Isolation Transformer for Vintage Amplifier

[Mr_bibbles]

Hey Y'all! Have a dangerous Estey 401 that's impossible to upgrade to a three prong plug!

http://www.magnatoneamps.com/schemat...ne_401_412.pdf

The filaments are all fed directly through wall AC and there is no Power transformer, which means our best remaining option is an isolation 1:1 Isolation Transformer, right?

I have never installed one before so I wanted to stop by and see if you had advice on sourcing one, and also if there was anything to watch out for.

Thanks!

[The Dude]

Yes, you need an iso transformer for the amp to be safe. I also notice there is no fuse drawn in the schematic. I'd also add a fuse. The easiest way to decide transformer and fuse size is to measure the current the amp draws as is, since there is no existing mains fuse.

[nevetslab]

Is that a Field Coil speaker, with it's primary coil being the 'choke' for the supply?

[The Dude]

I don't think so Nevets. There's no choke drawn in the power supply of the schematic and the chassis layout diagram shows what appears to be a regular OT. I don't see any indication of there being a field coil speaker. Looks like a "standard" SE- OT- speaker configuration to me.

[Mr_bibbles]

Thanks dude, just measure the amperage in on the AC line from the wall? Then I could use that information to find a 120VAC 1:1 Transformer that's at or above that current rating?

And the second transformer in there is an Input transformer; The OT is normal and the speaker is just a conventional speaker.

[The Dude]

Yes. Start by measuring the total current the amp draws from the line. The amp will draw more current when operating at high volumes, so you'll want a buffer, but at least that gives you a starting point.

[Enzo]

SIngle ended means class A, and the current draw shouldn't vary widely. The schematic calls it 18 watts down by the power cord. SO a 25-30 watt iso ought to suffice.


Wiring couldn't be simpler. Connect one side of the iso to the mains, and the other side to the power plug of the amp. If you want to move the power switch over to the mains side, go ahead. Also adding the mains fuse is a good idea as mentioned. Put that on the mains side.

[The Dude]

......The schematic calls it 18 watts down by the power cord.....

I must confess. I didn't even notice that, and it's hard to miss!

[Enzo]

Ah... you have Mt.Rushmore and that governor of yours to worry about. You get a pass.

[Mr_bibbles]

"Meth; They're on it!"

Political agreements aside, thank you guys this is very helpful and the owner of this Magnatone will be very happy! I'll put the switch on the mains side, that way it's interrupted before the transformer right? Should be safer.

As always I really appreciate your help guys. MEF is the best resource I have when stumped, and it's because of you cool dudes

[Mr_bibbles]

Found this lil' guy, should work great! Doesn't have a wattage rating but 70% of the 50VA would equal like 35Watts?

https://www.mouser.com/ProductDetail...L8G7F80A%3D%3D

[Enzo]

Ought to work, but note they have zero in stock now, expecting some on July 28. So are you in a hurry?

[Mr_bibbles]

Eep! Coulda sworn it said 6 available now. Here's a link to the same transformer model if anyone else needs it: https://www.masterelectronics.com/tr...-12696291.html