punkin said:
Good points there Easychair. I've never seen an amp equipped with a polarity reversal switch like that. I'm with you...it seems like a crazy idea. I still don't see how from the diagram that was posted how that switch actually cuases a polarity reversal though. The switch looks like it's DC/continuity isolated from ground through a capacitor. You've obviously seen this sort of set up before. I'm thinking the schematic isn't correct or the amp isn't exactly as the schematic appears.
The switch doesn't cause a reversal. I may have been on a crack binge. All it does is switch the cap from one leg of the service to the other.
The cap was necessary back in the day, it grounded noise from the chassis. Tube gear components, most notably transformers, produce eddy currents in the chassis. This makes voltage, and noise from the chassis not being a ground potential. There is an imbalance. It is also a natural part of tube gear. Some trannies and designs do this more than others. The chassis needed to be tied to ground, and through the neutral was the only way. I'll admit I don't fully grok this, but the cap makes it a high-impedance connection? Apparently this is important. The three wire system gets rid of the problem by providing a direct signal connection to ground without going through the neutral. Some makers still provide the switch, though. All I can figure is that...well, I can't figure it. I'll ask THE MAN WHO KNOWS next week.
A side note- some tube amps don't like to be on GFI outlets. GFIs trip when they sense a current imbalance on the legs. If enough current is being shunted through the ground wire, the GFI will sense the imbalance, interpret it as a fault, and trip, even though the amp is working fine. It's mostly old ones, as now there are strict rules about how much current and voltage can be on a chassis.
Another- lots of old tube stuff, radios, TVs, amps, didn't have the switch. You just plugged it in the other way if there was a problem. The switch was a convenience, then a necessity when polarized plugs came along.
Back to it-
All I really know is that you can get shocked if the switch is set wrong, and the hot is connected to the chassis. And you can get shocked dead if the cap has shorted and the switch is in the wrong position. That will put full line voltage on the chassis. And the amp can seem fine. Most people who get a jolt haven't really got the whole thing, the cap is protecting them to an extent.
You are right, Toki's amp is different than that schematic. His has a three wire system, with ground connected to the chassis. The cap and switch are after the accessory outlet.
At least, that's what pretty much every other late '70s Fender has. It can most likely be removed without affecting the amp at all. And even if it has a noise issue, it can most likely be resolved another way. Many noise issues in more modern amps can be traced to design and layout and component compromises.
I find it interesting Mesa still provides these switches. And that they say "if the switch is left in the middle position, any other shocks come from other gear" without ever saying that using the switch could shock you.
Happy Thanksgiving.
) and he is correct that this is the best way to verify a circuit. I do agree on this. The author of the article also represents a company providing professional power quality services and maybe in my mind he might slightly tend to bias information that would prove beneficial to promoting his interests.
