Isolation transformer.

[bluzmn]

I have a vintage Crown Guitars Japan widow maker that would make a perfect harp amp to take to jams (with a line out added,of course) that I would like to make safe for human consumption; it's small, it's light and it sounds fantastic. The Triad Magnetics N-68X isolation transformer has been suggested as a good candidate; I've attached the data sheet. What do you guys think?

[sgelectric]

Please post the specifications of the Crown Guitar Japan widow maker to get a response.

[bluzmn]

Which specifications are you referring to? I don't know too much about this amp; if I had known what a widow maker was at the time, I would not have bought it.

[sgelectric]

The voltage and the current(amperage) of the amplifier.
These will allow you to calculate the load on the isolation transformer.The product of the voltage X current(amperage) plus a safety margin will tell you how big an isolation transformer you need..

[Enzo]

Exactly. Does teh amp have some printing on the back that says something like 120VAC 1.6A or whatever?

[g1]

That transformer is good for up to 50 watts output. The ratings given are for 115VAC input. Modern AC line voltage is closer to 125V so that would give you about .4A max current. If you can replace the amp's fuse with a .4A, and not have it blow while cranked, it will probably work. Otherwise, it will be too small.
Like the other guys said, the power input current or VA rating should be listed on the back of the amp somewhere, otherwise, what size is the amp's fuse?

[bluzmn]

There are no marks of that kind anywhere on the amp; it had an internal fuse, so I had to take the whole damn thing apart. It turns out that it's a 1 amp fuse; does that mean that this particular tranny is insufficient for this application? If it isn't, what should the minimum specifications of a tranny for this task?

[bluzmn]

I don't know if there is such a thing as a .4 amp fuse, but I certainly have a .5 amp. Are you saying that if I put in a .5 amp fuse and it doesn't blow, that this tranny is OK to use in this amp?

[g1]

No, I said .4A fuse . When pushing the amp hard. A .5 amp fuse would allow 60Watts (.5 X 120) at 120V line voltage, some places have line voltage higher than that, at 125VAC the .5A fuse would allow 62.5 watts. The isolation transformer is rated for max. 50 watt output and you want a bit of safety margin.
That being said you could try the .5A fuse and see if it blows while playing the amp hard. If it doesn't you might get away with that isolation transformer if it doesn't get too hot.

[bluzmn]

By following 1 of the "Similar Threads" at the bottom of this page , I was able to find the article that suggested the Triad N-68X to me in the first place, and he explains very well why it is suitable. That thread is http://music-electronics-forum.com/t17598/ . The reply that mentions the article is the 8th one down.

[R.G.]

You can always use a bigger isolation transformer than you really need. No penalty for that, except for any weight and/or cost issues.

It's using one too small that makes things smell smoky.

[bluzmn]

Have any of you read that guy's article on Instructables - Isolation transformer upgrade for old guitar amps - is there any thing wrong with his reasoning? After more than a year the only change he makes is to reduce hum - there's no mention of smoke.

[Enzo]

I tried to read it, but it said I had to join to read the rest of the thing. I don't join web sites unless I plan to be there often.

What reasoning did he make, and abot what specifically?

Isolation transformers can go in front of ANY amp, even one that doesn;t need it. The only reasoning required is how much power will you amp need, and can the transformer provide that much. Just because a particular iso transformer works for some 5 watt amp doesn't mean it will also work for some 15 watt amp.

[Enzo]

I just read the Supro thread. I am not sure who the "he" was that explained the N68X was right. The only mention of the N68X was by the last poster, who had not made any other comments in the thread.

This is why we needed to know how much power your amp needed to operate.

[Enzo]

The Supro 6611 has one 50L6 tube and one 12AX7 tube, that's it, not even a rectifier tube. It is a very small, very basic amp. What type and how many tubes are in your amp?