Long time owner, finally noise free player

20 Studio, 50+, 50PA, 50R, L100, B100, L120, L120PA, L120R, 120 SLAVE, B120, B150, L150, L200, B200, Concord, and SMF

Long time owner, finally noise free player

Postby tucsonsound on Mon Sep 30, 2013 12:45 am

I've had this SC L120 mkIV since '97 or so. Never been serviced, always been noisy. After the internet arrived, I found out this was pretty common for the active preamp on mkIVs. Didn't get much use, but it was a bit of a family heirloom so it hung around anyway.
I'd been playing with slaving other preamps into the SC's power section via the reverb return (marked 'out' actually) and ended up pulling 4 of the 6 original Mullard EL34s out, keeping the best matched pair in the center and just using it as a dedicated 50w EL34 power section. It had a bit of hum but all the noise was gone and it really bettered most any amp's power section I put it up against.
Recently I started toying with the preamp again and discovered that when using the reverb send (marked 'in') it was actually fairly noise free playing through another amp's power section, but you really had to crank the gains up. That got me thinking, so I ran a cable from the send to a passive volume pedal and back to the return. I cranked the gains, bass and treble wide open and just brought the volume pedal up a bit. There it was, a completely noise-free, bright clean sound that I'd never been able to get out of the head by itself. I plugged my SG into the brilliant channel wide open, ran a jumper from the attenuated input to the normal channel and brought that gain up to about 7, left the bass and treble around 7-8 with the mids still off and it sounded like a brand new amp. Popped a Rat in-line and it was a glorious, tight distortion machine. It's been sitting there for 16 years and finally I basically added a master volume allowing me to run the preamp wide open and leaving the noise floor well below audible levels. What a great sounding amp, I actually get the Hiwatt comparisons now, huge and clean all the way up.
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Re: Long time owner, finally noise free player

Postby Papa Dog on Tue Oct 01, 2013 5:23 am

seems as though you have isolated the noise issue to V4a. Putting a volume control in the "reverb" loop between 3b and 4a would indicate that there is an issue in either V4 or one of the coupling components. Either a bad resistor or coupling cap. Dropping the volume there would minimize the noise introduced that way, and it should be an easy fix.
OTOH, you have found one of the "hidden features" of the Mk4. Putting a volume control in the loop will indeed allow you to run the pre wide open at a reasonable volume.
I would recomend the "slave" input for jumpering. You can have more gain, and it lifts the grid leak resistor off the channel you jumper from, thus mitigating any possible ground loop issues.
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Re: Long time owner, finally noise free player

Postby satamax on Sat Nov 16, 2013 6:26 am

Hi guys.

Well, some guy dragged me back here, sending me a PM. And i bump onto this.

Tucsonsound, that a good find. Prety sure i tried this on a L100. But can't remember having done it on a 120 And i have neither on hand anymore.

There's around the second section of V3 and V4 a pair of white and yellow wires which seem to be one of the culprits of the L120's noise. I think they are somewhat picking up radio frequencies/background noise. Moving thoses around reduces the noise. Tho, i've replaced thoses by shielded wire iirc (that's a long time ago) with no noticeable effect. You go figure!

There's the Mati Adolfensen mod, putting local nfb on the amplified tone controls, which helps.

I've checked the sound city site, and my annotated schemo isn't there anymore! Nor i can find it in my computer! May be someone will still have it around here. Showing the two or three denoising mods for the 120.
Huh?
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Re: Long time owner, finally noise free player

Postby jaywalker on Sun Nov 17, 2013 1:58 am

Here's one (I think it's one of them anyway)

Image

I have a couple of others that are .pdf
One is a noise reduction mod in the power supply. The other is more 120 mods.
PM if anyone wants them

EDIT: SEE REVISED/CORRECTED SCHEMATIC POSTED BELOW BY PAPA DOG AND NOW UP ON THE S.C. SITE
Last edited by jaywalker on Sat Dec 14, 2013 4:31 pm, edited 3 times in total.
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Re: Long time owner, finally noise free player

Postby GregEdge on Fri Dec 06, 2013 4:05 pm

jaywalker wrote:Here's one (I think it's one of them anyway)

Image

I have a couple of others that are .pdf
One is a noise reduction mod in the power supply. The other is more 120 mods.
PM if anyone wants them



would this work on a 50+?
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Re: Long time owner, finally noise free player

Postby Mr. Foxen on Fri Dec 06, 2013 4:57 pm

Don't think the 50+ has the same active preamp.
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Re: Long time owner, finally noise free player

Postby Papa Dog on Fri Dec 06, 2013 5:21 pm

GregEdge wrote:would this work on a 50+?


Yes...for the most part.
The active pre-amp is virtually the same.
there is a link to the 11-73 revised 50+ schematic here
You will notice tha, while the input is slightly different, from V1 on back it is exactly the same as the 120+ until you get to V4, which the 50+ is lacking.
Here is the corrected 120 mod schematic
Image
which is locted at Sound City Site schematics
Thanks Max
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Of cabbages--and kings--"
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