amp advice of the fixin' kind.

stewart · Post by **stewart** » Mon Dec 14, 2009 3:35 pm

right, i spied a SOUND CITY PA120 head going cheap in the local classifieds, and the guy lives about 2 minutes from my house. i went round for a look today, and he explained that it'd been sat in a cupboard for about 30 years. one of the tubes has definitely blown (it takes 6 EL34s!) but the rest look ok...

the problem is- he says that according to the sound city vintage site there's a very good chance the power transformer will blow if you switch it on without speakers plugged in, which, naturally, he doesn't have. does this seem likely? basically it's a gamble, i spend Â£100 on the amp, then maybe another Â£40 on getting a couple of matched tubes hoping it works... or i spend Â£100 and the thing asplodes when i turn it on. or i end up having to fork out even more getting caps and other components replaced, etc... there's a voltage switch on the back, and it seems to have a selector switch for the ohmage (8, 16 etc). i have a number for a guy who repairs amps and shit, do i call him and drag him over to have a look at this and risk him charging me money for it? would he even be able to do anything without me going away out to my parents and hauling a 2x12 over here (total pain)?

what should i do? take a chance on it or leave it be (bearing in mind it's christmas and i'm unemployed, and that money could be better spent on fags, booze and raspberry trifles)? and also, would there be any application on earth that i'd need a 120w tube amp for?

here's what they look like (pics from sound city site):

► Show Spoiler

Post by **Mike** » Mon Dec 14, 2009 4:16 pm

Never ever, ever, ever power on a valve amp without an attached speaker or load. Why not take your combo over and plug that speaker into the amp if it has the right impedance?

Fran · Post by **Fran** » Mon Dec 14, 2009 4:22 pm

I've been watching vintage Sound City stuff for about 3 months and i can tell you now; Â£100 is a good price for that (if its just a few tubes needs replacing). I'm watching a 120 Head on ebay at the moment but i think it will go high seeing as its got Mullards, a partridge transformer and all that malarky.

The transformer blowing without speakers plugged in seems a bit ridiculous to me but im no expert, maybe mike or filtercap will know more on that.
That Laney head i use is actually a tube PA amp, sounds really good, brilliant cleans. In fact if you use dirt pedals i think this is the best way to go, it is the ultimate blank canvas.

I'd probably go for it but i'm in a different position to you obviously, at worst i reckon you'd get at least Â£70 back on it sold as spares or repair.

Fran · Post by **Fran** » Mon Dec 14, 2009 4:24 pm

Mike wrote:Never ever, ever, ever power on a valve amp without an attached speaker or load. Why not take your combo over and plug that speaker into the amp if it has the right impedance?

Really? I never knew that.

Post by **Mike** » Mon Dec 14, 2009 4:28 pm

Dude. I thought you had been reading about Valve amps, that's rule 1. If the Output Transformer has nothing to drive, the energy will be contained as heat aand the windings will fuse to themselves, melting it.

Generally power tubes, fuses and some other components will all die at the same time. It is the worst thing you can do to a valve amp (i.e. playing through it with no load attached).

stewart · Post by **stewart** » Mon Dec 14, 2009 4:30 pm

Mike wrote:Never ever, ever, ever power on a valve amp without an attached speaker or load. Why not take your combo over and plug that speaker into the amp if it has the right impedance?

could take my HRD round i s'pose, it's 4 ohm.

Fran wrote:I'd probably go for it but i'm in a different position to you obviously, at worst i reckon you'd get at least Â£70 back on it sold as spares or repair.

i am tempted, just don't want to open up a big can of worms at this time of year if i can help it...

Fran · Post by **Fran** » Mon Dec 14, 2009 4:31 pm

Not read that, i must dash out and buy the Valve Amps for Dummies book!

chisa · Post by **chisa** » Mon Dec 14, 2009 4:32 pm

Mike wrote:Dude. I thought you had been reading about Valve amps, that's rule 1. If the Output Transformer has nothing to drive, the energy will be contained as heat aand the windings will fuse to themselves, melting it.

Generally power tubes, fuses and some other components will all die at the same time. It is the worst thing you can do to a valve amp (i.e. playing through it with no load attached).

yeah the amp always needs to see a load, whether it be a speaker or a dummy load. i suspect fran is being sarcastic

stewart · Post by **stewart** » Mon Dec 14, 2009 4:37 pm

another thing- if one of the valves has been taken out and you switch it on, will that cause bad things to happen?

looking at this photo it's the one on the far left (dunno if that helps).

Fran · Post by **Fran** » Mon Dec 14, 2009 4:37 pm

stewart wrote:
Mike wrote:Never ever, ever, ever power on a valve amp without an attached speaker or load. Why not take your combo over and plug that speaker into the amp if it has the right impedance?
could take my HRD round i s'pose, it's 4 ohm.

Fran wrote:I'd probably go for it but i'm in a different position to you obviously, at worst i reckon you'd get at least Â£70 back on it sold as spares or repair.
i am tempted, just don't want to open up a big can of worms at this time of year if i can help it...

Well, looking at that pic of the insides if yours is the same it sounds a good deal. It seems value drops when folk start swapping parts out of them, which i assume they do because i've seen Partridge transformers and all that mojo stuff on ebay.
I know what your saying though, i gambled on the Laney head but it was only Â£50.

Post by **Mike** » Mon Dec 14, 2009 4:40 pm

stewart wrote:another thing- if one of the valves has been taken out and you switch it on, will that cause bad things to happen?

looking at this photo it's the one on the far left (dunno if that helps).

It is always best to have a full compliment of valves, because again the OT can be put under undue stress.

You can remove pairs of valves but they have to be two sides of a Push-Pull arrangement (with an appropriate impedance setting adjustment). I would not turn that amplifier on without a full complement of Power Tubes in there.

stewart · Post by **stewart** » Mon Dec 14, 2009 4:44 pm

so basically i'd just have to take a chance and buy it without turning it on?

Post by **Mike** » Mon Dec 14, 2009 4:46 pm

Personally I would never do that. I would source some EL34s from a friend's amp and test it properly.

Post by **Doog** » Mon Dec 14, 2009 5:01 pm

stewart wrote:

I totally used to have the 50w version of that! Really wish I knew what happened to it.

Post by **Mike** » Mon Dec 14, 2009 5:03 pm

Somehow in the mists of time I lost my OC-2 pedal. I have no recollection of selling it or giving it away.

Fucking weird.

stewart · Post by **stewart** » Mon Dec 14, 2009 5:05 pm

i'm going to have to make a decision fairly quickly... potential bargain or potential bag of spanners. gah!

in full working order i reckon it'd be about Â£500 worth.

Post by **Mike** » Mon Dec 14, 2009 5:09 pm

stewart wrote:in full working order i reckon it'd be about Â£500 worth.

I don't think so bud. Stuff like that just doesn't hold it's value, and 120W is not a high rating for a PA, at all. Most people go for 300W.
Plus valve PA's are heavy as all get out.

Put it this way - my '81 JCM800 is one of the most sought away amplifiers ever made, and was in perfect nick and cost me Â£499 second hand.

Fran · Post by **Fran** » Mon Dec 14, 2009 5:31 pm

No one is buying them for PA purposes as far as i can gather, like i said, these old amps make an excellent clean head for guitar. Are you buying this for PA purposes Stewart?
Theres some similar on ebay for around Â£400 BIN, theres a few with bids at around Â£150 current.

Sound City

Sloan · Post by **Sloan** » Mon Dec 14, 2009 6:35 pm

If it's been sitting for more than like 5-10 years, the capacitors probably need to be 'reformed'.

http://www.vmars.org.uk/capacitor_reforming.htm

Usually, people just hook it to a Variac and bring the power up slowly watching to make sure nothing's getting too hot or whatnot. Sometimes it only takes a few minutes, sometimes it takes forever, sometime you can't get full voltage and you have to put new caps in.

With something older like this, it may be good just to get it recapped anyway if you plan on using it. if some of the old caps are still good, sell them on ebay as vintage shit so retards buy them thinking they sound better.

stewart · Post by **stewart** » Mon Dec 14, 2009 7:49 pm

aye, i suppose Â£500 might be a bit off the mark...

but anyway, it would be as a clean amp head, not a PA. i'm really swithering on this, by the time it's all fixed up i'd probably be as well buying a second hand laney head or something... and i dunno if i can be bothered with the hassle, to be honest... :S

amp advice of the fixin' kind.

amp advice of the fixin' kind.

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