nothing here

[ampcabinets]

?

[terminalgs]

i had a SC120L.   

an SC120L is LOUD  and clean.  the preamp doesn't overdrive much  so what you should be getting is a very clean signal all the way up.   If yours isn't LOUD and it distorts,  your 6L6's are probably worn out.

for the turret strips, check with Watts Audio http://www.turretboards.com/ under "circuit boards>  hiwatt style",  those look close to me (you should measure).  I looked at those once when  I thought about turning my SC120L into a Hiwatt until I realized the B+ was too low and it wasn't as simple as replacing SC preamp with a hiwatt preamp.  I thought the 'stacked two high' was simply 2 singles mounted above one another and it had more to do with the fasteners and spaces than the actual boards.

[RicharD]

There's a couple of different LB120 schematics over at Larry's.  Here's 1:

http://www.schematicheaven.com/bargainbin/soundcity_120.pdf

Sound City amps look like Hiwatts on the outside but not even close on the inside.  I couldn't find a close schematic for the one I worked on.  It was a grounding disaster on the inside.  I never got it right although the owner was happy in the end, I certainly wasn't.

[Ritchie200]

Hey Ampcab, good to see you back!

Looking at the schemo, do you have the reverb jacks?  Are they in use?  If not are the contacts clean and properly tensioned?  For that matter, if they are in use, are they clean?

Good luck, cool amp!
Jim

[Fresh_Start]

Keystone makes terminal strips with turrets:
http://www.keyelco.com/pdfs/M55p131.pdf

Mouser stocks some IIRC and "doin' the Google" may reveal other possible suppliers.

They don't have one that's exactly 23 turrets but you may be able to use the 26.

HTH

Chip

[gldtp99]

I stopped trying to repair problem SC 120 Mk4's years ago--- i've converted several to Hiwatt circuits in different ways (SC power supply/Hiwatt preamp----and different versions of an all Hiwatt circuit) but have kept the orig Partridge iron and EL34x6 outputs.
I've never seen a SC 120 circuit layout--- only schematics----and it's not uncommon for a given amp to not match any of the schematics exactly---- the factory builds weren't completely consistant---some had partially cathode biased output sections and some did not----many different preamp changes, too.
We A/B'd one of the SC 120/Hiwatt conversions with a Sound City 120 Tour Series (later channel switching/MV version of the 120 head) and realized that the Hiwatt circuit is better sounding and much easier to dial in----- the active SC 120 Mk4 preamp is very "touchy" in that a small turn of a pot makes a large difference in tone and there seems to be some interaction between tone pots (easy to lose a decent tone, hard to dial it back in)--- the Hiwatt circuit sounded basically good/great wherever the pots were set.
I also converted one SC 120 to the Soldano SLO lead channel----used only EL34x4 on this one---it's a great sounding hard rock/metal head................gldtp99

[LooseChange]

And.....

Am I missing something?

EDIT: Yes, there was something missing....the rest of the thread! 
I merged them together....Geezer

[gldtp99]

gldtop99,
I was really waiting for you to chime in on this one.

Do you just eliminate the reverb leg(s)/effects loop?

I have found the diagrams for the 100 4-tube 103, but:
1) are you using the 103 layout/schem as is?
2) do I just add the other 2 tubes in, in tandem, like it is now?
3) does the hiwatt use the dual bias pots?
4) do you use the 6L6's or prefer the EL34's, and which is cleaner in the hiwatt? (with 6 tubes it will be loud regardless.)
5) how much of the original would you keep besides the iron, sockets, etc??
6) how would be the best way to eliminate that master volume (I prefer none)
7) do you have any other suggestions for tonal changes?
8) do you have a favorite diagram?

Thanks,
rob

1) Some have been Hiwatt circuits except with EL34x6--- others are hybrids of Sound City power supplies/bias circuits and Hiwatt preamps
2) Yes
3) Hylight Hiwatts have no bias pots--many are modded with a single bias pot
4) Always EL34's
5) Depends--- i've done them in different ways
6) Keep stock MV and turn it up--- your diagram has signal running to ground, no signal would pass that way
7) Depends on what you're looking for--- use high quality old production preamp tubes
8) Hiwatt info at the mhuss Hiwatt site
Here's one SC 120/Hiwatt conversion job--- new sockets, boards, switches, pots, etc--orig chassis and Partridge iron--- EL34x6:

[gldtp99]

The reason i use EL34's on these Sound City 120 conversions is that the PT bias winding is wimpy----it usually puts out just enough to properly bias the EL34's.
I'm too lazy to build a voltage doubler for the bias circuit (there's a diagram at mhuss for this)--- and the EL34's give a midrange punch that i like in these amps.
Most 6L6GC's and KT88's would require more neg bias voltage than the typical SC 120 bias circuit is capable of supplying---- the voltage doubler would most likely be needed----other than that the 6L6GC's or KT88's could be used in these amps----maybe some matched sets of 6L6GC's or KT88's (low drawing sets) could be used without the bias voltage doubler, i don't know--- i like EL34's in these amps so i've never tried different output tubes.
If my responses seem vauge it's because i try to give the best info i can----- i've done these SC 120/Hiwatt conversions in different ways and all of them are good sounding, properly working amps---i don't know for sure if there's a better way to go about this than what i've already tried.
I have one more SC 120R (inop) here that will get some kind of rebuild in the future---- if it becomes a Hiwatt conversion then i will probably approach the job differently than the others simply because i will be at a different (more advanced, hopefully) place in my tube amp build knowledge.
I try to not give out bad information--- i did give out bad info (because i didn't know any better) about SC 120's yrs ago on another forum and i'd rather avoid that now.
Good luck on your project and let us know how it comes out--- your SC 120 is most likely repairable--- just because i like to gut SC 120's and build other circuits into them doesn't mean that this is the best approach for everyone..........................gldtp99

[Ritchie200]

The other were off by more than 10ma or more. Not even a close match.

so much for matched sets of quad.

Let me guess, Groove (your wallet) Tubes?  Sorry, old scar tissue....
Jim

[Ritchie200]

MHO?  Would involve KT88's regardless!  I would probably lean more towards the Hiwatt.  Who (sorry) am I to argue with the genius of Pete Townsend or Glenn Hughes?  An awesome beast for either guitar or bass.  Limited cliental, but a cool rebuild regardless!  Also MHO, the ~C~ KT88s seem to hold up best and sound pretty darn good, although the Hiwatt design is a lot easier on tubes than mine.  That's a pretty hefty investment to retube a sextet.  It will keep you warm in the winter!

Jim

[Ritchie200]

A silly question - what purpose does the bank of (7) 1000pf/1M serve?  Seems like a long way around a better idea.

Jim

[gldtp99]

This is called a "ladder circuit"-----You'll find them on Mk4 active Sound City preamps---not sure of the origin, maybe from hi-fi circuits.
We've been comparing some big amps lately at my shop-----'72 Hiwatt DR103, '67 Fender Showman, '64 Gibson Titan, '78 Marshall 2203, and a '77 Sound City 120 Tour Series.
Many have stopped by and played these amps and nobody has picked the Sound City 120 as their favorite---- the clean channel on this amp is very close to a regular L/B 120 Sound City Normal channel, except with no Presence pot--- the factory added a MV and dropped the Presence.
The Sound City 120's aren't bad sounding amps but in A/B tests against others they often aren't picked as favorites---- and they are complex (needlessly ?) and prone to noise/reliability problems.
The Fender AB763 BF Showman preamp is very simple vs the SC 120 Mk4 preamp but everybody here thinks the Fender sounds much better---- the different versions of the classic Hiwatt preamp are more involved than the Fender AB763 but less so than the SC 120 Mk4.
My goal with having many players compare all these different big amps was to see which big clean circuit (the Marshall 2203 was run on Lo input for comparison) most people prefer in direct A/B/C, etc testing--- so far there's been no clear "winner", as i'd hoped for---- most prefer the Hiwatt or the Fender but some like the Marshall--- two guys liked the Gibson Titan the best.
But nobody picked the Sound City as favorite--- So move on with your Sound City 120 conversion project !!! It seems that most players would prefer any preamp circuit other than the stock SC Mk4--- the SC 120 chassis is already punched for many preamp tube socket possibilities---and the front panel has plenty of holes for jacks, pots, switches, lamp.
See what you can come up with........................gldtp99

[gldtp99]

A JTM preamp--- one of the great sounding classic circuits, a fine choice---- are you sure you don't want a Master Vol on an amp with as much power as what you are building ? The Lar/Mar PPIMV would be a good choice for this circuit---easy to build and it works well.....................gldtp99

[gldtp99]

A couple of things------ the 5.6k grid resistors for the output tubes are on the preamp board--- it's best to have grid resistors as close as physically possible to the tube----- they are commonly mounted directly to (or on) the output tube socket.
Maybe i'm looking at your preamp board backwards but are you wiring the board/faceplate with the inputs over on the PT side of the amp by the Power and Standby switches? Usually the sensitive input jacks/1st gain stage are kept as far away from AC Power Supply components as possible (isolate sensitive inputs--- keep inputs and outputs seperate, no crossing)---- the bias circuit seems to be wired where the inputs usually are--- running the bias supply on a slant across the whole chassis just to get to the bias circuit itself doesn't seem to make sense.
If i'm looking at this wrong, i apologize in advance--- just trying to offer help on the layout so that the amp functions as noiselessly as possible...................gldtp99

[gldtp99]

As far as using a pair of 220uF (110uF total in series) vs. a pair of 100uF (50uF total in series) cap cans for the first filtering stage for this amp----- the only Marshall JTM45/100 schematic i can find (JTM45/100T) has 32uF total for the first filtering stage (32uFx4 in series/parallel).
So if you want the amp to have an overall feel closer to a real JTM45/100, then use the pair of series 100uF cans.
Both the SC 120 and Hiwatt DR103 use 220uFx2 in series and both amps do have a tighter/faster feel than many amps.
Remember that you're dealing with six output tubes and larger iron than a JTM45/100 (and lower B+, thankfully---the JTM45/100's 560V B+ is on the high side).
Either 220's or 100's will work but i'd probably go with the 100's for this build if i was doing it.
Also, i'd just go ahead and re-do the preamp board since nothing has been actually assembled yet----don't worry-- you're not the only one who has done things like this.
I'd also suggest not mounting the MV pot over by the inputs---avoid interaction between inputs and later (stronger) gain stages (don't mix Inputs/Outputs)----- mount MV pot in a position on the front panel where the lead runs to/from the MV pot can be as short as possible and still not interact with other stages of the amp............................gldtp99

[gldtp99]

Here's how i did a Sound City 120/ Soldano SLO lead channel conversion----- the SLO lead channel is Marshall-based (has one extra tube)----- the MV (Power Dampering in this one, not a true PPI MV) pot is next to the power indicator lamp.
Also notice how i stayed along SC 120 lines with the location of the full wave bridge Rectifier (except with 1000V/3A diodes) and bias circuit (dual bias pot as in stock SC 120).


This SC 120/SLO is not the same as a SC 120/JTM45/100 but there may be enough similarities (both having Marshall-type preamps and SC 120 output sections) that this pic may be helpful...............gldtp99

[bigsbybender]

Clean work Rob!
 :icon_thumleft: :icon_thumleft: :icon_thumleft:

j.

[Ritchie200]

Ditto on the nice job.  Although I was crackin' up over the '57 Chevy sparkle tuck and roll job on the Kustom cabinet in the background!  They were in world only they understood!  Not laughing at it - just laughing because this was right in the wheelhouse of the time I was playing out, and growing up (still doing that!) - just brings back memories!

Jim

[phsyconoodler]

Very nice work Rob!

[DummyLoad]

nice work AC.   

--ISO