Mod Identification. 1985ish JCM800 2203

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spin
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Mod Identification. 1985ish JCM800 2203

Post by spin »

I was hoping someone with more knowledge would be able to tell me what's going on here. A friend gave me this amp to look at, says it's real quiet (one tube looks burnt). I haven't fired it up yet, I always like to open it up and see what's been done before applying power. I asked what mod was done and he said he did not know, he'd just been using it as a second master volume. It doesn't look like the examples of PPIV I've been able to find. It looks the most similar to this: http://site.triodestore.com/MASTERVOLUM ... lume-1.pdf, but not exactly. Any recommendations would be greatly appreciated.


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Re: Mod Identification. 1985ish JCM800 2203

Post by JMPGuitars »

Ignore the resistance values, and look at this: storage/18wattMV.gif
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Re: Mod Identification. 1985ish JCM800 2203

Post by spin »

JMPGuitars wrote:
Sat 11/09/19 2:53 pm
Ignore the resistance values, and look at this: storage/18wattMV.gif
That makes sense as a volume control, voltage divider to ground. In what I have, the bias feed resistors are variable and omitted at full rotation. I've heard of lowering them down to 100k, but not to zero. I'm just not following what's going on here. The owner doesn't need bedroom volume, so I may just revert this back to factory and call it done.
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Re: Mod Identification. 1985ish JCM800 2203

Post by JMPGuitars »

Assuming I read your drawing correctly, you are correct, that you can drop it all the way to 0 which would mute the amp. It's a really simple way to add a MV. How useful it is, is a matter of opinion. I generally prefer attenuators over MV knobs in basic circuits, but I did use a similar MV in a Lite 2b build, and it sounds great.
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Re: Mod Identification. 1985ish JCM800 2203

Post by spin »

Yeah, seems I just wasn't understanding how it was working, I get it now. Found a few more examples as well. Looks like it is recommended to install a 2.2M on the pot (wiper to bias) as well to bring the 250k pot back to normal 220k and to protect against wiper fail. Fired the amp up at 100V on the dim bulb and gave it a once through, voltages look fine. Played a few chords, no issues. Still not sold on leaving the mod in place, shielded cable was not used to implement as it's recommended in what I've read.

Thanks for the info, Josh!
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