Date: Fri, 03 Jun 2011 13:25:47 +0100
From: John G3UUT <amps@grebe.plus.com>
Subject: Re: [Amps] Should a blower run after amp shutdown?
Another caution, this time about constant voltage transformers. I had
one of these and tried to use it to stabilize the mains to a signal
generator to improve drift. It sort of worked but the main problem with
it was that it distorted the mains waveform quite badly, sufficiently so
that a normal moving coil meter read very inaccurately. I'm not sure if
all constant voltage transformers have this problem but some certainly
do so if you want to set the filament to a particular voltage make sure
you use a true RMS reading meter.
73 John G3UUT
## The Sola brand constant V transformer's, made in the usa, have 1%
regulation,
and are harmonicly neutralized. They will take dirty power [ loads of
harmonics on the
input to them] and output a clean sine-wave, free of harmonics and other
trash. You can
also lay a dead short across their output, for hrs on end, with no ill
effects. They will kill
spikes and transients going into their input side. The freq doesn't change
either.
## The V readings between an analog meter and a true rms digital didn't vary
either, so
I'm assuming the output wave form was free from distortion.
## W7IUV told me a while back, he's convinced that a few of his 4-400 fil's
opened up
on him because of the high surge capability of the oversized fil xfmr he was
using. If a
correct sized fil xfmr is used..say a 15A xfmr on a single 3-500Z..and not a
30A xfmr...
and the fil xfmr is of the high reactance type, desigmed to limit fil inrush to
no more than
2 X rated xfmr current ouput, it should be fine.
## Not all fil xfmr's are like this though, and sometimes you have to use what
is avaialble.
On my 3CX-3000A7 amp... it requires a 7.5 v @ 50A xfmr. The one I use is a
8.25 vac
xfmr @ 80A CCS. It also has a single phase 208 vac input. So a variac has
to be used anyway.
## On the 3CX-6000A7 amp... it wants a 7.0 vac @ 79 A xfmr. I use a dahl
7.5 vac @ 75A xfmr
on that tube..also with a small variac.
## Even with a 60A CCS rated bifilar, I still lose .4 vac across the
bifilar itself, so on the 3x3 tube... I have to crank
7.9 vac into the input of the bifilar..to get 7.5 vac at the socket. In
normal operation, the fil V is way on the low side,
since the new tube has loads of emmission. The 3x6 is rated at 7.0 V It's
run at 6.0 vac
## On any new or rebuilt tube, I run the fil + blower only..at rated fil
V..for 48 hrs straight..since the getter's
in both the 3x3 and 3x6 are fil heat activated. This gets rid of any residual
gas. Tube is then put into normal operation.
After 200 hrs, fil v is reduced till the PO drops off a few watts [knee
effect], then the fil V is increased by .1 Vac
## I measure the fil v at the socket itself... with a 200 uh choke in each
leg..with a fluke 87 or Hp. I'm sure that the
dahl hypersil fil xfmrs [ not his EI types] are more than capable of excess
current, with a virtual dead short on their
output. I should try this sometime, on all my fil xfmrs, with perhaps a clamp
on ac ammeter, or an analog ac ammeter, the
type that uses the external doughnut slipped over one leg.
## These tubes are pricey enough as is, and my idea was to prolong their
life..hence the reduced fil V in normal operation..and
also both ramping up..and down... both slowly, the fil V, excess cooling, etc.
A lot of fm/am broadcast TX I have seen, don't use
a Sola constant V xfmr..and some do..and is an option on yet other's. Some
use a simple rehostat, some use auto ramp up SS devices.
Some use motor driven variacs. Most have no provsions for fil inrush at
all, since they are on 24/7.
Later..... Jim VE7RF
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