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Re: [Amps] GRADUAL FILAMENT warm up . . . .

To: Amplifier Mailing List <>
Subject: Re: [Amps] GRADUAL FILAMENT warm up . . . .
From: Gene May <>
Date: Mon, 26 Dec 2011 18:15:22 -0500
List-post: <">>
At the risk of mentioning something that is already familiar to the readers of 
this thread, but since it has been discussed for a while now (and because 
making an idiot of myself is not an unfamiliar action or experience . . . ), I 
point out devices known as "negative temperature coefficient thermistors".  If 
you "Google" this topic, you will have available more than you need to know 
from WikiPedia and commercial sources as well.  These inexpensive (less than a 
few dollars each) devices look like a ceramic capacitor, are about the same 
size, and behave in a manner opposite from filaments -- they have resistances 
about twenty to over a hundred times higher when cold than they have when they 
are hot.  I have put them in the primaries of filament supplies, and they work 
like a charm to slowly ramp up the filament current.  I suggest putting a 
bypass switch in parallel with it that can form a direct connection to the 
filament circuit after a minute or so after the thermistor warm
 s up and filament voltage is about normal.  This will save having extra heat 
in your chassis from the thermistor and a slight extra voltage drop in the 
primary circuit of the filament supply transformer.
I have never tried them in a plate supply circuit (to slow the current surge 
that charges the filter caps upon turn-on), and I don't think they would work 
in that role because they would never get hot enough to have low resistance.  I 
have put simple resistors (say, five watts, 20 - 100 ohms) with such bypass 
switches in the primaries of transformers of plate circuits.  Throw the bypass 
switch after a few seconds, shorting out this resistor, after the filter caps 
are charged (plate voltage reads normal).
Gene May

> From:
> Subject: Amps Digest, Vol 108, Issue 32
> To:
> Date: Fri, 23 Dec 2011 11:21:22 -0800
Warm up and on time question... (Radio WC6W)

Date: Wednesday, December 14, 2011, 7:38 PM
Speaking of warm up times and filament life got me to thinking about a ramping 
switch system for the whole power supply.
As I understand it there are devices which will chop up the AC sine wave into 
pieces like a PWM and slowly add more and more pieces until the full sine wave 
is present on the load.  I also understand they can be used in reverse. Sort of 
like having an electronic Variac on the input side of the supply.  Do any of 
you have a circuit diagram for such a system or could point me in the right 
direction?? I am not even sure what to call what I am looking for.  It sounds 
like a ramping soft start system and that would solve a lot of problems with 
initial turn on of the filaments and the Plate supply.
> > 
> > Thanks in advance.
> > 
> > Mike Baker? K7DD
> >

> End of Amps Digest, Vol 108, Issue 32
> *************************************
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