BF Grounded plug causing problems

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BF Grounded plug causing problems

Postby nicothebassdude on Tue Mar 01, 2016 3:55 am

Hey all, long time reader, first time poster. Lots of good info to be had in these forums!

Anyway, I've tinkered with amps for several years now, mainly just doing easy repairs mods and maintenance to friends amps. One such amp I am working on is a 66 Super Reverb. Kinda been giving it the ole over-haul so to speak, and one of the things I changed was adding a grounded power cable. I did this as I have on all other amps I have swapped the cord on, bypassing the ground switch and death cap all together, and tying the ground to a lug on the power transformer.

Amp fires up, is quiet, sounds amazing as expected... except when I kick on the vibrato I get the tick/click/pop, but it is REALLY loud. I did the mod where you add a .01uf cap from the 10meg resistor across the opto-isolator to ground. It did change the behavior a bit, but not much. This tick is so bad it will damage the speakers. It does change speed with the intensity knob. On a whim, I un-lugged the ground for the power cable and no more tick! no tick at all in fact, just perfectly working vibrato! Any idea what would be causing it to tick really loudly with the ground wire attached?

Am I missing something obvious here? This one has me stumped and all I seem to find on the googles is the mod to add that cap to filter out some of the noise in the vibrato circuit. :dunno:

If I remember I'll try to post up pics and what all I did once I get it back together. This is the one last thing that I haven't been able to figure out and remedy yet. I want the vibrato to work, but also the amp to be safe :?
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Re: BF Grounded plug causing problems

Postby eolon on Tue Mar 01, 2016 11:17 am

The easiest thing to try first is to switch the position of the black and white 120 VAC cord wires. It might be causing the problem, since the white wire is at the same potential as the green ground wire. If the Super still has the ground switch, just flip the switch and see if that helps. If not, you may have a ground loop somewhere, which unfortunatley can be hard to track down. Lead dress can contribute to this problem, so check the solder points on the vibrato caps and make sure that the caps are not lying down ( they should stand up).

Hope this helps,

e
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Re: BF Grounded plug causing problems

Postby nicothebassdude on Tue Mar 01, 2016 1:44 pm

Thanks for the response. My house is from the early 50's and mostly 2-wire. But I do have a grounded plug at my work bench. However, it could easily have polarity reversed, or some other fun things going on behind that plate. Regardless, I kinda wondered if polarity might be the issue.

At first I thought I can just use the original ground switch to lift the ground of the power plug, but that then presents possible danger to the user. And now that you have made your post, I believe I do recall doing this mod once before and changing out the spdt ground switch for a dpdt switch that would reverse the hot/neutral polarity. The ground switch is still in the super, I just completely bypassed it though since it was the style that shunts to chassis via the "death cap". I'll try swapping the wires tonight and reconnect the ground wire to see what I have. If that fixes it, I think changing the old ground switch to a polarity switch just might be the answer.

You never know what you might get out of the wall at some of the older dingy clubs either :cheers:
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Re: BF Grounded plug causing problems

Postby eolon on Tue Mar 01, 2016 2:20 pm

Yep, a lot of clubs and other questionable venues have amateur-wired outlets. That's why whenever I install a three conductor cord I try to use the ground switch so you can still switch polarity (safely). (There was something to be said for the old days when you could just rotate the cord plug and see if that reduced the ground hum.)
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Re: BF Grounded plug causing problems

Postby nicothebassdude on Tue Mar 01, 2016 2:27 pm

I think what was confusing me the most was the fact the amp was otherwise quiet... UNTIL I kicked on the vibe. Just hoping polarity reverse doesn't result in working vibe, but also a buzzy amp :)
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Re: BF Grounded plug causing problems

Postby PA100 on Tue Mar 01, 2016 10:03 pm

I wouldn't go swapping the phase and neutral wires around! I'd also check that the fuse is on the phase wire and not the neutral... Mine was on the neutral.. meaning that if the fuse blew because of a fault then the amp remained live... not good. Swapping around the eyes will have the San effect. Earth wire should go to the chassis close to where the cable enters.
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Re: BF Grounded plug causing problems

Postby nicothebassdude on Wed Mar 02, 2016 12:06 am

aha! I found the problem to be how I had the ground hooked up. I had it tied to the same tranny lug as the ground wire going to the filter cap board. I also had extended the ground wire to do that. I removed the wire I extended it with and tied it to the lug closest to where it enters the chassis as suggested, directly opposite where I had it lugged down before, and all is happy. No need for a potentially dangerous polarity reverse or ground lift switch! YAY!

Think I'll take a few more pictures and start putting it back together tonight. I wanna get it back to its owner so I can hear him melt my face with it :) More of a bass player myself if you couldn't tell by my board handle ;)
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Re: BF Grounded plug causing problems

Postby nicothebassdude on Wed Mar 02, 2016 12:46 am

Also, I have an electrical engineering background... so I understand the circuits and how they work... but there is still a certain amount of "voodoo" to tube amps that I haven't quite mastered yet...
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Re: BF Grounded plug causing problems

Postby pdf64 on Wed Mar 02, 2016 8:32 am

Well done!
Bear in mind that the PT fasteners often work loose; a dedicated fastener / lug for the safety ground is best practice, and is the requirement for new equipment of this type.
It's the only safety essential connection in the amp, and could prevent a fatality.
My band:-http://www.youtube.com/user/RedwingBand
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Re: BF Grounded plug causing problems

Postby stratele52 on Sat Mar 05, 2016 9:31 am

nicothebassdude wrote:Also, I have an electrical engineering background... so I understand the circuits and how they work... but there is still a certain amount of "voodoo" to tube amps that I haven't quite mastered yet...


There is no voodoo there all is electric/ electronic theory.
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