Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Homebrew on September 09, 2016, 08:34:05 am
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Hey guys
i have a ef86 preamp into a 12ax7 pi into a 2xkt66 UL power amp. Currently using a 5u4gb rectifier but will be going back to GZ34. It has a horrible buzz/hum in the power amp. i've narrowed it down to AC ripple on the screens. i isolated the power amp and it still hummed as soon as i ac ground the screens the hum went away. my plan of attack is to add another rc filter before the plate/screen node to clean up the ripple, before it hits the plates.
At max current draw (depending on bias) I’m seeing 430 ma (I put my meter in series between the standby switch and the first filter node and cranked the bias). I plan on running to a 60u filter cap then two 100 ohm 10 watt wirewound resistor in parallel then to my original plate/screen node.
According to my math 430ma X 50 ohms =21.5 volts
Each leg will take 21.5 volts x 215ma for a total of 4.7watts dissipation per resistor (well below its max wattage)
I chose this method because I was able to get the power resistors locally for 2 bucks. But I am open to another method that would be better. Any help would be appreciated. thanks
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430mA seems extreme to me. If that's real, you may have other issues. If you put a 1Ω resistor between the KT66 cathodes and ground what voltage do you measure across it?