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Old 3rd May 2018, 5:38 pm   #1
Viewmaster
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Default Valved singing plasma rig.

Just finishing off a plasma generator using two Russian pentode valves strapped as parallel triodes, the GU81's.

The plasma will be audio modulated via the centre tap of the directly heated valve filaments.

Circuit has been lifted off the net from a Czech web site.

The audio quality will be very 'toppy' but I do love big valves with plenty of
EHT, so thought I would build it.

EHT via a dangerously deadly micro wave oven TX, bridge rectified and smoothed.

13v 20 amp filament supply via a 12v toroidal TX with an extra few windings added to boost to 13v.
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Old 4th May 2018, 9:16 am   #2
kalee20
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Default Re: Valved singing plasma rig.

Can't wait! Hopefully you will put up a YouTube video!
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Old 4th May 2018, 1:30 pm   #3
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Default Re: Valved singing plasma rig.

It looks interesting! Please keep us updated.
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Old 4th May 2018, 3:05 pm   #4
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Default Re: Valved singing plasma rig.

Quote:
Originally Posted by Viewmaster View Post
Just finishing off a plasma generator using two Russian pentode valves strapped as parallel triodes, the GU81's.
Wonderful! Welcome to the club! That's the big transmitting pentode that was candidate for my fourth Tesla coil- the one I shelved recently because the components, including a specially wound 1.8KV transformer - were too heavy for me to move safely after my complex recovery.

That coil looks like it will be self-resonant at around 5-8MHz?

It's a neat plan of -I think it was DanyK - to use light bulbs for the grid leak circuit. 1,000/R to 1,500K seems to be in the range.

I'd make sure you have a good RF bypass capacitor there, from your EHT+ to ground.
If you have one, ignore this, but can't see one in your pic, but it makes all the difference. It works hard, so make it a good one!

Maybs also safety earth your TX core? I only see phase and neutral in your photo?

It's also a good plan to have the heaters RF bypassed, too.

Also obvious precautions with discharge caps on your MOT capacitors.

For (safety) illustration purposes, I once put a piece of rare beef steak across four charged 0.9uF Microwave oven transformer capacitors in parallel... It jumped and hissed and the way it cooked was an effective way to show caution!
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Old 4th May 2018, 3:49 pm   #5
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Default Re: Valved singing plasma rig.

Looks well Albert, those GU81's are nice looking valves. Any chance of a link to the article?

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Old 4th May 2018, 5:14 pm   #6
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Default Re: Valved singing plasma rig.

Andy,

It looks like this circuit - in any case , this guy (DanyK) is the original author - only with two GU81M’s instead of one.

There are some huge RF currents circulating and ground loops can be a problem.

I don’t know if there is a safety earth in Czech supplies - I don’t think so, I’d certainly safety earth the whole of the EHT TX case and ( isolated) EHT ground.

Apart from safety reasons , if there’s a flashover from primary to secondary of the air-cored coil, this will trip the RCD/ MCB before too much mischief is caused.
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Old 4th May 2018, 5:44 pm   #7
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Default Re: Valved singing plasma rig.

Thanks all for tips/suggestions.
Yes, it's the Danyk circuit.

http://danyk.cz/avttc_en.html
He has the ground side connected to mains so the audio input is live!
I don't like that.

In addition to bleeders across each cap I am in process of mounting 10x 15 watt mains pygmy lamps in series across the EHT caps (running at about 2.4Kv), so 240 volt across each one.
This will discharge them faster than bleeder R's alone if I see anything going amiss. I lifted that idea from the net too.

The caps and bridge diodes are not shown in my image as they are slung underneath away from wandering hands!

Using 2 EMF filters (belt and braces!) in series with primary of MOT and variac.
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Old 4th May 2018, 7:00 pm   #8
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Default Re: Valved singing plasma rig.

He does that set up to get the MOT windings in series aiding, for an extra few hundred volts EHT. If you instead wire the MOT comventionally you can isolate the EHT supply and safety earth the ground. I can tell you, it’s not worth the danger ; the results will be imperceptibly different.

I wouldn’t let the power supply float. A lot of RF will pop up where you don’t expect it to and can cause vicious mischief.

It might be an idea to further isolate your line-level signal in the modulation circuit to at least 6kV in case of a fault condition.
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Old 5th May 2018, 7:46 am   #9
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Default Re: Valved singing plasma rig.

OK, Al on all that. That extra 240v is not really required so will dispense of live audio to mains and take your advice re isolation.
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Old 16th Jun 2018, 10:18 am   #10
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Default Re: Valved singing plasma rig.

This project is continued here...…..No6
https://www.vintage-radio.net/forum/...d.php?t=147290
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Old 16th Jun 2018, 10:22 am   #11
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Default Re: Valved singing plasma rig.

Hi Albert meeting all my nbtv friends here !~ you have made some beast here i am going t follow your link and see where your at with it !
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