Wednesday, June 12, 2013


     It's been awhile since I traced the circuit and got it working:

     Naturally, it was not working when I took it down from the shelf.  Time to get out the signal tracer.
     At some point before I got it, the front panel was covered in wood-grain laminate.  I'm thinking a nice coat of plastic-friendly matte black would do it a world of good.  And perhaps some RCA-type knobs that had not been painted, too.

     (A word about the '2" Cannon' notation for the input connectors: that's what they are, great big old-fashioned microphone sockets.  A mic-connector roundup post is in the works to provide examples.)


  1. Still something clean and elegant about tube circuitry. Any photos of the wiring?

  2. Boy, that is one tidy schematic!

  3. Ah, an O pad!

    Used some of those in a theater sound installation once, just to confuse people. ;)

    Any idea what was intended with the terminal strip? I suppose it could have been accessory power or outboard metering, but I'm bothered by the (potential) ability to kill the filaments with the B+ still high.

  4. It's not a terminal strip, it's a 12-contact Jones plug with the pins in a 3x4 matrix. I decided a long time ago that life was too short to attempt semi-pictorial symbols for high-count, non-linear connectors that have numbered or lettered contacts.

    The original scheme appears to have been intended to allow changing from AC to battery operation by changing the cable, thus the two-pole switch with all the contacts wired to the plug, the B+ and ground connections and the jumper between the heater winding and the heaters -- though why the original builder bothered to light up the 6X4 in battery mode is a mystery to me. Subsequent owners jumpered the plug side. I need to clip the ones on the plug side and add the 1 - 3 and 4 - 6 jumpers to the socket (line cord) side.

    O-pads have several virtues, among them not needing an extra set of terminals in the middle like an H pad; sometime you can even just string one from one piece of equipment to the next. I think the resistor values end up closer to 10% and 1% resistor values, too. Plus, for higher values of attenuation, you can fake it -- 620R resistors at input and output and whatever 5k+ needed to make it work out for the series legs.

    Gewehr98, thank you! That's the second version. I may post the drawn-as-traced one for contrast. 20 years on, about all I would change would be to have the horizontal lines make the hop at the two (2) crossing points -- and don't think I'm not proud of getting it down to only two!

    eiaftinfo: Possibly. Mostly not my work and I don't recall much about it.

  5. Roberta, love your work!
    Minor quibble: The 6X4 heater is NOT powered on battery according to the schematic. It has its' own transformer winding separate from the "working" tubes.
    Dumb question: What's the purpose of the pad on the output? Impedance matching?

    Harvey, WB5MCT

  6. You're right about the 6X4 -- and to think, I drew the schematic and had it in front of me when I typed that! ::blush::

    The pad on the output is impedance matching and, in the case of a "dry pair" connection, helps equalize level when feeding cue audio or talkback down the line from the studio to the remote. (Without the pad, they'll be louder in their own headphones than what you feed to them from the far end. Yes, they could turn their mics down, but they usually don't)

    --I worked at a station that was still doing that, very early in my career. During halftime and before the game, you could listen to the comments made by the play-by-play and color-commentary talent, which I don't think they ever grasped. My, how they would have blushed! (Three-position key switch on the board: down put the remote into a mixer, middle was talkback, them routed into the cue speaker unless you pushed the TALK button, up fed program audio down the line.) The Z-matching trick here is that the effectiveness of a loading coils on the phone line depends on you feeding it from a good, low-reactance source and terminating it in a good load. The pad improves the match by the value of the pad. 4 dB, not too big a price to pay.