A really fun little circuit

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strelok
Posts: 276
Joined: Wed Mar 26, 2014 5:46 pm

A really fun little circuit

Post by strelok »

A few weeks back a friend asked me to do some tweaks to this Grommes little jewel (LJ4) that was given to him a long time ago. It had been converted into a guitar amp by someone else but he never really liked the way it sounded. I tended to agree and once I opened it up and saw what had been done to it I began to see why. For starters there was a strange local NFB loop from the second gain stage to the first which was not from the original circuit, once I removed it I found out why as the amp oscillated without it there. However this was due to the fact they had used unshielded wire from the input jack on the front to the first gain stage in the rear of the chassis. Replacing with shielded wire cured that problem. After that I moved the volume/Bass control from the output of the second gain stage to the first as the amp wouldn't play completely clean, it was originally meant as an audio amp and was intended for lower input signals. After that it was just a matter of tweaking some of the various component values to make it more suitable for guitar. Had some large bypass caps that caused it to have a fair bit of blocking distortion, and oddly 500k plate resistors in the PI. Additionally the PI didn't appear to have a cathode resistor. Instead I believe the bias was achieved through the NFB resistor getting a dc reference to ground through the secondary of the output transformer. Both halves were coupled together via a 100 ohm resistor giving a bit of positive feedback between the two. Quite a bit different from the usual circuits we're used to, most of this was part of the original design. I took that out as well and just added separate 1k's. I kept the NFB but moved it to the 4ohm tap. Other than that I added a 500pf bright cap to the volume knob and screen and grid resistors to help protect the output tubes. Was pretty limited to what I could do given the lack of tie points throughout the circuit and I didn't want to drill extra holes or drill out rivets. I was going to add a more conventional tone stack back given the proximity of the tone controls to the output section and again lack of tie points I figured it would be best to leave it as is and not risk further instability issues. Removing the NFB in the PI and/or global NFB to the PI caused the amp to be unstable. The tone controls that are in there are reasonably effective though. I re-used whatever components I could and the rest was just stuff that was on hand.

The end result however was quite surprising. I wasn't going for anything in particular just to make it sound good. I gotta say though, played through a weber 1225 in a closed back 112 it gives a pretty damn good EVH fair warning type sound. Certainly made me not want to give it back. :lol: There's a couple resistors that don't really need to be there including one that's not on the schematic on the grid of the second gain stage that was leftover before I moved the volume knob. However it was sounding so damn good I decided to just call it done there.

Anyway just thought I'd share the circuit and a few pics, its a little different and might be something fun to try. I'm certainly curious to build another and see if its the circuit itself or the really nice iron that happens to be in this amp, tubes as well.

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