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Old 12th Dec 2021, 9:20 pm   #1
The General
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Default HP 711A Bench Power Supply

This project started with my having a need for a bench power supply which could deliver up to 500V at a current of ideally up to 200mA while also having a 6.3VAC output for valve heaters. Additionally I needed a unit with a narrow front but deep case profile in order for it to fit more easily on my workbench. Trawling ebay turned up some rack mount monsters at ambitious prices which were too large & heavy for my needs without me replacing some of my storage boxes with a table & also the cat would lose his bed. He wouldn't like that but I guess the power supply would keep him warm. Ideally I'd like to get something like a Lambda 71 but they are all in the USA & shipping 60lbs across the Atlantic undamaged is crazy money. I thought about building one & was considering getting a transformer wound with the necessary HT & heater windings when this HP 711A came up on ebay. OK, only 100mA output but good enough for most purposes. I bid & won it.
The seller had packaged it reasonably well but when uncasing it I found the two 6L6s had worked free from their holders with one having a broken octal spigot. Not a great start but fortunately everything else including the two meters was in good order, the unit clearly having been well looked after. First job is an inspection, one of the rectifier filter electrolytics had oozed some nasty pus, someone had inserted some series/shunt resistors in the two meters - need to check that out later - & there are some black beauty caps that need replacing. The HP manual is available on the internet & so I printed it out. The unit can be wired for 115V or 230V power, the slot in the back panel is large enough for a 13A plug to pass through so I fitted a new UK spec power cable & wired it for 230V.
To be continued...
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Old 12th Dec 2021, 9:24 pm   #2
The General
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Default Re: HP 711A Bench Power Supply

The circuit is pretty straightforward with a couple of nice touches, the transformer HT secondary has three switched voltage taps feeding a full wave voltage doubler. The switches are operated by a camshaft coupled to the output voltage control, switching to a higher voltage tap at each third of the rotation. A 5651 regulator tube provides an 85V reference for a pair of 6U8 triode pentodes, one of which comprises a triode series pass element plus pentode DC amplifier to supply a -190V reference bus; the other 6U8 triode acts as a cathode follower for the 5651 while its pentode drives a pair of 6L6 series pass elements for the main output. A series relay coil pulls in at currents exceeding 150mA, its contacts then apply a negative voltage to the 6L6 grids, shutting them down & acting as a brick wall current limiter.
So on to the refurbishment. Both voltage doubler electrolytics were restuffed as one had leaked some pus & I gently reformed the others which turned out to be OK. I changed the paper black beauty caps for some new 630V polyesters & checked the carbon composition resistors for drift, these were all within spec.
Now I'm not in favour of redesigning or modifying kit for the sake of it (really not), but as this was a power supply I needed to rely on I decided to change the selenium rectifiers for silicon as I'd had past experience of seleniums going 'soft' i.e. high resistance (haven't we all?). So the main rectifiers were changed for 1N5408 1kV 3A diodes & the two small rectifiers for the internal supplies were replaced with 1kV 2A bridges. With the higher performance rectifier in mind I took the opportunity to change the value of the voltage doubler caps from 40uF to 100uF as I restuffed them, adding a 47ohm wirewound in series with the transformer secondary as a current limiter as the transformer secondary was quite low resistance at around 4 ohms.
I have an Avo Mk4 so i tested all the valves, one of the 6U8s had a slight heater-cathode leak so I replaced it. Surprisingly the metal 6L6s tested OK despite their unscheduled mystery tour around the cabinet innards so I glued the broken spigot back on & put them into storage. I had other plans for the series pass elements, I replaced the 6L6s with 5881s which have larger anodes & higher dissipation than standard 6L6s.
So time to switch on & see where we are at; fortunately everything works well, the reference bus is steady at the right voltage, small tweaks were needed to the two tracking adjustments so the output control covers the full 0-500V.
Now the meters needed some attention, they weren't reading properly but when I removed the series & shunt resistors which a previous owner had added to the voltmeter & ammeter respectively, they came into line. So why did someone add them? I really have no idea.
The final job was to check the overcurrent relay, I did this with power off using a separate current limiting supply. There is a bodge on test resistor which sets the current limit, this needed a little tweak to achieve the correct 150mA tripping point. It's the 180ohm resistor which can just be seen on the tagstrip in the photo showing the new rectifiers.
So now it lives on my test gear shelf, it's giving me good service I'm happy to say. Slightly hot rodded but still retaining its essential heart.
Actually, I may just bite the bullet & build myself that higher current supply one day, I fancy it may be a nice little project...
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Last edited by The General; 12th Dec 2021 at 9:40 pm.
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Old 12th Dec 2021, 10:22 pm   #3
Silicon
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Default Re: HP 711A Bench Power Supply

That is a lovely power supply.

Is the transformer happy with a 50Hz supply?

The tall electrolytics are very close to the 5881s.
I would fit some thermal insulation on the side facing the hot valves.

More fuses or current trips would protect that mains transformer from 'accidents' on the heater output.
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Old 14th Dec 2021, 9:01 pm   #4
The General
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Default Re: HP 711A Bench Power Supply

hi Silicon,
Thank you for your comments. The transformer seems fine at 50Hz, it doesn't get hot & it seems to be oversized for the application if anything. Certainly the secondary which has 260V across its full winding has a surprisingly low DC resistance so I guess is wound with relatively heavy gauge wire. The two electrolytics near the 6L6/5881s are in the voltage doubler circuit & are quite close but have cardboard insulating sleeves & don't seem to get particularly warm. The large aluminium can is a 4uF (possibly paper in oil) cap across the output, again this doesn't seem to get warm. There is a lack of fuses, the only one being in the transformer primary but I'm leaving the unit pretty much original apart from my minor hot rodding, it will never be left unattended in any case.
Mark
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