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edsmith 11th February 2005 04:51 AM

Lowering HV secondary on Power Transformer
I seem to have bought a PT too large for my homebrew 50watt plexi amp. It cranks out 430VAC on each side of the HV secondary, which I have been told will jump to 560V after rectification.

I need to lower my HV lines by about 100V each. How do I go about doing that? Can I do it with some sort of resistor? I need all the suggestions I can get.

Thanks heaps,

Giaime 11th February 2005 07:05 AM

Tube rectifier? Choke input? These are the usual things to reduce HV. You said 560 after rectification, probably the recto is solid-state. Try Duncan amp PSU desginer, it will help you.

martinab2 11th February 2005 07:16 AM

How about a choke input filter? Can give excellent regulation and sound a bit better than the usual capacitor input if designed properly. Its disadvantages? You get quite a big voltage drop - just what you are looking for.
If you don't already have it, I recommend you download PSU designer 2 from Duncan Amps (free software). This clever little program will allow you to simulate the power supply under different conditions.
Its always worth simulating your supply ito a stepped load (current drawn changes by a few milliamps as in real operation) and choose capacitor values to avoid any oscillation as the load changes.

software at

planet10 11th February 2005 07:28 AM

Cap input... 430 x 1.4 ~600V

Choke input 430 x 0.9 ~ 390V

If you need 460 V then choke input won't do ( but it does sounf better -- you do need to have a minimum current draw which makes it unsuitable for Class AB amps in general)

With the cap input you can throw voltage away with a tube rectifier, and more CL or CR sections.


Giaime 11th February 2005 07:38 AM

I strongly suggest a high-drop inefficient tube rectifier (and those things are beautiful to see...) and a cap input.

Remember that Jimi Hendrix made his plexis modded so the B+ was even higher... he said that they sounded better...

Higher B+ should you point more towards the Class B zone, and that's not good...

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