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Old 14th June 2010, 12:58 PM   #1
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Default Transformer questions

So I'm working on a project to convert my old solid state guitar practice amp into a simple single-ended tube amp. I'm working on the power supply and it's getting there. I got the filaments going but now I need my B+ supply.

It's a beginners project and I'm looking to spend as little as possible on it. I'd like to do it with parts I have if possible rather than buy new components (just for fun - as a challenge I guess) so I know I'm going to have Macgruber some things and I don't mind that. I'm doing it for fun more than for sound.

I've seen other projects where people used two ordinary step-down transformers hooked up secondary to secondary to get a suitable voltage for the tubes. I have two transformers I want to use but I've run into problems.

The first transformer puts out 32v unloaded (sags to about 27v under load). It's center tapped 16-0-16. The other isn't tapped and puts on 12v on the secondary. When I hooked them up secondary to secondary (outer wires on the first transformer to the secondary of the second) I get 160v for a few seconds until it drops off.

The first transformer starts getting warm at that point so I guess I'm saturating the core? Flux density is highest when there is no load on the transformer so I figure it must be working the first transformer pretty hard? I unhooked the second transformer and the first works fine so I didn't fry it. I switched the wires thinking they might be out of phase but that didn't work out.

I'm new at this and sort of learning as I go and inductors and transformers are a complex subject so questions:

-Do the two transformers have to have the same rated primary and secondary voltages to work in this configuration?

-Is there a way to make this work? The amp will have a standby switch and I haven't tested it with a dummy load yet. Will it still do this once I hook up a load to the second transformer?

-If this just isn't going to work where can I find some cheap iron? I seem people on here score some nice stuff from time to time. Ebay is cheap until you factor in shipping. Is there some easy to find devices that have a good transformer I can salvage? I'm shooting for a plate voltage of ~250vdc so something that puts out around 180vac would be ideal I think.

Thanks in advance!
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Old 14th June 2010, 07:02 PM   #2
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If you are applying 16 VAC to the 12 volt winding, the second transformer will probably saturate. if you apply 32 volts to it, it will surely saturate.
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Old 14th June 2010, 09:54 PM   #3
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It's usual to plan the voltage scheme and current requirements before you start with a project like this, and pick transformers that will both allow you to develop a heater supply and have sufficient current for the HT.

What is your local mains voltage?

w
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Old 15th June 2010, 06:56 AM   #4
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Quote:
Originally Posted by wakibaki View Post
It's usual to plan the voltage scheme and current requirements before you start with a project like this, and pick transformers that will both allow you to develop a heater supply and have sufficient current for the HT.

What is your local mains voltage?

w
120v 60hz

Obviously I'm a noob and sort of figuring things out as I go. But it's been fun so far.
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