[Amps] Should a blower run after amp shutdown?

[Jim Thomson]

Date: Fri, 03 Jun 2011 13:25:47 +0100
From: John G3UUT <amps at 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