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Re: [TCML] First VTTC - Some help needed choosing tubes

Good morning Steve, many thanks for the reply.
Using a single 811 would definitely simplify things. If there's not much benefit to putting two in parallel, I may go that route. 
I found a few other tubes I'm curious about, and whether or not they might work even better than an 811. One is the GU-81M, which is actually a pentode. I've seen these used in VTTCs, though with the additional screens the circuit gets a little more complicated (unless I tie them together?). I found a couple of those available, each less than $40 delivered. I also found some Eimac 4-125A tubes, $32 for a pair, delivered. I'm actually leaning heavily in this direction - even if I use just one of them for the coil, it'll be good to have a spare. These are tetrodes, but once again I imagine I can simply tie the grids together (?). 
Matt, Fairlee VT

-----Original Message-----
From: Steve White <steve.white1@xxxxxxxxx>
To: derstrom8 <derstrom8@xxxxxxx>; Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Wed, Dec 26, 2018 10:55 pm
Subject: Re: [TCML] First VTTC - Some help needed choosing tubes

Hello Matt,

I have only built one VTTC, based on a single 833A, so my experience is limited to that coil. There are others on here with more experience with VTTCs so maybe they will respond.

The 811 is a very commonly used tube for VTTCs and can handle decent power. I would stick with a single 811. Dual tubes just complicate matters having to do with matching, dual filament transformers, complexity, etc. I have also heard from other that dual tubes really add little to the streamer length over a single tube. I would use Steve Ward's 833A schematic as a starting point. Just about every VTTC is a variation of it. One thing to incorporate is a way to soft-start the filament. This will extend the life of the tube greatly by avoiding current in-rush to the filament. There are two ways to do it. The best way is with a small variac for the filament. A cheaper way is to have a switchable dual voltage power supply for the filament. A current limiting resistor is used to generate the lower voltage. You then use a 2-position switch to select between the low voltage and nominal voltage supplies.

I wouldn't worry about damaging a tube too much. They are electrically very rugged and can take a lot of abuse. They are not all like IGBTs or MOSFETs which will explode if you look at them the wrong way.

Steve White
Cedar Rapids, Iowa
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