Voltage help

Bob, a step up transformer is a 1:2 device, meaning it will put out twice as much as you put in.

Line voltage can vary a bit, here in my shop it tends to be high, around 123-125 Volts. In other words, if I hook up a step up transformer here, I get well over 240 Volts secondary.
This may be different where you live.

The best thing is to run the old Marshall on its highest tap, 240V.
Find a good Variac on ebay. Plug the Variac into the wall, the step up into the Variac, the amp into the step up, Measure the secondary voltage (what's coming out) of the step up, and adjust the Variac until you get 240V. You're good to go.

If the amp is new to you, have all the voltages checked out by a pro. These old transformers can sometimes have partial shorts resulting in secondary voltages that are too high. Make sure the voltages are within specs at 240V.
An easy way to find that out with some certainty yourself is to check the heater voltage. Make sure it's around 6.3 to 6.5 Volts, adjust the Variac accordingly.
If you don't know how to do it, don't do it, there are lethal voltages on the tube sockets.

So, not to make this too complicated, you want to run the amp at 240 Volts, you want to make sure your step up doesn't put out more than 240V, and you want to get your amp checked out to make sure the voltages are fine.
Then you never think about it again and enjoy that killer amp.

Hogy

 

Maybe it's easy for me to say this being somewhere where the line voltage is 240V, but I would simply measure your line voltage, and set the amp's own voltage selector to that (or the next highest if it's between two) - "then you never think about it again".

The transformer has multiple voltage taps for a good reason - it was designed to run like that.

Just my opinion, but I would always go for the simplest solution with the least extra stuff in the power supply chain.


Like Hogy says, it is a good idea to have internal voltages checked too though, in case there is some problem.

 

John, those old Marshalls actually do sound significantly better on 240V, because of how the power transformer is wound.
Try it sometime and tell me what you think.

Hogy

 

I have - sort of. Although here's the difficulty... I'm on 240V. This means that to run an amp at 110V, I have to use a step-down transformer, or a Variac. I've done this several times when setting amps up for overseas customers who want them for 110, and I think they do definitely sound worse like that.

But... is it the running the amp at 110, or the step-down transformer in line, that does it?! I can't tell, from where I am, since I have no way of doing one without the other! I had been inclined to think it was the transformer, much in the same way that using a crappy thin power cable spoils the tone.

So without moving over to the US or Canada, I'm still none the wiser... although if you think they sound better at 240 with a step-up, I'll take your word for it. Question... does the size/quality of the step-up make a noticeable difference?

 

The size of the step up is very important. If it's too small, it will sag under load and cause bad tone. 500W minimum for a JTM45 / AC30, 1000W for a Super Lead.

The Marshall's 240V primary uses the entire primary wind and makes for better bass, quicker response, better headroom and stability.

I'm not a transformer designer, but I was told the primary is simply tapped for the different (lower) primaries, meaning you're only employing part of the power transformer at lower settings.

Modern amps like the Komet use two independent 120V primaries. Wire them in parallel for 120V operation, in series for 240V. Either way, the whole transformer is "on".

Whether this explanation holds water or not, as someone who has serviced, collected, and gigged old Marshalls in both parts of the world, I can tell you from personal experience that every one of them sounded best at 240V.

Hogy

 

I believe it has to do with the current sourcing of the pwr tranny, and not the fact that there is a variac/step up tranny in line. The multi taps might give the same secondary voltage, but not at the same current rating. The 240 tap has the twice the primary winding of the 120 tap, and only has to step up 1/2 as much to reach the same secondary voltages. I'd like to find a tranny spec sheet for these that showed current ratings for the different taps.

One way to tell is to measure the secondary voltage from idle to full load, and see if there is a different amount of sag from the two different taps.

Later, PaulC

 

Done that, and yes, there is.

Hogy

 

Cool - I'd love to see the numbers if you've got 'em handy! I've always wanted to make a JTM 45 ish tranny with a current rating putting it closer to 240 performance. But it's been awhile since I've had a real one on the bench...

 

Thanks Hogy! That is useful information. I had based my dislike of outboard transformers on bad thinking perhaps... I always thought that the less you have in the way from power socket to amp the better - and that the simplest, most straightforward solution is the best... or - "why step up the voltage twice, when you can do it once?"

My Variac is only rated for 3A, which is plenty for what I normally use it for, since that's 720VA at 240V... but certainly running an amp at 110 via it doesn't sound nice - mushy and a bit muffled. The largest simple step-down I have used was 500VA, which I didn't think sounded as bad, but still not great.

Perhaps I need to get an industrial-size step-down to be my '110V mains supply', then experiment with stepping it back up again, or not...

...nah, I think I'll just take your word for it! They start to get expensive and heavy very quickly.

Interesting about the different voltage sag. I assume that's because the current in the primary is twice at 120V what it is at 240. That might also make the difference between 50Hz and 60Hz more important too.

 

It's the other way around - the current sourcing would be twice at the 240 tap giving less sag. It would also have less voltage fluctuation on the secondary due to changing line voltages. Call it an even an even 1to2 and 1to4 ratio for the two taps. A 5vac change in the line would be 10vac fro 240, and 20vac for 120.
More current availability, and a more consistent secondary voltage. But that's only for places with 240. A 240 step up tranny would just double the fluctuations...