[Amps] Checking Filament Voltage

[Jim Thomson]

Date: Mon, 20 Feb 2017 23:59:27 +0000
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: Re: [Amps] Checking Filament Voltage

Apparently your filament transformer is separate from other
transformers (plate). 

I'd suggest running the AC line outside the amp and using a Variac. 
The drawback to taps is your line voltage can vary with the season or
other factors and you'd be stuck. 

As others have suggested, bring the filament connection out to a pair
of easily accessible banana jacks and you're all set for whatever
happens. Be sure to use an accurate meter such as a Fluke. This is not
an area to save a few bucks on a cheapie. 

Used Variacs are widely available. Go first class!

73, Bill W6WRT

##  He said he was using an oversized  fil xfmr....with taps.  Be real careful
when using an oversized fil xfmr.  W7IUV tried that when using a 5 v at 30A
fil xfmr on a single 4-400A.  Ended up with a lot of tubes with shorts developing.

##  Typ cold resistance of a tube is only one tenth   vs the hot resistance.    
With a rheostat  in the pri,   he could not only set the fil V, it would also limit fil inrush current.

##  You can do the same with a small surplus  1-2 A  variac, provided you use the variac to
bring the fil V up slowly each time.   If the variac was pre-set to correct fil V, then a simple toggle used
to switch on,  dunno if the  small variac would have enough Z to limit fil inrush.   In that case,  you could add
a simple step start...... or remember to turn the  variac down to zero  after the amp is shut off each time. 

##  I use a fluke 87A, its been calibrated, and is a true rms reading type.   It saves a lot of grief when used for
testing equipment, or trying to calibrate stuff. 

##  Your typ  5 v  at 30 A  fil xfmr, will  read 5.5 vac, no load.  And  5.0 vac, with a 30 A CCS load.   IE:
typ fil xfmr  will  read 10 % high, with no load on it. 

Jim   VE7RF