Amp started humming very loudly

[Analog Assassin]

Went to my friend’s house, where I had my trusty old Super Reverb. Plugged in, and it was humming. LOUD. Tried to play through it, the bass notes sounded horrible, distorted, actually sounded like it was generating some weird harmonics because it made my guitar sound out of tune.
I’m thinking the filter caps went south? I changed them last around 1996 or 1997. The big caps were Mallorys, the smaller ones are Atoms. Visually, they look good. No ruptures, no bulges.

The power tubes are 7581’s, I bought them around 2001. I tapped them, no loud sounds from that, a little blue glow, looks normal.

The amp still hummed loudly even with the amp volume turned all the way down, so it’s apparently not in the preamp, I really think it’s a filter cap went bad. Or could it possibly be the choke? It's kind of weird, when I turn it on "standby" and let it warm up, then I turn it on, the hum comes on right away, but it sounds like the hum "rushes" into the amp. like "mHMMMMMMmmmmmmmmmmmmmmmm"

[Mach Schnell]

If no redplating is evident from the power tubes (lack of bias/bad tube) then the caps are the most likely culprit.

[Analog Assassin]

Yeah, it really seems that the caps went south. I'm just surprised that I don't have any "visual" evidence. Then again, those Atoms and Mallorys are very well constructed.

I think I'm going to change the power tubes as well. I was looking at JJ's KT66 tubes, as I've been intrigued by KT66's for a while. Any recommendations? I play moderate volume stuff, I use my BSIAB2 for distortion, and run the amp to where it's still clean but punchy and dynamic.

[billyz]

First check the Power Supply caps and Bias cap . It would not be unusual for them to go bad in 10+ years. They can visually look fine, measure them. I have seen the choke go bad too. The hum was higher than normal but not as loud as a bad cap. Check the Tube Bias and Current too. Mismatched Power tubes can hum, especially if one is really drawing current.

[billyz]

For power tubes in a Super Reverb I would be more inclined to find a nice set of RCA's 6l6's. Even good Pulls of the RCA 6l6 will make that amp sound like Audio Nirvana, use a good NOS 12at7 to drive them. The RCA's will cost a bit more but the reliability and much longer life span in addition to the superior tone, makes them the KING of power tubes for the Super Reverb.

The recent JJ power tubes I have been seeing are very unreliable right out of the box. The potential damage they can do makes me uneasy too, especially in a nice vintage amp.

[Analog Assassin]

First check the Power Supply caps and Bias cap . It would not be unusual for them to go bad in 10+ years. They can visually look fine, measure them. I have seen the choke go bad too. The hum was higher than normal but not as loud as a bad cap. Check the Tube Bias and Current too. Mismatched Power tubes can hum, especially if one is really drawing current.

I replaced the bias supply cap and diode not long ago, maybe a little over a year.
I'm not sure how to test the power supply caps. I've got a fluke multimeter, not the real fancy ones, but it's got most basic functions. I tested them with ohms while still in the circuit, all four PS caps had readings in the megaohm range.

[M Fowler]

Did you check to see if the reverb wires are plugged into the reverb in/out jacks or they have shorted or the reverb pan has shorted.

When the filter caps go bad they usually give an off a warble sounding like playing out of tune sound. Other than inspecting the amp carefully for breaks or ground problem, not sure at this time exactly what the problem would be.

Mark

[Analog Assassin]

The reverb plays fine, no reversing of leads, etc...

I'm pretty sure it's the filter caps. I was pricing them. I found that I can get the Ruby brand caps a lot cheaper than Sprague Atoms. Are they up to snuff? I know to stay away from Illinois caps.

[billyz]

I would recommend the F&T caps . The Ruby's are rebranded something, probably chinese made like the Ruby tubes.

One test of the caps is to measure the impedance and then reverse the leads and measure the impedance. It will measure very high , IE 6.** M and the other way should read a Negative impedance. The cap will be charging so the measure will be moving. But if it's leaking, it will measure the same or shorted. Another way is to measure to see if it's leaking voltage to ground. I don't think these are conclusive by any means, except if shorted, which would probably have taken out the fuse. These are some real quick checks though. If one is bad , I would change them all out.

good luck, nice amp

[M Fowler]

F&T are a very reasonably priced filter cap choice as well as the Atoms and Illinois are great value. No idea about the Ruby's.

Could go high class with Mallory

Mark

[Phil_S]

If your SR has the caps in the dog house, buy caps that have good stiff leads. I know the Atom's are like that. Make a pig tail curly q as a stress relief before attaching the lead to it's solder point. Don't stretch the leads "tight" as that can lead to premature failure. This amp has lots of power and that means lots of vibrations to shake the caps.