# Kalamazoo Reverb 12 - HUMMMMMM!



## fishin' musician (Jun 19, 2008)

Recently acquired a garage sale find Kalamazoo Reverb 12 that has more than just a hum, it goes GRRRRRRRRRR! and in the brief moment that I plugged an instrument into it the sound of the guitar was barely audible. The hum remains constant regardless of the volume or tone settings, however turning on the tremolo did make it go GRRrrrRRRrrrRRRrrr. I am of course going to change the filter caps and swap out the tubes, any other ideas as to what may be causing the issue?
The chassis itself looks lovely and clean with no signs of loose wire, broken solder joints or burnt resistors or capacitors.

Thanks, Dwayne


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## Greenbacker (Mar 29, 2007)

Those are the two things I thought of immediately as well. On an older amp like that with an unknown history, you can only improve the amp by doing this anyway. It's just going to be troubleshooting from here and this is a good place to start. It sounds like you might have already checked to ensure the amp is properly grounded as well.


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## gregsguitars (Aug 16, 2010)

One like this one ?


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## fishin' musician (Jun 19, 2008)

gregsguitars said:


> One like this one ?


Yes, exactly like that one, only the vinyl cover for mine got lost along the way. I'm still waiting for parts to arrive for the rebuild but I have swapped out the two prong cord for a three prong and I've taken the on/off switch away from the tone control. I'm excited to hear what it sounds like.


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## WCGill (Mar 27, 2009)

Likely filter caps. It could also be something ungrounded but with an amp this old, from a garage sale, it's likely it hasn't been used in some time and that's a death sentence for filters. Have fun, it'll be cool when it's up and running.


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## fishin' musician (Jun 19, 2008)

*Hum 90% gone*

So I changed the 2 prong to a 3 prong, took the power switch away from the treble pot and then I changed the filter capacitors. Dry, the amp sounded lovely but it still had a slight but noticable hum. The tremolo was nearly invisible and the reverb was thin so I stuck a fresh 12ax7 in the reverb driver. No change
So, I changed all of the orange drops and remaining electrolytics and voila, the reverb and tremolo sound lush and tasty.
But 10% of the hum still remains. What can I do to make the amp dead quite. Any suggestions?


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## WCGill (Mar 27, 2009)

Check to make sure your input jack is shorting when nothing is plugged into it. With a non-conducting probe (chopstick, etc.), move the wires slightly inside the amp listening for a change in the hum level. Piggyback a good filter cap (same voltage ratings please) on top of your existing filter caps and see if there is a reduction-careful here, the cap can stay charged upon removal. Are your heater wires twisted or is one side of the heater supply tied to ground? Lots of things to check here. Many of these older amps didn't have overly-generous filtering and the build-level wasn't what it is today. Good luck.


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## fishin' musician (Jun 19, 2008)

WCGill said:


> Check to make sure your input jack is shorting when nothing is plugged into it. With a non-conducting probe (chopstick, etc.), move the wires slightly inside the amp listening for a change in the hum level. Piggyback a good filter cap (same voltage ratings please) on top of your existing filter caps and see if there is a reduction-careful here, the cap can stay charged upon removal. Are your heater wires twisted or is one side of the heater supply tied to ground? Lots of things to check here. Many of these older amps didn't have overly-generous filtering and the build-level wasn't what it is today. Good luck.


Thanks for the help, plenty of room for tinkering with this one. Had some nice NOS GE 12ax7's that I was saving for my Fender Bandmaster, but I stuck them in the Kalamazoo and it quietened right down. Playing a guitar with humbuckers helps too, but I do love my P-90's! Oh well, it's a trade-off.


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## fishin' musician (Jun 19, 2008)

Finally found the source of the hum! Turns out that it was a burned out resistor that jumpers between the terminals on the input jack. I also re-twisted the filament heater wires and moved them over the other wires and away from the chassis. My vintage Kalamazoo Reverb 12 is now as quiet as a church mouse. Of course, now my reverb tank has crapped out with the input transducer giving up the ghost. Luckily I found an exact replacement Gibbs C type reverb pan on fleabay. I'll post some pictures once everything is up and running.


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