advice on grounding a vintage amp

Tube or solid-state, this section goes to eleven!
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THE_SCORPION
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Post by THE_SCORPION »

I've just scored a magnatone m8, and I would like to ground it with a new 3 prong iec socket.
what is the proper chassis grounding procedure for old tube amps?
here is the original schematic for reference;
magnatonem8.pdf
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KindaFuzzy
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Post by KindaFuzzy »

Wicked, with reverb too. I like to ground the IEC ground right beside where it enters the chassis with a dedicated bolt and one of those star washers with solder lugs on them. The black should go to the fuse holder, then the white to point #4 in the schematic, which goes to the power switch. Hope that helps, and post some pics if you get a chance.

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THE_SCORPION
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Post by THE_SCORPION »

ok, thanks
but what about the .047uf cap from power switch to chassis ground - shouldn't that be removed?
it's suggested on this page: http://www.rru.com/~meo/Guitar/Amps/Kal ... /safe.html

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KindaFuzzy
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Post by KindaFuzzy »

For sure take that cap out and save it for something else if it's a good cap. Then you should be good to go.

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THE_SCORPION
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Post by THE_SCORPION »

got around to doing the iec mod today.
had to cut socket opening with dremel to get iec to fit
Image
then used sheet metal screws to fasten iec to chassis (old receptacle was held in place by spring mount clips)
Image
used cloth covered wire to piggyback on an existing ground lug and soldered fuse holder and neutral leads
Image
after removing the cap from power swich to chassis , i fired it up and the amp has less hum and oscillation noise.
recommend this mod to anyone wanting to breathe new life into a vintage amp
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iec_wire.jpg
iec_wire.jpg (21.17 KiB) Viewed 1654 times
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dread pierat
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Post by dread pierat »

The IEC chassis ground is supposed to be on its own bolt with nothing else attached. This is to eliminate the possibility of forgetting the chassis ground after working on the equipment. Remember: chassis ground is a user safety feature, while the circuit ground is there to make the electrical circuit work. There should be no currents in the IEC ground under normal operating conditions so it doesn't make sense to connect it to a node in the circuit anyway.

The IEC ground cable should be the longest wire of the 3 connected to the IEC plug, to prevent a live chassis in the event a mechanical failure where the IEC plug leaves the chassis.

I wouldn't use self-tapping screws to fasten the plug, considering the abuse it gets when plugging/unplugging, or when someone pulls the wire.

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THE_SCORPION
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Post by THE_SCORPION »

dread pierat wrote:Remember: chassis ground is a user safety feature, while the circuit ground is there to make the electrical circuit work.
Well, Shiver Me Timbers!!! the chassis ground is the circuit ground in a vintage amp.
BTW, self tapping screws through 1/8 in. steel ain't goin nowhere, i don't care how many times you unplug it

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dread pierat
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Post by dread pierat »

THE_SCORPION wrote:i don't care
Look, I can also quote out of context.

I was stating two proper chassis grounding procedures for IEC Class I, not giving a talk on the relative merits of chassis currents in vintage amps.

There should be no currents in the IEC ground under normal operating conditions. It should be wired and connected as a safety feature - because it is.

:horsey:

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DougH
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Post by DougH »

THE_SCORPION wrote:I've just scored a magnatone m8
Cool amp! 8) How does it sound?

I'm building a Magnatone 107 Starlet clone right now (with a safe isolated power supply, 3-wire cord, etc). Looking forward to hearing it when it's done.
"You have just tubescreamered or fuzzfaced yourself " -polarbearfx

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THE_SCORPION
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Post by THE_SCORPION »

Hmmm... check out the Coasters' "the shadow knows"

You can hear the sound of magnatone vibrato on the guitar.
True pitch-shifting vibrato is really something else... especially on a hollow-body.
the amp even has a "mellow" tone setting, great for dirty, low-down retro blues flavor.

Could you post the schemo for Magnatone starlet?
You might want to daisy chain the input stages if it's similar design to the M8, where each instrument input
takes only one half of the preamp tubes power. By wiring them in parallel you get double the output.

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