Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jack1962 on September 23, 2013, 09:29:52 am
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anyone with any ideas why this Hot Rod Deluxe would have a measurable -20 at every point that should be ground potential on the main board :dontknow:
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anyone with any ideas why this Hot Rod Deluxe would have a measurable -20 at every point that should be ground potential on the main board :dontknow:
In reference to what? Chassis? Maybe a high resistance connection to chassis.
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sorry I should have been clearer , from any cathode in the amp the cathode side of the cathode resistors a -21vdc can be measured .
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sorry I should have been clearer , from any cathode in the amp the cathode side of the cathode resistors a -21vdc can be measured .
In reference to what??? You have one meter lead on the cathode of a tube but where is the other meter lead? What do you measure if you connect one meter lead directly to one side of a cathode resistor and connect the other lead directly to the other side of that cathode resistor?
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I have one end the black ( or common on most meters ) to the chassis ground , the other lead the red one to either cathode on the tube or on the cathode side of the resistor I get -21Vdc . from the ground side it depends on which stage of the amp it's between -18 and -21Vdc . What I don't understand is no where (that I see anyway on the schematic or from inspecting the amp is there -21Vdc I see the bias voltage -50Vdc , the voltage for the solid state reverb + and - 16Vdc . I have attached the schematic.
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I have one end the black ( or common on most meters ) to the chassis ground , the other lead the red one to either cathode on the tube or on the cathode side of the resistor I get -21Vdc .
I'm suspecting a poor ground connection on the pcb. May be caused by a loose connector, broken pcb trace, etc.
What voltage do you measure if you connect one meter lead directly to one side of a cathode resistor and connect the other lead directly to the other side of that cathode resistor?
Turn the amp off and measure resistance from the ground side of a cathode resistor to chassis. Should be zero ohms, but I suspect it is some higher value.
Is the amp sick? What are the symptoms? Do you trust your meter?
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found the problem resistor R63 the 10 ohm resistor that is between the PCB and the chassis (well actually the ground loop on the pots ) was seriously fried. Thanks for the help Sluckey :guitar1
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And how did that ^^^^^ happen?
Not sure that solves it... There should be a direct connection from the pc board to ground. Should be either to the right of the input jacks or dead center on the board. (Different models).