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Old 12th September 2012, 07:33 PM   #1
mogliaa is offline mogliaa  United Kingdom
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Default 600V bench variable supply to test transmitting valves

Tired of my HT (+600V) variable bench power supply to suffer collateral damage when accidentally shorted whilst testing transmitting valves for output stage (i.e. FQP3n80c MOSFET passive regulator blowing out), I decided to make a simple but effective valve stabiliser. As nothing comes for free, here are my design constrain factors:
1. Input raw supply is +620V @ 100mA
2. Filament secondary winding is 15V @1.5A
3. No additional secondary winding is available for a floating screen supply (e.g. pass valve is pentode)
4. Output voltage ideally should be 0-600V

So with the restriction of not using a pentode as pass valve, I looked out for candidates to match my requirements and instantly thought about GU-50 in triode-strapped mode. Yes, I know that UG2 limit is 250V, not 1,000V as anode max voltage. But, in triode strapped specs are not shown.

I recently checked this with the 814 triode strapped, and seems to be ok UG2=Ua in triode mode. 7N7 also said this was ok and Morgan Jones previously tested this as well with similar valves.

So, question here is: can the GU-50 withstand 600V in triode mode or do I need to look out for options?

What about adding a zener string across G2 and cathode with a resistor between G2 and anode? Not ideal, but perhaps I can get a compromise solution that will allow me to get 0-600V swing here.

Thoughts?
Cheers,
Ale
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Old 12th September 2012, 07:39 PM   #2
tomchr is offline tomchr  United States
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I would recommend using an actual error amplifier rather than just a cathode follower for the supply. Unless the 620 V is regulated, you won't have much ripple rejection and the output voltage of your circuit will vary depending on the incoming line voltage.

Obviously, this doesn't answer your question, but I thought I'd point it out.

~Tom
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Old 12th September 2012, 07:45 PM   #3
mogliaa is offline mogliaa  United Kingdom
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Hey Tom,
Thanks, yes I know the rejection of this stage is worse than a simple RC stage as Merlin Blencowe clearly pointed out in his book.
Raw supply is well regulated. I'm trying to reuse what I have here, otherwise I should be looking for a bigger chassis and a different power transformer. I know that the floating screen supply is the best choice in this topology, but that will require an additional transformer and I don;t have the space to fit it in here...

What about an LND150 cascoded CCS feeding a zener string to ensure G2 stays below 250V? This should only kick in when output voltage is below 350V..
I may need to build this and try out whether the sturdy GU-50 have the stones to survive the 600V in triode mode!
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Old 13th September 2012, 12:41 AM   #4
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Hi Ale,
Why go through the frustration of using a regulated HV supply only to have it damaged by a gassy, arcing or shorted tube? I have been testing high power tubes for many years and learned that regulation just isn't needed. The plate supply in the rig pictured below is a very simple LC combination plus a bleeder. Nothing is regulated, but everything is metered. This setup is good for 1500 watts input class A, the limit of my mains. A 1KHz 1v sine wave is amplified for Gm measurment.

In order to have a tube regulated supply supply adjustable between 0 to 600 volts, you will need a variac in the primary of the plate transformer to limit voltage drop and dissapation across the pass tube(s). Usually this is about 150 to 200 volts. The small voltage adjust pot can be mechanically coupled to the variac to rotate with it. This is the way Kepco, Lambda, HP and others did it.
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Old 13th September 2012, 07:43 AM   #5
mogliaa is offline mogliaa  United Kingdom
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Very interesting and wasn't aware of this!
Not sure if I understood your suggestion. From what I read, you are suggesting a variac in the primary, classic LC filtering stage in secondary and the voltage setting pot mechanically coupled with the variac?
Thanks
Ale
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Old 13th September 2012, 09:22 AM   #6
palmas is offline palmas  Portugal
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Quote:
Originally Posted by mogliaa View Post
Very interesting and wasn't aware of this!
Not sure if I understood your suggestion. From what I read, you are suggesting a variac in the primary, classic LC filtering stage in secondary and the voltage setting pot mechanically coupled with the variac?
Thanks
Ale
Thats how I do it. Have a 1250V PSU connected to a high power variac, so I get from 0 to 1250V 500mA, easy and rugged.
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Old 13th September 2012, 04:44 PM   #7
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I think you also need a supply of negative grid bias if you want to be able to cut off the tube fully. Possibly you could use a small second transformer for the required filament voltage and use a tripler on the 15V to get the required grid bias. The variac idea on the primary side of a plate transformer is an excellent idea to limit power dissipation in the pass tube(s) but would require a dedicated transformer providing only plate voltage.
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Old 13th September 2012, 04:47 PM   #8
palmas is offline palmas  Portugal
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Originally Posted by kevinkr View Post
I think you also need a supply of negative grid bias if you want to be able to cut off the tube fully. Possibly you could use a small second transformer for the required filament voltage and use a tripler on the 15V to get the required grid bias.
For that I have another psu (an old Farnell PSU 0-350V with a EL34)
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Old 13th September 2012, 04:51 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by palmas View Post
For that I have another psu (an old Farnell PSU 0-350V with a EL34)
I meant in the B+ supply the OP designed, I also used a separate supply for controlling the grid bias when I tested high voltage transmitting tubes. I used an Isco electro-phoresis supply to provide B+ of up to 1.5kV. Recently gave the set up to a knowledgeable friend as it took up too much space..
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Old 13th September 2012, 06:27 PM   #10
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What about solid state regulation

something like this might work (correct me if im wrong)

Click the image to open in full size.

opamp goes High. transistor turns on. cuts the tube of.
opamp goes low tube conducts fully.

The reference depends on divider ratio but could be powered from a RC Filtered PWM signal omitting a Dac

For a current limiter.
You could use a low side current shunt and compare that voltage against the reference. (could also be a dac) and Pull the inverting input of the first opamp Low. this will make the output high turning the transistor fully On . and cutting Of the pass tube.

Heck if it works you could use this to make LF Sawtooth shapes for a curve tracer. (whit current limiting)


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