annoying hum

[bruce50]

I have a '79 Hiwatt DR504. One day I turned it on, warmed it up, and hit the standby. There was an audible "Pop" followed by a 60 cycle hum that is not affected by volume (gets neither louder or softer when turning knobs) or whether or not anything is plugged in or not. Amp works normally and sounds great but will not stop humming. Replaced all the filter caps but the biass cap. Could it be this? If not, what other component could fail (the pop) and cause what sounds like a grounding "Hum"? Thanks.

[Enzo]

One dead power tube.

ANd why not the bias filter when replacing all the others?

[bruce50]

My amp guy said the bias cap tested good?

[WholeToneMusic]

Coupling caps

Did the tech replace the coupling caps also? Ususally 60 cycle hum is from the filament supply or a source outside of the amp.

[bruce50]

annoying hum

Everything tested good, including the coupling caps. We are stumped.

[WholeToneMusic]

Grounding

Make sure none of the ground points have lifted by using a meter on them set to resistance, should be close to or read 0. Sometimes even a loose input jack will cause the amp to hum, make sure all the screws are tight.

[Bruce / Mission Amps]

My amp guy said the bias cap tested good?

Hmmm... I've never refurbiushed an old Orange amp yet that didn't need the two bias filter caps replaced too.
I suspect he didn't feel like doing it because it is a little tricky.
To do it right, you have to unmount the circuit board to unsolder the two 10uF caps and reattach two new ones. It's not a fun job.
If you do it, you don't need to use any thing bigger then 16uF@100v to 22uF @100v for the bias filter caps.
I doubt that this is the cause of your brand new hum but which came first the hum or the filter caps you changed out?

[bruce50]

annoying hum

The hum came before the cap replacement. This is on a '79 Hiwatt DR504. Are they similiar to Orange?

[Enzo]

All amps are similar inside, your amp hummed before, it still hums after the cap job, but the cap job ignored the bias caps. One important rule in my shop is NEVER think up reasons NOT to check something. Pull the phase interter tube. If the amp still hums, it is coming from the power stage, and those bias caps would be prime suspects.