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noisey on 4ohms nothing on 8


umph
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recently bought a sound city120 it was fine when i was trying it out on 8 ohms but as soon as i took it home and put it on 4ohms its really noisey like static what could be causing this? and can anyone recomend any good amp techs around sheffield?

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[quote name='umph' post='159623' date='Mar 18 2008, 02:06 PM']recently bought a sound city120 it was fine when i was trying it out on 8 ohms but as soon as i took it home and put it on 4ohms its really noisey like static what could be causing this? and can anyone recomend any good amp techs around sheffield?[/quote]

Did the 4 ohm cab have a horn/tweeter and the 8 ohm cab not?

Try the amp with a variety of cabs before concluding it's the amp.

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[quote name='umph' post='159623' date='Mar 18 2008, 02:06 PM']recently bought a sound city120 it was fine when i was trying it out on 8 ohms but as soon as i took it home and put it on 4ohms its really noisey like static what could be causing this? and can anyone recomend any good amp techs around sheffield?[/quote]

Been passed on a contact for an amp tech in South Kirby (not TOO far from Sheff'). Aparently a lover of all thing valve amp, detailed in his work an well priced.

[email protected]

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damn was using it at band practice today and it started making bad noises and when i touched the standby switch it felt like holding on a electric fence, think its defo amp tech time except i can't afford it :/ haha anyone want to buy a soundcity 120?

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no, i changed it to 4ohms when using my 4ohm cab and changed it to 8ohms when using the cab at practice. i think thats pretty obvious i do have prior experience with tube amps and i do know how fickle they are and easy to f*** up.

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This is interesting.

Mine was set up for 8ohm when I bought it and I've switched down to 4ohm to match my Epifani 2x10" and it is noisy.

When I get home I'll try it back at 8ohms and report back on the outcome.

Presumably at 8 ohm into a 4 ohm cab the output will be reduced?

I'll not be cranking it up either way.

Peter, who's not an expert on valve amps and doesn't feel in any way diminished by this :)

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well the cab at practice was 8ohms, and after i played for a little while and tried to put it into standby the switch gave me the worst electric shock i've ever had. i tried to contact the seller i got it off but he's ignoring me :) i've been jipped

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[quote name='GreeneKing' post='160719' date='Mar 20 2008, 11:25 AM']Presumably at 8 ohm into a 4 ohm cab the output will be reduced?[/quote]
No the amp will be destroyed. Valve amps have to be used into the correct load and will give the same power into the correct load. Thats what the output transformer does.

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My Sound City 120 is a Mk4 and does allow switching to 4. 8 and 16 ohms on the output transformer. It's been a little noisy (when I'm not playing) in an annoying way in the house.

I tried swiching from 4 to 8 ohm and it makes no difference. It sounds great when I'm paying and I have no concerns about anything else. I'm led to believe that the capacitors may need some attention/replacement. Does anyone know of a good repair person in the NW (I'm in kendal)?

Peter

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[quote name='umph' post='160672' date='Mar 20 2008, 10:14 AM']no, i changed it to 4ohms when using my 4ohm cab and changed it to 8ohms when using the cab at practice. i think thats pretty obvious i do have prior experience with tube amps and i do know how fickle they are and easy to f*** up.[/quote]
You would be surprised.

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[quote name='GreeneKing' post='160758' date='Mar 20 2008, 12:42 PM']My Sound City 120 is a Mk4 and does allow switching to 4. 8 and 16 ohms on the output transformer. It's been a little noisy (when I'm not playing) in an annoying way in the house.

I tried swiching from 4 to 8 ohm and it makes no difference. It sounds great when I'm paying and I have no concerns about anything else. I'm led to believe that the capacitors may need some attention/replacement. Does anyone know of a good repair person in the NW (I'm in kendal)?

Peter[/quote]

Hi Peter.

Using a valve amp set to 8 ohms into a 4 ohm cab is a pretty quick way to destroy it. If you've got away with it so far, all well and good, but I wouldn't recommend you carry on doing it even for a second. Valve amps shouldn't even be fired up unless they're under the correct load. Oxblood made a useful post about using valve amps into too high a load, and though not as bad as using it into too low a load, it will burn out your valves much quicker and can take the OT with it when they go.

Take a look at [url="http://vintageamps.com/plexiboard/viewforum.php?f=9&sid=34986776f7f9070d4713c0acc74c1611"]the PlexiPalace Sound City boards[/url]. Sound City amps have a reputation for being noisy, but there are mods that can be made that will quieten them down.

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Thanks Musky. I was very gentle with it as I didn't want to overload the amp. I use solid state theory I'm afraid and putting a load of half the impedance onto an amp means to me don't crank it up beyond a very small amount.

I bought the amp from brycee and it's been looked after. I'm not sure if the buzzing is excessive but Oxblood's heard it and seems to think it is.

I see the one mentioned here is for sale, could be a good project.

Peter

Edited by GreeneKing
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Hi folks,

Was just mooching on net and found the site during a search for something and this was the post that brought me to the site :)

"Using a valve amp set to 8 ohms into a 4 ohm cab is a pretty quick way to destroy it."

This is not strictly true, that will depend on the ouput transformer... apparently, this tends to be the case on many British made amps... However, it is a good practise and rule of thumb to follow...

Using oversized transformers or conservatively rated units, an impedance mismatch will reduce power output and bandwidth... I have done this on several amplifiers I have...

This tends to be the case very much so with solid state amplifiers as unlike valves, they are not limited by the power and output transformer specs/ capacities...

valves can often be the cause of a lot of noise / problems in valve amps and if you have been sold an old sound city valve amp that is suspect, it is unlikely the power tubes have been changed for a long period...

Old electrolytic caps can also be a problem, particularly if the amp has been unused for a long period and then the caps are hit with HT...

The safest thing to do is take it to an experienced/ qualified person to look over for you, it is quite likely a service would revitalize your amp and if the above is not the issue, if the issue did not come up during the service, then it could be looked into at the same time...

With regard the switch and the shock, as you say this is obviously time to take it to someone who knows what they are doing, it may also point to a fault with earthing, which could also be something to do with the problem... best thing is get someone to take a look and see :huh:

enjoy! ;)

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