WD88 heater fuse

[Riggers]

Hi all
So I switched on the amp tonight and small valve heaters did not light up. I opened up the amp and the heater fuse had blown.
Firstly, i wanted to confirm the correct part to buy, on my pack list i have 3A antisurge MCF06G-6. When i checked the Farnell site the code gave me a 6.3A antisurge fuse.... is this ok? If not, am i after 3A fastblow/slowblow??

Secondly, this is the first fuse that has blown after running the amp for almost 2 years...anything to worry about? I did notice C14 and 17 sit rather loosely in their plastic holders, i might wrap a zip tie or something around each of the to secure better.

Cheers

Phil.

[bikerhifinut]

ok Phil.
First of all the dc heaters supply 2x5687 at 0.9A each plus 1xECC83 at 0.3A = total steady DC current draw of 2.1A. Now there will always be a fairly hefty current surge as Valve filaments warm up and achieve their "hot" resistance. So you should always buy a "T" type fuse sometimes known as slo blow although thats not strictly true. T satnds for time delay to allow for momentary current surges.
now as the total current draw is 2.1A plus a few mA for the LED then I personally would try something like a 2.5A fuse first and certainly nothing bigger than 3A. Fuses can blow for no obvious reason especially if subjected to a series of on/off cycles over time as each surge will cause a stress to the fuse wire. But as always when a fuse blows for no immediately apparent reason it's best to have a look and check voltages, physical wiring for visual defects, etc. another thing MIGHT be if a valve heater fails and possibly short circuits another part of the internal structure as it melts/breaks. So you might have a duff valve when you switch on again.
Hope that helps.
You must have the XL version as the standard KT88 amp doesn't fuse the heaters with the bridge rectifier bolted to the chassis.

A.

[Stanton]

Hi Phil

Had the same thing on the DC heaters of my WD KT88 XL after 4 years use. The original fitting was a 20mm T3.15A L 250V slow blow fuse. I bought a packet of replacements off eBay. It was cheaper than the parking charge would of been to shop in town. Checked the wiring for shorts and the valve heaters too. All voltages were good on switch on and all has been well for the last ten months since. So assume the fuse just wore out.

Regards

Frank

[bob orbell]

I personally can't see the need for a heater fuse on the XL version, mine does not have one and Iv'e had it from almost the first one out of the door with no such problem, although, if it was me I would fit a 4 or 5 amp fuse, if there was a short even the 5 amp would pop, or remove the fuse altogether. BOB

[Riggers]

3A slow blow fuse purchased and inserted and all well again. For the tech-minded, is "slow-blow" the same as antisurge?
I did also take the opportunity to tie down C14 and 17 a little more securely. In my 88VAXL they are held by 2 plastic "U" shaped clips which allows them to sit a quite loosely. A couple of zip ties looped around each have fixed this.
Thanks for the replies
Cheers
Phil

[John Caswell]

Hi all,
Slo Blow, Anti-surge and "T" (Trage) are one and the same.
I would have expected to see a T fuse in the heater supply chain as the valves are lower resistance when cold, and certainly on the WD88s I built they were.
Fuses do become "tired" after a time and just fail. No fault, a bit like humans "I have had enough" and just give up and fade away.

John