Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: okguitarman on January 04, 2016, 09:28:22 pm
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I have a Blackstar 5 watt head. bad board they will only deal with authorized repair shops for replacing so I would like to use it on a 5w EL84/12ax7 build.
The PT @124v input the sec. is 382vac no center tap, after full wave 350vdc add 16uf cap measures 533vdc
is it this high because it is unloaded? How much voltage drop should I expect?
Should I get another trans. with lower voltages?
They were using a 12bh7a output the voltages @ tube were
Pin 1 plate 442v
Pin 2 grid -20.81
Pin 3 cathode 77.5mv
Pin 6 plate 440v
Pin 7 grid -20.66
Pin 8 cathode 77.2mv
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> is it this high because it is unloaded?
My house rises to 125V if I don't turn anything on. When I turn on a lot of stuff it sinks toward 110V.
EVERYTHING sags. Even Hoover Dam generator sags when Las Vegas turns on all their lights.
> How much voltage drop should I expect?
It depends. Small transformer, 20% is a fair bet.
> voltages @ tube were plate 442v
Hair less than 20%. If you aim for around the same current (did you determine that?), then expect similar voltage.
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Thank you very much and happy new year!