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Sound City 120

Discussion in 'Amps and Cabs [BG]' started by ianjunior, Mar 31, 2014.

  1. ianjunior


    Mar 30, 2014
    Hopefully someone can help me with a problem I have with fixing my amp. It’s a sound city 120 which I bought about fifteen years ago, I didn’t know the first thing about amplifiers when I bought it and can’t remember any specific details about its demise or its condition prior to the day it died.
    I remember turning it up as loud as it would go and thinking at the time the 100 watt cab might not be able to handle the power but I did it anyway and after a few minutes of deafening power chords the sound faded away and some smoke was coming from the back. I turned it off and let it cool down, then turned it on to see if it was working only to find a very weak splattery sound.

    The amp went into storage for ten years and then went to a local guitar shop to see if the transformers were dead, in this ten years my knowledge of amps and valve equipment had increased slightly and I knew that as long as the transformers were still healthy I could maybe fix it.
    A year passed and when I finally went to get it back they told me the transformers were fine but it wasn’t economically viable to repair it and they turned down my offer of letting them have it for £100. When I next turned it on it was totally dead, no indicator light, not even a hum.

    Now I have the amp here and armed with still more knowledge I have started to restore it.

    First thing I noticed was the guys at the guitar shop had snapped one of the fuse holders. I sourced two more, fitted them, fitted new fuses and now the thing powers up. The original valves are probably not much good and one has since been smashed but I put two el34s in the middle two sockets and attached it to a 8 ohm speaker and plugged a guitar in. It made a very faint sound, distorted and crackly.
    I consulted the forums etc. and started to replace likely sources of the problems. Several preamp resistors (cathode resistors and grid leak resistors) have been changed, several caps in the pre amp too, all values in the power amp tested and found to be correct, the main big caps all changed, the 100uf cap in the bias circuit replaced, still a very faint sound. Some mods which have been done have also been undone and these included some weird alterations to the mid pot, presumable to alter the eq effect… I have restored this to stock.
    Running an external source into the reverb loop on the back of the amp also yields a faint sound so I’m guessing the problem isn’t limited to the preamp. I tested to output transformer and found it to be giving the correct values, also the b+ after the bridge rectifier is plenty high enough, if a little high so I think the guitar shop people were right about my transformers. However at almost every point on the schematic where a voltage is specified the actual voltages in the amp seem way off. Seems that one side of the PI is about right, about 180v and the other, while specified to be different, is actually about 400v if I remember rightly. Also voltages in the preamp seem wildy wrong.
    As the reverb loop comes into the circuit just before the phase inverter I replaced this ecc81 with a new one, still a faint sound. I mean really quiet. Almost the same volume as the guitar unplugged. (Quite a nice sound incidentally, fluid and harmonic overdrive not a harsh or intermittent distortion but way quieter than it should be even with only two tubes in the output.)

    I have ordered a pair of cheap el34s to see if this is what I should’ve done at the start and they arrive tomorrow along with 4 new diodes for the rectifier. The reason I’ve replaced the valves last is due to cost - I decided to replace components in price order figuring that most of what I’ve done should really be done anyway - and if new valves had brought the power back I probably wouldn’t have bothered recapping it out of laziness. I have a horrible feeling that new valves isn’t going to solve the problem hence my writing this, and I will post this if and when that happens.

    Any ideas I haven’t had already would be most welcome, and advice, links, etc. likewise.
    In summary, I bought an amp with a unmatched and presumably poorly biased set of output tubes, killed it by running it flat out, then left it for fifteen years under a bed, Replaced everything apart from the pre amp valves and don’t have enough experience to know why it could sound so quiet and not be totally silent, I’ve assumed that dead valves would mean a silent or at least crap sounding amp.
  2. Bassmec


    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    What a shame I can't tell if you don't live near our Mr Foxen (posts regularly here)as he is a worldwide authority on these great old amps.
    It could be anything from your description from a damaged heater supply in the pre amp to an open circuit resistor, damaged coupling cap or a wire just fell off somewhere.
    Still not quite sure unless we have a plate voltage measurement on the power amp tube bases via the output transformer.
    Output transformers are difficult to test without resorting to the old heater transformer + resistor in circuit backwards ploy.
  3. First, do not continue to try to turn it on to play with it. It's not going to magically heal itself, and continuing to mess with it may very well fry the transformers after all, making it completely cost-prohibitive to fix.

    Then take it to a real tech, not just your local guitar shop. Here's the schematic here: http://bmamps.com/Schematics/Sound City/soundcity_120.pdf

    Which shows it's a pretty simple amp, and likely could've been repaired (at the time) relatively inexpensively-- if a competent tech had looked at it. However, you have continued to turn it on and mess with it, and probably have done even more damage. Once smoke comes out, it needs repair, not new valves!!
  4. CL400Peavey

    CL400Peavey Supporting Member

    Nov 7, 2011
    Grand Rapids Michigan

    If it doesnt work, and you are not comfortable diagnosing and repairing it on your own, then take it to a tech. The more you fuss with it, the more likely you are to break it more.
  5. Lo-E


    Dec 19, 2009
    Brooklyn, NY
    If you're fairly confident working under the hood of this thing, and it seems you are, I'd suggest disconnecting all loads from the power supply and see if all your B+ points have the correct voltages. They'll all be on the high side because the load won't be there and so there won't be any current draw, but it will give you an idea of the health of the power supply. From there, reattach each section of the circuit to it's B+ point one at a time, checking voltages as you go. That will narrow down your search to which part of the circuit is causing the problem. Start with the power section and work your way back. When you start reconnecting sections, put the tubes in so the amp is drawing the current it should.

    Output transformers can be intermittant. Measuring may not reveal how it will act under load and under full signal.

    ALWAYS be sure to have a speaker load on the power section when testing. It can be a speaker or a fixed resistor, but running the power tubes without any load can kill the output tubes, the output transformer or both. Really.
  6. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    The transformers in that amp were made by Partridge. it has six EL34s so replacing a pair is pretty meaningless. The amp needs to be seen by a skilled tube tech. Basically it's a simple tube amp with a unique EQ section.
  7. Usually when smoke comes out and sound dies you have a shorted output tube that takes out a grid resistor. But 6 grid resistors in this one, could also be something in the power supply. Based on that schematic, I would be looking at the 100 ohm resistor that feeds the output tube screen grids and preamp possibly being open or much greater than 100 ohms. And also the bias supply. If the bias supply goes bye bye then all tubes will try to draw maximum current and you will either blow a fuse or burn up some resistors. A check of the voltage at pin 5 of the output tubes should show something like -40VDC or maybe greater, every amp is different. Good luck!
  8. ianjunior


    Mar 30, 2014
    Thanks. Im gonna check these things tomorrow, tonight ive noticed that pulling any pre amp valve out results in silence, which is what i'd expect, but for some reason pulling V2 out doesn't make any difference. I wonder if there's a problem in the preamp and another problem with the power supply. all resistors in the output stage seem very close to what theyre meant to be, i,e within a couple of percent.
  9. ianjunior


    Mar 30, 2014
    that 100ohm resistor measures 110 ohm....
  10. Ok it's not the 100 ohm resistor then. Sounds like you're on the right track though. Rereading your original post about the voltage on the phase inverter plates - something is wrong there. They should be fairly close. Possibly a bad tube or open plate resistor. Did you measure those 82k resistors?
  11. ianjunior


    Mar 30, 2014
    ive put all this in another forum and someone's pointed me towards the phase inverter. It was one of the first things i swapped in fact, i bought a brand new tube, it was fairly cheap electro harmonix one. I'm probably going to use an external preamp anyway so as long as i canb get the PI and what comes after working ill be happy. I'm well aware after the past two weeks reading everything i can find online that the pre amp in this amp and its eq set up is odd and complex and although nobody would post to a forum about how well their sound city preamp performs they seem to write abut ones which are noisy/shagged, i'm more interested in having the power amp working and this means sorting the PI out too as the reverb loop comes in before the PI.....
    So, I just measure the voltages on the pins of the PI and theyre all totally wrong. 380 on the plates, cathodes 105 not 90, grids at 50v and 96v .
    im pretty sure i checked those 82k resistors early on but I'll have to double check i guess.,
    one thing i did notice is testing the plates with my voltmeter makes a very (refreshingly) loud buzz through the speakers.....
  12. ianjunior


    Mar 30, 2014
    ...yeah 81k and 83k
  13. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    A dodgy socket can cause plenty of fun, cleaning all contacts really well often does a lot for an amp that has been stored and now has quiet output, along with refreshing all the solder joints. You can swap one of the known good ECC83 into the PI slot, that should be an ECC81, it means you won't get full output power before loads of distortion, but it will tell you if its just the ECC81 that is being weak. Find some interdental toohbrushes and alcohol (denatured/methylated or isopropyl) and clean all the pre sockets, use pipe cleaners for power ones.

    If the caps are all done and the resistors have been measured, I'm thinking its going to be a connection, also check the fx loop and the trim pots and switch for the half power, those are a weak point for dirt. You can also put signal into the fx return to check the output stage from the PI, I use a Sansamp for this.

    Transformers in Sound City 120 were not always Partridge, some used Electro Voice ones, look the same without lacquer, and often have a heat shield bolted to one, every time one is on ebay with these they claim to be Partridge though.

    Lots of Sound City inside pics on my blog for guidance, and there is a restoration thread linked in the stickies of a couple I did on here: http://ampstack.wordpress.com/tag/sound-city/


    Edit: Also, those blue dubilier caps are the devil, replace them.
  14. You might have a leaky coupling cap if all the resistors in the PI check out. Also, the stage before the PI is a cathode follower that is directly coupled to the PI. If that's messed up it could throw the grid voltage off enough to where the PI won't work properly.
  15. ianjunior


    Mar 30, 2014
    thank you , i just noticed there's an open cap in the preamp as well. ill sort that out after work then look at what the PI and cathode follower are doing.
    cheers folks
  16. ianjunior


    Mar 30, 2014
    your picture of the pristine sound city has been very helpful the past couple of weeks, there's a cap in mine and in that picture which i cant find on the schematic, it's a little silver cap going to ground near the presence pot. i have a feeling mine needs changing but i dont know what value it should be. do you?
  17. ianjunior


    Mar 30, 2014
    think it might be. there's a cap right near there going to ground that i cant find on the schematic. it appears to be in other peoples amps though....
  18. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Dunno value, and not in one at present, one to tidy is close to front of queue though so will pay attention if I remember.
  19. ianjunior


    Mar 30, 2014
    ive actually found a different schematic to the one ive been using, not only does it have values for all the caps but it has a mystery looking 500pf cap going to ground off v4. so thats that,
    now im gonna have another look at those picture sof the pristine one you posted to see whats going on in my preamp. i think the person who's been in here before me has altered it quite a bit.. i have a feeling he was trying to make the eq behave differently. wish i hadnt started this...
  20. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack