Author Topic: Usable vacuum tube?  (Read 1092 times)

Offline kamelryttarn

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Usable vacuum tube?
« on: November 06, 2018, 12:56:44 PM »
I think I have a 5867 tube with the necessary transformers (heater and HV) and was wondering if it's possible to use it for a VTTC. I found a schematic that use a pair of 811 tubes but I am unsure if it's possible to use a single triode. Any thoughts?

Offline Mads Barnkob

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Re: Usable vacuum tube?
« Reply #1 on: November 06, 2018, 02:10:34 PM »
It even easier to use just one tube for a VTTC, since you do not have to do any impedance matching between the tubes or adjust grid-leak circuit feedback between them.

A 5867 ~ TB3/750 should make a nice VTTC.

Are you going to use the classic Armstrong VTTC circuit?
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Offline kamelryttarn

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Re: Usable vacuum tube?
« Reply #2 on: November 06, 2018, 04:04:55 PM »
It even easier to use just one tube for a VTTC, since you do not have to do any impedance matching between the tubes or adjust grid-leak circuit feedback between them.

A 5867 ~ TB3/750 should make a nice VTTC.

Are you going to use the classic Armstrong VTTC circuit?

Haven't got that far in the planning yet. I need to get a better understanding of the circuit first. And also try and measure my transformers. The HV should be interesting. Is a crude voltage divider ok to use for lowering the voltage to my multimeter?

[edit]
I connected the HV-transformers primary to the secondary of a transformer I had laying around. I also found an old multimeter I could sacrifice should it turn out to be WAY too high voltage.  When I fed the primary with ~33VAC I got around 145V so when using 230V I should be able to get close to 1kV AC. Not as much as I hoped but it will have to do. DC voltage should be close to 1,3kV under load I suspect. The filament transformer on the other hand looks to be broken. I will have to make some measurements on it.
« Last Edit: November 07, 2018, 09:42:08 AM by kamelryttarn »

Offline Mads Barnkob

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Re: Usable vacuum tube?
« Reply #3 on: November 07, 2018, 12:23:30 PM »
You got it right, either do a low voltage input test and just multiply it up to full input voltage or you can also look up the input impedance of your multimeter and add 9 or 10MOhm to get a 1:10 division.

You could make a 5VAC heater transformer yourself, its most likely not more than 5-6 turns on any regular mains transformer where you remove the secondary winding for a new high current winding.
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Offline kamelryttarn

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Re: Usable vacuum tube?
« Reply #4 on: November 07, 2018, 02:31:19 PM »
You got it right, either do a low voltage input test and just multiply it up to full input voltage or you can also look up the input impedance of your multimeter and add 9 or 10MOhm to get a 1:10 division.

You could make a 5VAC heater transformer yourself, its most likely not more than 5-6 turns on any regular mains transformer where you remove the secondary winding for a new high current winding.

Sounds like a great little project. I actually think the filament transformer I got with the tube was made that way but it looks like the windings overheated. This will be my first ever rewind so it will be exciting and educational, although not very high tech. :)

Offline kamelryttarn

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Re: Usable vacuum tube?
« Reply #5 on: November 12, 2018, 12:57:26 PM »
I stripped the transformer bobbin of the secondary windings. Beneath the filament winding there was an unknown winding but since I will only use it for the tube filament I removed that one as well. Can anyone explain why this bobbin has two separate sections for primary and secondary windings when I can usually only find bobbins with a single space for both windings? This one seem much easier to deal with as a hobbyist.
« Last Edit: November 13, 2018, 11:23:18 AM by kamelryttarn »

Offline sjsimmo

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Re: Usable vacuum tube?
« Reply #6 on: November 22, 2018, 01:17:32 AM »
The separate sections for primary and secondary are generally to increase the safety of the transformer. The bobbin acts as an isolator, and is a lot easier to manufacture to code. Of course, having windings on the same bobbin increases coupling, and thus efficiency (in most cases), but it's cheaper and safer to use seperate sections for mains isolation.  :)
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Offline Mads Barnkob

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Re: Usable vacuum tube?
« Reply #7 on: November 22, 2018, 11:36:49 AM »
It all comes down to the class of transformer that you are willing to invest in, so its all a design option what kind of physical size or temperature rise you will allow.

If you really need to make a small transformer, you are forced to design it for a higher temperature and thus you need to use materials from a higher insulation class, making it more expensive, but space was your priority. The other way around you can as seen with MOTs dont care about size and you can manufacturer it cheaper.

See more here: https://en.wikipedia.org/wiki/Insulation_system
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Re: Usable vacuum tube?
« Reply #7 on: November 22, 2018, 11:36:49 AM »

 


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