Excessive hum with 1960's amp.

[Rotus623]

I posted a thread in here about a 1964 Sano that I picked up the other day. Its got a really nice vintage tone, and I love it!! The downside is this, when the amp is turned on, it makes just a bit of hum. When I turn the main volume up, past half way it starts to hum a lot louder, and it continues to increase as I turn it up. If I turn up the secondary volume, and turn down the main volume I can get the amp a lot louder before the hum kicks in.

The amp has a two prong power cord, and it has a hum/interference knob on the back along with a polarity switch. The polarity switch changes nothing, and the hum dial corrects the hum a bit.The amp is plugged into a monster power supply/surge protector. I was thinking that there is either a bad main volume pot, or I need to convert to a 3 prong.

Another strange issue, when my buddy and I were playing our electrics,I touched one of his strings and got the hell shocked out of me. He touched my guitar string and it zapped him too. What the eff??

[DigitalJunkie]

Does that amp have a power transformer?
If not ADD AN ISOLATION TRANSFORMER -My guess is that it does not have a power tranny,and thus no isolation from the AC line,very dangerous as you discovered.


Edit:
DO NOT add a 3-pin power cord until you verify that it is not a line-operated circuit,or else you will pop the breaker on whatever outlet it is plugged in to.
Add an isolation transformer,and then a 3-prong cord,once it's isolated from the AC mains.

[Rotus623]

Actually it does have a power tranny...... I am working on getting the schematic right now.

[prairiemystic]

Rotus623, you need to be careful... you can take an AC voltmeter reading from earth ground to the amp's chassis (1/4" jack on guitar) and see what you get, also while trying the hum controls.

All ungrounded (two-prong powered) tube gear floats above ground depending on the leakage of the power transformer and polarity switch caps. It can be enough to just "bite" or enough to kill - usually when musicians grab the mic (grounded) while touching their guitar. Similar to Leslie Harvey's death in 1972...

I would measure the resistance and leakage current from the amp chassis to its power cord pins. If it's a shorted power transformer, the amp should not be used until fixed.

[Rotus623]

Gotcha, so you think that the power tranny is shorted? What should my voltage reading be from the chassis to ground?

[Rotus623]

Holy **** Prarie!! I am getting 118.2 VAC from the chassis to earth ground. What does this mean?!

[taj]

Now you know why KISS wear those boots.

But seriously, turn the plug around in the wall. Does the voltage go away?

[Rotus623]

Hahahaha!! Yes Taj, good point, when I reverse the polarity (it actually has a switch to do so) the voltage goes down from 118.2VAC to 2.xx VAC, so phew, that's a lot better. BUT, there is still a hum at loud volumes even when the polarity is switched. Any ideas??

[jjman]

It's time for a properly wired 3 prong power cord and new filter caps. That's just to get the ball rolling. Better safe than sorry.

[Rotus623]

Worrrrddddd... Thanks to all guys, I'm gonna get crackin' on this big boy. I'll let you guys know when I get her all converted.