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Voice of Music 722 reel to reel conversion

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  • Voice of Music 722 reel to reel conversion

    I bought a Voice of Music 722 reel-to-reel late last year for the purpose of recycling the transformers and tubes and making a Champ-ish amp. I've repaired before, but this is my first build. The donor used a single 6L6GB and I chose to follow the non-tremolo schematic for the AA764. I tested the donor with a guitar plugged in before disassembly. It worked and didn't make any funny noises or smells. I did continuity tests on the PT before installing and everything looked good. After firing it up with no tubes, I found a little over 5VAC at the rectifier heaters. I measured 396v at the high voltage leads at the rectifier socket. My research showed that was correct for this PT. It's definitely over-sized for the application.

    When I plugged in the 5Y3 rectifier tube, I had B+ but it was still in the 390's. I expected that to increase slightly. I installed the tubes, attached a speaker and took some quick voltage readings. I saw 533v on pin 3 of the 6L6 and 529v at pin 1 on the 12ax7. Yeow! I powered up momentarily to get these numbers. The can cap segments had these numbers as well. So everything looked okay until I plugged in the preamp and power tubes. I'm guessing the 500v cap is not well at this point. I was quick with my readings and no smoke or smell present. I did, however, get the beginnings of a squeal before i shut the power down on the last reading. I put it away for a few days and did some research.

    A second look at the 5Y3 found a rattle inside. Cooked by me or pre-existing problem- I'm just not sure. I installed another one. I stepped the voltage down before the first cap with a 5w 250 ohm resistor, which is apparently common for this conversion. This morning started with 460v of B+ and now adding the resistor gives me 434v at the first cap. I used the AA764 schematic-specified resistors between the two remaining caps. 1K and 10K respectively. The rest of the tubes installed and a speaker connected, I get 431v at the second cap and the third cap is at 428v. I expected larger drops than this. Quick readings show 446 at pin 3 of the 6L6 and 418 at pin 1 of the 12ax7. I checked resistance between cap 3 and pin 1 on V1 and I see the 100K of resistance the AA764 schematic shows me. Pin 6 on V1 is showing 430v. I'm seeing similar results on the 6L6 - 446v on pin 3 and 441v on pin 4. Could these readings be a result of damage to the can cap on first start? On another note, I am using the VoM OT. The black secondary is labeled 32ohm on the schematic and was attached to the cathode of the 6L6 on the donor. I found info online from others who have done this conversion. They used the yellow 8ohm secondary and the green ground wire for the output jack. I taped off that black wire according to what I read. Now I have doubts, but I'm don't see how that would be contributing to the high voltage issue. I have another resistor to step down the voltage another 30v or so. I've compared my build to the AA764 schematic and can't see why this is happening. Does anyone have thoughts on where I should look next?

  • #2
    Well, after spending a considerable amount of time on this over the weekend, a light bulb came on. Not one over my head, rather the one on the current-limiting device I had built for this project. Because the 150w bulb never came on at all when I first turned on the amp, I figured there was a problem with the bulb and since I was seeing high voltage numbers, I began to think it just wasn't working at all. "current". I have a switched outlet before everything that is used to take quick voltage readings and I just forgot about the limiter. That is, until I left my DMM on the capacitor test setting accidentally and that bulb lit up. So after feeling like an idiot, I looked on the bright side and realized I didn't have a short. Plugged in directly, pin 1, V1 shows 308v. Still not there, but half way there and I know the big PT needs a step down to close that gap. The Xicon ceramic resistor I installed today is coming out. Zener on the HT center tap? Researching now.