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 Need an Odd Low Current Voltage?

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FrankB
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FrankB

Join date : 2010-11-22

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PostSubject: Need an Odd Low Current Voltage?   Need an Odd Low Current Voltage? Icon_minitimeFebruary 17th 2019, 10:28 pm

You can easily get low current, odd voltages by simply plugging an old octal socket into your tube tester, setting the switches to positions for a known tube pinout, and adding a few adjustable voltage regulators to the socket.

For instance:
 Need 22.5V bias to test an old radio? Yup. It will work just fine. Set fil selector to 25V, (Setup for 25 Volt fil tube- like 25Z5, 6, W4, etc. tube), add a bridge and filters, the VRIC, set it and you have the voltage.

 DO NOT use this method for more than a few minutes, as tube testers were designed to be  intermittently used.

 But it can help you determine if a piece of equipment is useable.
 This method can be used to save you the money on very expensive and hard to find  batteries to see if the equipment is worth the expense. Yeah, its bulky and cumbersome, but it works.

If you have a really quality tester, you can find out what the different bias voltages are for tubes the set provides and use them, if they are what you need.

Of course, you can simply buy a busted up tube tester, pull the transformer out of it, and build they entire adjustable setup into a nice case, with different voltage outputs either fixed or variable. I would not recommend any long usage of the voltages, as the transformers were not designed to continuously be run, but for a quick test, should work just great.
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Rod Clay
Senior Member 75+ Posts
Senior Member 75+ Posts


Join date : 2018-08-01

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PostSubject: Re: Need an Odd Low Current Voltage?   Need an Odd Low Current Voltage? Icon_minitimeMarch 20th 2019, 3:13 pm

Here is another source for a bench power supply with various output voltages: Homebrew LV and HV power supplies from hamfests. These were often built up to be rack panel mounted. I picked up a couple of these at our Lincoln hamfest. One was complete with panel and the other was just a chassis with only the HV oil caps remaining and a few other parts. The iron was all gone as well as the 866A rectifiers. Even the panel light bulbs were gone!  

The other chassis/panel is complete with a 5Z3 LV rectifier and 1970s era solid state 866A replacements. Just recently I got this one cleaned up and checked out. No front panel controls or pilot lamps on this one. Everything is on the back lip of the chassis. Separate power cords for HV and LV. I disconnected the HV power cord for now so I won't plug it in by accident. Not sure of the HV available here as I can't make out the markings on the Inca HV plate transformer. There are (2) sets of taps for the HV. The HV oil filter cap voltage rating would be a clue but its paper label has come off and long gone now. I will measure its capacitance later-that and the physical size is a clue-also the voltage rating can be estimated by the size of the terminal insulators on the top of the can. Also the builder had used a filament transformer for the 866A tubes that was not rated high enough (most have a 10,000V insulation rating) so I disconnected it as it was not needed with the replacement rectifiers anyway.

Well, I fired up the LV side and took a voltage few readings with a DMM after having done some Ohm meter checks of the wiring, etc.. The unloaded DC voltage was rather high at about 450 volts and that is just with a 5H filter choke in the circuit and no filter caps as yet. I plan to use the LV side with a variac to vary the output voltage as needed. I may have to replace the 5Z3 with a solid state rectifier as the filament winding is also on the LV power transformer. But I just remembered that there is separate 5V filament transformer on the chassis that I could rewire to power the 5Z3 filament. 

Gotta go to church now to help out with our "Dinner at Six".
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FrankB
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Join date : 2010-11-22

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PostSubject: Re: Need an Odd Low Current Voltage?   Need an Odd Low Current Voltage? Icon_minitimeMarch 20th 2019, 5:22 pm

Remember replacing the 5Z3 with a solid state rect will increase the output voltage.
But you can rub a filament transformer in buck/boost config in the primary to drop the voltage instead of adding a huge power resistor.

Those filters can knock you across the room, even after a week. I had some and tested that by arcing them across for up to a week. nice snap, but diminished as time went by.
 If it's (cap) not labeled voltage wise, I'd replace it for safety & be sure its all fused.
 Somewhere I have a bunch of 866 SS repl, and maybe some of the 872 tubes (Maybe 6-10) & solid state stored away, if you end up needing any.
 The 866 & 872 tubes are mercury vapor rect and should have the filaments on for 30 minutes first time, and 15 min.2nd time before firing them off. I replaced dozens of them in vinyl welding machines, where the workers wouldn't wait till they had fully warmed up & the mercury vaporized.
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Rod Clay
Senior Member 75+ Posts
Senior Member 75+ Posts


Join date : 2018-08-01

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PostSubject: Re: Need an Odd Low Current Voltage?   Need an Odd Low Current Voltage? Icon_minitimeMarch 20th 2019, 10:39 pm

Yes. A dropping resistor may be needed. I am hoping to get 400 volts out of the LV supply with a choke input filter. The builder had a pair of 40uF 450V caps wired in series for the filters. This suggests the B+ was running in excess of 500 volts with a capacitive input filter. 

There are no bleeder resistors present in this power supply so I will have to come up with one for the LV side to start with. I figure a 20K Ohm unit rated at 20 Watts will do. 75K to 100K Ohm resistors rated at 100 Watts or higher is common for HV supplies of 2000 Volts.

I did some checking this afternoon and found the HV filter cap has a capacity of 2UF. The physical size of it plus the insulators on the terminals suggest it has a 2000V working voltage. The cap checks good for quality and for leakage up to 500 volts-the limit of my tester. There is a 20-5 Henry, 400 mA swinging choke present as part of the HV filter circuit. This choke is suitable for 2000V working Voltage.

I want to mention that the 5 volt filament transformer was used by the builder to light the 866A filaments as they were wired in series-a somewhat unusual arrangement. Adding variac control to the LV supply power transformer now seems to be getting a bit complicated. I hope to get this supply ready for use soon.

73, Rod  WB6FBF
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FrankB
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Join date : 2010-11-22

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PostSubject: Re: Need an Odd Low Current Voltage?   Need an Odd Low Current Voltage? Icon_minitimeMarch 21st 2019, 6:21 pm

I know that last I looked RF Parts, https://www.rfparts.com, had filter caps in the 600V range. 
I ordered from them both when I was working on cable company equipment and  on old radios, as some of the earlier radios even used 800V filters in the P.S. in their receivers.
 If you have to series filters, please don't forget the equalizing resistors across them. (For the uninitiated, these are NOT bleeder resistors in the normal sense). Sometimes they added a ceramic cap. across the filters too, in the range (typ.) of .001-.01 mfd.
 IIRC, this was to kill any RF oscillations in the P.S.

It's amazing to think that many early radios used from 1 mfd to 4 mfd as the main filter caps, and I remember repairing an old Transformer Company of America aka TCA, radio that used nothing smaller than 1 and 2 mfd caps as coupling caps in the radio.
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