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24kilo 5th April 2006 12:25 PM

Old audio amp change
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I have a old Olympic 812, ( which I was given the scmat's to, Thank you) I want to change the rectifier with Diodes, Of course I need a dropping Resistor. But if you look at the circuit the plat Voltage on the 12ax7 is only 65 and the 50c5 is 145-150. Will the extra %20 Hurt me?

SY 5th April 2006 12:51 PM

I'm not sure I understand the question. Are you replacing the half-wave recitifier with a full-wave bridge?

Tom Bavis 5th April 2006 01:53 PM

I would change to a full-wave bridge and if necessary, increase the cathode resistor for the output tubes. Less hum, and transformer will run cooler. Extra voltage won't be a problem as long as the output tube dissipation is kept within ratings.

50C5 is rated for 5.5W, IIRC. If cathode resistor is 100 Ohms, then cathode voltage / 100 is cathode curerent. Say 8V: 80 mA. 150V * 80 mA, is 12W, too much. So increase cathode resistor and measure again.

Tubelab_com 5th April 2006 09:00 PM

The first time I read this I thought, what the schematic already shows a diode. Then my brain rewound about 50 years and I remembered the selenium rectifier. In most cases you can just replace these with a 1N4007 and gain about 10 extra volts. Check to make sure that you are not running too close to the maximum ratings on the electrolytic caps. They might need replacing also. If so use capacitors with the same value (microfarads) and the next higher voltage rating. The tubes will probably be OK. You are already getting more voltage since todays line voltage is higher than it was 50 years ago. The cathode resistors on the 50C5's could be changed to the next higher value if the tubes start to glow red.

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