Connecting a 110V amp to 240V - possible?

[mfwalker1]

Hi.
I suspect the answer is a great big NO, but I thought I ask.
I have a friend who's bought an American amp on eBay for use here in Australia.
The amp (Silvertone 1484) is designed for 117VAC. In Australia we use 240V. 250V on a good day!
The amp uses a voltage doubler (silicon diodes) on the secondary side of the power transformer to get to 480V (the schematic is easy to find on the internet).
I'm wondering if it be possible to connect the primary to 240V and re-arrange the secondary(ies) to a straight half bridge instead of doubler?
I'd need to mod the filament and -ve bias too, but this is not difficult as they have center taps. I can use a resistor network and pot to add 'adjustable bias'.

1. Would this be dangerous? I.e. exceed the primary insulation breakdown voltage?
2. Would there be issues, i.e. core saturation etc.
3. Forget it. He's got a step-down transformer anyway.

Interested in your thoughts.
Matt, Melbourne, Aust.

[booj]

3.
Those boogers are hand wired meaning the leads are actually wrapped around the connections.
3.

[chipprogr]

Why reinvent the wheel. Seriously.#3. Forget it. He's got a step-down transformer anyway.

[Bruce / Mission Amps]

Not sure about this myself.
Many of those old amps were built at a time when the line voltages in the USA ranged from a low 110vac to a high under 120vac, with the average some where around 115vac-117vac.
Now, with that in mind, that power transformer, who's primary is expecting 115vac-117vac would be seeing 122vac to as high as 125vac there with a step down transformer, right?
It has been my experience that with the higher primary voltages, these old amps end up running pretty high B+ and plate voltages on the power tubes with a tricky bias setup, that leaves a lot to be desired with respect to making it adjustable and or more deeply negative.
So I'd go with the easier method of using the step down tranny and two or three, reverse biased, 5 watt, 9v to 15v zener diodes in the bottom power tranny's ground lead.

[knausz]

I think it's possible to do what you describe. I wouldn't worry about the transformer, it can handle the extra voltage on the primary. As long as the secondary voltage is about the same as it was before the mod, the power should be about the same also. Your B+ will be higher when you get rid of the doubler, because there is some extra voltage drop due to the extra diode. Couple more notes: I wouldn't connect all the filaments to the same half of the filment winding, try to balance the currents. Also, note that your bias voltage will double, so you will have to install some more voltage division, not just for the output tube bias, but also for the -1.5V bias.

That said, I wouldn't do it for two reasons. It's nice to keep the old amps unmodified and secondly, it's a lot of work to fix something that ain't broken.

Best Regards,
Imre

[tedmich]

as long as you're at it, add a ground and make the thing less of a death trap, variable bias should be easy too

[chipprogr]

Bruce, then why not just add a zener after the rectifier , before the 1st capacitor? The amps transformer will handle the extra 20 volts or so and so will the rectifiers. This would lower the B+ back down and would only affect the B+ , not filaments.

[mfwalker1]

[QUOTE=knausz;141095]I think it's possible to do what you describe. I wouldn't worry about the transformer, it can handle the extra voltage on the primary. As long as the secondary voltage is about the same as it was before the mod, the power should be about the same also. Your B+ will be higher when you get rid of the doubler, because there is some extra voltage drop due to the extra diode.

Interesting.....Thanks for the replies.
To be honest, at this stage I haven't even seen the amp. So I'm just fishing for ideas. I'm curious as to whether the Xfmr primary can handle twice the voltage. Also whether the extra voltage causes core saturation. And then there's the 50hz/60hz....

Yep. Adding a ground and removing the death cap would be a must.

[Albert Kreuzer]

I'm curious as to whether the Xfmr primary can handle twice the voltage. Also whether the extra voltage causes core saturation. And then there's the 50hz/60hz....

This.
You could try it on a variac with the transformer disconnected from the amp.
Measure the current through the primary. Should be almost zero.
If the primary current rises significantly before you reach 240, you've got a problem.

Cheers,
Albert

[Steve Conner]

Hi.
I'm wondering if it be possible to connect the primary to 240V and re-arrange the secondary(ies) to a straight half bridge instead of doubler?

This makes no sense! :P

If the primary can be set up for 240V, then you don't need to do anything to the secondaries.

If the primary can't be set up for 240V, then you can't use the amp on 240V. The 120V tap will NOT take twice the voltage it was designed for! The transformer will saturate and go up in smoke.

The American domestic market was large enough that most US amp makers didn't bother installing a universal transformer. They'd have a 120V only part for the domestic market, and if they exported, they'd put a different transformer in the export model.

[mfwalker1]

Update.

I've given the idea away.

In the end he bought a 500W step down transformer and the amp works great (except for the reverb - which is normal for this amp!).
No evidence of 50Hz/60Hz being an issue.

At the moment the amp is stock with the 117V cord, 0.05 cap (C31) and ground switch. I will mod this by removing the cap (c31) and providing a 3 pin USA style plug and cord. The step down TXFR provides an earth connection.

Regards,
Matt

[J M Fahey]

Hi Matt/mfwalker
If some day that you have nothing better to do you could take a few pictures of that infamous reverb tank, I would be most grateful.
TIA