ground switch

teamo · Sea Monkey Joined: 15 Nov 2007 Posts: 13

Fender 75 combo tube amp (1981). The ground switch is the old a/off/b type. It appears to do nothing. I thought that it reversed the polarity of the hot and neutral but it does not. Is it supposed to? Thanks.
Jim

tw001_tw · Posted: Wed May 28, 2008 9:25 am Post subject:

Not really reverse polarity, but picks which side goes to ground, if any,
on a 2 prong power chord. No need for it (ground switch) on a 3 prong chord.

I like pictures. Do you?
http://www.unclespot.com/3prongconversion.JPG

-tw

teamo · Sea Monkey Joined: 15 Nov 2007 Posts: 13

I know that in some situations this switch is supposed to cancel some of the hum. I am thinking of converting this to a three prong outlet. Will this make the hum cancel feature unusable or will there be less hum due to the chassis being grounded? Also in the diagram it looks like the accessory outlet is now wired using this switch. Is this a conversion to make the switch an on/off switch for the outlet? Thanks.

teamo · Sea Monkey Joined: 15 Nov 2007 Posts: 13

On further review of the drawing it looks like the original wiring has the white wire going to the fuse and the black wire going to the switch. Normally with a/c wiring as in home wiring the switching and fusing is always on the black (hot) side. The conversion looks like it corrects this.

tw001_tw · Posted: Wed May 28, 2008 3:27 pm Post subject:

teamo · Sea Monkey Joined: 15 Nov 2007 Posts: 13

Thanks for the help. Only one thing I didn't get: Good idea! (especially if the amp is just a players amp) What is meant by a "players amp"?

tw001_tw · Posted: Wed May 28, 2008 7:10 pm Post subject:

Some people, for collection and/or purist reasons want to preserve the
amp (OR guitar) in as much its original condition as possible. (to preserve
its value and/or factory specs - this goes beyond simply taking care of your
equipment). But if you use your amp as a tool to make music, or value its
'functionality' over its 'preservation' - then its 'just' a 'players amp' (or
guitar, or whatever the case). (loose definition, but it'll work.)
(Do I have enough parenthesis ??) (I do tend to use them a lot, huh?) Laughing

If I were you, I'd consider changing it to a 3 prong chord.

And I want to clarify something;
I said "green grounded on power transformer mounting lug/bolt "
That's where I put it. others will have their own places or thoughts on the
matter, but I've had good luck with it grounded there.

good luck.

teamo · Sea Monkey Joined: 15 Nov 2007 Posts: 13

I haven't used this amp for years and I finally brought it home the other day and it already has a three prong cord. I found the schematic online and it shows that in each position the different legs of chord are grounded. So if I have the hot side switched to ground why doesn't it just blow the fuse? Here's the link:

http://www.el34world.com/charts/Schematics/fender/75.pdf

Another question: I have been reading a little about biasing. How do I know what type of biasing this amp uses. I don't understand it all but I've seen some stuff about fixed bias and cathode biasing or something like that.

tw001_tw · Posted: Tue Jun 10, 2008 4:59 am Post subject:

Hey there teamo. ok.. a ground switch on a 3 pronged chord. I've heard
about them and read very little about them. Its for the same reason,
to reduce hum/noise. If I were you, I'd leave it in the middle pos. unless
there was a problem. Sorry I can't help you much there.
Regarding the biasing - yours is a fixed bias amp, which means you can
adjust it with a potentiometer which is inside the amp. I would
advise you to leave this part alone unless you are very careful with step by
step instructions next to you - otherwise you or the amp could get hurt.
Normally, you only consider adjusting the bias if you replace the power tubes.