[AMRadio] filament voltages

Merz Donald S merz.ds at mellon.com
Fri Aug 20 11:06:17 EDT 2004

I do the exact same thing on my single ended 250TH rig with a start and ending voltage of 3V. 
73, Don Merz, N3RHT

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of Geoff/W5OMR
Sent: Friday, August 20, 2004 10:57 AM
To: Discussion of AM Radio
Subject: Re: [AMRadio] filament voltages

> Filament voltage regulation is a good idea. Ref:
> http://www.vt52.com/diy/tips/tt_filaments.pdf
> But, be careful not to make the turn-on shock situation worse.  Basically, I
> suggest one or two big constant-voltage transformers and a few small
> filament transformers, possibly with a series resistance of a few ohms in
> their primary circuits.  The little transformers will overload on turn-on,
> limiting thermal shock to the tubes, yet the system will maintain operating
> filament voltage nicely, over a wide range of mains supply voltage.  Then
> you can tweak the filament voltage to extend emission life, and the voltages
> will stay put, and turn on will be gentle.  The gentler, the better.
>   Bacon, WA3WDR

Good information, Bacon and quite helpful.  With the cost "these days" of
the big triodes, preserving those that I do have has become a key issue.

With regards to the variac on the filament transformers primaries, what I've
taken to doing in the last week or two, is when I'm through radioing for 
that session, I reduce the primary voltage, so that the secondary voltage
drops down to about 1.5V (according to the meter) and then leave 'em
set there for about a min (milking down the filaments while listening out of
the round table), then hitting the 'big switch' which turns all equipment off.

This way, when I go back out for the next AM session, and turn the big
switch, then the filament voltage is down to 1.5v, and I let 'em come up
nice and slow, until I've found a QSO that I'm ready to break, or start
calling CQ.  Either way, before the plate switch is thrown, the tubes are
warming up, along with the rest of the equipment, for about 15 mins, and
another full minute or two, after the full 5.0v is applied to the filaments of
the 250TH's.

Another suggestion by John/WA5BXO, for equipment, is to avoid Zero-
Voltage cross-over, putting a thyrister(?) across the AC line, avoiding 
those rare times when you hit the switch at the highest arc of the 60cycle
sine wave, and hitting the transformers with what they think is a direct short.

A Thyrister may not be the device, as it's been some time since we talked
about stuff like that, and (quite frankly) I've slept since then ;-)

Thanks again, Bacon!

73 = Best Regards,

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