Supro Bantam

[beasleybodyshop]

Had one of these Supro's on my bench this weekend. The guy wanted a 3 prong cord installed.

Where is the PT? haha. Doesnt have one. Never seen an amp that just runs straight 120V wall voltage.

I don't think I can actually do a proper 3 prong cord on this amp. Tried to ground the earth wire on the OT nut - ends up making a mad buzzing sound regardless of polarity. It sounds fine with just the line and neutral wires hooked up.

Just thought I would share this one with you. Cool sounding little amp. Has some wierd buzzy tones on lower notes, but the speaker seems to be good. Going to try to replace the dual 50 can cap and see what happens.

[M Fowler]

Only way to make amps like this one safe is an isolation transformer. 120v to 120v.

Be very careful working on this amp.

[martin manning]

I think this one will supply enough current: http://www.mouser.com/ProductDetail/Tri ... L8G7F80A==

[Leo_Gnardo]

Ground and neutral are to be attached - "bonded" - to each other within the service box for the building AFAIK. Re-doing at the amp, yes that would set up a helluva ground loop.

Isolation transformer is the only way around it with "hot chassis" amps.

[Structo]

I know it is dangerous to use a transformerless amp but wouldn't a polarized plug provide some measure of safety?

That is of course if the receptacle that it is plugged into is wired correctly.

[Phil_S]

I know it is dangerous to use a transformerless amp but wouldn't a polarized plug provide some measure of safety?

That is of course if the receptacle that it is plugged into is wired correctly.

I'm not an electrician, so I'm speculating here. Provided there is a fuse on the hot leg, I guess a two prong polarized cord is an improvement, but it fails to address the real problem. That fuse protects the house and maybe the amp as a secondary consideration. The user of the amp still has the basic danger of a potentially hot chassis and nasty sparking mic to lips, etc. There is only one way to fix this and that's with an isotranny. I don't think you could pay me enough money to even flip the power switch without it.

[xtian]

You could add some indicators to the front panel that show when the polarity is correct, or about to murder you:

[img:400:610]http://images.monstermarketplace.com/el ... 00x610.jpg[/img]

[pops]

New here, but with a two prong cord i plug it in and check the chassis then when it is not hot i make a notch in the plug where the ground should be. It the outlet is wired right and it is plugged in with the notch at the ground, it should not have a hot chassis. There are other considerations with these amps and i would add the iso tranny too.

[beasleybodyshop]

I ordered the ISO tranny Martin suggested (thanks Martin)

Why in the hell was this ever made this way?? Further research shows that it is common for these things to electrocute people. Makes you want to slap the engineer who designed this, but he's probably dead by now.

[martin manning]

I guess people just weren't afraid of 110V back then, and they weren't afraid of lawsuits, either ;^)

I'd see if that isolation transformer can be screwed to the end block near the power inlet. It has a lead to ground the frame to the chassis, so that will be covered.

[M Fowler]

I have a transformerless amp like this too it was given to me recently by a good friend.

His dad used to play through it his whole life.

[pops]

They were cheap to make, that's why. No power transformer.

[beasleybodyshop]

Update - ISO trans that Martin suggested worked like a charm! Thanks guys.