Levi #34 spec for JCM800..anyone tried this take?

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Kapo_Polenton

Kapo_Polenton

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http://www.mediafire.com/download_repai ... oc0fvfk8h7

Nice little document which goes through the mods without drilling holes.. I like that. I ran what i thought was the #34 spec at one point. Sounded great to my ears.. this one takes the mod a few steps further than the info I had. Just wondering if anyone has tried this ..my stocker with 6550 sounds well.. flat. Even with a boost I feel it is missing something as for one i think the bias was set quite low and the power tubes are old. I was thinking of going to EL34's (the cost of 6550's is high) but thought i'd revisit this mod while I was inside and biasing a new set of tubes. If i need a bit more, I'll use a little boost to push me over the edge. Right now, I barely get enough gain with reasonably hot pickups and a bad monkey.
 
No problem.. i was doing a google search on mods and found this. Someone took the time to put together a nice guide. I think i will prob give it a go.
 
here are some old forum things i came across awhile ago. special guest appearance by cameron.lol
 

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Just an update for anyone looking to mod a JCM800 for a tad more bite. I'm talking 3 parts swapped and you have a killer tone machine. I borrowed two changes from the site above and added one additional cap as per an old psychodave post I found on another forum. This amp is seriously impressing me now. Preamp pushed to 10 it gets a tad flub/thick in bass but keep it at 7 or 8 and you have crunch and definition. Slight push from a screamer or other OD and you are even more into flavour country but I am loving this for taht old school VH sound and Ratt type tones (out of the cellar). To me it has that 1984 grind with a Charvel or any type of strat with a humbucker.. plenty gain to solo with if you have hot pups too. I love it. Nothing new, people have been doing this flavor of a mod forever but sometimes the simplest ones are the best. Handful of components and EQ pedal or light OD for the rest.See below:

2. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor. This will beef up the gain. This seems to be the "sweet-spot" on these amps. If you want insane gain use a 1.5K.<-- Here is where it is lacking if you use a resistor alone. Sounds a tad buzzy and like bad distortion however if you bypass the 4.7 K resistor with a .47 UF cap. AWESOME!

3. Locate the 820ohm cathode resistor (R9 found on V2A) and place a .68uf cap across it. This will increase the gain a little. But mostly it gives you the nice "Plexi" midrange boost. <-- I used a 1 uf here, a little extra sizzle.

Anyway, that simple.. no major mods and easy enough to do without disconnecting all your leads to the tubes. Helps to have someone hold up the circuit board as you get underneath it but that is what wives and gf's are for right?? Enjoy ! I'll try to get a clip up and gut shot later this week.
 
34 36 39 got em all. tried em all. there what marshall should have done themselves a long time ago. Just would have required a little more time making sure they were stabel. BUT then again people would prob complain anyways. The usual kid grabs the gain and dimes it.. If you di that with any of the original 34 36 39 they would prob scream and feedabck. Its all about walking on the edge. That sound thats just about ready to fall apart but doesnt.
 
I couldn't have said it any better... people nowadays expect to dime preamps and record with gobs of gain rather than actually developing technique and digging into the strings. The 36 mod can get a bit messy with stages crossing over one another but something simple like the above or even adding half a tube to a plexi circuit could have taken them a long way. Instead, they went diode clipping. Not that the 2205 isn't a kick ass amp in it's own right, it just goes to show how uninterested they really were in R and D.
 
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