The "Cigar Box Tube Amplifier" show on this page is unsave when used with a guitar!
"negative side of the cuircuit as ground"
Just my opinum - maybe ask someone else...
Gobi

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Yes, This is a very unsafe amp. All you have to do is touch another piece of gear while holding your CBG and get full 115 VAC mains current across your heart. Probably kill you !

You need to install an 1:1 isolation transformer on the AC power input. Then connect a 3 wire power cord with ground. You can get transformer from Antique Radio Supply or similar. Any amp tech can help you.
Yes this one. Because you HAVE TO get in contact with the ground on an electric guitar this really not to recomand!
The name Cigar Box amplifier is just because of the verry small size (in these days)
...and it's not "potentially dangerous " - in my poit of few its ABSOLUTELY DANGEROS
if you want a small amp just do a search for "firefly guitar amp" it runs a solid state rectifer and 2 12a?7's
What tubes do you have in that amp ???

I was checking out the schematic from the one-tube amp that was posted here http://www.guitarworld.com/diy-musician-build-your-own-one-tube-amp... and have a few question. In the explaination of the build the writer says to add a .05mfd 200V Cap in the position shown on the schematic by an X. Will doing what is highlighted (in the attached picture) along with adding a grounded plug to the amp prevent electrical shock? Is this a way around the Isolation transformer? Or what exactly where they trying to do with adding this cap?

Attachments:

there are plenty of safe amps out on the internet to make. this looks very dangerousAC mains direct to the heater element of the valve.

No, there is no way around an isolation transformer in this amp.

Whilst not being an amplifier, for a first time introduction to valves I like the Matsumin's valvecaster boost pedal - low parts count and if you wish you can wire it direct to the valve. Best of all the circuit uses a 9v supply so no worries about the high voltage normally needed for valve projects. If you use it as a boost pedal between your guitar and amplifier it adds a valve sound to a solid state amp.

http://miaudio.com/wordpress/wp-content/uploads/2013/11/12AU7-6111_...

thanks for the link David. what a nice simple circuit.  might have to rethink the preamp in my amp rebuild...  

have you built one? and if so does it have a nice crunch tone?

You have to know what your doing with tube amps. The tubes can produce 300 to 400 volts that can stop your heart very quickly. Read up on tube amps circuity before building anything with a tube in it.

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