Alternative Transformer Fender Champ 5f1

For those building and repairing amps on their own. Learna and share ideas.

Alternative Transformer Fender Champ 5f1

Postby downunda on Thu Mar 05, 2015 10:58 am

First post to the forum, although have been browsing for a while.
I am putting together a 5F1 champ from scratch and on a minimal budget. I live in rural Australia, and electronics parts are hard to come by, or expensive to order from overseas due to postage. I have scavenged many passive parts, chassis, turretboard, switches, pots, resistors etc, from some old 1950's electronics (radios,scopes etc) and have come across a transformer 0-240 Primary, 6.3v, 5v, 360-0-360 HT ( a Cossor KA30246 ? )
Getting a tranny from the US is a killer for postage due to the weight, so I am wondering whether this transformer would be usable for the Champ. :|
My electronics knowledge is fairly basic but can read a circuit. I am using the valve rectifier WY3GT, not the SS alternative.
The secondary is a bit higher than the 650v of the standard tranny. What issues does this create, and more relevant, how can I overcome them :?:
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Re: Alternative Transformer Fender Champ 5f1

Postby gusfinley on Sat Mar 07, 2015 5:02 am

The first question is: Would it provide enough power?

Your best bet on this is to know what type of amp the transformer came from - What tubes were in the original amp? If you add up all of heater current and plate current of the individual tubes than you should have a good idea of what kind of curren the transormer can provide.

Being that this is a Champ, you should be good. The 5V winding is usally just for a rectifier, and most amps are going to have at least one preamp and power tube.

The second question is: Can you reduce the voltage to be in the same range as a vintage Champ? The answer is likely yes.

The quickest and dirtiest way to do this put a resistor before the 10K "choke" resistor to drop the voltage. Duncan Power Supply Designer is a great tool to use for something like this.

I put in the stock fender champ circuit into duncan's PSU designer, then changed the transformer to the 360-0-360 that you have. By trial and error, it looks like a 470 ohm resistor will drop the voltage enough to get it close to vintage values. This resistor would dissipate about 4 Watts of power, so double that to a 10W resistor for safety. Of course, this is just a mathmatical simulation, but it will give you somewhere to start from.

Best of Luck to you on your champ build!
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Re: Alternative Transformer Fender Champ 5f1

Postby ThisLifeILead on Wed Apr 08, 2015 8:34 pm

im with gus. i normally "cheap out" and make whatever i have work. ask lotsa questions and we will do out best to help :highfive:
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
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Re: Alternative Transformer Fender Champ 5f1

Postby PeterS on Thu Apr 09, 2015 1:44 am

An alternative to using a resistor to drop the voltage (which will work too) is to use zener diodes. If you want to drop say 50 volts, instead of looking for a 50 volt many watts zener, just buy a quantity of for example 5 watt 10 volt zeners and mount them on a terminal strip in series. You can adjust the voltage by having more or less of them, and it's cheaper to buy several 5 watt zeners than one 20 watt zener.
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