I love mystery chassis!

[KD1SH]

   I picked up this beauty at a local hamfest last week. I was free and contained some useful parts, so I couldn't resist. Though it wounds my heart, I'll surely wind up parting it out or totally reworking it, since it's probably not much use to me in its present configuration.
   Before going that route, though, it's often fun to try and figure out what this thing was for, and maybe who built it. It's obviously a power supply, of course. The Thordarson power transformer is rated at 435V on either side of the center tap, at 250ma, and the two Thordarson chokes are 24/13 (swinging) Henries at 250ma and 12 Henries (fixed) at 150ma. The 5U4 rectifier is no mystery, and the 0C3 regulator tube, as well, but the 6SJ7, 2A3, and 45 are tubes I'd normally expect to find in audio gear. Yet, it's clearly not an audio device since it only has outputs for high and low DC voltages, bias ( the bias jack is hidden under the ground stud in the picture), and inputs for AC line and a relay control line. Maybe those tubes are used for a somewhat sophisticated regulation scheme? Adjustable, maybe? (There's a pot on the rear of the chassis.)
   Check out the construction! I'm not convinced that a ham built this. If a ham did build it, he was very professional and meticulous. The laced wiring bundles are reminiscent of military work, or at the very least, high-quality commercial gear. The holes for the tube sockets and such were neatly made with a punch, and everything is laid out precisely, suggesting commercial production. It was obviously made to interface with a rack of other gear, since it has no on/off switch, indicator lights, or metering.
   Mystery boxes and chassis are some of my favorite hamfest finds!

[Pete, WA2CWA]

How many hams have the ability to emboss the tube numbers directly into the chassis?

Thordarson had designs in the back of their transformer catalogs and issued separate audio and amateur radio transmitter designs in separate documents over the years.

As far as the use of "audio tubes" in what looks like a power supply design, Heathkit used 6L6's in several of their kit power supplies.

[KD1SH]

How many hams have the ability to emboss the tube numbers directly into the chassis?

I suppose they might look embossed in the picture, but they're actually ink or paint; probably rubber-stamped or stenciled. Still somewhat unusual for a ham-build.

Thordarson had designs in the back of their transformer catalogs and issued separate audio and amateur radio transmitter designs in separate documents over the years.

I've got PDF's of some of those Thordarson catalogs and guides; I'm going to look through them to see if any of the circuits look like they might be relevant to this chassis.
 

As far as the use of "audio tubes" in what looks like a power supply design, Heathkit used 6L6's in several of their kit power supplies.

Yes indeed, I've got a Heathkit power supply that uses two 6L6's in parallel as series-pass elements and, I think, a 6BH6 pentode as an error amplifier.

[Pete, WA2CWA]

Back in the day, techs within the various telephone companies, WECo, and Bell Labs had printing tools which consisted of wooden sticks and at one end, a rubberized letter, number, and maybe a few punctuation symbols. There was also a thick inkish-tube of black ink. This stuff was used to stamp metal frames, chassis's, metal cabinets, etc. A steady hand generally was required; no drinking prior to doing an application.

[W1DAN]

Hi:

I wonder if this was a re-purposed audio amplifier from the 1940s? Note the covered holes that probably once held tube sockets.

Very nice wiring!

Dan

[KD1SH]

   That's a thought. However, there are no other holes in the chassis for audio inputs or outputs. I also figured that maybe those covered tube socket holes reflect the fact that the same chassis might have been used in several models of the manufacturer's products.
   To take a wild guess, I'm thinking that the two 2A3 triodes are connected as parallel series-pass elements for the low-voltage supply ("reg" is marked over the low-voltage millen) with the 6SJ7 pentode as an error amplifier; the 45 triode as the series-pass for the bias supply with the 0C3  
setting a stable reference. The high voltage probably comes right off the filter caps with no regulation.
   Or something like that; just letting my mind wander amongst the possibilities. It's certainly not worth the trouble of ringing out all the wiring and drawing up a schematic just to satisfy my curiosity. I'm curious, but not that curious. It pains me to cut up all that skillfully done wiring, but this thing will most likely live a second life as a handy bench supply, with the rectification and regulation redone with sand-state components. I spared it a certain scrap-bin fate, so I'm happy.

Hi:

I wonder if this was a re-purposed audio amplifier from the 1940s? Note the covered holes that probably once held tube sockets.

Very nice wiring!

Dan

[W3SLK]

That whole chassis looks to have been repurposed. There are additional plates that were made to accomodate the Thordarson transformers. I like the tie wrapping done. That is evident of art from a bygone age. Using 2A3 as regulators? What a waste of a fine audio tube! . Those who were elmers to us knew how to save a buck by reusing chassis that once held a different duty altogether.

[KD1SH]

   Indeed, those 2A3's are prized by the audio guys these days. I'd almost thought of restoring the full tube complement on that chassis, so I could power it up and see what it does, and then I saw what those tubes are going for these days.
   My wife used to do that sort of cable lacing back when we worked together at an aerospace company in the 80's. I haven't seen the inside of any modern aerospace equipment lately, so I don't know if it's still done. It was time-intensive and required skill, but it was an excellent way to secure wiring harnesses in equipment required to survive in harsh environments, including vibration.
   The only problem was troubleshooting; the (mostly) girls in the cord-room would yell at the techs, "Don't you dare cut up my lacing!"

That whole chassis looks to have been repurposed. There are additional plates that were made to accomodate the Thordarson transformers. I like the tie wrapping done. That is evident of art from a bygone age. Using 2A3 as regulators? What a waste of a fine audio tube! . Those who were elmers to us knew how to save a buck by reusing chassis that once held a different duty altogether.

[Opcom]

It's interesting that the 2A3s are inserted in opposite directions.

It's interesting that two oil caps in series are used for connecting the presumed anode and a presumed filament pin, of the two 2A3s. two oil caps in series on 2A3s seems overkill or did I miss a wire at that midpoint?

I vote for a small cathode or grid modulator, and/or a bias pack.