intermittent 60Hz problem -- ideas? suggestions? Help

[jimipage]

Hey fellas--

I built an amp for a customer a few months ago. It's a single channel cathode-biased dual 7591 design loosely based on a Super Reverb. He plays it loud and proud on his weekly gig, and has been for the last few months with no problems. A couple of weeks ago he turned on the amp and got a 60Hz buzz/hum. I've been checking it out this week, but can't find the source. The buzz/hum kicks in 10-30 seconds, sometimes even a minute, after the amp goes from 'standby' to 'on'. It literally 'clicks' on. The buzz/hum is affected by the Gain, Treb, Mid, and Reverb controls. Bass doesn't seem to make a difference, and Master only varies the overall volume. I CANNOT figure this out. I'm ready to pull my hair out. One guess I have is that it's a capacitor issue, maybe? How would I check caps for this?
If anybody has ANY idea what to do or what to check for, PLEASE help.

--thanks!

[Andy Le Blanc]

Not familiar with the amp but theres a few things to try

short the input to the phaseinverter to see if its the pre or power

is it affected by tube replacement or say you pull the first gain stage tube

double check bias and bias supply circuit

if its a bad cap or fault in the power supply it will grind really loudly
no matter what

but a bad coupling cap will break up the amps sound when played

[CaseyJones]

It literally 'clicks' on. The buzz/hum is affected by the Gain, Treb, Mid, and Reverb controls.

Assuming your build was solid to begin with I'd swap the preamp tubes for known good tubes. In a Fender influenced circuit I'd make sure the input jacks are tight, the switching contacts in the jacks are clean and the preamp "sees" a good solid ground. The "click on" part makes me lean towards a faulty tube.

[Phil_S]

When a problem presents itself like this, I think it is important to be extra methodical in trying to troubleshoot it. This means keeping notes about everything and anything you observe, even if it looks normal. Sooner or later, you will be able to isolate and address the problem. I'll say it again, being methodical, layout out a well thought out plan, following the plan, and keeping notes is they way to tackle any problem (in any discipline) that is difficult to solve.

Annoying as this is, going back to basics is always a good practice. For this I'm attaching an couple of pages from Tom Mitchell's book (sorry, I think you know all this, but it can't hurt). Mitchell says, "Most common causes of hum are faulty filter capacitor, loose ground connection, bad output tube, and bad rectifier". I'd go there first.

You are fortunate that there is an obvious clue. At 10-60 seconds, the problem clicks on. At roughly 60 seconds, most circuits will come to full operating temperature or pretty darn close. So, think about what's affected by heating and cooling. I'd look at the tubes. I'd look at caps. I'd look for a bad solder.

Since it's easy to swap tubes, just do it. I have no reason to say this, but I'd swap the rectifier first. Also, pulling tubes one at a time may help you to narrow your search to a particular section of the amp.

Assuming tubes are good, I'd get a chopstick and magnifying glass to start looking for a broken solder. I'd start by examining every ground connection, and yes, I'd start with the filament string and the artificial CT if you used one. I would also concentrate on tube sockets because the tubes themselves generate so much heat. I might just reflow any solder that is the least bit suspicious, as this is a relatively quick task.

If you get this far and there is no change, I would then suspect individual components that are affected most by heat or the circuit becoming fully charged. As you know, the usual suspects are caps and resistors. At this point, I suppose you've got to start lifting C's and R's from the circuit to test them.

Good luck.

[ampdoc1]

A can of coolant is also a big help in finding "thermal" problems. You don't even have to pay the big bucks for coolant sprays, Just buy some of the "air" spray for cleaning computer keyboards, and turn it upside down when spraying your circuit.

ampdoc

[jimipage]

Thanks for the helpful and informative replies, gents. I certainly feel like I've got a better jump on this today
Here's what I've done so far:

changed out preamp and PI tubes
replaced entire PI with new components
replaced entire reverb circuit components
replaced some pre components
replaced first pre filter cap
replaced most pots

Amp has SS rectification, so I doubt the problem is there. I guess I'll try new output tubes and reverb tube. I'll look to replacing the rest of the filter caps if need be, as well. And, of course, look to reflow joints.
Funny thing is, the tone of this amp still sounds great. It was, IMO, a solid build from the get-go. My customer has been using this amp exclusively since he got it 3 months ago. At least he's still happy

One thing I didn't describe: yesterday he's playing a gig at a blues club. I can hear the hum/buzz only slightly while the band's playing(of course, I'm actually listening for it). In the middle of a song I get up to go to the restroom, which is in a hallway around the corner behind the stage. I'm in there and I can still hear the noise. This has now become more than annoying. Then I look up and see the flourescent light in the restroom. I go over and turn off the switch and the buzz goes away. Same exact buzz. Of course, I was hearing the light and not the amp all the way back there, but it was the same noise/frequency. Is that 60HZ or 120Hz?

Anyways, thanks again for the assistance, guys. I'll report back when I know more. And thanks, Phil, for the link. I need to print that and hang it up in the shop.

[Structo]

I think fluorescents are 120 hz.

[Firestorm]

You might try taking voltage measurements on the preamp plates during the 30-60 seconds before the hum clicks in. Then measure them again to see if they change. If a coupling cap is leaking DC, it will make an amp hum. I don't think power amp hum would be much affected by gain and tone controls, so I'd concentrate on the preamp sections first. Check the mid cap: you say that the bass control doesn't seem to affect the hum -- if the mid cap has gone wacky, it would manifest itself in the Bass control (does the bass control actually work to alter the bass?)

How old is the filter cap feeding the preamp. That could hum if it's on its way out.

[Andy Le Blanc]

another thought.......
if its 60 cyl hum.... check your fil. hookup
also double check the amp rating of the fil. winding Vs. the total draw from the tubes

if youve over drawn you might have over heated the winding and it might be
introducing the buzz there

[jimipage]

Here's the progress so far:

replaced all preamp PS filter caps
replaced lead from first stage blocking cap (treble cap) to treble with shielded wire
replaced lead to first stage plate

The new shielded wire and the new plate lead did help a little. PS caps were just fine, I suppose, because nothing changed upon replacing them. Seems that the noise now only kicks in if a guitar is plugged in. OK, fine. I checked my input ground and everything seems solid. Then I happen to look at the filament wiring and see that some of the wires have fused together! I found where one was not cut short enough at the socket and was barely making contact with the socket mounting screw. OUCH! So now I've gotta rewire the filament -- UGH. That's has to be the worst part of ampbuilding for me. To top it off, in trying to get the little bits of filament lead out of the sockets, I ended up breaking a pin on three of the four sockets -- I've only got two replacements on hand at the moment, but I've got more in storage. The saga continues...
thanks again, guys. I'll be reporting back soon.

[CaseyJones]

So now I've gotta rewire the filament -- UGH. That's has to be the worst part of ampbuilding for me.

HOOoooLLLEEE CRAP, dude! I can see you bent over the engine bay just rippin' parts outta this thing! Sounds like at this point it's a complete overhaul less that one silly socket. Replace that one more part and it's a brand new amp!

Oh yeah, I think you found yer hum problem.

[jimipage]

So now I've gotta rewire the filament -- UGH. That's has to be the worst part of ampbuilding for me.

HOOoooLLLEEE CRAP, dude! I can see you bent over the engine bay just rippin' parts outta this thing! Sounds like at this point it's a complete overhaul less that one silly socket. Replace that one more part and it's a brand new amp!

heh! it wouldn't be the first time

[ampdoc1]

Minus the fairly obvious fact that I wired up the wrong pins on my power tubes (I oriented the tubes off 90 deg on my last "scratch" build, but this is a kit), using buss wire is SOOO much simpler than twisted wire. The socket pins are tied to the buss with pieces from resistor and cap scraps.
0 hum on my last build.

ampdoc

[Andy Le Blanc]

hey you found it......
had a bassman in for repair once with flaky fil, winding
I was to the point of insanity and wonderment when I accidentaly
shorted one side of the fil. to ground and discoverd the fix
fil. wiring can be a chore... been liking rails lately too

[nickt]

Minus the fairly obvious fact that I wired up the wrong pins on my power tubes (I oriented the tubes off 90 deg on my last "scratch" build, but this is a kit), using buss wire is SOOO much simpler than twisted wire. The socket pins are tied to the buss with pieces from resistor and cap scraps.
0 hum on my last build.

ampdoc

I like the look of your buss except for it's not being insulated (unless you're using clear heat shrink?). Might give it a try! Thanks for posting!

Edit: Oops sorry for the hijack...