2-prong cord - how to make it safe?

[Greg Ellis]

I've got a very old three-tube amp, likely a department store model from the 60's, I think. It might even be a Garnet.

12ax7, 50C5 (=el84), 35W4 (rectifier)

There's no power transformer at all - the heaters run AC, in series, and the B+ comes straight off the filter caps, post-rectifier.

Currently, I run the hot wire to the fuse and the neutral wire to the switch, although obviously these will swap depending on how I fit the plug into the receptacle. That circle beneath the 100K resistor is a pilot light.

Here's a rough schematic (sorry, there's a few mods in this already; some ideas I've been kicking around; the original amp has a single pre volume and tone; it's pretty close, though).

Is it possible to make this thing safe? I have a three wire plug waiting, but I'm not sure what to solder where.

http://i64.photobucket.com/albums/h178/GregEllisToronto/el84_schem.jpg

edit - sorry, does this belong in the amp tech section?

[traynor_garnet]

The easiest way is to buy an isolation transformer. Plug the amp into the isolation transformer and then plug the isolation transformer into the wall. Viola, you are now grounded and have something between you and the juice coming form the wall.

I actually have one I am not using if you are interested. It's very heavy and pro grade, but if you dig these old little amp it's a must. Maybe we can work out a trade of some sort?

TG

[jfk911]

now im no electronics expert but isn't this an AC circuit? If so cant you just bond to a piece of metal thats not part of your circuit? Becasue you Neutral is already a bonded conductor? Im not an amp expert so i may be wrong

[sysexguy]

You need to be very careful with an amp that's connected directly to the wall. You may want to go to group DIY and look up how they deal with it for their Pultec type EQ's (using 2 transformers) http://www.prodigy-pro.com/forum/index.php . A 1:1 iso transformer would be fine as noted above.

Andy

[greco]

The easiest way is to buy an isolation transformer. Plug the amp into the isolation transformer and then plug the isolation transformer into the wall. Viola, you are now grounded and have something between you and the juice coming form the wall.

I actually have one I am not using if you are interested. It's very heavy and pro grade, but if you dig these old little amp it's a must. Maybe we can work out a trade of some sort?

TG

TG told us about this transformer (specs, etc) in past threads in this forum. It appears to be a quality transformer that is spec'd quite high from an electrical perspective.

These amps are known as "widow makers" ....they can be dangerous without some form of protection...according to the discussion in previous threads.

Cheers

Dave

[traynor_garnet]

TG told us about this transformer (specs, etc) in past threads in this forum. It appears to be a quality transformer that is spec'd quite high from an electrical perspective.


Yep, I got it from Ripper who is an amp tech and forum member. It's a professional grade iso transformer that techs use while testing and poking around. I no longer have my "widow maker" amp and just want to make sure anyone with such an amp can enjoy it safely; if you play one of these thing regularly, pick one of these iso trannies up.

TG