Mains power dissipated by a 100 watt valve amp?

[Mark]

Hi

I’m looking to buy an auto transformer as the mains voltage at my residence is typically 230vac or 235vac instead of 240vac. I’m fixing a JCM-800 2204 and the rail voltage of the amp is 360VDC instead of the 380VDC on the circuit diagram.

I’m thinking many 100W amps have a 3 amp fuse thus at least 720VA spec would be required.

Any thoughts on the specs for a suitable auto transformer?

Thanks for your help.

[sluckey]

Search eBay for 240 volt variac. This 240V/1000VA variac or similar would cover all your needs.

[Helmholtz]

If the amp has a voltage selector, try the 230V setting.
Also check the heater voltage.

(Power consumption/demand is not the same as dissipation.)

[Mark]

Search eBay for 240 volt variac. This 240V/1000VA variac or similar would cover all your needs.

Thank you again for your advice, it’s much appreciated.

[Mark]

If the amp has a voltage selector, try the 230V setting.
Also check the heater voltage.

(Power consumption/demand is not the same as dissipation.)

Thank you for your assistance.

The next voltage on the dial is 220vac, it’s a bit higher than 230vac but worth a try. As long as the heaters aren’t any greater than 6.9vac it should be fine.

I will report back if it makes any significant difference.

[pdf64]

Just to note that 230V is, and always has been, well within the acceptable range for a 240V nominal supply.

[Mark]

Just to note that 230V is, and always has been, well within the acceptable range for a 240V nominal supply.

The funny thing is 15 years ago I had the opposite problem, that the mains voltage was 248vac.

[Helmholtz]

Actually 8V more or less shouldn't be a problem at all.
That's just 3.3% of your nominal mains voltage.
I'm seeing such variatons over the course of a single day.
Depending on country, mains voltage can vary up to +/- 10%.
On many old Fender schematics there was a note saying that circuit voltages can vary up to +/- 20%.

[Roe]

the rail voltage of the amp is 360VDC

I'd look for the the 220/225v primary

[Mark]

the rail voltage of the amp is 360VDC

I'd look for the the 220/225v primary

I will be trying that tomorrow. Thanks for your input.

[timrobertson100]

Search eBay for 240 volt variac. This 240V/1000VA variac or similar would cover all your needs.

Is there anything in particular one should look out for with an older second hand one please? Presumably the wiper can fail, but that would fail open rather than full voltage, right?

(Don’t mean to hijack the thread, and thought this seemed appropriate to raise in this discussion)

[sluckey]

Is there anything in particular one should look out for with an older second hand one please? Presumably the wiper can fail, but that would fail open rather than full voltage, right?

It's an electro-mechanical device so I would mainly judge by appearance. Don't buy if it looks beat up unless you can test before you buy. The knob should turn smoothly with no looseness and voltage should vary smoothly without any dropouts. Buy a quality name like Staco or Powerstat.

[Mark]

the rail voltage of the amp is 360VDC

I'd look for the the 220/225v primary

I gave it a go. The mains voltage was 236vac which I didn’t think was too bad. The heaters were a bit low at 6.0vac so I thought it was safe to try it. The heaters rose to 6.6vac so that was okay.
The rail voltage was measuring 360VDC initially and when the voltage selector was set to 220vac the rail voltage rose to 400VDC.
I was initially measuring the output as 35WRMS, with a 400VDC rail the output was 43WRMS. I don’t think the increase in wattage is worth leaving the voltage selector on 220vac.

During all this I found out that my CRO isn’t measuring correctly and a friend’s CRO was giving me a similar output to what I was reading on my multimeter.

[Roe]

the rail voltage of the amp is 360VDC

I'd look for the the 220/225v primary

I gave it a go. The mains voltage was 236vac which I didn’t think was too bad. The heaters were a bit low at 6.0vac so I thought it was safe to try it. The heaters rose to 6.6vac so that was okay.
The rail voltage was measuring 360VDC initially and when the voltage selector was set to 220vac the rail voltage rose to 400VDC.
I was initially measuring the output as 35WRMS, with a 400VDC rail the output was 43WRMS. I don’t think the increase in wattage is worth leaving the voltage selector on 220vac.

During all this I found out that my CRO isn’t measuring correctly and a friend’s CRO was giving me a similar output to what I was reading on my multimeter.

both will work but go for what sounds and feels best