Tube Amps / Music Electronics
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|3/11/2003 4:15 AM|
|tlister||Overheating output tubes|
I just rewired an old mono hifi amp to be a guitar amp (Blues Jr meets 18 watt) and everything seems fine except that the output tubes overheat. The amp is has two EL-84 tubes in push-pull and was cathode biased (150ohms). I am confused as the voltage at the cathode is 9 V and using ohms law I get 60 mA for both tubes, should be OK. I also added in series a 125 ohm and I get 13 V at the cathode, or 47 mA, should be really cool. Still get the plates glowing orange. Eventually the volume drops (3-5 min) and I power down. No sign of tranny heat. Looking the old monoamp schematic, I notice that B+ should be 335 V for this amp, but it is more like 360 V. The screens are connected through a 500 ohm resistor from B+. I have the phase invertor isolated from the grids using 0.022 uf orange drop caps. I have tried 6 different tubes in the two slots. Retensioned the tubes and cleaned with Caig. Running out of ideas.
It sure sounds good while it lasts, maybe I'll name it "one shot deal".
|3/11/2003 6:32 PM|
|tlister||Re: Overheating output tubes: found problem|
Found the problem, one of the coupling caps was wired after the grounded 220kohm resistors.
double check your work!
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