Date: Thu, 6 Feb 2020 11:55:07 -0600
From: Joe <nss@mwt.net>
To: peter.voelpel@t-online.de, amps@contesting.com
Subject: Re: [Amps] 3xx3000 at For tubes like this, BIG currents on the
filaments.
<Has anyone made a switcher supply for it?
<Joe WB9SBD
## 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 V..at 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.
Jim VE7RF
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