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Eico HF-60 Amp Line Votage Protection

Posted by Chris Haedt (A) on November 1, 2003 at 07:53:52

I just completed restoring a pair of Eico HF-60 amps for a customer and they really do sound wonderful. The manual is quite specific about the amps being designed to operate at 117 VAC line voltage and that damage will occur at voltages above 124 VAC. This higher voltage is common in my area, and does result in 500+ volts going to the first filter--which is its design limit operating voltage. Ideas to drop the voltage and protect the amp? I have replaced the filter caps with 500 volt rated I/C caps under the chassis. CL90 Thermistor/inrush current limiter is an option, as would be a variable output isolation transformer. I'd like to know if anyone has done a good working mod to protect these fine amps.

Also, the amp design relies heavily on output tube balance to null 120 hz. power supply hum. There is the first filter mentioned above, and then B+ goes right to the output transformer. The particular tubes in the units are not well matched for mutual conductance. When the output tube balance is set to 0 VDC at the meter jacks--there is still minor PS hum present at the ouput. I can observe this on the scope and null out the hum by rebalancing the tubes. Maximum hum reduction does not happen when the DC balance is at 0. Now this problem may go away when better tubes are put in--doing so tomorrow. Anyone have these amps and experiment with hum reduction?

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