Safe heater supply voltage for valves?

4114Effects

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. 

ICBM

Yes, that will work fine. It will also reduce the power-up surge because the resistors don't go to near-zero resistance when cold. Make sure they're a fairly high power rating - preferably a small wirewound type.

ecc83

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.

ICBM

ecc83 said:

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.

I've done that a few times with 60s/70s Japanese or European amps which frequently only have 220V primaries and run very (dangerously, often) hot at 240V.

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.

ecc83 said:

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.

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.

4114Effects

Cheers for the advice guys. For £20 delivered from China I'll take a punt and run some tests on one. 

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. 

ecc83

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.

ICBM

ecc83 said:

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!

I did . But they're cheapskates, of course.

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.

ecc83

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.

Modulus_Amps

Not sure who you are buying from, but I have tried transformers from a well known Chinese supplier and ended up putting a bunch of them on the bin with no chance of getting any money back, They seemed to work fine but caused pops and crackles in the amp, they could also not say that the transformers would meet EU requirements for safety.
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.

ICBM

ecc83 said:

"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?

Proper regulated voltage output units... cost about £2K each I think, one for each group of rooms. Cheaper than constantly repairing the gear - there were other more serious failures as well as the nuisance blows, but none since they bought them.

ecc83

Modulus_Amps said:

Not sure who you are buying from, but I have tried transformers from a well known Chinese supplier and ended up putting a bunch of them on the bin with no chance of getting any money back, They seemed to work fine but caused pops and crackles in the amp, they could also not say that the transformers would meet EU requirements for safety.
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.

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.

Cirrus

This sounds like a job for...

DA-DA-DAAAAAAAA

DADADADADA-DA-DAAAAAAAAAAAAAA

ICBM

Cirrus said:

This sounds like a job for...

DA-DA-DAAAAAAAA

DADADADADA-DA-DAAAAAAAAAAAAAA

Bucking transformer!

(Not a typo .)