Another Use for 6AV5GAs; Pass Tubes in Regulated Power Supply for SE Amps
From FleaBay I bought a mediocre Heathkit PS-3 Power Supply, the predecessor to the much better PS-4 and IP-32. Turns out is wasn't quite up to "mediocre"; the seller didn't remove the tubes, and both 1619s fell out and got their guide pins busted. Plus, the original builder apparently used 1/4" solder and a heavy hand, so every solder joint had big globs of solder (poor kit builders are always a risk with Heathkits, of course).
Never liked the PS-3 design anyway (the pass-tubes are 1619, which are directly heated beam tetrodes with 2.5V/2A (!) filaments; Heathkit likely got a huge stock from WWII surplus), so I decided to rebuild it as an SE or small PP prototype Power Supply using my favorite Black Sheep tube, the Sylvania 6AV5GA.
Kit power supplies of the era (the later Heathkits, plus the EICO 1050, for example) often used 6L6s as pass-tubes, usually the GCs in later models, but I figured somebody had to have used the 6AV5GA, probably triode strapped. as it was (and is) a lot cheaper than the 6L6GC (audio) tube, and was close to ideal as a pass-tube when triode strapped.
Indeed BOTH General Radio AND HP used either the 6AV5GA or its cousin the 6AU5GT as pass tubes. The best implementation was the P/S in the HP 202A Function Generator, which used a 5U4GB + CLC filter, which I preferred.
Tubes were 2 x 6AV5GAs + 6AU6 error amp + 0A2 cold diode ~150V voltage reference tube. I have several dozen 6AU6s, and two 0A2s came with the PS-3, and I have half a dozen well-seasoned 6AV5GAs.
I removed the old 6SJ7 octal error amp and the 6X5 bias rectifier, which left enough room for a Triad CX-14 choke. Reworked the reference voltage divider in the HP circuit, including a 100K 10 turn pot, so the circuit would be centered on 300 volts, and adjustable from about 250-350VDC output, depending on current. Otherwise it's the stock HP circuit, except of course that the Heathkit transformer is 380-0-380 instead of the 475-0-475 HP PST.
I stayed with the 0A2 tube voltage reference because (a) it's a TUBE (duh) and (b) it makes a nice purple glow (see (a)). A zener would likely be more consistent but subject to thermal drift; cold cathode tubes apparently can have slightly different values each time they are started-up, but don't drfit as much with temperature. Besides, Zeners need to be decorated (ie bypassed) with a couple of caps to eliminate the avalanche noise; well, I guess unless you use a string of 6.3 volt ones. Purple Glow Tube it is.
I set the pot exactly mid-span and powered it up with a 2K ohm load; after about 10 seconds, the meter needle swung-up to 300 VDC, spot-on, and stuck there. Current was spot on at 150ma.
Subsequently I've been heating the shop with my little resistor bank at 310V and 160mA just to see how the 6AV5s like the continuous high current. No embarrassing redness after the first hour. This isn't surprising, since I've run Sylvania 6AV5GAs (including these exact tubes) at 80 mA in a Simple SE without problems.
Unfortunately, the PST is barely warm pushing 160mA, so I am obviously a woosie. I need to scrounge-up some more load resistors.
See the photos below.
I completely disassembled the badly-built PS-3, salvaged the sockets and the needed power resistors, cleaned-up the chassis, and rewired. All told, took about ten hours. Only new significant part was a $12 Bourne 10-turn 100K pot (the blue thing on the panel); one turn equals about 10VDC (a little too fine) so I might adjust the voltage divider resistors to make the adjustment more coarse.
The power supply schematic is in the HP 202A manual at www.hparchive.com/Manuals/HP-202A-Manual-SNprefix-037.pdf if anybody is interested.
I figure I'll add a S/S bias voltage supply for fun; S/S seems to be a really good idea for a bias supply; bias turn-on is practically instant, so there's a bias voltage on your grids long before the B+ hits the plates.
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