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Re: Sagging JCM800 pre-amp or blocking distortion proble

10/5/2005 9:13 AM
Doug	Re: Sagging JCM800 pre-amp or blocking distortion proble Mine had the 680k resistor as well, it was a different color and looked as replaced, I think it's original though, a work in progress at the plant before it went out the door. This circuit didn't last long before Marshall reworked it. I've made the boost channel a jcm 800 2204 minus the cathode follower. 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. This was my first amp mod, but it's turned out surprisingly well. The boost channel screams, and I put in a switch on the normal channel so I can lower the cathode resistors to 820ohms (plexi-ish)(1a & 3a) or up to 2.7k and 4.7 k very clean. My problem now is some instability in the boost channel, strangely not at full gain but between say 4 and 8, I get a squeal. from 9-10 it sounds great, and I can back the guitar off but I'd like to have a full usable range on the gain control. I put in a 100pf cap after boost volume to ground and it helped narrow the squeal from say 5-7 on the control, but I'm just stabbing in the dark. By the way what I thought was "blocking" distortion was a problem with rewiring the cathode follower to a gain stage, the switching circuit connects the plate to the grid between 2a and 2b. I moved the switching blue or purple wire to the plate on 2b and that fixed the problem. This channel still hits the post reverb tube really hard, I upped the 680 ohm to a 820 ohm to settle the gain a little. What's an HPF?

10/5/2005 9:13 AM

Doug

Re: Sagging JCM800 pre-amp or blocking distortion proble

Mine had the 680k resistor as well, it was a different color and looked as replaced, I think it's original though, a work in progress at the plant before it went out the door. This circuit didn't last long before Marshall reworked it. I've made the boost channel a jcm 800 2204 minus the cathode follower. 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. This was my first amp mod, but it's turned out surprisingly well. The boost channel screams, and I put in a switch on the normal channel so I can lower the cathode resistors to 820ohms (plexi-ish)(1a & 3a) or up to 2.7k and 4.7 k very clean. My problem now is some instability in the boost channel, strangely not at full gain but between say 4 and 8, I get a squeal. from 9-10 it sounds great, and I can back the guitar off but I'd like to have a full usable range on the gain control. I put in a 100pf cap after boost volume to ground and it helped narrow the squeal from say 5-7 on the control, but I'm just stabbing in the dark.

By the way what I thought was "blocking" distortion was a problem with rewiring the cathode follower to a gain stage, the switching circuit connects the plate to the grid between 2a and 2b. I moved the switching blue or purple wire to the plate on 2b and that fixed the problem. This channel still hits the post reverb tube really hard, I upped the 680 ohm to a 820 ohm to settle the gain a little.
What's an HPF?

Replies:

Jon Blackstone JCM800 4210 - post-PI MV, switchg Xtrs -- 10/5/2005 12:17 PM