Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jalexquijano on October 27, 2017, 03:42:30 pm

Title: 1990 Red Knob twin Hum when standby off!
Post by: jalexquijano on October 27, 2017, 03:42:30 pm
Im experiencing a loud hum whenever i turn my 1990 Fender red knob twin standby switch off. This only happens when the amp is cold and has not been used for last couple of hours? Just to mention all of the Illinois branded filter caps were replaced in both the power section and the preamp section including the bias filter cap. Should i call my tech to solve this or can i fix it myself? The loud hum just lasts 20 secs. After that the noise starts to decrease and goes back to normal operation.

Just to mention i bought the Illinois preamp filters from Hoffman amps!

 Any clues???
Title: Re: 1990 Red Knob twin Hum when standby off!
Post by: PRR on October 27, 2017, 07:57:56 pm
Are the bias caps installed backward?
Title: Re: 1990 Red Knob twin Hum when standby off!
Post by: 92Volts on October 29, 2017, 01:27:24 pm
Note with bias caps, the + end is connected to ground because it's storing/smoothing negative voltage within the bias supply... different from most other caps where the - end is connected to ground. PRR likely noted them because they're easy to install backwards for this reason.

In this schematic: http://www.blueguitar.org/new/schem/fender/the_twin.gif (http://www.blueguitar.org/new/schem/fender/the_twin.gif) which I think is right for your amp, both C305 and C282 are involved in the bias supply.

In general, it sounds like something is taking a while to fully charge/stabilize in the power supply. There could be tons of reasons... but during a cap replacement job, the changes which might introduce new problems are improperly installed caps. Backwards, too small, faulty, or not soldered in correctly.

Was it doing this before the cap replacement, and you hoped the replacement would fix it, or is it new?
Title: Re: 1990 Red Knob twin Hum when standby off!
Post by: sluckey on October 29, 2017, 01:32:26 pm
Bias caps installed backwards would not likely cause a loud hum that just lasts 20 secs and then play normally.
Title: Re: 1990 Red Knob twin Hum when standby off!
Post by: 92Volts on October 29, 2017, 01:35:02 pm
Quote from: sluckey on October 29, 2017, 01:32:26 pm
Bias caps installed backwards would not likely cause a loud hum that just lasts 20 secs and then play normally.

Yeah it might be more like a hum for 20 seconds followed by an explosion  :icon_biggrin:
Title: Re: 1990 Red Knob twin Hum when standby off!
Post by: sluckey on October 29, 2017, 01:41:08 pm
...and then melting glass!  :icon_biggrin: