120Hz hum starts when amp warms up?

[premiumplus]

I'm working on a Super Reverb that has a 120 Hz hum, isolated to the PI or output section. It's quiet until it heats up.Anyone ever encounter a heat related hum? It's not tubes, I've replaced them with known good ones. Next step is component cooler when it's warmed up, I guess.Also wondering if the fiberboard could be getting conductive.I'd appreciate any thoughts you all may have...thanks fellas.

[Garrett335]

Howdy!
What do you mean by heats up? Like after a minute of the heaters warming up? Or after playing for an hours?

[premiumplus]

It starts humming after ten minutes or so.

[Latole]

Filter caps  ?
Are they new or originals ?

You'd be surprised how old filter caps can cause strange problems.
If they are original, I would replace them.

[tubeswell]

Filter caps  ?
Are they new or originals ?

You'd be surprised how old filter caps can cause strange problems.
If they are original, I would replace them.

Including replacing the filter caps for the bias supply voltage (which can cause hum at the output tube grids)

[Latole]

Filter caps  ?
Are they new or originals ?

You'd be surprised how old filter caps can cause strange problems.
If they are original, I would replace them.

Including replacing the filter caps for the bias supply voltage (which can cause hum at the output tube grids)

Right , it is a filter cap too

[premiumplus]

All the electrolytics have been replaced, including the bias cap, and the work looks fairly recent. I did scope them looking for AC, and I also paralleled them with known good caps and that didn't help. Everything filter capacitor related looks ok, including the bias cap. Also, there is a 3 wire power cable and the 'death cap' has been removed.
Whoever did the work also replaced the coupling caps with some nice yellow film caps. When I first opened up the amp and saw that my heart sank, thinking that he had harvested the nice blue coupling caps that Fender used in the mid-late 60s. But I think by 1975 they had changed to the brown turd caps, so I'm assuming they are okay.
Usually I am really bummed when I see a previous "cap job" but whoever did this job attended to detail, and nicely dressed the leads. Everything is very nicely done.

[Latole]

We like pictures, can we have some ?

I will take some AC ( rippple ) reading at power supply ; Voltmeter and scope
1- Before hum
2- With hum

Don't truss looking new filter caps

[66Strat]

All the electrolytics have been replaced, including the bias cap, and the work looks fairly recent. I did scope them looking for AC, and I also paralleled them with known good caps and that didn't help. Everything filter capacitor related looks ok, including the bias cap. Also, there is a 3 wire power cable and the 'death cap' has been removed.
Whoever did the work also replaced the coupling caps with some nice yellow film caps. When I first opened up the amp and saw that my heart sank, thinking that he had harvested the nice blue coupling caps that Fender used in the mid-late 60s. But I think by 1975 they had changed to the brown turd caps, so I'm assuming they are okay.
Usually I am really bummed when I see a previous "cap job" but whoever did this job attended to detail, and nicely dressed the leads. Everything is very nicely done.

When were the caps replaced? What are the date codes on the caps? If a capacitor is leaky, paralleling it with a good cap is not a reliable test method. A leak down test to see how long the cap holds a charge would be a better test method. Additionally, you could measure the voltages and voltage drop across the resistors in the the B+ rail for anything out of whack. A leaky cap will draw excess current and will show up as a larger than expected voltage drop across the immediately preceding resistor..

[premiumplus]

I'll check for date codes and shoot some pictures when I get down there tomorrow. Hopefully early afternoon.I agree that just looking for AC across filter caps is no good test, and I did verify that they all look as good as one can tell while in-circuit.

I'll take readings down the supply rail and let you know what I see tomorrow. Thanks for the thoughts, fellas...I've been doing this for a while (retired EE and longtime amp builder/repair) and I can't remember seeing one that was quiet until the chassis heated up.
Something else that I will do, RG Keen suggested "re-melting" the wax coating on the fiberboard with a hair dryer. Makes sense...a leaky board can cause all sorts of grief.

[premiumplus]

Things are looking up. I replaced the phase inverter input coupling cap and the hum is nearly gone. Way, way better. I've been playing it for a couple of hours and things are good. I'll let it cool off overnight and see what happens in the morning.

[premiumplus]

Thanks fellas, it looks like things are working as they should. In addition to replacing the PI input coupling cap I tried something that RG Keen wrote over at geofex some years ago. He suggested using a hair drier and "remelting" the wax that impregnates the fiberboard Fender used, in order to cure "Fender-itis". You're probably aware, but it is a condition where the board became conductive.  So I did that, and took Q-tips to clean up the board while it was hot. Problem solved!

It makes me happy, because this particular year of Super Reverb, the '74-'75 era was the first real tube guitar amp I had. I traded a keyboard player straight across for it. His Super Reverb (which was like new, 1 year old) for my Acoustic Control Corp. 150 head and 4x12 cab, which was also like new. It was a great trade for both sides, the keyboard guy got a great sounding solid state amp and me the guitar player got a nice new Super Reverb. I gigged that amp in the late '70s and early 80s...fun times.The amp we were just working on was modded and the infamous "Pull Boost" circuit was removed, which didn't break my heart. He changed all the coupling caps too, for whatever reason. I'll never know. Also, the amp has got the nice CTS Alnico 10" speakers and is in great shape.
Here's a couple of photos, before and after the wax 'Remelt'. Also, the last picture shows a little mod I did today to make the 3-way ground switch into a Negative Feedback selector. To the left is standard 820 ohms, center is no feedback, and right is 1500 ohms. The amps sounds great and takes me back 40 years ago. Thanks again for the help!

[66Strat]

Congrats on getting things sorted out. Now all you need is a roadie to carry that thing around for you. It hurts my back just thinking about carrying it. I've got an 83 Concert that weighs about 75 lbs. It likes to stay in one spot.

[mresistor]

Man that is one CLEAN super reverb..   Great that things worked out for you as I think SRs are some of the best sounding amps out there..  I think many performers would agree.


66Strat  there are plenty of dollys these days to take the load off your back ..       

[66Strat]

Man that is one CLEAN super reverb..   Great that things worked out for you as I think SRs are some of the best sounding amps out there..  I think many performers would agree.


66Strat there are plenty of dollys these days to take the load off your back ..   

It likes to stay in the basement where it can wail really, really LOUD. I like to play it while I'm upstairs (think Albert Collins long cord) where I can sit and sip Bourbon whilst I play.

[sluckey]

Are you using that 400E for a door stop?   

[premiumplus]

Are you using that 400E for a door stop?   

It's a foot rest. I use an old Sprague TO-6A for doors. 
You can't see it, but the SR has a set of casters on it. Still, I doubt I'll be slogging it around like I did the other one 40 years ago! I've got a new speaker grille with aged cloth on order from Mojotone...the one that came with it had a big hole and was in bad shape. The tolex is great, though. I'll put up some pics when I get this one all finished. It's going to look like a well loved original. It'll probably end up semi permanently onstage at church for praise band duties. That amp really sounds nice now. Almost as good as my Z amps.