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Old 21st June 2006, 06:21 AM   #1
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Default Bench PSU for tubes

Hi all!
Im looking for a suitable benchpowersupply for tubes, HT should be variable from, say, 90V to 400V and a bunch milliamps, enough to drive like an EL34 or more..

I guess that HT-transistors from old TV-sets could be used with success, i'd prefer transistors as regulators.

i was browsing the web with no luck..any ideas?!
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Old 21st June 2006, 07:08 AM   #2
rdf is offline rdf  Canada
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Scour E-bay for electrophoresis units, such as the Helena Laboratories Titan Plus. Still in current production and I picked mine up for ~$50 USD. Dual outputs (one adjust though), digital display switchable for either output current and common voltage, from memory good for 800 Vdc at 300 ma. Don't take that last one to the bank but it's in the neighbourhood.
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Old 21st June 2006, 08:56 AM   #3
EC8010 is offline EC8010  United Kingdom
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If you're forced to make your own, you need to gang a variac and a potentiometer onto the same shaft. The variac is used to control the AC going into the transformer feeding the regulator and the potentiometer controls the regulator output voltage. In this way, the dissipation of the regulator doesn't become ludicrous at low voltage/high current.
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Old 21st June 2006, 09:19 AM   #4
ilimzn is offline ilimzn  Croatia
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Also, use HV MOSFETs for pass transistors, MUCH less concern about SOA. These can commonly be found in switching power supplies for various equipment. Look for the 800-900V max variety.
If you don't want to use a variac arrangement, you can also try doing the regulation in two ranges - by speccing the transformer for half the secondary voltage, and then using a switchable voltage doubler arrangement. This circuit is very simple and appears in PC PSU units connected with the 115-230VAC select switch
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Old 21st June 2006, 09:40 AM   #5
corbato is offline corbato  India
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I made a simple solution for myself.

I got a transformer wound with tapping at each 50 volts from 0 to 450volts at 350mA with a SS bridge rectifcation. Same transformer has a 6.3v/6A filament winding and an adjustable -100volt/100mA bias winding. Everything is housed in an old UPS box with a front mounted voltage selector switch and terminal posts. Metering is with external DMM.

Not really helpful if you need a very precise, specific voltage but suitable nevertheless for a host of projects.
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Old 21st June 2006, 10:57 AM   #6
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Another idea to keep costs & dissipation low is to use several cheap 100VA isolation PTs.

Build several separate fixed stabilized voltage PSU units with them, then "stack" those units on each other to one unit below the desired voltage, and stack another variable regulated PSU on top for fine voltage adjust.

Be aware that safety relies on the isolation of each unit from the mains! Be sure you know what you are doing!

Neverteless, I often use this method on the bench, from zero up to 4 units stacked as needed, and a variable Gossen/Metrawatt 0-160V/300mA PSU unit on top.

Attached the circuit for one of the fixed & stabilized voltage PSU units.

The MPSA92 + surrounding parts forms a temp-compd CCS for the Zener string (very good voltage stability, low temp drift). B546A, R4 and R5 form a current limiter.

Tom
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Old 21st June 2006, 12:35 PM   #7
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I use a little Heath IP32 regulator for preamp benchwork - for amps I have a massive HP 500V/200mA regulator. Got both of them on E-scam for a pretty good price - but shipping was killer on the HP!
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Old 21st June 2006, 01:27 PM   #8
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I use several bench supplies. For most amplifier development work I use an old Knight tube regulated supply it goes from 0 to 400 volts at 200 mA. There were thousands of these built in the 50's and 60's by dozens of different companies. They can often be found at flea markets and hamfests for cheap. I got 3 different ones for under $25 each. They can be found on Ebay but they go for over $100.

For big projects I use a Fluke 407D. This is big, heavy, and puts out up to 555 volts at up to 300 mA. It uses 4 807's for pass tubes. I got mine on Ebay for $25 and $50 for shipping!

I have a home made power supply that is nothing more than an industrial control transformer that is fed by a Variac. I have a bridge rectifier and CLC filter on the output. Voltage range is 0 to 600 volts at 1/2 Amp. Yes this is unregulated, but it is representative of the power supply in most amplifiers. A standard 6.3 volt transformer is used for filaments.

Industrial control transformers can often found surplus for very low cost. They come in sizes from 50VA to over 1KVA. I use the 500 KVA and 1KVA sizes. They come in several voltages, you want the one that has a 240 / 480 volt primary and a 120 / 240 volt secondary. Turn it around, feed the secondary with your mains voltage, 120 or 240 using the appropriate connections. Connect the primary for 480 volts, and use it as the secondary. You get 480 VAC at lots of current. Attach rectifier and filter circuit. 600 volt versions exist but they are rare.

I use this method to get 600 volts DC for killer P-P amps. I also have one of these transformers feeding a voltage doubler circuit using two 5AR4's in my 845 amp I get 1100 volts.
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Old 21st June 2006, 06:38 PM   #9
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Isco 464 electrophoresis supply - upto 600V @ 300ma, up to 2KV @ 90ma, no filament or bias. Meters both volts and amps, can current limit, voltage limit or wattage limit. Krohn-Hite UHR-220 tubed supply - 500V @ 2000ma, plus filament and adjustable bias. Both tough as nails, and both found cheaply on fleabay.

Both are tougher, safer and cheaper than anthing I would build myself. Lots of good choices if you are patient.
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Old 21st June 2006, 09:48 PM   #10
rdf is offline rdf  Canada
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Isco 464 or 494? Have to agree with the Isco recommendation, the power limit set alone is worth is a godsend. 50 watt plat diss. max.? Set it to 40 and forget about it. The one downside is lack of repair info, I want to rebuild mine but haven't had any luck finding a manual or schematic.
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