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Re: [Amps] 3CX-3000A7 at 144

Subject: Re: [Amps] 3CX-3000A7 at 144
From: Steve Thompson <>
Date: Thu, 6 Feb 2020 22:48:39 +0000
List-post: <>
GS35B, 8877 and the like have separate cathode - they don't need a CT for the B- return. DC switcher supplies work just fine as long as they go into constant current rather than tripping out on overload.

Steve G8GSQ

## Dont waste your time. 7.5 vac at 50A fil xfmrs are a dime a dozen. (center tapped)..with a myriad of primary voltage taps. My dahl is tapped at 198-208-218-230-240-250 vac. ## Buddy tried using a switcher supply for his hb 2 x GS35B 6M amp... 4.7 kw out. Separate switcher for each tube fil. Problem was, even though each switcher has...soft start built in, the cold resistance of a directly heated cathode is typ only .1 to .083 of the hot resistance. So even though each switcher was rated for DOUBLE the ccs rating of the tube, the switcher would still go into instant shut down mode. So in went an even bigger pair of switchers, now rated for QUADRUPLE the CCS tube rating. Ok, works, no more shut down. New problem. The B- return on a GG triode is via the CT of the fil xfmr. Switchers dont have a CT ! A DC to DC converter complicated setup was engineered. Worked fine until one day we lost the B- connection.... problem with the BLACK Millen HV connector. All hell breaks loose when the B- is opened off on a GG Triode. The cathode will try and assume full B+ potential. Of course, both HD switchers and the elaborate DC to DC isolator setup....all blew their brains out. ## Plan C.... installed 2 x new hammond fil xfmrs, one per tube, each with its own CT. Metering rewired, such that one plate current meter can be switched to read individual plate current, or combined. 1,2, 1+2. Idividual W4ZT adjustable bias boards used, one per tube.

## On my own 160-15 M 3CX-3000A7 amp, A sola constant voltage xfmr.... feeds a regulated 240 vac into the input of a small 0-240 vac at 5A varaic. Output of variac feeds the input of the dahl fil xfmr. I use the variac to bring up the fil voltage slowly. RF chokes on each leg of the 3x3s cathode. Wired to both a 0-10 vac panel meter...and also a pair of pin outs. Fluke 87A plugged into pinouts for precise reading.. and also to calibrate the panel meter. Measure fil the socket, with a 87-A. Make sure there are NO Vdrops across the chokes.... due to loop current from any panel meter. ## On buddys 3x6 HF amp, no sola used, but a step start resistor + timer used to shunt the resistor....which precedes the variac. His variac tweaked, then never touched again. ## RF parts sells an excellent socket for the 3x3. The F7 version does not require a socket. RF parts now sells a superb HD chimney, made from .25inch thick acrylic. Top lined with .125 inch thick solid teflon doughnut.....whose ID just fits the OD of the 3x3.
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