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Post subject: 1969 Super Reverb- 60 Cycle Hum
Posted: Thu Jan 05, 2012 10:46 am
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I recently was offered a great deal on a '69 Super Reverb. It's got what to me sounds like a 60 cycle hum. How dangerous is this, and could it bite me in the $@! later? THanks for all of your help.

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Post subject: Re: 1969 Super Reverb- 60 Cycle Hum
Posted: Thu Jan 05, 2012 11:50 am
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Hi captainsmooth,

It could be a number of things (including normal). Is it barely audible, or loud? Do you hear it without a guitar plugged into the amp? Is it only noticeable when you turn the guitar volume up? Most any tube amp will have a small amount of hum while using, more or less depending on guitar and pickups, more hum with single coils.

If it is an unusually loud hum, it could be a bad tube(s), or amp problem. If an amp problem, I would first want to be sure that the three prong plug is intact, then check the power supply filter caps.

The only "danger" that I can see is if the three prong plug has the ground pin broken off. But as with any tube amp that has some years on it, good idea to give it a good once over, caps being common suspects as amps age.

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Post subject: Re: 1969 Super Reverb- 60 Cycle Hum
Posted: Thu Jan 05, 2012 4:31 pm
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Post subject: Re: 1969 Super Reverb- 60 Cycle Hum
Posted: Fri Jan 06, 2012 3:37 am
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Just as a bit of trivia. 60Hz hum is like below an open low-E string. 120Hz in like around 3rd or 4th fret of the A-string. 60Hz can be so loud, even with volume low--that you immediately worry about your speakers. 120Hz hum oftern is volume control depedent.

60Hz is primarily due to the primary side of the power tranny or the heater line. If you pull the amp look at the 100-ohm, 1/2 watt resistors tied from the lamp to ground. Often one is broken or has bad solder. These resistors act as pseudo-CT for the heater line---making twisting more effective in lowering hum. Check the twisted green lines to the tubes for crossing over signal lines. Try to keep the line away from any other line.

120Hz is primarily due to ground loop. This is more of a pain in the arse to fix. Could be a bad solder point to ground. Mis-wired shielded cable to the input tubes (one side should only be grounded). Cables crossing over each other, esp the power supply wiring coming from the rectifier to the power supply caps. Dried out power supply caps. Color of your sox &/or the phase of the moon. In other words, you need to really check the wiring from input to output.

Anyhow, I like to keep the heater line lifted above the chassis, signal wires, & toward the back of the chassis. And the signal lines lifter and centered over the breadboard. Routing of wiring and ground issues are the usual 120Hz bug-a-boos.

Good luck with that great amp & the hum issue.

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