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Re: [TCML] 7.5V 52A Filament Transformer for 3CX3000 Needed
The biggest advantage of using a variac in conjunction with a filament
transformer is the ability to "dial-in" the correct filament voltage.
Depending on when they were made, you may find filament transformers with
115, 117, 120, or 125 volt primaries.
With a variac, you can set the correct filament voltage regardless of
transformer input voltage rating, variations in local line voltage, or line
voltage "droop" in your equipment when everything is operating at full load.
Obtaining maximum tube life is VERY dependent on maintaining correct
----- Original Message -----
From: "S&JY" <youngs@xxxxxxxxx>
To: "'Tesla Coil Mailing List'" <tesla@xxxxxxxxxx>
Sent: Sunday, June 14, 2009 7:19 PM
Subject: RE: [TCML] 7.5V 52A Filament Transformer for 3CX3000 Needed
Regarding in-rush current control, hams have long controlled the inrush
current on their larger transmitter plate power supplies, when first
on and charging their filter caps. Typically, they use a relay activated
a timer to add a big power resistor in series with the plate transformer
primary. This allows the filter caps to charge for a few seconds, then
power resistor is switched out by the relay.
You could provide a similar "soft start" circuit with your filament
For example, let the filaments warm up at half-voltage for a minute or
then the timer-relay would activate to apply the full filament voltage.
relay-timer circuit ought to be in the primary of your filament
where the current being switched is much lower than in your filament
circuit. I believe such an arrangement would contribute to significantly
longer tube life, as well as eliminating the need for a variac.
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Sent: Sunday, June 14, 2009 1:20 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] 7.5V 52A Filament Transformer for 3CX3000 Needed
Another option (regardless of what transformer you use) is to use a variac
on the primary side of a filament transformer. This allows you to have
superior control over inrush current as well as being able to precisely
adjust filament voltage at the tube socket.
I appreciate your desire to use a commercially-made, appropriately-rated
transformer as it just makes a "professional" appearing setup, better than
any "home-brewed" transformer could.
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