Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on March 10, 2011, 08:54:24 pm
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In another post someone suggested to use a ground lift when connecting two amps. I just want to make sure I understand the ground lift concept correctly. Say you have two amps that are both grounded. You connect one to the other with a cord. Now you have a ground loop because they are both grounded and the chassis are connected. The idea is to lift the ground of one amp but that amp is still grounded through the cord connecting the two amps. Is that right?
So what happens when your buddy says "Hey man, let me try that out with my amp." So you mute your strings with one hand and unplug the cord going to the ungrounded amp with the other? Now you're connecting the two chassis together.
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http://www.freewebs.com/valvewizard2/grounding.html
check out around page 13 after d/l-ing
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You do NOT want to lift the power ground. Ever. If you are having a problem with a ground loop, it is MUCH better to lift the signal ground on the amp (can be difficult, if the amp wasn't designed from the ground up to do so), or open one of the ends of the patch cable, and cut the ground on the cable. Then, make DAMN sure you mark the cable, or you are going to be very, VERY confused the next time you try to use that cable!
(My tech kit for sound gigs has a half dozen ground lift barrels for XLR cables, and a couple of TRS jumpers which are similar.)
Gabriel
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I recently watched a video by Taylor guitars about ground loops between amps/effects that were powered from seperate sources. Big hum. Their solution was to put an adapter on one of the amps/effects, removing it's earth ground. And, leaving the signal ground as the only source of ground for that equipment. I asked them if they considered this safe, and never got a response. I look at tube amps (and the same applies to any pieces of equipment tied together electrically) like I look at old test equipment (meters, scopes, power supplies). Back in the analog days, all our test equipment had an external ground to be used to connect each together, as well as to earth ground. Of course, most if not all had ungrounded power cords. It's still a good practice. I'm wondering why they didn't just reconnect the equipment to the same power source, to see if that solved the problem.
Jack
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Man that peice from valve wizard is a deep section on grounding Im working that into my next build.
I must read it a few more times to get fully wrapped around it.
Bill
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I'm gonna go +1 with G. Hoffman. You want all your equipment properly earthed. If you experience a loop, break the ground on 1 end of a patch cord, preferably the load end. Equipment ground = safety. Signal ground = noise shield.
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Man that peice from valve wizard is a deep section on grounding Im working that into my next build.
I must read it a few more times to get fully wrapped around it.
Bill
Yes, it's takes a few readings w/ time in between to fully assimilate it all. With a number of varying ideas out there w/ respect to this issue it doesn't make it any easier. It can actually take several builds to get complete understanding (it did me).
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Or get one of these HE-2s and keep it in your cable bag for just such occasions.
http://www.ebtechaudio.com/hedes.html (http://www.ebtechaudio.com/hedes.html)
We had this situation just this past Sunday with a fellows pedal board and amp. Plugged straight into the amp, fine, through the board to the amp, buzz. We tried lifting the ground on the pedal board power supply, but that didn't help, then I remembered we had an HE-2-XLR in the tool box. Hooked it in between the pedal board and amp -- ah, no buzz.
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I recently watched a video by Taylor guitars about ground loops between amps/effects that were powered from seperate sources. Big hum. Their solution was to put an adapter on one of the amps/effects, removing it's earth ground. And, leaving the signal ground as the only source of ground for that equipment. I asked them if they considered this safe, and never got a response. I look at tube amps (and the same applies to any pieces of equipment tied together electrically) like I look at old test equipment (meters, scopes, power supplies). Back in the analog days, all our test equipment had an external ground to be used to connect each together, as well as to earth ground. Of course, most if not all had ungrounded power cords. It's still a good practice. I'm wondering why they didn't just reconnect the equipment to the same power source, to see if that solved the problem.
Jack
I'm somewhat shocked that Bob's lawyers let them say that. It lets them into all sorts of liability issues. I would NEVER lift a chassis ground. People have died from using equipment which isn't grounded correctly, and while it isn't all that likely, it is too easy to avoid, so why risk it! But do make sure you mark the patch cord, if you go that route!!!!!
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Man that peice from valve wizard is a deep section on grounding Im working that into my next build.
I must read it a few more times to get fully wrapped around it.
Bill
FWIW I have done this with several recent amps as well as my 6G15 and some of my earlier amps that I use with the stand-alone reverb unit, and it certainly makes for a fairly noiseless set-up.
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I'm somewhat shocked that Bob's lawyers let them say that. It lets them into all sorts of liability issues. I would NEVER lift a chassis ground. People have died from using equipment which isn't grounded correctly, and while it isn't all that likely, it is too easy to avoid, so why risk it! But do make sure you mark the patch cord, if you go that route!!!!!
In all fairness, and so that I don't rely on memory for what is demonstrated in the video, here is the link for all to view:
&feature=related
The video ends, leaving the viewer with the notion that it's ok to leave a piece of equipment with earth ground lifted, though it's not stated.
Jack
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I build my amps with grounding similar to Valve-Wizard and London Power. I have two grounds. One is earth and the other is the entire amp circuit. Everything is isolated from the chassis. I can lift the ground to the audio and leave the earth connected. I don't usually add a switch unless it's required.
FWIW... All my builds are extremely quiet.
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+1 LC
My last chassis build demonstrates the differences for these two to make things a bit more clear. I didn't build in a switch but if I needed one, I could always add a switch btwn the point shown of the green/yellow signal ground wire & tag strip or btwn the green/yellow wire and tranny bolt point.
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Do you use insulated input jacks? I don't understand. In order for the input tube to work your strings(cord) are grounded to the same point as the cathode so you're grounded to the power supply ground. Are you saying that you lift the chassis from the 3rd prong?
Unless you use an isolation transformer aren't your strings grounded to the circuit ground? If you lift it at that point the chassis is grounded but you're strings are not.
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Do you use insulated input jacks?
Yes, you have to or your still grounded to the safty/chassis connection.
Are you saying that you lift the chassis from the 3rd prong?
No. You have to leave that for safty.
Their using --two-- seperate grounds. Look at what LooseChange wrote again........
I build my amps with grounding similar to Valve-Wizard and London Power. I have two grounds. One is earth and the other is the entire amp circuit. Everything is isolated from the chassis. I can lift the ground to the audio and leave the earth connected. I don't usually add a switch unless it's required.
FWIW... All my builds are extremely quiet.
Read the link that jojokeo posted, it will help you better understand what their saying.
http://www.freewebs.com/valvewizard2/grounding.html
check out around page 13 after d/l-ing
And look at jojokeo's photo. that he posted a few posts back.
It's a little confusing at first but you'll see the differance after you think about it for a little while. :think1: It all seemed the same to me at first but as I read more and thought about the -- actual physical -- wiring connections it started to make sense.
Brad :icon_biggrin:
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I've got a Fender Blues Deluxe on the bench and even these are grounded in just two places... Earth and everything else. All the jacks are insulated from the chassis.
Oops... One exception and don't know why.. The Speaker jack is not insulated but it does have a ground wire back to the board and probably could be insulated too.