6GV8 / ECL85 based 250 volt tube regulator PCB
Lately I've been on a linestage kick, and it seems I start every project by designing an appropriate power supply. Most end up in the 200 - 300v range. It seemed to make more sense to develop a small power supply module that could be recycled into each project.
About 2 years ago I purchased a PC board off of Ebay that was a clone of the Marantz 7c linestage portion. While the linestage was reasonable, it contained a really neat regulator circuit based on an obscure 6F3 tube that I've never seen. It turned out that this was similar to the very common ECL85/6GV8 9 pin triode/pentode. Both the ECL85 and it's higher filament voltage cousin the PCL85 are dirt cheap and easy to locate.
I've taken the basic schematic from that regulator, and made some simple changes to allow for a more flexible design. I've been very pleased with the performance on the work bench. It regulates tightly over 0-40ma of current draw.
Some construction notes:
D1 75v/500mw zener diode
D2 62v/500mw zener diode
D3 Diode - Bridge, 1.5 amp / 1000v (GI/Vishay W10G, DigiKey W10GGI-ND)
C1 100uf/400v radial electrolytic cap
C2 100uf/400v radial electrolytic cap
C3 47uf/400v radial electrolytic cap
C4 0.1uf/400v film cap
C5 0.1uf/400v film cap
C6 0.1uf/400v film cap
R1 470 ohm 3 watt resistor
R2 100 ohm 1/2 watt resistor
R3 200K ohm 10 turn potentiometer Bourns series 3009P
R4 100K ohm 1/2 watt resistor
R5 150K ohm 1/2 watt resistor
R6 100K ohm 1/2 watt resistor
R7 82K ohm 1/2 watt resistor
R8 (optional bleeder resistor, nominally 470K ohm 1/2 watt)
V1 6GV8/ECL85 - ok to use higher voltage versions such as 9GV8 or 18GV8, but adjust filament voltage accordingly.
Misc parts - 9 pin PC mount socket, 4 corner standoffs, 3 each 2 pin connectors for inputs and output
For a target voltage of 250vdc:
1) Use a B+ transformer of 225-250vac. The Hammond 369AX 40VA, 125-0-125 @ DC ma 100, 6.3VCT @ 2A. works nicely with this circuit, just ignore the center tap of the B+.
2) Place two zener diodes in series for a total of around 135v (75v and 62v in series work well).
3) C1, C2 and C3 need voltage ratings approximately 1.5x the input AC voltage, so 400v or 450v parts will work nicely.
4) Adjust R3 for target voltage, use an insulated screwdriver for safety.
1) Exercise common sense and caution, this circuit involves significant voltages. Make sure the PC board is electrically isolated from the chassis and user. Do not build or use this circuit unless you have the necessary experience and expertise with high voltage circuits.
2) This circuit can supply sufficient B+ for most preamplifier circuits and regulates nicely with current draws between 0 and 40ma.
3) A small filter choke can be substituted for R1.
4) A voltage regulator tube such as 0C3 could in theory be substituted for the Zener diode reference and mounted off of the board. I have not yet personally tried this change.
5) Fuse the primary of the transformer appropriately - a 1 amp fast blow fuse will typically be reasonable.
My initial PC board had a single error, hence the jumper at the lower edge.
If there is sufficient interest I'd consider having a run of PC boards made up. Let me know!
Schematic file as a PDF
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