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Old 22nd March 2013, 12:51 AM   #1
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Default Mojo fix

I could tell I was running low on mojo...not getting the shakes yet but my eyeballs feeing a bit hairy. So I set a Bidnip snipe.

You can see somebody over-tightened the input jack and now it's shorted, but he says something else is wrong. Heaters glow and speaker hums. Of course it needs a 3-wire cord and 3-prong plug, and a re-cap job too. Heck, it's only got a dozen parts. The back panel looks pretty crummy; as I understand it this was actually considered a bass amp back then (the guitar version had a tremolo. Can anyone make out the tube numbers from the tube chart?

Vintage Decca Amp Tube Guitar Amplifier DMI 61 Parts or Repair Made in USA | eBay

Last edited by cyclecamper; 22nd March 2013 at 12:55 AM.
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Old 22nd March 2013, 01:13 AM   #2
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Talked to the seller. Tube compliment:
35W4
12AU6
50C5
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Old 22nd March 2013, 01:37 PM   #3
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Quote:
Originally Posted by cyclecamper View Post
Talked to the seller. Tube compliment:
35W4
12AU6
50C5

Will need an isolation transformer on the mains otherwise its death in a box.
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Old 22nd March 2013, 04:23 PM   #4
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I was looking for that in the pics too! I'm hoping there's a power transformer out of sight behind the chassis. Otherwise, indeed, it needs one.
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Old 22nd March 2013, 04:48 PM   #5
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I see the 2-wire line cord (which also needs to be upgraded to 3-wire) come in and one line goes direcctly to what I assume is a rectifier tube pin. The other goes up out of sight, I assume to the control panel power switch....

Oh my gosh, just look at the schematic. Indeed, there's no power transformer. I don't know if it's 'death in a box' but it sure doesn't meet UL standards (or the standards of this forum for that matter). Geeze, even the heaters are just in series with each other and a resistor...

It looks like it just half-wave rectifies the AC line. That will drop the line voltage to "minor heart attack in a box" LOL. That can't be much B+ for a tube amp!

Is the 12AU6 a single pentode? That might sound cool, but even a champ has 2 stages (of a dual triode 12AX7) before the power tube.

So what's the power rating of a 50C5 anyway?

It's looking to me like it would make a lot more sense to rebuild it into a Champ. Keep the chassis and maybe the output transformer. And the cabinet.
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Old 22nd March 2013, 04:54 PM   #6
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Check out the "tone" control in the schematic. It's just a cap (and pot) across the output transformer primary.
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Old 22nd March 2013, 04:57 PM   #7
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Low B+, low impedance speaker. Not sure whether the output transformer will even scale up to a higher B+.
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Old 22nd March 2013, 05:03 PM   #8
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Looking at the "Angela" schematic online, I'll bet the "Angela" single-ended parallel dual 6V6 would be nice in this big box.

Last edited by cyclecamper; 22nd March 2013 at 05:05 PM.
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Old 22nd March 2013, 05:04 PM   #9
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I guess even Mojo addiction CAN kill.
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Old 22nd March 2013, 11:16 PM   #10
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discussion of direct mains connected equipment is against the rules. These devices are very dangerous. It looks like the images on Ebay have been withdrawn. Unless you intend to use this with an Isolation transformer this thread should be closed.
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