[Amps] Checking Filament Voltage

[Jim W7RY]

I just put the jacks on the back of my 8877 amplifier. I just don’t drive 
the amplifier when I'm measuring filament voltage. The leads are only 2 
inches long.

Simple!

73
Jim W7RY


-----Original Message----- 
From: Jim Thomson
Sent: Sunday, February 19, 2017 11:34 AM
To: amps at contesting.com
Subject: [Amps] Checking Filament Voltage

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




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