3 vs 2 prong power cord when restoring old tube gear??

[Bill Ferris]

 

[crispycircuit]

It's a safety issue, just do it ! It's extremely rare for it to cause any hum or noise.

 

[Bill Ferris]

It's a safety issue, just do it ! It's extremely rare for it to cause any hum or noise.

Kudos crispycircuit, been my experience for decades servicing and safety upgrading older 2 prong tube gear as well.. No lawsuits either or dead/injured customers !! Single AC circuit power feeds for all connected audio equipment always results in zero ground loops & low to no AC power based noise.. Take care Sir. Regards, OKB

 

[Johnnykuz]

I've done 3 prong cords on all (7) of my vintage guitar amps and have no issues with ground loops, never have. Although I get that a polarized two prong cord will do the trick fine, that assumes that the building wiring where you are is done correctly. That's fine if you're at home but if you're out on a gig with your amp, you have no idea if the receptacles & building wiring are properly grounded. If you've ever played guitar standing in front of a hot mic & touched it only to be knocked on your keister, you'll want to do that with all of your eqpt. I just completed a particularly tricky one with a 1967 Ampeg Gemini II guitar amp. Ampeg uses a DPST switch for the main power switch and not only switches the line into the transformer primary, the second set of contacts switches the amp pilot light on between one of the transformer secondary center taps & ground. The standby switch was a DPDT switch that switched one of the secondary center taps to ground AND because it could be thrown both ways, would switch the death cap between ground & one leg or the other of the incoming line. The amp had been converted to 3 prong when I obtained it but it wasn't done correctly at all. The standby switch would only work in one position and if you tried the other (opposite polarity) position, it switched the hot line side directly to ground tripping the circuit breaker in the house. Obviously this cord had been installed by a "clever" guy who didn't see the necessity to remove the death cap & fix the standby switching properly. I had to replace the DPDT switch with a SPST switch to switch the secondary center tap to ground to switch from standby to operate mode as well as remove the excess wiring & death cap. Works great & no ground loops.

 

[Patrice B]

I've done 3 prong cords on all (7) of my vintage guitar amps and have no issues with ground loops, never have. Although I get that a polarized two prong cord will do the trick fine, that assumes that the building wiring where you are is done correctly. That's fine if you're at home but if you're out on a gig with your amp, you have no idea if the receptacles & building wiring are properly grounded. If you've ever played guitar standing in front of a hot mic & touched it only to be knocked on your keister, you'll want to do that with all of your eqpt. I just completed a particularly tricky one with a 1967 Ampeg Gemini II guitar amp. Ampeg uses a DPST switch for the main power switch and not only switches the line into the transformer primary, the second set of contacts switches the amp pilot light on between one of the transformer secondary center taps & ground. The standby switch was a DPDT switch that switched one of the secondary center taps to ground AND because it could be thrown both ways, would switch the death cap between ground & one leg or the other of the incoming line. The amp had been converted to 3 prong when I obtained it but it wasn't done correctly at all. The standby switch would only work in one position and if you tried the other (opposite polarity) position, it switched the hot line side directly to ground tripping the circuit breaker in the house. Obviously this cord had been installed by a "clever" guy who didn't see the necessity to remove the death cap & fix the standby switching properly. I had to replace the DPDT switch with a SPST switch to switch the secondary center tap to ground to switch from standby to operate mode as well as remove the excess wiring & death cap. Works great & no ground loops.

Well, necroing this thread.

Being new to guitar/bass tube amps (5 since last year...) here is my take on it:

- Instrument tube amp: I make sure to do the three prongs conversion as guitar/bass strings + two prongs amp and maybe a mic on the way could equal:

- However, on hi-fi tube gear, unless previous owner already converted to three prongs, I replace the death cap (verifying at least there is no AC on the chassis) and keep the original cord...

 

[Mr Boochie]

Well, necroing this thread.

Being new to guitar/bass tube amps (5 since last year...) here is my take on it:

- Instrument tube amp: I make sure to do the three prongs conversion as guitar/bass strings + two prongs amp and maybe a mic on the way could equal:

- However, on hi-fi tube gear, unless previous owner already converted to three prongs, I replace the death cap (verifying at least there is no AC on the chassis) and keep the original cord...

Patrice, all it takes is one knucklehead having his amp's 2-cord power line flipped, compared to the others in the band. Have a hot chassis. And plugged his guitar into same daisy chain or mixing board.

 

[Patrice B]

Patrice, all it takes is one knucklehead having his amp's 2-cord power line flipped, compared to the others in the band. Have a hot chassis. And plugged his guitar into same daisy chain or mixing board.

Hello Mr Boochie,

As noted, only my hi-fi tube amps stays on two prongs (Scott, Fisher, etc...). I’m not using them with guitar, bass or mic (except one Eico via my audio interface but it was already converted back when I bought it).

My (now five) guitar/bass amps are all converted to three prongs.

 

[Kalamazoo]

Interestingly, as the OP, I am still exploring this topic. Love to see the continued interest. One of my restored Magnavox 196s is two prong with safety cap.The other is three prong. I do not hear an appreciable difference in noise floor between the two.

Tonight I just converted one of my old Tweed Fender Princeton to three prong after I got tired of it shocking me. I will bag up the death cap and old cord as collectors seem to value old parts.

Again, thanks to all of the knowledgeable and always helpful AK mentors! Jim

 

[Patrice B]

Yes, mandatory on musical instruments amps... unless you want to be killed by a guitar/bass!