What did Mark A. do to this JCM 800?

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Ronniejackjones
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What did Mark A. do to this JCM 800?

Post by Ronniejackjones » Fri Feb 02, 2018 12:29 am

*Sorry for posting twice; saw this section after posting the other-thought this one more appropriate*

Hi Folks: Can you tell me precisely what modifications were performed on this JCM 800 50w? Sounds especially fantastic and I would like to know why! Can make out some of the specs on what I think are the replacement parts, but help to that end would be swell. Regards. 
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Elad E
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Re: What did Mark A. do to this JCM 800?

Post by Elad E » Sat Feb 03, 2018 3:34 am

Not a 2204 expert but I see a Lar/Mar PPIMV installed, the .1uf Mallory caps also look like a retrofit and I can't see the NFB resistor (R24) but it doesn't seem like a 100k and is connected to the speaker jack instead of the 4ohm tap on the impedance selector. R12 bypass cap looks like a retrofit too.

Also not sure about the all the jumpers (R6-R8) and the two Metal Oxide dropping resistors - values are right but their installation seems a little a bit ad-hoc.

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neikeel
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Re: What did Mark A. do to this JCM 800?

Post by neikeel » Sat Feb 03, 2018 10:58 am

Your pics are missing the back panel but looks like a post phase inverter master volume has been added.
There has been a small electrolytic bypass cap soldered across the 820R cathode resistor of v2a. There will be a boost in certain frequencies that will depend on the value of the cap. A smaller (0.47uF) will be upper mid focussed and big cap (330uF) passes a very wide spectrum (often ‘v. muddy’ in my opinion)
From the look of the input jacks it looks like there has been cascade of the two triodes of V1 but wiring under the board.
Do both pairs of input jacks work normally and both pots or does one work as a pre-pi mv?
Now that I know that is was Marks work all the mods fit in place.
Neil

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Re: What did Mark A. do to this JCM 800?

Post by Ronniejackjones » Sat Feb 03, 2018 12:14 pm

neikeel wrote:
Sat Feb 03, 2018 10:58 am
Your pics are missing the back panel but looks like a post phase inverter master volume has been added.
There has been a small electrolytic bypass cap soldered across the 820R cathode resistor of v2a. There will be a boost in certain frequencies that will depend on the value of the cap. A smaller (0.47uF) will be upper mid focussed and big cap (330uF) passes a very wide spectrum (often ‘v. muddy’ in my opinion)
From the look of the input jacks it looks like there has been cascade of the two triodes of V1 but wiring under the board.
Do both pairs of input jacks work normally and both pots or does one work as a pre-pi mv?
Now that I know that is was Marks work all the mods fit in place.
Neikeel: I greatly appreciate you taking time to help me w/ this. It seems everything you suggested is in keeping w/ Mark's common mods. A few things still have me scratching my head: what do you think those yellow capacitors are on C2? That a silver mica wired to them? All the jumpering has me confused as well. I am trying to collect as much info as possible for my tech who will be doing the work. I am using this as the foundation and then adjusting thereafter. Best regards.

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Re: What did Mark A. do to this JCM 800?

Post by neikeel » Sat Feb 03, 2018 4:36 pm

I am afraid the pics of the preamp are a bit 2-D but essentially the pre amp is set up as a 2203/4 (so cascaded V1 triodes with a treble peaker (500pF/470k) between stages. Mark seemed to find that dropping the bypass cap on what were the mixers (470k with cap and 470k to ground) to 100pF IIRC (vs 470pF as stock. Laney used 250pF on their plexi-likes.
Neil

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