Tube Amp 110/120 to 220/240 Step-up Transformer Concern

[teuchter85]

I'm in the US and recently acquired a Luxman MQ-3600 Tube Power Amplifier. It was originally a Swiss unit and as such is rated for a 220V 50-60Hz power input. Obviously, I need a step-up transformer, but most of the ones I have looked at state that they are 110/120V to 220/240V which would indicate that they are simple voltage doublers. The voltage in my house is fairly consistently around 119/120V, which would mean that I would get around 240V out of the step-up. My concern is that as all the voltage rails inside the power amp are unregulated I would get ~9% higher voltage rails by running a 220V rated unit at 240V. I'm sure that some degree of headroom is designed in, but having, for example, a nominal 495V rail running at 540V worries me. Any thoughts/suggestions?

 

[Palustris]

Normally, we expect Japanese equipment to have a 100V power transformer input, but since the power transformer has a 220V input, one has to wonder if there is also a 120V input. Have you got a photo of the power transformer?

 

[gadget73]

^ thinking the same thing, often 240v transformers have two 120v primaries that are either wired in series for 240 operation or in parallel for 120 operation. If its got that arrangement a simple re-wire would be the way to go vs an external step-up.

still going to have the same issues though, transformers aren't regulated so a swing up in voltage at the wall will bring the internal voltages up by the same percent.

 

[teuchter85]

I don't think a transformer rewire is an option. There is only one primary winding that I can see - pins 17 & 18 marked "AC LINE 0 220V"

 

[gadget73]

Unfortunate but at least its definitive


One thing i really appreciate about Japanese trafos is clear markings of voltage and current for each winding

 

[Bill Ferris]

You could purchase the selected wattage with headroom step up transformer then hook it up and measure the device`s voltages, and if too high, put a rated for the device`s expected wattage draw with some headroom variac between your 120 v outlet to the step up transformer, and adjust the voltage down to bring in device`s voltages down to your comfort level.

More complicated feeding power to the Swiss unit, but doable.

 

[TriodeLuvr]

If you use a 240V:120V stepdown transformer in reverse, the output voltage will be less than 240V. Here's a discussion explaining this:

Backward Power Transformer

Other options would be to use the 120V:240V step-up with a bucking transformer on the primary, or with a variac as previously suggested.

Bucking Transformer

Many of us have to deal with this even though our gear is US-made. Much vintage tube equipment was designed for 115V-117V mains. My line voltage occasionally exceeds 125V, and that pushes the B+ in my Citation very close to the 500V rating of the main filter caps. I'm in the process of wiring up a bucking transformer that will reduce system voltage by 5V or 10V to alleviate this.

Jack

 

[Bill Ferris]

If you use a 240V:120V stepdown transformer in reverse, the output voltage will be less than 240V. Here's a discussion explaining this:

Backward Power Transformer

Other options would be to use the 120V:240V step-up with a bucking transformer on the primary, or with a variac as previously suggested.

Bucking Transformer

Many of us have to deal with this even though our gear is US-made. Much vintage tube equipment was designed for 115V-117V mains. My line voltage occasionally exceeds 125V, and that pushes the B+ in my Citation very close to the 500V rating of the main filter caps. I'm in the process of wiring up a bucking transformer that will reduce system voltage by 5V or 10V to alleviate this.

Jack

I considered suggesting a bucking transformer before posting the variac suggestion, but thought if the OP was so concerned about the Swiss made tube amp`s being subjected to higher internal voltages, that he could more tightly dial in the incoming ~120 v AC to give him the device`s desired internal voltages comfort level he seems concerned about.

If he has the technical comfort level he could also select and install a properly chosen Thermistor instead, to be installed in the units primary for some small voltage drop and also soft start, if the tube equipment is SS rectified.