Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: CCZ on May 14, 2012, 03:12:57 pm

Title: Changing supply power
Post by: CCZ on May 14, 2012, 03:12:57 pm

Hi Everyone,

I have a Randall RG100sc G2 Series.  The power is 240V, 50 Hz.  I would like to convert it to 110V, 60 Hz.
What would I need to do this?  I am guessing that I could get there with just a power transformer, but I am having a hard time figuring out exactly what that should be and where to get one. 
Randall was not very helpful when I spoke with them.  They do not support that amp and more and the closest authorized shop is a couple of hours away. 

Thanks for your thoughts!

Chris

Title: Re: Changing supply power
Post by: PRR on May 14, 2012, 06:01:51 pm

Does the PT have multiple primary leads?

Title: Re: Changing supply power
Post by: CCZ on May 14, 2012, 08:25:52 pm

No, just a hot and neutral. 

In a different area of my life I have seen solder tabs on transformers so that they could be set up for different input voltages, but alas, not here.

CCZ

Title: Re: Changing supply power
Post by: CCZ on May 15, 2012, 07:47:53 pm

Thanks for the schematic.  Unfortunately, it does not apply to my amp at least so far as the PT is concerned. 

I dug into it a bit more this evening.  My PT has two different, two phase outputs.  The first should be about 32V line to line and 16V line to ground.  The second should be about 56V line to line and 28V line to ground. 

Any thoughts about where I can find such a transformer?  Can I assume it should be 100VA since it is a 100 Watt amp?

Also, what do you think will happen if I give up and just plug it in at half of its rated voltage? 

Thanks,

CCZ

Title: Re: Changing supply power
Post by: PRR on May 15, 2012, 09:34:16 pm

> I assume it should be 100VA since it is a 100 Watt amp?

No amp is 100% efficient. PT rating should be 200VA 56VCT 4A.

> just plug it in at half of its rated voltage?

Probably won't smoke. Will make at-best about 25 Watts.... disappointing.

You do know there are voltage conversion transformers?

http://www.amazon.com/Goldsource%C2%AE-Voltage-Converter-Transformer-ST500/dp/B0022QOSDK (http://www.amazon.com/Goldsource%C2%AE-Voltage-Converter-Transformer-ST500/dp/B0022QOSDK)
http://www.amazon.com/Voltage-Transformer-Converter-Worldwide-Included/dp/B000W9T5SW/ (http://www.amazon.com/Voltage-Transformer-Converter-Worldwide-Included/dp/B000W9T5SW/)

Title: Re: Changing supply power
Post by: CCZ on May 16, 2012, 09:39:56 pm

Thanks for the help everyone.  I think I am just going to get a boost transformer as PRR has suggested. 

The 56VCT is still not quite right.  It has dual, center tap outputs at the same voltage.  What is in there now has them at different voltages.

Chris

Title: Re: Changing supply power
Post by: PRR on May 17, 2012, 10:42:09 pm

> different voltages

You need +40V and -40V at BIG current (can be dirty) for the Power stage, and +/-15V at low current (must be clean) for the preamp stuff.

The plan cited in this thread gets the little power from the big power through dropping resistors and Zener regulation. That works but can be hot, meaning toasted PCB around the dropping resistors. Alternatively the designer can specify a second low-power winding on the main PT (plus rectifier and caps) to get closer to the right voltage for the preamps, less excess heat. If you were to try to replace the PT, you could use a large PT just for the Power section, and add a small PT of the right voltage for the preamps.