Studio Tech’s Corner: Odyssey and Oracle

Thursday, July 12th, 2012

A couple months back, Calvin asked me to take a look at a couple broken amplifiers he had lying around. One was a Peavey Bass Amp with some broken pots; pretty simple. The other was a Fender VibroChamp that kept blowing its fuse. Well actually Calvin didn’t remember what was wrong with it, but I quickly discovered it was blowing its fuse. Little did I know how much work I would end up putting into this little guy.

The vibrochamp has a super simple circuit, and its old enough that there were schematics floating around the interwebs (although they’ve since disappeared, here’s a different one). However, the physical layout is pretty haphazard. It seems to have been manufactured before circuit boards, or even perfboard, and most of the components are soldered point-to-point to some sort of fiberglass or cardboard thing. At least it looks heat-resistant.

Anyway, first thing I did is replace all the electrolytic caps: so that was the can cap at the output of the rectifier and the caps on the cathode of each tube. Electrolytics are want to melt after a couple decades, so it is always good to replace old ones. Alas, no dice, still blew it’s fuse, and started sparking to boot. Feeling discouraged, I let it lie for a while.

When I came back to it, Bob had acquired a tube tester; although we all agree it looks more like something pulled from the cockpit of a spaceship, or maybe a miniature time machine. In-between the Hive Dwellers playing with the dials, I figured out how to use it and tested all the tubes. Lo-and-behold, the 6v6 output tube and one of the 12ax7s were fried. I dug up some replacements, popped them in, and voila, fuse doesn’t blow, amp makes noise, much celebration.

Tube Tester/Time Machine

But I’d been getting a rather high B+ voltage coming from the rectifier, so I started checking plate voltages and they were all crazy hot, well above their ratings. I poked all around the circuit trying to figure out the cause, I checked for shorts, I retouched almost every solder joint, I replaced a few resistors that had drifted significantly, I misread the schematic and replaced a resistor that was just fine and then had to re-replace it, but still the plate voltages were way way high.

Finally, I went all the way back to the transformer and checked the AC voltage on the high tension secondary. The schematic calls for 650VAC, it read…750VAC! Way to high. Huge bummer, transformers tend to be the most expensive part to replace. But still, we ordered a replacement transformer that was advertised as a proper replacement for a Champ. I hook it up and the secondary reads 700VAC. Still high.

Left: Bad Old Transformer, Right: Bad New Transformer

Well at this point, my spirits were pretty low. It seems the options were to a) keep ordering the replacement transformers until one was actually built to spec or b) buy a original champ transformer on ebay; both expensive prospects. Fortunately, I was saved by local amp guru Don Johnson. Don had been fielding my frustrated phonecalls throughout this odyssey and when I told him about my woes with the replacement transformer he started poking around his basement for something that might suit me better. A week later, I stopped by and he handed me a massive grey transformer that he had pulled out of a Newcomb amp chassis (he builds custom heads in them). We gave it a test and it ran out about 675VAC (it was rated for a 115VAC input instead of the now standard 120VAC). Close enough, I say.

But, it wouldn’t mount in the holes for the old transformer. So I drilled some new holes in the chassis and mounted it. Then I spliced extensions to all the leads since they had been cut short and soldered it up. To give the rectifier a little extra voltage drop, Don suggested using a 5r4 tube instead of the called for 5y3, which totally did the trick. The B+ voltages were pretty damn close to those indicated in the schematic and I was feeling great.

Newcomb Transformer and 5r4

I was about to close it up, but I thought I’d check the heater voltage first. The tubes call for a 6.3VAC heater voltage and they got…6.8VAC, 120v wall power strikes again. After consulting with Don once more I put a 1/2ohm 3w dropping resistor between the secondary and the tubes, dropping it down to a reasonable 6.2VAC.

It’s the little green thing

So the Champ is back! And will hopefully be appearing in recordings here in the near future.

About a week ago I was watching Beat Happening videos on the youtube and what do I see? The very same Champ that electrocuted me 15 times, rocking Black Candy live on TCTV in 1988.

It seems to have been stabbed slightly less back then.

5 responses to “Studio Tech’s Corner: Odyssey and Oracle”

  1. Amy says:

    Awesome! Don’t understand a word.

  2. Jim Gray says:

    Reposting commentary from Facebook, as this seems like the better froum for the discussion.

    John Mcmurtry I would have left the transformer alone; what’s a little big voltage amongst friends?
    Jim Gray ‎650VAC is a scary big voltage and best not to share with anyone.
    Jim Gray ‎100V here, a 100V there pretty soon your talking about some real voltage.
    John Mcmurtry Not to get too technical; but it’s actually 650 volts peak to peak or 325 volts center tapped, which is a pretty common voltage in tube amps. Some of my amps run much higher. The amp in question may have a short in the primary windings of the power transformer, or possibly a shorted rectifier tube.
    Jim Gray I like technical. I can see that if the primary winding was shorted it would increase the turns ratio of the secondary to the primary and boost the voltage. Wouldn’t a shorted rectifier tube blow the fuse?
    Amy Levinson Gray What the f are you two talking about?

    John Mcmurtry That’s why I said I would leave the transformer alone; there was nothing wrong with it…
    John Mcmurtry As for the rectifier tube it depends upon which elements

    • Jim Gray says:

      I am just curious as to how there would be a higher voltage. If the rectifier element fails open it will will look like a half wave rectifier and the voltage will be lower, if the rectifier element fails short it will blow the fuse.

  3. Max says:

    I think that the secondary voltage measured high for two reasons: the transformer is probably rated for 110/115 on the primary, and the voltage probably sags significantly when the amp is driven. Old amps tend to run the tubes really hard, and the old tubes would take it, but for the sake of compatibility with new production tubes the steps you took were prudent. It is likely you could have skipped the transformer replacement and just run it as is with the voltages a little high. As a rule you can almost always get away with a 10% variation in voltage in a tube amp with no trouble at all.

  4. Kate says:

    Nice work repairing a little sliver of history! Hope there is a long life ahead for this amp.

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