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Myke

Tue Jul 15 2008, 03:03PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Well I tried the loading capacitor and it made it so that there was no breakout at all. It also drew more current (could tell by the sound of the MOT). Hrm... maybe I connected it incorrectly. I was thinking that the cap would go from output to ground. But would the cap go from plate to output?

I tired the var resistor and found that the less resistance there is from grid to ground, the bigger the discharge is. Also I put a 390pF cap across the var resistor and it didn't seem to do much.

I'll mess around with it later.
Thanks though.

Steve Conner

Tue Jul 15 2008, 03:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The loading capacitor goes from plate to ground.

Mads Barnkob

Tue Jul 15 2008, 03:53PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Hope you dont mind I add a question instead of a new thread :)

Resistors for the choke at plate supply, would it be a problem if its housed in a heatsink?

these are among the only ones I can find in Denmark at a fair price f.ex.:

Myke

Wed Jul 16 2008, 03:36AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Ok, so I tried the loading cap with ~75pF and ~65pF (5 and 6 390pF 1.5KV caps in series) and with both, they drew tons of current and there was no breakout... Should the loading cap be even less because of the high frequency? I don't have many small caps that are HV. I have a couple 20pF 1KV caps that I could string in series but I donno...

For cooling would I just put a fan under it? I am worried that if I put a fan on one side of it, it would cause the glass to crack.

MadsKaizer: I'd assume they can't handle as much voltage because the metal casing is closer to the terminals.

EDIT: Bleh, my dad is worried that the exposure of radio waves might do some physical damage to me... He said that the person who discovered x-rays didn't know that they were dangerous. I tried explaining it to him that the radio waves won't do anything but he told me to research it... I also told him that the high voltage was more dangerous than the radio waves.

Mads Barnkob

Wed Jul 16 2008, 02:34PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Myke wrote ...

MadsKaizer: I'd assume they can't handle as much voltage because the metal casing is closer to the terminals.

Arg! I'm too used to work with under 1000V in my job, so I didn't even consider that sparks could jump to the housing, I was only thinking about how the housing would affect the choke wound around it. Unfortunately ceramic resistors ain't sold from new at those high wattage.

Myke

Fri Jul 18 2008, 07:09AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Here are some pics of the coil and the setup

After about a minute, the tube's plate starts to glow a dull red (not very brightly). Is this bad? Is there any way I can make it so I can put more power through it without worrying about overheating? I still don't understand how reducing the grid to ground resistor would cause the flame to get bigger.

Proud Mary

Fri Jul 18 2008, 08:36AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

No it shouldn't be glowing red at such a low anode voltage, Myke - it means the valve is drawing too much current.

My best guess without all the data to hand: you should make the grid much more negative so the valve is cut off - or almost so - in the non-oscillatory state.

To adjust for this, you will have to increase the feedback coupling by winding a turn or two over or around the anode coil. I don't think the amount of feedback provided by the little hook-over pick-up is sufficient for such a big valve, which is much less sensitive to changes in grid voltage than the valve used in the German design.

Shaun

Sat Jul 19 2008, 07:48PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

When I saw this thread I was surprised that building an HF coil could be so easy using tubes, and decided to build one myself.

However I can't find an EL519 tube, and I don't know vacuum tubes very well so I'm not sure what a good replacement would be. I've done a little googling and have found that a common replacement for EL519 is the EL509. I can't find any sites that sell either, tho. Except for one site that ships from Russia for like $30 extra.

There are a bunch of tubes that turn up on Ebay when you search "EL509" but they are all 6P45S tubes which the sellers claim to be equivalent to the EL509, but are labeled as tetrodes, when I know that EL509 is a pentode.

I'm pretty confused at this point, so can anyone suggest a common tube that will work in this circuit? If its easier to just leave out the audio section that's ok too.

Myke

Sat Jul 19 2008, 08:02PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Tetrodes work fine with this circuit. The 4-400A is a tetrode.
You can look for a 6KG6A because it should have similar ratings.

So I added more feedback and the flame got bigger. I need to find a new material for the output because the copper wire that I have now is melting :P. To measure HV current, can I just use a analogue meter?

Shaun

Sat Jul 19 2008, 09:52PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Ok thanks a lot Myke. Just for clarification on my part, is the reason I can use a tetrode because the third grid in the schematic is just grounded anyway?

Also, I think the best way to measure current would be with a CT hooked to an oscilloscope. Anything directly connected to the circuit, like a shunt or analogue meter, would drastically change the coils characteristics due to the very high frequencies.

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