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hugeshows hugeshows is offline
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Default Restoring a Sherwood S-5000

While I wait for JRC to update us on that, I thought I would post on some progress I've made. I've re-done the bias supply and finished the HV+ filter sections, for starters... But I am too lazy to write that up right now! Let's pause that part of the thing and go over the electrical safety for this amp.

Lots of people think that automatically putting a 3-prong plug on a vintage amp is a good idea. There is partial truth to that. It is certainly a lot more important on a guitar amp than it is a hi-fi amp, however. On a guitar amp, you can be holding an instrument that is at whatever potential the chassis is at, and if you touch a grounded microphone with your mouth it can be enough to rattle your fillings! That is a very potentially dangerous situation, especially considering the wide array of generally decrepit electrical situations one typically finds on a stage. So folks who automatically put a three-prong cord on a guitar amp will find no quarrel with me.

Putting a three-prong outlet on an integrated tube receiver/amp however is a different animal. For starters, chassis ground is signal ground on these amps. If you ground the chassis to your house ground, you have introduced a level of safety perhaps, but you've also put a potentially very noisy and dirty ground on your signal ground. This raises the probability of ground loop related issues, where AC travels your signal ground, and just about guarantees that your phono stage will be a complete pain in the ass to get quiet. Modern amps that are designed to have 3-prong plugs are a different beast, though not immune to this problem either. It's not uncommon to see "ground" floating a few volts above actual ground, and in lots of areas in the US, ground is merely bonded to the neutral bus in the breaker panel. So essentially, you've just connected your amp chassis to neutral - the same neutral that connects to every appliance in your house, and gets dirtied by any number of motors, compressors, dimmers, etc.

I think that converting to 3-prong on a the S-5000 is a mistake and not at all necessary for safety. If somebody feels otherwise, no reasonable argument will be ignored, so speak up. Safety is more important than tone.

Does that mean you should leave the stock power cord on your amp? No. For starters, the original cord is not polarized. Putting a polarized plug on your amp will do more for safety than just about anything else. First off, it guarantees that the hot leg of the AC goes through the fuse and not the neutral leg. That's very important. There's a twin capacitor in the S-5000 located near the convenience outlets. It's a ceramic and not at all likely to fail. But, if it DID fail by shorting, and you just happened to have the un-polarized plug still, then there's a 50/50 chance that the chassis now has full 120AC on the it, and that same 120AC is also bypassing the fuse now too!

On top of that we have the convenience outlets themselves, which while not terribly unsafe, are also unpolarized, but moreover using them tends to dirty the noise floor of the amp. So we need to deal with the electrics. Something has to be brought up to date. Here's what I do:

1) Replace the original cord with a polarized two-prong lamp cord.
2) Disconnect the courtesy outlets and all extra wiring for them
3) Wire the mains cord to bypass all that original extra stuff
4) Replace the dual .002 cap with a .01 cap, and move it behind the power transformer where the center-tap grounds.
5) Move the vintage Sherwood tag to the new cord!
6) CHECK THE VALUE OF THE FUSE!! SOME DIMWIT STUCK A 10A FUSE IN THIS UNIT!!

Here's what the unit looks like gutted of all the unnecessary AC cordage and with a new power cord:

https://www.dropbox.com/s/j6si8yqs91...%20%282%29.jpg


See how nice that cleans up? All of the wires to the receptacles is gone, the chassis noise cap is now behind the power transformer (and is now ALWAYS on neutral!) So by forcing the chassis cap to be on neutral, and the fuse to be on hot, we have improved the safety of this amp significantly without messing with ground or screwing up noise levels. All while maintaining a fairly stock look:

https://www.dropbox.com/s/u5dfec453z...%20%283%29.jpg


So that leaves the question of the AC receptacles... There are a couple choices here. For starters, remember how I mentioned that I've added individual cathode resistors? Well, we now have 4 external terminals unused! These AC receptacles can be re-used as bias current measurement points! If you're fearful about what would happen if somebody plugged a tuner into the socket (not much) you can solve that by stuffing the sockets with cut-off AC prongs, allowing probe insertion, but blocking an actual plug!

If you don't mind altering the chassis, those holes line up perfectly with certain styles of slider switch. You could add any number of options, either with the bias or even put a standby switch if you wanted (not recommended).

In any event, the amp will be safer and possibly less noisy after you do this.

More to come!


-forkinthesocket