69 Fender Bandmaster Reverb ohm switch?

[benindavis]

HI Fellas! (Gals too)

There is a switch on the back of my amp head that .. Well I think its a switch that changes the ohm output of the speaker or something like that .. but.. hell maybe im waaaaaaay mistaken..Anyway.. if someone knows about this switch and which position means what ... LET ME KNOW! OH.. its a 3 position switch.
THANKS!!!

[Enzo]

Do you mean the ground switch? Is it next to the main fuse holder and the convenience outlet? What does it say in the print around it?

[benindavis]

hmmm lemme look........

Yeah ... there is a tag there that says "GROUND" duh.. ok ... well, exactly what does that switch do.. Im pretty green .. first tube amp and all.. hey it was free! cost me 50 bucks for new tubes and boom baby! it works! I just want to know what all the switches etc do.I got a couple new Jensen speakers and Im gonna build a cab for it . ( its just the amp head) I want to make sure when Im wiring the speakers that I have the right impedance set ya dig?
Ben
btw THANKS in advance!

[Enzo]

If the amp has a three prong power plug, then the switch does little or nothing. It is conceivable that one setting might hum a little less than the others.

If the amp has only the two prong AC plug, then the switch will determine if the chassis is "hot" electrically - that is whether you will get shocks when touching it and something else. How to set it then? Set it so you don't get a shock.

If it is only a two prong AC cord, then I highly recommend replacing it with a three pronger.

[MWJB]

The ground switch hopefully stops you getting zapped with an ungrounded amp (2 prong AC cord). If you have a 3 prong AC cord check continuity from the ground prong to the amp chassis with an ohmmeter (you're looking for "0" ohms or the meter default, maybe "0.3" ohms?), if this is what you get, you can ignore the ground switch.

If you have a 2 prong AC cord get a tech to replace it with a 3 prong cord, if you must play it in the meantime and you get a shock, throw the ground switch to the other position. Make sure that you always plug in the AC plug the same way round (mark it to preserve polarity - though you might find some AC outlets that are wired differently which will throw things out) and avoid touching anything that might be grounded like PA mics, water pipes, radiators etc.

You'd be very unlucky to be seriously hurt from this kind of shock, but when it happens it's not a nice surprise, especially when getting zapped in the mouth from a PA mic! In short, if you have a 2 prong cord, do the right thing and change it.

BF/SF Bandmaster heads are 4ohm.

[benindavis]

ok but..........

The switch has 3 positions. what is the reason for that? .. anyone?
thanks

[TD_Madden]

let me try...in the middle-position switch is basically "out" of the circuit. Switching to either of the other positions connects a capacitor (.047mf??) from one side of the A/C or the other to the chassis. That cap is sometimes referred-to as the "death cap", mainly because if it shorts, the chassis is 'live" and you become a potential ground.

I've clipped that cap out of all my amps that have such a switch, and installed a properly-wired three-conductor line cord.

[TD_Madden]

the switch's intent is hum-reduction, as mentioned above.

[benindavis]

Thanks

You guys have been a great help to me! I do have a 3 prong plug on it and it doesnt seem to make any difference in the HUM what position the thing is in.
Thanks again!
Ben