Transformer replacement advise needed

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sliberty
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Joined: Fri May 26, 2006 5:03 pm

Transformer replacement advise needed

Post by sliberty »

A few years ago, I built a 6G2 Princeton, and used a Musical Power Supplies PT325.2 for the power transformer. This amp has seen only light use until 2 months ago. Then I started using it at the Wed blues jams, so it at least got a few hours per week of use by me (and maybe more when I wasn't there).

Last night, the amp died. In my troubleshooting, I liberated all of the secondary wires from the circuit and the amp still blew a proper fuse within seconds of startup. So it would appear that the PT is toast.

So, obviously I need a replacement PT, which will be a problem due to mounting centers, etc. (this exact model is no longer available). But once I work that out, what do you recommend as far as getting the amp going again? Should I just wire in the PT, check a few voltages and then plug in some tubes? Or should I be doing some further testing of the circuit before completely hooking everything up? I guess I am wondering if there is a significant risk that something else caused the PT to fail (rather than it just failing on its own), and will the amp as-is kill the new PT as well.

Any advise is welcome.

And if any of you happen to have one of these MPS PT325.2's laying around gathering dust, let me know if you would like to part with it.
Rockwell666
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Re: Transformer replacement advise needed

Post by Rockwell666 »

I would wire in the new PT, No tubes, and fire it up with a bulb current limiter to check for shorts. I bet it was the heater winding that shorted. was the Pt matched for the circuit current wise?
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TUBEDUDE
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Re: Transformer replacement advise needed

Post by TUBEDUDE »

Test the tubes if you can. Look at the sockets for carbon tracks or signs of overheating. Look over the bias circuitry, making sure there's no short to ground. Check the power supply filter caps, rectifier and resistors. The usual suspects.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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sliberty
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Re: Transformer replacement advise needed

Post by sliberty »

The PT was 100mA on the high voltage, which is more than enough for a Princeton, 3A on the heaters, and 2A on the 5V tap. I think spec wise I should have been fine.

I'll probably play it safe and replace the main components in the bias supply (cap and diode) and maybe the filter caps as well. As for tubes, I'll probably replace the rectifier and maybe the power tubes too as I don't have a good way of testing them. I don't see anything unusual on the tube sockets - no obvious carbon tracks or burn marks. And no obviously burned resistors either, although I may replace the 100 ohm faux center tap resistors for good measure.

Is there anything else I should be thinking about?
Stevem
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Re: Transformer replacement advise needed

Post by Stevem »

Something to think about in terms of failure might be thermal conditions as in how close the output tubes are to the PT and air flow around it?

In last small combo amp I built ( 4 -6V6 ) I mounted the PT on the bottom of the amp due to my concern for thermal issues .
I extended the wires and used a 12 pin Molex connector to make the harness unplug easy , as a side benefit I had the most AC hum free amp I have ever had or heard!

Also note that as a added life extender for your new PT I would wire in a AGC type 100 ma fuse into your B+ center tap!
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nickfl
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Re: Transformer replacement advise needed

Post by nickfl »

While you are in there it's probably not a bad idea to add rectifier backup diodes as more cheap insurance.
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