Isolation transformer.

[bluzmn]

Enzo, you don't have to be a member to read the whole article; membership bestows certain privileges, one of which is being able to DL the article as a PDF, but I've read it several times without being a member (I just read it again to try to answer your questions). I'm not as technically astute as you, so my answers might be a little twisted. His little Gregory amp has the same tube configuration as mine; 35w4, 50c5, 12au6. He first makes the point that VA only matches wattage (IxE) for resistive loads; for inductive loads he says wattage is roughly 70% of VA. I think he guesstimates the power consumption by comparing it with other, larger amps he owns - "It's a safe guess that most 3-tube amps don't consume anywhere near 40 watts of power, and probably not 30 watts. Since more than half the load of a small amp is resistive (the tube filaments), and 70% of 50VA is 35 watts, then a 50 VA rated transformer would be fine." Of course, this is out of context and you should probably read the entire article, but that's the gist of it.

[Enzo]

Sorry then. WHen I opened the article it came up with page 1. WHen i went to click the next page, it came up something like sign up now to see all this material.

[g1]

He first makes the point that VA only matches wattage (IxE) for resistive loads; for inductive loads he says wattage is roughly 70% of VA. I think he guesstimates the power consumption by comparing it with other, larger amps he owns - "It's a safe guess that most 3-tube amps don't consume anywhere near 40 watts of power, and probably not 30 watts. Since more than half the load of a small amp is resistive (the tube filaments), and 70% of 50VA is 35 watts...

I guess what he was getting at with the 70% thing was that a 50VA rated transformer may only be capable of 35 watts. He probably mentioned this because some amps have their power requirement stated in watts.
But aside from all this, you read the entire article and are prepared to add the extra rectifier, convert the heaters to DC etc.?
The SS rectifier was required because the tube half wave rectifier caused overheating in the transformer due to imbalance.
"When I first installed it, I tried to use the N-68X with the circuit, as-is. But it immediately became obvious that the transformer became too hot, considering a current draw less than 30 watts."
Then he mentioned this would probably have not been a problem with a bigger isolation transformer
"Is all this absolutely necessary? Well, with a large enough Isolation transformer, maybe not. A 100 or 150VA rated transformer could safely deal with halfwave issues for a <50 watt amp, I'd say."
Is a bigger transformer external to the amp seeming like a better option yet? You can use it with other amps and they can all be kept bone stock.
If not, he then discovered hum problems which were solved by changing the heaters over to DC which required a big power resistor to "dump" some extra voltage which resulted in extra current draw through the transformer.
If you are up for the challenges, it should work. But what originally seemed like a simple bit of wiring got quite convoluted.

[bluzmn]

Would installing a 3-prong plug make this amp any safer?
@g-one - are you saying that installing a solid state bridge rectifier would make the use of the N-68X transformer feasible? How hard could it be to install 4 diodes?

[g1]

No, you can't install a 3 prong without the isolation transformer.
And no it's not hard to install 4 diodes, but it is only required because the N-68X was overheating without them, so I would consider it too small and in my opinion the author is a bit misleading for not making this clear right from the start. Please re-read the entire article to see if you think this qualifies as a simple rewiring job.

[bluzmn]

OK, g-one, I get what you're saying. But I kinda thought complicated wiring jobs is one of the things this forum is all about. Anyway, I'm thinking of selling this amp, and would like to do the ethical thing and make it safe first. But I've already paid too much for it, and not likely to get a price anywhere near what I've got into it; so when I found that article on instructables.com, I thought "Hmm, $11.41 isn't too much of a hit to take for doing the right thing." C'est la vie. I've got a question that I've been trying to figure out, though, but I just don't have enough knowledge of electronics to come up with a definitive answer; is there any way that you could use 2 of those Triad N-68X transformers together to adequately do the job?