Sound City 120

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Sound City 120

Postby Racing on Wed Mar 04, 2015 9:44 am

Have had this SC 120 around the shop since ages.
Belongs to a dude i´m involved with a bit within the PA scene... Anyways. Bjorn asked me to recap it as i had some spare time,and spare time never seemed to happen so..
Day arrived when it was simply time to dig in.

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What we´re talking here is a bare chassis,and what´s more one that´s been to hell n back. All the vitals are still there by any means,it´s just that the amp in case has seen fires and god knows what else. In turn the local Mr Handyman had been at it,with miserable results.

Bjorn was of the impression that fresh powertubes were in order,but after a quick check...it was anything but that. Amp was REAL low on power which was traced to the plate resistors for the PI being of 470k a pop. No...no drift. Just the work of Mr Handyman..

So. Replaced that per schematic,and indeed power was all there. But...so was hum. Like a sonofabitch.

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So. Measured the stock e-caps with one of my ESR meters and they were in essence all shot. Crashed n burned in short. As in the pic,replaced them all. The major ones with JJ cans,but they were all replaced sans the small cathode decoupling ones for the stages of V1.

Still hummed.

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Old Partridge irons are still there,and intact.

Anyway. Then took to digging a little deeper as far as pure design,and as it turns out the amp didn´t conform to "the golden rule". Took care of that. In turn the wire heading from the bridge neg vs chassis ground was burnt..replaced that and in turn checked integrity of chassis solderprong. Them two tricks sure made for a vast difference as far as hum.

Then took the CT for the heaters out and replaced with a 500 Ohm trimpot across the prongs of one of the tubesockets to be able to balance heater vs ground,which also helped.

Amp still hummed more then needed tho and TBH it´s such a mess under the hood that i had a real hard time settling on what was stock and not. The groundbus at the pots struck me as it was though,and that rendered that the preamp upon checking hit ground in like 10 places all in all! So..screw that bus. Redid it the "correct" way and let the entire thing hit ground at the stock point by the inputjacks approx,and indeed..the amp settled.

Had a powertube that showed crossover flashing when signal was applied,replace that. Installed simple 1 Ohmers for the powertube sockets vs ground... Going to redo that i believe and gather all the powertube grounds for one wire that heads for ground at the main junction point at the chassis by the negative fuse.

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Hiss went down in an appreiable manner while pulling these stunts too.

Then took to installing a piece of sheetmetal as "floor" for the chassis to act as a shield,and that certainly helped too. As is the amp is currently used for bass work by us in the two bands,and does so rather well. Ample power to say the least,and the span of the pots is on the ok side no argument.
In this case mods will be kept to a minimum,if anything per above we´re talking mods to straighten the thing out as i like amps that are well behaved in idle.

As i´m to revisit the amp shortly i´ll take some underhood pics to go with the story.

What i´ve concluded is that some of the single strand wire used has seen better days,and that i guess is just a receipt of that this amp has been REALLY run into the ground and put away wet.

We used it at a gig this weekend past and all of a sudden it started to crackle,and of course it did so at the end of soundcheck. Gave it a whack and it stopped (after replacing the offending preamp tube).
In short i presume this is due to a bad solderjoint at the tube in Q....
Racing
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Re: Sound City 120

Postby Racing on Wed Mar 04, 2015 8:37 pm

So.
Spent the time needed on that SC and...net result is...finally. NOW it´s quiet as a mouse in idle. Even with the dials at WOT.

Mind you,the pics to follow is AFTER a first good clean!!! Spent time desoldering the MASSIVE amounts of solder all over the place. Must have used up like 30ft of wick! :shock:

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Yep. At some point the poor thing has indeed been on fire. The soot ain´t no lie,and it´s on there tough enough that the only way to get it out is to do so mechanically. Ie;sand it out.

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Mustards wasn´t used throughout,but sure is now. Have a rather large stash of them around..don´t care for them much..so..might as well use them in instances as this.

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Preamp board after sanitizing. Component legs has been straightened out,MASSIVE amounts of solder has been removed..and in turn the whole thing has been cleaned once with rubbing alcohol. Needs to be like 2 or 3 times more tho..
Trimpot in case is balance pot of 500 Ohms for the heaters.

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Lots of the wiring had cracked and blistered. So,,replaced where needed with multistrand.

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That there running for the presence pot was a mess you won´t believe. Nope. The stock is it four or eight uF cap isn´t anymore. It´s been replaced by a 15uF one.

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´N here running for the inputjacks et al.


Word of advice. Seing how these are put together there is work to be done to a stocker. There is NO NEED WHAT SO EVER for them to omit the sounds most of them do in idle.
Reroute the bridge negative asf per usual (the "golden rule") and in turn redo the entire preamp grounding so it to the letter hits chassis at ONE point only. These two alone makes for a world of difference.
In turn cut the centertap for the heaters out of the mix and replace with a balance pot per pic above. The heater winding is NOT balanced for minimum hum stock,so for instance a pair of equal sized resistors vs ground from each side of that winding will NOT do if the idea is to make these SOB´s behave.

The stock grounding point by the inputjacks will do just fine. There´s a solderprong bolted to the chassis there,and that´s where the groundbus for the 1st tube and the inputjacks hits the chassis,and so should the rest of the preamp.

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In turn physically make sure that the fuseholder for the HT has good contact with the chassis via its wire. This is essential,and on this particular amp..this was in a bad cond. Remedied...and difference is like night n day.

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In turn these suckers really needs to be replaced. They´re worn beyond limit. Will check that out asap.

Next up on the agenda,foremost,tho is to take each pot apart and clean out...check integrity of rivets vs carbon track and retension.
Racing
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Re: Sound City 120

Postby jaywalker on Thu Mar 05, 2015 5:46 am

Thanks for the play by play. Keep it coming. :jam:
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Re: Sound City 120

Postby Racing on Thu Mar 05, 2015 11:25 am

Thanx.

Forgot to mention one thing that´s valid too. I rerouted the grounding of the "last" of the can e-lytes. Stock it grounds TGT with the negative from the speaker out to the chassis approx along the turret rail by one of the powertubes.
I rerouted the grounding of that electrolyte to the common groundpoint for the preamp,by the inputjacks,as that is what it serves. The preamp. Electrolyte smoothing caps should always ground at the groundpoint for the triodes or whatever it serves.
Think in "power circles".
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Re: Sound City 120

Postby Raw Power on Fri Mar 06, 2015 1:44 am

Racing -
Excellent posts on this site. Much appreciated . :thumbsup:
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Re: Sound City 120

Postby Harvey58 on Fri Mar 13, 2015 12:29 pm

Great work man! Im curious who was the one who solder all that "shit". If it was in the factory or by someone later at homework? How it plays now? Can you tell something about the output sound and settings (possibilities)l? What i remember of my sound city (before the preamp conversion) was that i was angry always when i touch any of the tone controls, because a little change means that i must tweak again overall volume controls. But the sound were interesting > juicy i can say.
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Re: Sound City 120

Postby Racing on Sat Mar 14, 2015 10:28 am

Well,i can try! :lol:

One of the features of these amps IMO is their tonal range. Being kind of quasi active EQ equipped that kind of comes with the territory. However,as lots more triodes are involved that will per definition raise noise floor. Also comes with the territory,within reason depending on how it´s been done,and as such it therefore also stands to reason that it is a real good idea to "redo" or "resto" the amp using all the examples available in the text book.
The controls of it are interactive tho,as you point out.

All of them small issues,or whatever you wanna call it,adds up. Hence,doing away with them according to text book simply hands you a better behaved amp. It´s that simple.

I try n explain what the issues are above to the best of my knowledge and interpretation... Adhering to "the golden rule" for instance is ALWAYS a good idea and there´s TONS of amps out there that does not...which is a rather profound error in design really if you ask me.
Further,it´s by no means limited to "old" amps. Just take a look at the Redknob Fender Super60 for instance (as well as other Redknob Fenders). By cutting ONE track on the PCB and replacing that with a piece of wire the amp basically turns completely numb in idle.

As for this particular unit i strongly suspect that "Mr Handyman" has been at it,and ergo..them LUMPS of solder has most likely been added to by said Mr Handyman. Make no mistake,this particular amp has certainly been leaned upon along the road.. When i started out it was about run into the ground..

Then there´s other considerations to take into account IMO. The amp uses single strand wire for the most and while it makes for an approach that might be more aestheticly pleasing initially...the simple truth is that single stranded wire won´t stand up to vibration over the long run as multistrand ditto will. Fact of life.

That also brings that there´s sort of a judgement to be made as i see it. How "stock" does it need to be? I´m all good as far as some amps being kept in as stock as condition as possible, but when an amp has been on the slippy slide for as long as the one pictured above..to hell with it. Reasoning behind that,the main one,is that the amp is still to be used.
To produce music,as god intended.

Would your average Joe see or even complain that the work above has been done? Most likely not if you ask me. The mods pulled have absolutely nothing to do with actual tonal performance but are mods that have been implemented to make the amp behave better. Noone appreciates noisy amps..simple.

So? Well. For those into music more then vintage value i´d say that what´s described above might very well be worthwhile. You do NOT detract from the original idea as far as sound in any manner. You simply make the amp way more well behaved..that´s it. Worth it? Yes. Yes it is cause a stock SC-120 is a rather noisy piece of equipment.
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