Peavey Power - Polarity Switch

[TomCarlos]

Friends...

I've been wondering about the older Peavey amps that have the DPDT Power (with center off) - Polarity Switch. If I am not mistaken, this switch reverses the polarity (power/neutal) going into the Power Transformer? Is that really necessary? I don't think new amps do this, do they? I also don't see lots of amps that have the .022uf caps on a switch.

Looks like power comes in (black), goes to the thermal switch, other side of thermal goes to the center of the Power Switch, polarity swapping happens, etc.

It just seems that if you had to replace a worn out power switch, you'd put in a SPST and be done with it... and loose the cap. Is there any harm in doing that?

By the way, I have read other posts that talk about Turn On, Turn Off Pops and a culprit of that can be these caps. The Bass Amp I just fixed has a nasty Pop (turning on and off). Not sure if it has anything to do with this switch or the .22uf caps.

Just wondering...

Tom

[Enzo]

Things have changed in the world in the last 30-40 years. In the USA, most wall outlets are now grounded. Trust me, when I was on the road in the 1960s and 1970s, that was not the case, clubs rarely had grounded outlets. So reversing the power leads was common to stop shocks and kill hum.

It is mostly not necessary now, but it might reduce hum in some cases still. So I see no reason to remove it. Frankly, if the switch died, I'd probably put the same type in. But if you feel like an SPST, go ahead.

Turn on pops are rare, but turn on thumps are not

Turn off pops are common and don't hurt anything, however they are not the same thing as a turn off thump.

When you flip a power switch off, many times it sparks inside. The power in the circuit takes a brief moment to die away, so the thing can still amplify during that moment. The spark in the switch gets amplified. Without getting into contact pitting and stuff, some switches spark more than others. So a new switch may help. On the other hand, ADDING a cap across the switch often reduces sparking. Now if a cap fails, isn;t there anymore electrically, THAT can increase popping, and a new cap might put it back to rights.

If the switch pops, try spraying some Deoxit down into it around the toggle. WHy not?

If you think it might be sparking in the switch, then leave the switch on. And turn it off by pulling the cord from the wall outlet. If it still pops, then it likely is not the switch, but if pulling the plug makes less noise, I then suspect the switch.

[TomCarlos]

Thanks Enzo.... I'll give it a try.

I realize that there are many amps that have what you once called the something like the "Ka whump" ... was I close? That is very different than a small (and reasonable) turn off pop. I'm hearing the "Ka whump" so I'll dig deeper. I found other threads where you addressed this too. I will review those again.

The other odd thing about the switch is the cap that goes to the output jack. There's 120v across that thing... I don't get it. Perhaps this has nothing to do with "the switch" and more along the lines of a reason for the cap to go from the output to the power source. I'm sure there is a plausible explanation.

Tom

[soundguruman]

The switch reverses the polarity.
ONE of the incoming power wires connects to the chassis thru a capacitor, this stops some buzz noise.
Flip the switch, and the HOT or neutral is grounded thru the capacitor, to the chassis. The switch selects which one is grounded.

If you have 3 wire power, with a real ground, you do not need the reverse switch.

If you have no ground, and just 2 wire power, then the switch will help to reduce the buzz, in one of the two positions.

That's the basic idea. In case your ground wire is not available from the power source.

KA WUMP: the POP that occurs when the unit is turned on or off...
This has nothing to do with the grounding or the reverse switch, or the ground capacitor.

It's the power supply capacitors, discharging...there is NO mute circuit.

More modern amps...have a MUTE circuit...that prevents the POP noise.
Early Peavey amps do not have this MUTE circuit.

[TomCarlos]

Thank you SGM... that makes sense.

[g1]

The other odd thing about the switch is the cap that goes to the output jack. There's 120v across that thing... I don't get it.

It is only using the output jack's ground connection, it has nothing to do with the output. Electrically it is going to chassis ground.
The output jack ground was probably the closest and most convenient chassis connection.