intermittent 120 cycle hum - Fender DRRI

[pjd3]

Thanks for stopping in.

My band member has a 7 year old Fender DRRI that began making a 120 cycle hum. I took it home to investigate and found that the hum is intermittent - it can gradually fade in and fade out. None of the panel controls affect the hum which since it is a 120 hz I suspect it is a power supply or power grounding issue somewhere.

Is anyone aware if bad filter caps can be intermittent when they first begin to go south? One thing I did find is that you can make the hum go away for a while if you flip the standby switch, wait a few seconds and turn it back on. This will work sometimes, but not all the time. Sometimes its on the first standby flip, or it can take a few times. Perhaps this could be a clue into the source of the hum. I was able to repeat this tactic enough times to have confidence that it was definitely connected to making the hum go away. And the hum will stay away once it is gone, until you turn the amp off for a while and comeback. Then you have to play standby games once again to eliminate the hum

I should mention that I also put in a new GZ34 but, same issue persisted.
Also I've built a couple of tube amps and know amp safety and basic trouble shooting.

Any help on this would be great, perhaps there are some particular ground points that need addressing on these amps?

thanks very much,
Phil Donovan

[pompeiisneaks]

Often intermittent noise like that can be due to a poor connection, either side of the filter caps since you're hearing it at 120hz... one of hte filter caps is 'good' until it heats up and then gets better when on standby for a while and current slows and things cool. Not a guaranteed cause, but worth looking for bad solder joints.

~Phil

[pjd3]

Thank you Phil, sounds like worthy advice and makes sense.

Before I chose to dismantle all the printed pcb's (intense gigging schedule starts tomorrow) I thought I would do some preliminary experiments with tube pull outs and substitutions. First was a GZ34 substitute with a new JJ, hum persisted. Then, thinking that pulling out the phase inverter might tell me about the source of hum, I found that the hum seemed to disappear without the Phase inverter tube installed. That, much to my surprise led me to believe that the 120 cycle hum was a preamp issue. (wouldn't the hum still be there if was purely a power supply issue? ) The hum persisted with a new Mullard 4024 PI tube so, removed the tube driver and send tubes - hum persisted. Now, I pulled V1 And V2, and the hum disappeared which I really don't get. How would the input tubes be cause for a 120 cycle hum? Arent they only receiving a somewhat smaller plate voltage that has been filtered many times before reaching them? I put the RCA anos 12ax7 back in V2 (which is the utilized channel) no hum - put V1 in (a Ruby) and thought I heard the hum just starting to sneak back in. Replaced the Ruby V1 with a grade B Sylvania from KCA. No hum, even after a hand ful of power cycles of the amp power, and standby. Usually, the hum would always come back after a power cycle. So, I was left with a modicum of confidence that this somehow had addressed the hum. Or, I was just unlucky and caught the amp in one of its "No hum - intermittent phases". I suppose if a preamp tube is failing and its characteristics are changing it could translate back into the power supply in some fashion or another but, that fashion just isn't clear to me.

Well, thank you for listening and after work today I'll go put the DRRI through some more paces and see if the hum with return with the alleged V1 tube replaced.

Best,
Phil Donovan

[Stevem]

If during this condition the amp also drops some volume and distorts, then you have a 6V6 power tube that is intermittently making a bad connection in its socket and loosing bias and red plating.

The bias adjusted pot may be going intermittent also.

[pjd3]

Thanks everyone,

So update: much to my surprise, the hum has not returned over 3 gigs since I replaced V1. For a million dollars I couldn't explain how V1 could be a cause of a 120 cycle hum, but, it ain't over, but I'm glad the hum is over.

Of course, I'm still waiting to see if it comes back, then I could at least search for something more obvious for the hum!

Best,
Phil Donovan

[didit]

Hmm --

Filament.

Best .. Ian

[pdf64]

How was the 120Hz frequency identified?
Did any of the amp’s controls affect the hum?
Was it a buzzy type of hum, ie with lots of high frequency content?

[pjd3]

Hi pdf64,

None of the panel controls had any effect on the loudness or eq of the buzz. I was nearly positive it was a 120 cycle buzz but to be certain called up a youtube video playing a 120 cycle buzz from a bad cap. It also matched the pitch of the first fret on my A string which is 110 hz, and one fret brings it up to nearly 120 Hz.

I wonder if a compromised filament condition could somehow translate into a signal wire or element. At my stage of the game I can't figure how but I still have a lot to learn. But, I did do the tube pulling and substituting in the most methodical way I could think of and V1 (the unused channel) was the final step to a non-returning hum.

Another gig last night and zero hum. I'm grateful in a baffled kinda way.

Best and thanks,
Phil Donovan