Sound City B120 MKIV restoration

[Mr. Foxen]

Figure you all saw the big pile of Sound City 120s I bought. 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.





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.



Now for the actual electronic bits, here they are:



Bit neater and much less f***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:



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 f***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:


Others less so:


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. Anyone wanna buy an Japanese OC2 for £65? Would cover the power valves.

[EdwardHimself]

This looked like quite an interesting thread but unfortunately you're on my ignore list so i can't see it

[pantherairsoft]

[quote name='EdwardHimself' post='1267990' date='Jun 13 2011, 10:11 PM']This looked like quite an interesting thread but unfortunately you're on my ignore list so i can't see it [/quote]

Well that was a pointless comment! Thanks for the input.

I'm interested to see how this turns out. Well, actually looking forward to [i]hearing[/i] how this turns out...

Shep

[Mr. Foxen]

Fairly sure it is going to mostly sound like a lot of swearing at various points.

[Pentode]

[quote name='EdwardHimself' post='1267990' date='Jun 13 2011, 10:11 PM']This looked like quite an interesting thread but unfortunately you're on my ignore list so i can't see it [/quote]



These 120's are great amps - I'll be following this with interest, thanks for putting it up Mr F.

Edited June 15, 2011 by Pentode

[ShergoldSnickers]

*Spots large pool of nostalgia and goes for a wallow*

Great stuff Mr. Foxen.

[Mr. Foxen]

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.

Edit: Just spotted I used naughty words in the diary because they are ok on the first forum I posted this on, sorry about those.

Edited June 14, 2011 by Mr. Foxen

[Mr. Foxen]

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:


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:



Actually, now I see it next to the original photos, it makes a fair bit of difference.

[Slipperydick]

Looks nice.

Few tips if it aint too presumptuous. When I was an aprentice in a repair shop, donkeys yonks ago, Rexine (like tolex) used to clean up a treat using 1001 carpet cleaner, diluted about 10-1 with water, and put on and rubbed in with a brush. looked new afterwards.

Most hard surfaces cleaned up with a mix of cloudy ammonia about 10-1 with water, plus about a teaspoon of fairy liquid, put it in an old plastic sprayer, spray it on, leave it a minute and wipe off. stinks, but its amazing stuff. Cloudy ammonia is a bit hard to get now though I think.

[Mr. Foxen]

All tips are appreciated. OK to copy them over to the other forums where these threads are, if I use them? Electronics stuff is most useful, I'm not super clued up.

[3below]

Keep the details coming, I need to do some work on my (unmolested, bedroom condition, bargain of a lifetime,) SC 120. Will be interested to see how the noise level pan out, mine is noisy when not playing.

Tip 1: get a temp controlled soldering iron and damp sponge pad
Tip 2: Solder suckers are good, de-solder braid is also excellent for final clean up of tricky parts
Tip 3: Flux paste of the correct type

[Mr. Foxen]

Make sure there is an ECC81 in the phase inverter slot, loads seem to have 83s in there which make them really gainy and up the noise floor a lot. If it is unmolested, replacing the electrolytics will be the main thing to do. I've sent a list to a electronics parts supplier, hoping he gets back to me, and I can get a kit for you too.

[Subthumper]

Hi, good work. I use the mini tooth cleaning brushes for cleaning valve sockets. Also well worth getting is some deoxit or peavey funkout spray. Servisol on pots tends to wash out and dissolve the grease inside and makes the pot feel loose, and can also dissolve some plastics and the crackle will come back after some use. Gimme a shout if you get stuck for any parts, we're in the same town.
Cheers Just

[Slipperydick]

Not an amp I'm familiar with, but tbh anything with ECC83s in,I'd swap the anode load resistors - all of em, theyre cheap enough and it often makes a huge difference.

[Pentode]

[quote name='Subthumper' post='1269508' date='Jun 15 2011, 12:40 AM']Also well worth getting is some deoxit or peavey funkout spray.[/quote]

Ooooo.... Is this Deoxit stuff as good as it's made out to be?

Might get some if it is.

[Mr. Foxen]

[quote name='Slipperydick' post='1269534' date='Jun 15 2011, 01:14 AM']Not an amp I'm familiar with, but tbh anything with ECC83s in,I'd swap the anode load resistors - all of em, theyre cheap enough and it often makes a huge difference.[/quote]

Care to explain more?

[Slipperydick]

[quote name='Mr. Foxen' post='1269668' date='Jun 15 2011, 09:07 AM']Care to explain more?[/quote]

The (usually)100K Resistors (Brown Black Yellow) connected to the anodes - pin 1 & pin 6 of each ECC valve. Assuming this is the correct schematic they are 220Ks on the first valve (Red Red Yellow).

[url="http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg"]http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg[/url]

Edited June 15, 2011 by Slipperydick

[escholl]

[quote name='Slipperydick' post='1269897' date='Jun 15 2011, 12:02 PM']The (usually)100K Resistors (Brown Black Yellow) connected to pin 1 & pin6 of each ECC valve.

[url="http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg"]http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg[/url][/quote]
I think he meant, why would replacing them make a huge difference.

[Slipperydick]

[quote name='escholl' post='1269898' date='Jun 15 2011, 12:04 PM']I think he meant, why would replacing them make a huge difference.[/quote]

Cos they have a hard life, usually discoloured, and its a common problem with a lot of amps. I'd check or change the 1K Screen feeds to the EL34s as well, cos if one or two are O/C you might not notice that the amp is not running on all of the output valves.

Only my opinion, based on experience though.

[escholl]

[quote name='Slipperydick' post='1269919' date='Jun 15 2011, 12:15 PM']Cos they have a hard life, usually discoloured, and its a common problem with a lot of amps. I'd check or change the 1K Screen feeds to the EL34s as well, cos if one or two are O/C you might not notice that the amp is not running on all of the output valves.

Only my opinion, based on experience though.[/quote]
No, seems perfectly sensible. I've not got a huge amount of experience with valve amps myself, mostly solid-state and/or small-signal, so I shall keep this in mind too.

[Big_Stu]

[quote name='Mr. Foxen' post='1267953' date='Jun 13 2011, 09:46 PM']care taken to not scour off the partridge stickers on the trannies, I have the screening cans for the pre valves put aside.[/quote]

I take it this was a dry scourer? If not you're a braver man than I, not saying it's wrong - but I'd be scared of switching it on ever again. I also read that the rust on the tranny's assists them in some way, can't remember the details.

[Mr. Foxen]

[quote name='Big_Stu' post='1270009' date='Jun 15 2011, 01:10 PM']I take it this was a dry scourer? If not you're a braver man than I, not saying it's wrong - but I'd be scared of switching it on ever again. I also read that the rust on the tranny's assists them in some way, can't remember the details.[/quote]

[quote]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.[/quote]

Partridges are varnished anyway, it is a moist sponge rather than a dripping wet one. The 'theory' of the rust is the insulating oxide layer between the laminations breaks up eddy currents in the magnetic flux, but I have no idea if that is snake oil or not.

[Slipperydick]

[quote name='Mr. Foxen' post='1270023' date='Jun 15 2011, 01:14 PM']Partridges are varnished anyway, it is a moist sponge rather than a dripping wet one. The 'theory' of the rust is the insulating oxide layer between the laminations breaks up eddy currents in the magnetic flux, but I have no idea if that is snake oil or not.[/quote]

Seems about a hundred years since I was taught about that, and as a (mostly) service engineer, even longer since the information has been much use. But iirc, and I might not, its a special grade of low conductivity soft iron with a high proportion of silicon in it, thats coated in some kind of soft laquer. assembled in alternate E`s & I`s so that there are no shorted turns. Take an old transformer apart, you'll see what I mean.

Cant see rust being part of the design, but it might help.

[Happy Jack]

[quote name='Mr. Foxen' post='1270023' date='Jun 15 2011, 01:14 PM']... the insulating oxide layer between the laminations breaks up eddy currents in the magnetic flux ...[/quote]

Y'see, [b][i]this [/i][/b]is why I never even try to understand electronics. I'm sure that's a line from [i]Back To The Future[/i].

[Slipperydick]

Eddie Current... What a great name for a 50s Rock n Roll singer !