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-   -   How to drop 10v from mains for 300b amp. (http://www.diyaudio.com/forums/tubes-valves/55709-how-drop-10v-mains-300b-amp.html)

protos 19th April 2005 03:45 PM

How to drop 10v from mains for 300b amp.
 
OK I moved here to brazil and the mains is 125V.This means that for my 300b(actually eml 520) amp on the 115v windings the B+ and of course the heater voltages are way over the top.The draw I calculate is about 200/ 220 VA.
What is the best way to get back to specs?
1.Put a huge bunch of about 5ohm total power resistors or ideally one 5ohm 50w power resistor just after the mains socket -drawback = lots of heat
2.Put the calculated resistors on B+ plus heaters - lots of work and calculation .Thatīs eight different resistors.
3.Use diodes or zeners.I havenīt really thought about it thru yet.Obviously the diodes will have to be after the rectifiers for the B+ and will have to drop around 90v.But you canīt use them for the ac heaters.
Up to now I was using a step down autoformer that gave about 220v on the 230v windings of the amp.However I generally don´t like the idea of using autoformers before the power amp.Experience has shown that they muddle the bass.

dhaen 19th April 2005 03:58 PM

What bass?:D

Seriously, the transformer option sounds the best. Proprly implemented - with an adequately rated transformer, and neutral continuity, I cannot envisage any sound deterioration.
The transformer could be one with 115v and 125v taps, or a 10v transformer with the secondary used in anti-phase to subtract 10v. The transformer size using this arrangement will be very small.

I certainly wouldn't get involved with nasty zeners ;)

John

thoriated 19th April 2005 04:00 PM

You could get a 15A-20A 10V secondary transformer and buck down your line potential to 115v or you could add medium value chokes to the input supplies of the affected equipment (assuming they're not choke input already). The former tactic is more practical.:)


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