As I understand it, valve heaters are 6.3v AC with a +/-10% tolerance. Although running them at those extremes would probably reduce valve lifespan somewhat.
I ask because I've found a decent source of HT transformers from China. Very cheap for what they are, look well made but they're set up for 220v primary. Im not worried about the increase in HT, but this would put the heaters about 6.95v assuming the mains is 240v
Would working out the current draw and putting suitably rated resistors in series with the heater wires be a good enough method of bringing the voltage down? I'd probably use a half-value resistor on both leads to avoid messing with the heater balancing.
Cheers.
Comments
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
If you are going to all that trouble you might as well build a simple rect and regulator circuit for the pre amp valves? How many LT windings are there on the transformers?
Since they are wound for 220V I would be a bit worried as to how they will take to 230V +10%?
Do you have a variac? If not you really should get one. Then check the primary magnetizing current at 220, 230 and 253 volts. If it shows signs of a sudden increase at 253 you could be in trouble.
All problems would be solved by a suitable resistor in the primary circuit but you can't really get a value for it until you have built the amp! One of the Allyclad jobs under the chassis will be fine and will reduce inrush current a bit and make fusing easier.
Dave.
You can get a rough guess at the value if you assume the amp's power draw is four times the rated output power, which won't be too far off usually, although it will be higher for very low-powered amps.
I'm fairly sure this is the cause of the frequent nuisance blows of mains fuses in a studio near the shop in Glasgow where the voltage is right on the limit. (I think over, often - although Scottish Power denied it.) The amps most affected are some modern Chinese-made ones which although they do claim to have a 240V setting, run very hot. I did think about putting resistors in line, but decided not to since it's not really regulations… I told the studio they should get power conditioners instead, which they eventually did and which does seem to have fixed the problem.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Specs are pretty random- 220v primary, ht 230-0-230 at 100ma, and two non centre tapped 0-6.3v windings at 1.5a each.
IC, B's bigger amps have a surge limiting thermistor (BIG mother, never been a problem) in the mains input circuit. Now the company are ANAL about regs and safety so you can be sure it is a kosher thing to do. Maybe investigate fitting such a component to the troublesome amps?
Or of course tell them to buy better amps...You know whose of course!
Dave.
I don't think it's the inrush surge that's the problem, I think it's the transformers going into saturation as the voltage reaches 253 (or more, I suspect - I measured as high as 262V, admittedly not with a calibrated meter) - a thermistor won't stop that.
It wasn't just these amps, they have a couple of Mackie PAs go down as well - the power conditioners seem to have stopped that as well. It's the better solution overall if Scottish Power won't do anything.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Ah no! I did not mean that inrush WAS the problem just that a thermistor would add a bit of resistance and, because they are designed for such use, they are "legal".
"Power Conditioner" covers a lot of ground IC! From a £1.99 RockBottom strip to a continuous converter or CV transformer. What have they got?
Dave.
Also keep in mind that you may have to pay customs handling fees and VAT.
But go for it, I tried it and decided using a better known group of EU based companies was the better way to go an I get much better after sales service.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
In my last month at B's I was involved in testing transformers from alternative suppliers (eggs and baskets, don't want them all in one!) a task I had been involved in, ongoing ever since starting.
I had the one and only, in 5 years failure of a (HT-60 IIRC) OPT after just a few hours of torture. It does seem that "fings ain't what they used to be"!
Caveat bloody emptor!
Dave.
DA-DA-DAAAAAAAA
DADADADADA-DA-DAAAAAAAAAAAAAA
Bandcamp
Spotify, Apple et al
(Not a typo .)
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein