1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
     
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

Sound City B120 MKIV restoration

Discussion in 'Amps and Cabs [BG]' started by Mr. Foxen, Jun 13, 2011.


  1. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    I picked up some rough Sound City amps, figuring they would make a good investment. I like them because they are made of good components on turrets, so they are easy to work on, and 6 EL34 is loads of power. The pre has a bad rep because it is much more complex than most valve amps, but it is actually very flexible, just a bit unfamiliar, and it can get pretty noisy with the amount of components that can drift in it. Hopefully that can be fixed with a good service. I'm not too techy on the electronics side of things, but I'm learning. I did just break my multimeter, which isn't ideal.

    Step one, after selecting one that looked basically complete, was give it a wash. Damp scourer sponge and some washing up liquid for the tolex, and the top bit of the chassis. Nothing is gonna be passing current soon, so a bit of moist in there is going to do no harm.

    DSCF0001.

    DSCF0015.

    Top of chassis after a wipedown, this was pretty seriously grubby, bit dust/grease and corrosion, the scourer side was applied to that, care taken to not scour off the partridge stickers on the trannies, I have the screening cans for the pre valves put aside.

    DSCF0002.

    Now for the actual electronic bits, here they are:

    DSCF0004.

    Bit neater and much less ****ed with than the previous ones, snubbers on the diodes are installed in a much more fancy twisty way (smarty looking jobs).

    Essential tools for this part:

    DSCF0013.

    Keep water away from electrics, so alcohol is the way, meths for bits you can get at, servisol (aerosol alcohol) for the bits you can't, like inside pots. Don't use WD40 for this, WD40 is an oil in a solvent, the solvent part acts like servisol, dissolving grease, but the lubricant oil part sticks around making a pain of itself. Plus it dissolves/corrodes some things. Servisol is safe and nice. Cotton buds dipped in meths for cleaning the jack sockets and contacts, and the grease with dust stuck in it around valve bases, enough dust stuck together and you can get a conductive path that shorts out your valve, high voltages around there make this very much more likely. The dust is burnt to carbon, which is more more conductive, and makes things worse, you get more flashes until something properly explodes or catches on fire. Everyone should clean out their amp once in a while, even just the jacks and valve bases can make a fair bit of difference.

    Also in that pic, you can see some of the main thing I was trying to achieve, a list of electrolytic caps (the ones that look like batteries, full of rolled up stuff and wet paste) I need to order. They degrade with age, and are a fairly major thing to be ****ing up in amps. I am just going to replace them all because modern ones are plain better, not point in messing with trying to keep things original at the cost of being noisy and not working right. You can mess with testing and reforming them, but I reckon it is pointless for a working amp. I decided to read all the values off the amp, because I suck at schematics, and these amps are a bit 'custom' and not necessarily consistent.

    Some were easy to figure:
    DSCF0005.

    Others less so:
    DSCF0010.

    Had to get the soldering iron out to get at this, did a pretty bad job of it, as only have lead free solder hand and it totally sucks. Some lead solder has been ordered.

    Some of the pots feel pretty dodge, stiff bits, but a good servisol spray and twiddle has freed up most of them. Won't be an update for a while as gotta wait for parts to arrive, and some money to come in so I can afford a set of valves, and a new multimeter.

    Latest amp pile pic:
    DSCF0049.
     
  2. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Wild! Can't wait to see how this thread turns out!
     
  3. my dad would love you. when he played bass he would only play sound citys. he had about 10 diffrent ones through the years
     
  4. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    One of these was probably his then, dude accumulated them based in Edinburgh. I figured I'd recover them and bring them to the more accessible south. Wasn't sure this was the right place, but repairs is a bit more basses oriented.
     
  5. i know his fave for years was a 65 watt combo but i think he said for live use he modded his t-40 to run in stereo so he could run 2 of those heads. theres one good way to tell though. do any smell like dope?. he used to hide his stash in his gigging rigs
     
  6. AwkwardLoudness

    AwkwardLoudness

    Jan 11, 2010
    KY USA
    I've restored a couple Mark 4 SC 120 amps. The most important steps are to replace the original filter capacitors, bias pots, and screen resistors. I like to replace the noisy potentiometers as well, but that's optional.

    There seems to be a lot of variation in the circuits of Mark 4 amps. Some 120s have a lot more gain than others in my experience. The last one I owned had TONS of gain, and the others were far cleaner.
     
  7. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    The valve in the phase splitter position is the one that messes with the gain. Should be an ECC81 (or 2, can't remember), but most seem to have an 83 in, which basically means the tone controls (which are band passed gain controls) are giving audible crunch before half way. The dude kept all the valves from these, and he had 2 ECC81 in the whole pile.

    Are the screen resistors the big green jobs from the power valve bases?
     
  8. Sweet thread Foxen, makes me want to route about inside my Simms-Watts head, but I have no electrical experience (well, no servicing experience) and I hear Tube amps can get pretty fatal!
     
  9. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Simms Watts are pretty meat and tidy inside. Get some gutshots for amp guts threads. My Laney combo just sold on ebay, which will fund the valves. Shame no-one bought the Nexus though, would have funded all of the rest too.
     
  10. PunkRocker33133

    PunkRocker33133 Supporting Member

    Feb 11, 2008
    Northern VA
    If you ever wanna sell any of these, PM me. I will pay shipping to the States
     
  11. no. sell me one and ship it to glasgow. ill give £200 for it
     
  12. AwkwardLoudness

    AwkwardLoudness

    Jan 11, 2010
    KY USA
    Oh yeah, I agree that the phase splitter needs to be an ECC81, but that wasn't the issue on my high gain SC120. I did lots of tube swaps in it and finally ended up with ECC81s in all five preamp tube sockets....it still had a ton of gain.

    Every single Mark 4 era B120 or L120 amp I've seen is wired uniquely. Your amp is no exception, but it is sort of similar to a couple circa 1973 amps that I've seen.

    Here's one of 'em (that I since sold):

    SC120guts2-1.

    SC120guts1.

    SC120frontchassis.

    Yes. I can't see the power tube sockets in your pictures, but I can see the far end of a couple green screen resistors.
     
  13. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Sold my Laney combo, so I just pulled the trigger on the power valves. Ordering direct from Hong Kong works out fairly cheap, and I'm told the Shuguangs are pretty good for new production valves.
     
  14. 60jbass91

    60jbass91

    Apr 24, 2010
    UK
    I have a b200, once it had been for a full service and had some parts that very much needed replaced, it now runs sweet as a nut, the preamp isnt noisy at all now (different story when I first got it, mine too had the ECC83 in the pre). Whats even stranger is that I bought mine from evilbay and the guy who sold it lived in Edinburgh.
     
  15. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Bit of an update because I had some thoughts whilst wandering around town today.

    Firstly, pipe cleaners for cleaning valve base sockets, dippe in meths, they came out grubby so I'm gonna assume it works:
    DSCF0018.

    Still too big for pre valve sockets though.

    Second, I bought a can of this stuff for shining up car interiors. Seems to make things blacker and shinier, but means any dirt stuck in the basketweave shows up more. Should probably take a brush to it if I seriously gave a flying one:

    DSCF0017.

    Edit: Actually, now I see it next to the original photos, it makes a fair bit of difference.
     
  16. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    DSCF0029.

    Overkill filter capacitors (modern ones are laods smaller, same value equivalents in 35mm don't even poke out the top of the clamps, I figured more is more so these are more than double value of originals, more punch or something. I'm told it is a worthwhile upgrade for my purpose, of bass, and the guitarist I lent my Burman with overkill filter caps in was loving it, so think it should be happy.
     
  17. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    Good one! but get some new high speed diodes to be safe with huge caps, as the inrush current is going to increase a bit.
    I fiddle about with all sorts of amps from southend on sea.
    They are fooking great when you get them sorted out a bit with a few choice upgrades.:bassist:
    PS I even know the amp marked STACK is made from parts originated by Roost Amplification of none other than southend Essex.
    Does the Stack have the Original Partridge transformers?.
     
  18. how come you replace the screen resistors and bias pots? all the ones i've had have still been within about 1% tollerance.
     
  19. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    The Stak is marked Dorset inside from memory, I have the matching 1x18 W horn (acoustic 360 style) cabinet with it, which kind of makes it a giant ballache, but it is a shame to split them. It does have the Partridge trannies. Most stuff I have does, except the Carlsbros. What do I need to look for in diodes? The ones in there have snubber cabs, would the still be necessary, and what values would I need?
     
  20. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    Simms watts L 100 are especially near fatal!, the exposed wire wound resistor on the right hand side top surface! of the chassis next the mains trans, has had me while biasing one, the 200 Supers are instantly lethal at 671 running plate volts and as much currant as you like.:bawl:
    Stonking Great old Brit Amps though.
    [​IMG]
    That bloke was famous for his sound well before he switched to Hiwatt and then Sunn/Stramp.
    [​IMG]
    Chrickey you hep cat chaps! the fronts only made out of the same jolly old tough stuff they used for spitfire cockpits! Plexiglas, absolutely Groundbreaking, Don'cha Know old bean!