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Re: [Amps] Filament overwind on an EHT Toroidal

To:, Glenn McNeil <>
Subject: Re: [Amps] Filament overwind on an EHT Toroidal
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Thu, 23 Aug 2007 08:35:21 +0100
List-post: <>
Glenn McNeil wrote:
>I'm in the early stages of constructing a Twin GI-7Bt 144 Mhz 
>Amp...mainly for portable use, hence the smaller, cheaper tubes, and 
>I'm looking for only 400W Output.
>I am trying to make it as small as possible, and as light as posssible. 
>It will be self contained, and I have sourced a Toroidal Transformer 
>with two 800v windings that I will series up for 1600ac, with 68uf of 
>C, hopefully giving me around 2100vDC at 500ma. The Toroidal 
>Transformer arrived and has two 6.3v, 5amp ac windings, overwound on 
>the outside of the thing...if I series these up, I get my 12.6 Filament 
>requirement for the two tubes ( 2 amps each at 12.6v ). This would save 
>a seperate Filament tranny.
>Now, on Fault ( I predict one may happen ) , I usually use the G3SEK 
>System, and drop off the EHT Tranny Primary using a hefty Relay. Using 
>this method on my system above would also drop off the Filaments, and 
>really complicate things.Hitting Reset would see the EHT come back up 
>on Tubes that had cooled down due no Filaments.
>I propose to use a Vacuum Relay in the EHT Supply, downstream of the 
>Glitch Resistor, for Fault isolation.
>In case of a fault, the EHT is removed, but Filaments left going, as 
>the AC is still on the Toroidal Transformer, and in fact, the EHT is 
>still up, but isolated from the Tube.
>I can then decide on what to do next...hit Reset, or have a think.
>Am I cutting corners too much in trying to save a bit on 
>weight/cost/complexity, and should I drop in a toroidal to run just the 
>Filamnets and do things in the usual manner ?
>Any comments welcomed !

Just because those two windings exist on the HV transformer, it doesn't 
mean you have to use them - and there are very good reasons not to.

There has to be a second small transformer anyway, permanently energized 
to power the control board and the relays. For many reasons, it makes 
much more sense to run the tube heaters from that transformer instead.

Your proposal can be made to work, but it definitely isn't the one I 
would recommend. In particular, that relay in the HV+ line could be a 
major headache.

Under fault conditions such as a tube arc, the glitch resistor will 
limit the instantaneous current surge... but the arc is still being 
powered from the HV supply, so that supply must somehow be turned off 
within the next few milliseconds. The Triode Board has a fast electronic 
current trip which switches off the power to the HV control relay.

If the HV control relay is in the transformer primary, it only has to 
break 115/230V AC mains. A wide range of compact and inexpensive relays 
can handle that. But if that relay is in the HV+ line, it has to break a 
surge current of >20A, pulling a 2000V DC arc between the contacts as 
they open. A relay that you can *trust* to do that, reliably and 
repeatedly, will be larger and more expensive than you imagine - 
definitely a bigger complication than another small mains transformer!

That's why I strongly recommend that the HV control relay is in the 
transformer primary. Done correctly with the mandatory glitch resistor, 
that system will comfortably handle a direct crowbar short from the tube 
cap to the chassis, shutting down with a quiet 'tick'  and not even a 
mains fuse blown. Tube arcs will often not repeat when you power up 
again, so all you have to do is press the Reset button and you're back 


73 from Ian GM3SEK
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