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previous: Doug Re: Sagging JCM800 pre-amp or blocking distortion proble -- 1128525190 View Thread

JCM800 4210 - post-PI MV, switchg Xtrs

10/5/2005 12:17 PM
Jon BlackstoneJCM800 4210 - post-PI MV, switchg Xtrs
> Mine had the 680k resistor as well, it was a different color  
> What's an HPF?  
Sorry. High-pass filter. A 'T' of 2 caps in series with a resistor to ground between them is a formula for a steep bass cut. In the case of the post-PI MV, I think the 1M R to ground (the pot) is too large to hurt. But the bias resistors are 220K, and cutting the capacitance before them (by sticking in the 2nd .022 cap) could make an audible difference. 'Don't really want to be cutting bass at this point in an amp with a small, open-back cabinet. I just replaced 2 of the .022s with .1uF's.  
Hey, BTW... has anyone here talked about how a post-PI MV defeats the negative feedback?  
Another 4210 thing to watch out for: Beware the switching transistors. They can act as diode clamps on negative signal swings. I had a nasty distortion that took me a long time to figure out, and that's what it was. I think the transistor had failed. But when you think about it, isn't there a diode from collector to base in any simple NPN transistor? I may replace mine with darlingtons.  
> I've made the boost channel a jcm 800 2204 minus the  
> cathode follower.  
So you don't see the cathode follower as part of the Marshall 'magic'?  
> I've tapped the eq off the plate side of 2b, then around 3a  
> (previously the channel's third gain stage, shared with the  
> normal channel) through a .022 cap that pairs up with 3b's  
> .022 cap.  
You mean the boost channel bypasses the reverb?  
Of course none of this addresses your current problem. Sorry no insight there. Do you have a resistor between the gain wiper and the grid of v2a? I found this to be necessary. I do not recall the symptom that necessitated it. But the objective of my mod is to make the boost channel like a plexi, and a plexi would have such a resistor. In fact, a plexi would also have a resistance to ground at the grid of v2a in the form of the mix resistor from the 'normal' input. This cuts the level here quite a bit. Maybe your configuration is just pushing to much signal into v2a, and your 'bright switch' cap (my Fender roots showing) may be aggravating it.  
Jon Blackstone

Doug I don't have a resistor after the g... -- 10/5/2005 1:48 PM
Doug I guess I didn't notice that the ma... -- 10/6/2005 12:21 PM