply in a separate box) the black millen was not making contact.? ?With fil lit, then the B+ on next, all hell breaks loose...with an open B-.? ?The cathode indeed assumes B+ potential.? ?Those amps had aprx 4500 vdc no load on em.
I had the? exact same thing happen on my hb 4-1000 amps.? ? HV was in it's own box...... and separate? connections for the B+ ( millen)......and similar for the? B-.? ? One day, in super haste, zero patience....and only being 20 yrs old back then,? I wired it all back up....and proceeded to get in on the contest that night.? ? Like an idiot,? I completely forgot to hook up the B-? at the amp end.? ?All hell broke loose... with loud snapping and crackling going on.? ?Hooked up the B- and al was well.?
Next time I had it apart, the .01uf @ 1 kv disc caps used on the cold end of the fil choke were fried... with huge black smudge marks down the side of em.? ?With no B- hooked up, the cathode assumed all of the? B+....and arced across the bypass caps.? ? ?I was lucky it didn't arc across the coupling caps... ( between cathode and tuned inputs)..and start rfying the caps on the tuned input.?
These days, I take all that B-? wiring seriously.? ?The B-? from the supply goes to the negative side of the plate current meter.? Positive terminal of plate current meter is bonded to the positive terminal of grid current meter.? At that junction point,? it goes off to the bias cutoff resistor ( 2 x 100k @ 3 watt MOF's in parallel....for redundancy)...then through a mess of series bias diodes, then to the? CT of the fil xfmr.? ?
IF the plate current meter ever went open,? you have just opened up the B-.....and again, all hell breaks loose.? ?So in goes safety diodes across the plate current meter ( in addition to safety diodes wired directly between B- of? HV supply....and the HV supply chassis...and? also between? B-? and chassis? in the RF deck. ).?
On a similar note, drake used this stupid scheme? for cutoff bias? in the L4B / L7 amps.? ?They applied +120 vdc (+90 vdc when in? the lower 1900? voltage CW position) to the? CT of the fil xfmr...on RX.? On TX the center contact removed the +120/+90 vdc.... and instead? the CT? went off to the metering etc.?
Problem is...when relay contacts? in mid air? ( rich measures called that....'no mans land')? the CT has gone open.? ? ?And again, the cathode try's for a split second, to assume? full B+ potential.? ?Their scheme works.... but after years of use, the center contacts of the oem 3PDT TR relay were fried...and I mean burnt so bad.... it would not pull normal idle current.? With amp keyed, the CT was open.? ( The 100w input +? kw output contacts? were fine).?
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?The easy fix of course was to toss the +120 vdc bias junk..... and install a 100k / 3 watt resistor in the CT of the fil xfmr.... an re-wire the contacts of the center pole of tr relay so resistor is inline all the time...and shunted on TX.?
I also added 10 x 1n5408's for bias....and also grnded the grids etc, etc.?
