Silvertone model 1419 Widow Maker Amp - Installing isolation transformer

[DrGonz78]

Like I said every change on this damn little amp paints me in a corner. So I had to change the speaker and went to place it on the baffle board only to discover that a lip of the speaker hangs over. Of course it won’t fit in the cabinet. So now I will have remove a little dimple in the crap particle board for it to fit. This amp ain’t worth the time but my cousin will love it.

[DrGonz78]

Okay I have come across something that I have never encountered before. Let’s see if I can explain it clearly and find out what is wrong. I plug in a cable to the amps input, tap the tip and it is loud & proud. However if I touch the grounded sleeve part of the cable while tapping the tip it is significantly quieter response. No hum is present in either situation.

Like I have said this amp just won’t let me win. It behaves this way whether I have grid leak and/or grid stopper resistors installed. I have the input jack wired straight to 1M potentiometer then it goes to the grids of a 12ax7. The 12ax7 is wired up in parallel and if I am not touching ground (chassis) the signal is significantly louder.

Any idea what can cause this amp behavior? Much appreciated thanks

[DrGonz78]

One other thing to address is the amp was hum free out of the cabinet. When I installed it in the cab it was humming. Which makes no sense. It is not humming now and it could of been a bad input jack that I replaced.

Perhaps the amp is behaving correctly now. When I am grounded to the input would that reduce the static noise level my finger creates tapping on the tip of the plug? I am thinking that is all that is going on but I can’t remember ever noticing such a huge difference when testing any other amp before. Too late to plug in guitar tonight but will do that tomorrow.

[Helmholtz]

However if I touch the grounded sleeve part of the cable while tapping the tip it is significantly quieter response. No hum is present in either situation.

That's quite normal. You are using your body as a signal (hum) source. When you touch something grounded you're partly shorting that voltage.

Human body is part of a spatial capacitor and acts as an antenna for electrical noise in your house. You can see your body voltage when you touch the input of a scope.
It's a very high impedance source, though.

[DrGonz78]

I think I am use to higher wattage amps when you tap your finger on the input it’s just loud. Plus this amp has just been one thing after another which made almost deny it was working correctly. Good news is that I jammed on it today and for what it is it sounds good. My nephew will love it as his first amp.

[g1]

I think I am use to higher wattage amps when you tap your finger on the input it’s just loud.

Now that it's caught your attention, next time you do it on a higher power amp you will notice. Big difference with the volume depending on whether you are touching the sleeve or not when you touch the tip.
Also you can get a lot of weird glitchy noises when using the 'thumb' technique, so should just be used as a 'go / no-go' type test.