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[Mike Sokol]

I've got two different isolation transformers coming for this upgrade. The little 35-watt Stancor is a 120-to-120 volt open frame transformer which I'll mount in the bottom of the Danelectro case. That will allow me to add a proper grounded power cord and isolate the chassis from the incoming neutral line (or hot line if the plug is reversed ) I'll properly ground the chassis to the EGC without creating a secondary G-N bond.

Tripp Lite has finally agreed to send me a cased 250 watt isolation transformer with a Faraday shield. I plan to try this out for leaky back line gear that's tripping GFCIs. My question then will be do I want to actually ground the chassis of the guitar amp, or completely isolate it. I'm not sure the amp should be totally floating, so maybe this is another place for a stinger cap? More to try but this is certainly getting interesting. 

[Stephen Swaffer]

I've got two different isolation transformers coming for this upgrade. The little 35-watt Stancor is a 120-to-120 volt open frame transformer which I'll mount in the bottom of the Danelectro case. That will allow me to add a proper grounded power cord and isolate the chassis from the incoming neutral line (or hot line if the plug is reversed ) I'll properly ground the chassis to the EGC without creating a secondary G-N bond.

Tripp Lite has finally agreed to send me a cased 500 watt isolation transformer with a Faraday shield. I plan to try this out for leaky back line gear that's tripping GFCIs. My question then will be do I want to actually ground the chassis of the guitar amp, or completely isolate it. I'm not sure the amp should be totally floating, so maybe this is another place for a stinger cap? More to try but this is certainly getting interesting.

The original design called for a grounded chassis.  You have 2 conductors -hot, and neutral, aka, grounded conductor.  Probably not musician friendly with a hot chassis, so it would have to be grounded.

But, I don't pretend to be a musician-and I know better to get into an argument over the best way to do this    

[Mike Sokol]

The original design called for a grounded chassis.  You have 2 conductors -hot, and neutral, aka, grounded conductor.  Probably not musician friendly with a hot chassis, so it would have to be grounded.

But, I don't pretend to be a musician-and I know better to get into an argument over the best way to do this

This absolutely needs to have an isolation transformer, and the chassis should somehow be referenced to the earth via the EGC ground wire. The question is should the chassis be a bonded ground or connected to ground via a stinger cap? A bonded ground will protect the musician against a failure in the amp itself (in this case a short in the isolation transformer), but not protect them from a "hot" mic. If an integrated stinger cap at the isolation transformer is quiet enough (I'm guessing no buzz), then the musician will also be protected from a hot mic failure. I'm leaning towards building in the stinger-cap for the first try and seeing how it all works. If it operates without noise, then this experiment interleaves nicely with JR's stinger-cap experiments. 

Again, for you non-professionals out there DO NOT try this at home yet. I'm qualified to work on live circuits and take all safety precautions. Once I determine that a rewiring method is both safe and legal I'll make note of it. The same goes for JR's experiments.

[Stephen Swaffer]

The thing that concerns me with a stinger cap is that if there is a hard fault to ground in the isolation transformer, the current will be limited and no OCPD will open, leaving a hot chassis.  The reason I find this acceptable with JR's setup is that he is GFCI protected which deals with the safety issue.

I find it interesting that the original schematic you posted shows a fuse on one side of the line and a switch on the other-that is a close parallel to the way houses wired wired at one time.  Fuses on the hot legs and all switches on the neutral side.

[Mike Sokol]

The thing that concerns me with a stinger cap is that if there is a hard fault to ground in the isolation transformer, the current will be limited and no OCPD will open, leaving a hot chassis.  The reason I find this acceptable with JR's setup is that he is GFCI protected which deals with the safety issue.

You're correct, but I was planning to use a GFCI to feed the iso transformer. That would offer the best protection, I think.

I find it interesting that the original schematic you posted shows a fuse on one side of the line and a switch on the other-that is a close parallel to the way houses wired wired at one time.  Fuses on the hot legs and all switches on the neutral side.

Yeah, that's exactly how the incoming power is wired. Makes it doubly deadly because there's a non polarized power plug and no power transformer. So it's a true 50/50 chance the chassis will be bonded to the hot wire. And even if it IS plugged in the right way, when you flip the power switch off, the chassis is now energized through the filaments. Just crazy wiring, but the amp really does sound great. My iso transformer is supposed to be here this Saturday so I'll see if I can get it wired up safely.

[Kevin Graf]

I have a little isolation transformer that's a lot like the one in the photo.  But it's at least 30 years old, it's 50VA not 30 and it's a Triad-Utrad.

[Mike Sokol]

I have a little isolation transformer that's a lot like the one in the photo.  But it's at least 30 years old, it's 50VA not 30 and it's a Triad-Utrad.

Contact me off list if you want to sell it. [email protected]

[Mike Sokol]

Tripp Lite made good on their promise to send me a 250 watt (well, actually V-A) isolation transformer. So I now have two different ways to make this work. I'll set up the outboard Tripp Lite first as an experiment, even though its 250 watt rating is overkill for a 10-watt tube amp. But it should be a good proof of concept. I'll then do a permanent inside mount of a little 35 or 50 watt isolation transformer to make this safe for the musicians. In both cases I'm going to tie the chassis of the Dan to the incoming EGC rather than mess around with any on-board stinger GFCI thing. But this will be a good candidate for the full safety treatment including the GFCI stinger backline outlet. I'm going to have a little rehearsal time tomorrow (finally) so I'll try this out and see if it works as envisioned.

[Mike Sokol]

Any ideas for a temporary grounding connection that can be made to a backline amp that won't scratch up the chassis or require tools to loosen any screws? I've thought about hacking a standard 1/4" phone plug to remove the tip, then connect the sleeve to the EGC ground wire. Could then be plugged into a spare output or input jack on the amp. Because the tip of the plug would be gone, it wouldn't switch anything on the phone jack. But there needs to be some sort of spring action on the shaft to help make contact, sort of bigger 1/4" (6.35mm) version of the standard 4mm banana plug. Is there anything like that already manufactured? I've even thought about using a pin from a CeeForm connector and doing a slit in the end to expand it a bit for spring action. But I don't have an CeeForm to measure and can't find a chart showing contact diameters. If there was a version with 1/4" contacts that would be ideal. See picture for an idea of what I'm talking about.

The idea is that this iso transformer with a special "grounding wire" could be used on any backline amp to make it safe. So you wouldn't care if the stage amp had a missing ground pin on its power cord, or a leaky power transformer, or even no power transformer at all, like my Danelectro. But I don't want to use a big alligator clip to make contact with the chassis since that's going to scratch a potentially collectable amp.   

[Stephen Swaffer]

Welders often use magnetic grounding electrodes.  Should be able to find an Alnico magnent with a drilled and tapped hole.

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