[Amps] Checking Filament Voltage

[Jim Thomson]

Date: Sun, 19 Feb 2017 16:10:02 +0000 (UTC)
From: Roger Parsons <ve3zi at yahoo.com>
To: Amps <amps at 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