Sound City 120

[Ianjunior]

Hello.
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.

[terminalgs]

I'd focus on the PI.  If voltages are off, that's the first place to look.  Especially if its 400v vs. 180v. 

I'm looking at the "big" 120L schematic I found here: http://soundcitysite.com/schems.shtml

Check all voltages against the schematic, anywhere a voltage is off by more that 5%-10%, investigate.

Specifically, on the PI: the plates should be 180v on pin 6  and 170v on pin 1.  cathodes 3 and 8 should be 90v.  grid 7 and 2 look to be 88v.  If one of those plates is in the 300-360v range, then you have a problem with that tube or the socket, or a resistor. since the two triodes share a cathode resistor and one triode seems okay (180v side),, its either the 82K plate resistor, the tube itself, or the tube socket.

On a gain stage  (like each of these PI triodes) you should have a voltage drop across the plate resistor.  If the voltage is the same on both ends of a plate resistor, something is wrong (resistor, tube, socket).

If a socket is in poor shape (i.e. corroded) the current across a bad connection will cause heat, and might eventually go up in smoke.  resistors also go up in smoke.  either of these can also take a triode with it.   your problem could be any 1,2 or 3 of these problems.

I'd check all triodes for voltages.  make sure you have a voltage drop across all plate resistors, and make sure they are in the ballpark of the schematic's listed voltages.

(I had a SC120L. it was a clean machine!  I love those partridge transformers).

[stratele52]

Long storage ; start to clean all the tubes socket's contacts and do some tension to Power Output tubes 's contacts.

[Ianjunior]

Man this is great. thanks very much. i forgot to mention that pulling v1, v3 and v4 out of the preamp results in total silence however pulling v2 out seems to have no effect, still the same quiet output. does v2 actually amplify my signal or solely dead with shaping bass and mids?
ill get on looking at my phase inverter voltages later terminalgs
thanks again for now

[Ritchie200]

Ianjunior,

Do you have any other preamp tubes laying around that you can substitute?  Definitely clean all tube sockets, also check tension on your input jacks.  Check the easy stuff first.  (You may already have!)  But back to the main problem.  You said you let some magic smoke out?  It had to have come from somewhere.  Do you notice any arcing on sockets, burnt components, ohm selector for the speaker(s), etc.?

If all the voltages check out, you may need to build a listening amp (check library) to track it down.

Good luck!
Jim

[Ritchie200]

Sorry, just now looked at the schematic.  Does it have the reverb?  If so, unplug and check jack tension for send and return.

Jim