Custome Electra V 460A amp help

Discussion in 'Fisher' started by Degajoe, Mar 17, 2017.

  1. Degajoe

    Degajoe New Member

    I have a 460A amp that I have modified for stand alone use following advice from other AK threads. This amp was non operational when I started (bunt output transformer). I have replaced all electrolytic caps, coupling caps, the top hat diodes (1N007), tested every resistor in the amp, and have brand new tubes.

    The good news is the amp is now operational, the problem is my B+ voltage is 410v after warm up which is 25v to high. This causes my grid (pin 9 370v) and plate (pin 7 400v) to run high also.

    How can I drop my B+ by 25v to solve this problem or is it fine to run this way? I have not ran it for mare than 5 minutes since completion last night.

    Mods: C93B, C93C, R124, R127 Removed, 350 ohm 20w across C93A, and increasing R123 to 7k ohm 3W.



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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Ball Ground, GA
    This is not a big problem, but if you want to adjust it, first check the heater voltage at pins 4 & 5 of the output tubes. If it too is 5% high or more, then using a AC line voltage adjusting device (variac or bucking transformer) is the best approach to use. If it is less than 5% high, then I'd leave things alone. Good tube life starts with the heaters operating close to the design center voltage they are rated at. Once or if that is good, then check the plate dissipation of the tubes to see if that is reasonable as well. Once both of these things are good, then just enjoy!

    BTW -- R110 can also be removed, and I also recall R123 needing to be higher in value than 7K to bring the 290 volt source in line, now that it is no longer supplying power to the tuner chassis. You might double check the recommended values for that resistor.

    Good luck with it!


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