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Old 14th March 2012, 09:59 AM   #1
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Default Thyratron based emergency switch?

Hello all, I have a question related to my current build. Since I am using precious gu81m tubes and do not wish to see them prematurely depart this world, I'd like to implement a safety device.

I have separate boards and circuis for all voltages on board (bias, screen, anode for drivers) but I would like to have a safety device which will take out the B+ on the gu81m (1600v) if bias to mosfet stage/gu81m suddenly goes out.

Can I use a thyratron? I found almost nothing on operation but I gathered a firing voltage is necessary to have conduction from cathode to anode. Can I assume the absence of grid voltage (ie bias) will shut the tube off?

Please understand my concern

I can find and implement a thyratron for little effort and money so that side of thing is not a problem.

Also other questions:

1. the thyratron will behave like a standard gas rectifier with DC flowing through the filament transformer's CT? This would be DC however not rectified AC. Is it possible to saturate the heater core?
2. If the above has a positive answer can I solve the problem by taking the "processed" DC from the heater wires rather than through the heater transformer's center tap?
3. Are there sonic effects caused by thyratrons in the path of the b+?

Thanks
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Old 14th March 2012, 10:54 AM   #2
Doz is offline Doz  United Kingdom
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Unfortunately, a gas thyratron is like a thyristor. Once it is fired, it stays fired until it's supply is removed.
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Old 14th March 2012, 10:59 AM   #3
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I thought the state was voltage dependent hence the use in pulse equipment....

Just out of curiosity....what would be the effect of a thyratron on the b+ as opposed to an all ss powersupply? I don't mean as a rectifier but rather after the caps as a sound "colouring" device maybe?

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Old 14th March 2012, 11:02 AM   #4
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In pulse equipment, the "downside" of the pulse causes the thyratron to switch off. Some pulse forming networks employ a second "de-q'ing" thyratron to effectively short the input to ground to shut it off. You could use it as a crowbar, however, I doubt your rectifiers/ transformer will love you for long.
No idea about audio use.
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Old 14th March 2012, 11:38 AM   #5
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thanks a lot. that cleared the whole thyratron issue. I guess a redundant bias supply would be best for this application.

Thanks!
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Old 14th March 2012, 11:50 AM   #6
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Just use a correct sized fuse
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Old 14th March 2012, 11:56 AM   #7
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Quote:
Originally Posted by Alexontherocks View Post
.... I'd like to implement a safety device.
I hope you're designing in many many safety devices! This project has got hairs on it if it is being used as a learning curve
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Old 14th March 2012, 11:56 AM   #8
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My objective is to protect the gu81m from accidental bias voltage failure. where could I put the fuse?
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Old 14th March 2012, 12:03 PM   #9
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Quote:
Originally Posted by trobbins View Post
I hope you're designing in many many safety devices! This project has got hairs on it if it is being used as a learning curve
You are right! Many safety devces are in place.

1. Full grounding of chassis and boards
2. fuse on each transformer 220v line
3. separate transformers for LV, HV, filament supplies
4. 20kv insulation on all HV (>500v) lines
5. plexiglass protection panes to create internal layers inside the amp
6. oversized caps for all boards
7. separate voltage sources (+600,+1600,+300+500-300+150v)
8. at least 2 cap stages for every board (in case a cap stage blows)
9. Teflon cap machined to cover all exposed HV leads
10. sofstart, timers ecc (to avoid nasty arching with the huge rectifiers)

what I wanted was to protect the tubes themselves from bias failure.
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Old 14th March 2012, 12:25 PM   #10
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Quote:
Originally Posted by Alexontherocks View Post
My objective is to protect the gu81m from accidental bias voltage failure. where could I put the fuse?
In B+ line..
When bias falls away, GU81 will pull too much current (for a short period, wich it can handle) and fuse will blow.
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