Sound City questions

[drewl]

Yikes, got an L120 here that's hummin' like mad, especially around V4.
That is 88v on V4 pin 3 of the schem right?
This one has the cathode resistors on pins 8 of the output tubes.
Why are they 22ohm on one half of the OT and 68ohm on the other?


I had a 120 with reverb that was in pretty good shape, but this one's a mess inside!

[drewl]

Well it seems the answers to my questions came up under my post, these problems are quite common for these amps.

[jrfrond]

These amps are notorious for this kind of garbage, hence the nickname "Sound Sh*itty".

You can squelch some of the hum by using shielded wire to and from the front panel to the active tone control stage. This is where most of the hum emanates. Ironically, these tone controls don't even sound that good. Also by keeping the SENSITIVITY switch on the rear panel in the Low Gain position. Just switch it and you'll hear it.

Many of these L120's were converted B120 bass amps, and vice-versa, apparently done here in the states when Dallas Music Industries in NJ imported them. If they were short one model, they would convert it and slap another label over the existing label. It seems that they might have had chimps with soldering irons performing some of this work, because it's usually pretty sloppy.

[drewl]

Wow, this one looks like a whole gang of "chimps" were in it, especially the non shielded connections you mentioned, hideous!
Anyway, a couple caps, tubes and screen grids this one is sounding pretty good with alot less noise.

Note: I performed the modifications to V4 as drawn up on one of the schematics and the power went waaayyyy down, yes it was noticeably quieter, but it was only putting out about 60watts.
I changed it back and it's screaming out over 130w with a bit more noise from the tone controls.
Keeping the cap across V4's plates reduces the hiss dramatically.

I'm thinking shield some of the stray wires and put in some low noise tubes and it will be quiet and put out full power.