New Amp Advice: UK to US Conversion Content!

[MEC]

I just purchased a Sound City 120l.
It has a 3 prong British plug end on the power cord and I have a few questions about converting it to work in the US.

The back of the amp has a switch for adjusting the voltage (I'm guessing it adjust the Hz too?) and says to use a 5amp fuse for the US, as you can see in the pictures below.
Once I change the switch to 115v and put in a 5amp fuse, can I use a simple adapter?
I'm also curious about permanently changing the plug end.
Can I simply swap the British plug end out for a US one, so long as I match the wires to prongs correctly?
It seems as if I could, I just want to make sure I'm not overlooking anything that could damage the amp.

Also, when I change the voltage switch to run on 115v, will that make it necessary to re-bias the amp or does the amp take that into account and correct itself?

Here's pic's and the schematic.

Thanks,


Full Schematic:
http://www.soundcitysite.com/sc_webpages/sc_120_1.jpg

Back of Amp:


Mains Transformer Diagram:

[Grandnoise]

hey man I've already switch it over to 115v for ya, but yeah all the other questions are valid.


This is weird, a little bit like some posting pictures of an ex-girlfriend online...

[eatyourguitar]

chopping the plug is a maybe. if you use an adapter, your fine. you need to make an adapter though. you don't want to use a step down converter. one thing to consider that even though it is still going to work if you swap the left and the right, it may violate safety codes. look at the diagram you posted. common on the transformer needs to connect to common in the wall. I can't explain in detail why it is done like that but it is important for safety.

[McSpunckle]

Everything should be the same when you switch voltages. You really shouldn't have to do anything after that (aside from the plug and fuse). It just uses a different tap on the transformer so you can get the same voltage on the secondary side. There's some variance in mains voltage from place to place anyways, so small changes in voltage shouldn't hurt anything.

Frequency doesn't matter. At least not 50hZ vs 60hZ. It's just getting rectified to DC after the transformer anyways.

You can use a simple adapter, yeah. Or you can cut off the end of the cable and attach a US plug. Either way is totally fine. Changing the plug would be easy, so I'd probably go that rout since it's more permanent.

The transformer really won't care if you get the two mains wires mixed up, but it's good practice to match them up. The chassis grounds with the actual ground wire, so that's the only super critical one-- but, again, match them up just 'cause it's a good thing to do.

[MEC]

Everything should be the same when you switch voltages. You really shouldn't have to do anything after that (aside from the plug and fuse). It just uses a different tap on the transformer so you can get the same voltage on the secondary side.

^I believe this answers the re-bias question too but just want to make sure I'm picking up what you are putting down.

If I understand what you said, it seems like if the voltage was the same on the second side the bias would be close to the same too?
I figured the amp would have a way of adjusting for this but I'm not the best schematic interpreter. :/

[eatyourguitar]

some amps do not need adjustment. my mesa was cathode biased for example.

[MEC]

hey man I've already switch it over to 115v for ya, but yeah all the other questions are valid.
This is weird, a little bit like some posting pictures of an ex-girlfriend online...

I promise, I won't post the pictures I have of your ex-girlfriend!

Promise broken:

NSFW: show

some amps do not need adjustment. my mesa was cathode biased for example.

This amp has two bias adjustments one for each set of three power tubes.
I think so long as the output voltage on the amp side of the transformer is adjusted to the same level regardless of input voltage, I should be ok.
I like to set the bias a little cold though so I may need to tweak it anyway.

[McSpunckle]

^I believe this answers the re-bias question too but just want to make sure I'm picking up what you are putting down.

If I understand what you said, it seems like if the voltage was the same on the second side the bias would be close to the same too?
I figured the amp would have a way of adjusting for this but I'm not the best schematic interpreter. :/

Yeah. There's basically no reason (that I can think of-- I've been wrong before) that anything would have changed other than the input voltage.

If the transformer was putting out 350V, it's still putting out 350V... ish. Tube amp power supplies are really simple circuits, so small voltage differences from house to house (and venue to venue) will be translated through the transformer. So the amp isn't THAT sensitive to small changes anyways.

If you know how, it's good to check anyhow.

[devnulljp]

The guys on the vintage amp forum use step up transformers for their old British amps.

[eatyourguitar]

ok dev I have to ask a stupid question. does a marshall sound different @ 220VAC? like seriously, are they posting audio clips to show the difference?

[devnulljp]

ok dev I have to ask a stupid question. does a marshall sound different @ 220VAC? like seriously, are they posting audio clips to show the difference?

I think it has more to do with load on the transformer or something ... I'm not an EE so most of the details fly far over my head.
I'm running my old AC30 at 115 V but I might try a variac or somehting to try it at 240V. See if it sounds any different.