Isolation transformer for my beloved little supro?

[akimball442]

http://schematicheaven.com/bargainbin/supro_s6611.pdf

Please check out the above schematic diagram. This is one of those 'death' amps, which has no power transformer.

I understand an isolation transformer can be installed, can anyone recommend one, and also educate me on what modifications, if any, to the circuit will be required to make this amp safe?

I've got lots of vintage amps, expensive ones and cheap ones, and I've rebuilt and modified most of them with great success, but I've never done anything to this one.

I plug it into the living room wall with it's little puny lamp cord, and it sounds great turned all the way up, but I know it's not safe.

Thanks for any advice!

[Jazz P Bass]

Looks to me like the easiest thing to do would be to buy a 120Vac to a 120Vac transformer.
Install it right before the power switch.
It has to handle 2 amps x 120V= 240VA.
Here is a link:Triad Magnetics - N-66A - Allied Electronics

[JHow]

Why does the transformer need to handle two amps? Isn't that a lot of transformer for just two tubes? Not questioning your expertise, but trying to understand how to think about these things. Thanks!

[Jazz P Bass]

I went with the 2 amp fuse that was marked on the schematic.
If you really want to nail it down, you will have to measure the AC current draw of the amplifier from the mains.
(If the next question is "How do I do that?", do not attempt it)
I think 250VA is fine.
Please consider a three prong cord.
Green gets bolted to the chassis.

[JHow]

I was just trying to do the rough mental arithmetic about current draw through the heaters and plates. It didn't seem like it would need so much current to run the circuit. I won't ask about measuring current draw!

Also, what is that 270K R and .05 cap from the AC line to the chassis? Is that necessary if you use the transformer?

[olddawg]

Looks to me like the easiest thing to do would be to buy a 120Vac to a 120Vac transformer.
Install it right before the power switch.
It has to handle 2 amps x 120V= 240VA.
Here is a link:Triad Magnetics - N-66A - Allied Electronics

That's a bit pricy. If you have a couple of same spec junk transformers; 12v, 24v or whatever with enough current, just hook them up secondary to secondary. You will have a little loss but line voltage is higher nowadays anyway.

[JHow]

I think its pricey because it may be too large. 240 VA is a lot of power. For 12ax7 and 50L6 heater current should be around .15A. Heaters require the most most power here. Looking at the schematic, if we drop 55V over the two 200 ohm resistors, as suggested by the heater string, we're at about .1375A. The plates will pull relatively small amounts, maybe another 50-60mA. I'm not an engineer, but to my thinking you could probably get by with a smaller transformer. I don't understand the logic behind the orginal 2A fuse, perhaps so it doesn't blow on start-up, but this circuit doesn't seem that disimilar from your typical "champ" that would use a 1A slo-blo fuse.

[Jazz P Bass]

From my Googling it does appear that the 2 amp fuse rating is high & that a 50VA transformer will do fine.

Link:Choosing an isolation transformer

Time to research terms like series heater/transformerless/acdc/widowmaker amps on various forums and the web.

In answer to your question on the 270K & .05 uf capacitor.
Those two components are the tail end of the heater circuit.
The line voltage is dropped through the two 7Watt cement resistors, then through the tube filaments & finally to the 270K resistor.
I believe the .05 capacitor is for hum decoupling.

[otto pärt]

Here's what you need: Triad N68X from Mouser. $11.20.