Date: Mon, 20 Feb 2017 23:59:27 +0000
From: Bill Turner <dezrat@outlook.com>
To: Amps group <amps@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
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