K5881 Mk1 - Mains Tranny R.I.P.

[espidog]

Completely out of the blue, the mains tranny on my K5881 has bitten the dust. I normally leave the amp on 24/7 (not very green, I know, but good for the valves). This morning I came down to breakfast and noticed the amp was dead. Diagnosis has narrowed the fault down to the mains tranny. There's no doubt. It's buggered.

Don't know why it's died, though. It was only 9 years old, and had never been subjected to any undue stress. Maybe there was a nasty mains surge in the night, but surely anything bad enough to cause shorted turns would have tripped the household RCD, wouldn't it?

Anyhow, that's all academic. I'm now facing getting a replacement. Sowter's price for something suitable is around £130+VAT. - almost as much as I paid for the entire amp kit originally! On Monday I'll ring SP Wound Components (who I believe made the original) and see what their price is.

In the meantime, if anyone out there has a suitable unit they'd like to sell, I'd be very interested to hear from you! AFAIK, the relevant specs are as follows:

Drop-through style.
Length: 115mm
Width: 97mm
Height not important.
Distance between fixing bolts: 95mm x 77mm

VAC at secondary: 360
Current rating of secondary: 400mA minimum (450 would be better)
Heater winding: 6.3V Centre-tapped
Heater current draw: 4.8 Amps (so I guess winding should be rated @ a minimum of 5 Amps)
Power Rating: 200 Watts

Cheers all,
Ken

[Hardy]

I must admit I don't bother leaving valve amps on....especially if they have soft(ish) start.

I only leave SS pre-amps and CD players on as they sound vile for ages if you don't......

Rich.

[Richard]

Hi, 5881 HT consumption is 250mA so this one should do it but I don't know how the mounting dimensions compare, might be worth an email.

[Ianm2]

get in touch with Geoff at sp wound components, the original maker

[espidog]

Originally Posted by Richard

Hi, 5881 HT consumption is 250mA so this one should do it but I don't know how the mounting dimensions compare, might be worth an email.

Thanks Richard. Unfortunately, that particular one falls a bit short of the mark, ratings-wise. The K5881 PSU uses a bridge rectifier and a capacitor-input pi filter, so that 250mA figure has to be multiplied by 1.61 to find the correct rating for the HT secondary winding. This gives us a figure of 402.5 mA (call it 400).

Originally Posted by Ianm2

get in touch with Geoff at sp wound components, the original maker

Thanks Ian. As mentioned in my original post, I have considered that option, but on reflection I think I'll try other manufacturers first. The SPW product failed after only 9 years of normal use, and as this is the second of their transformers that has fallen down on me (I had to replace an OPT last year, which likewise failed for no apparent reason), I'm understandably a little wary of going back to that company!

[A Stuart]

When I got SP to replace my mains transformer for K5881PSE they offered an uprated version, as best I recall at little, if any, extra cost. It had more laminations, so could have less turns, so thicker wire. I understood the original was specified (by WAD rather than SP) very close to the limit. Because of the greater number of laminations it stands 3/4" taller than the original, but you would hardly notice. And it was the exact dimension to fit the original gold cap and case aperture/bolt holes.

Casual conversation with John Caswell suggested that transformers are just to be expected to fail. Like you, I had thought they were fairly inert lumps of iron and copper which you don't expect to change in 50 years.

Alastair

[espidog]

Hi Alastair,
Thanks for that info. If the original tranny was specified as you mention (in other words UNDER specified), it explains a lot. I remember the reassurances that were published in HiFi World at the time, telling us not to worry, that it was perfectly normal for the K5881 mains tranny to run as hot as it did. I never quite believed it then. It seems my scepticism was well founded.

As for the expected lifespan of wound components in general, while I have the utmost respect for John Caswell's experience, I can't agree with that analysis. When my radio ham father wound his own transformers, inductors etc, he always took care to build in sensibly generous margins, such that they would never be stressed during normal use. If he had to buy a commercially made item, he exercised similar judgement. Result: the equipment he built in the 1930s-1950s is still in working order today. I've had to replace the odd capacitor here and there, but the wound components have never given rise to a moment's concern.

P.S.: Can you recall how much SP quoted you for that uprated tranny?
Cheers,
Ken

[Ianm2]

I had an sp output tx go on me, too!! mine cost about £40 ish, Geoff was very nice, did a very good deal on it, was quick and it was well packed, I suppose with those prices, I wouldn't worry about 9 yrs. the mains may well be cheaper, but adding in the passage of time

Again, with deference to dr john, wound components are meant to be the most reliable, and some do go on forever, testimony to that is these old 1950s valve amps and transformers, everything else goes, but the tx's just go on and on, if well designed

I have been harping on about reservations about wad transformers for years, but seems like few took notice of me, as usual, however, here is the irrefutable proof. ahh well

[A Stuart]

It was £60 incl postage, but that was almost 2 years ago, before everyone started complaining about copper prices.
Alastair

My transformer never ran more than warm to touch. Oddly enough WAD always emphasised how they catered for the market in warmer parts of the world (Malaysia was mentioned as far as I recall) so they claimed to generously specify their transformers to run much cooler than was commonly done.

[espidog]

Ta for the info, chaps. I'll continue getting quotes from manufacturers, but if anyone has a suitable unit gathering dust, give me a bell on 020 8539 2786 or PM me via this forum.
Cheers,
Ken