Date: Sun, 19 Feb 2017 16:10:02 +0000 (UTC)
From: Roger Parsons <ve3zi@yahoo.com>
To: Amps <amps@contesting.com>
Subject: [Amps] Checking Filament Voltage
I am busily making a replacement amplifier using a metal/ceramic filament tube.
I would like to coarsely adjust the filament voltage before I install the
blower. (The amp is built into an existing cabinet and the filament transformer
is a fair way from the tube.) My question is: Is it permissible to run the tube
with only filament voltage and no air for a short period? If so, what is a
short period?
Thanks
Roger
VE3ZI
## what tube ? I would not do it myself. The fil can get very hot, esp on
direct heated tubes like 3CX-3000A7, 4CX-5000 etc. U would be better off to
temp install a pair of leads to the fil, and route them external, so you can
maintain airflow,
then use a fluke dvm to read the fil V. You can use real small gauge test
leads for this application. I used a pair of 100 uh chokes, anything up to
2.5 mh will also work, right at the socket, then route test leads to front
panel fil voltmeter. A 100 ohm
resistor in each leg just b4 the front panel meter. Resistors are safety only,
in case the test leads get shorted. Then I parallel the front panel voltmeter
to a pair of banana jacks, just below the panel meter. Then its an easy
way to plug in my fluke 87.
I used test leads on the fluke that have banana plugs on both ends. Then u
can also calibrate the analog 0-10 vac panel meter, via its set screw adj.
So the analog jives with the digital.
## In normal operation, drive RF into the cathode will not get past the
pair of small RF chokes at the socket. Both analog meter studs and also the
pair of banana jacks have a .01 uf bypass cap to AL front panel. You can
also install another pair of .01 uf bypass caps on the outboard sides of the RF
chokes. Again from output of each choke..to chassis. (so 6 caps in total).
I tweak my fil V with a variac, so I have to be able to measure at the
socket.
## You can do the same with indirectly heated tubes like 8877, YC-156,
YC-172 etc.
Jim VE7RF
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