WD88VA trouble

[Bj123]

Hi All

just need a spot of advice. I have been running a WD88VA since I built it in 2005 ish and it has preformed faultlessly until the other day when there was a loud bang and lots of smoke (but the music played on interestingly). I got to it fairly rapidly and switched it off at the mains and left it to cool.

On taking the back off I found Capacitor C7 & Resistor R16 completely blown apart & the entire innards covered in foil strip and fluff from the cap.

I did a little checking arround and found that no other components seem to be damaged and the output transformers still have the same resistance on each channel so they must be OK. Also the valve doesn't have any unwanted shorts across any of the pins.

As I am by no means an expert on these things will I be Ok just replacing these components and firing it up again or is there anything else I should check? and should I put a new KT88 valve in.

Also would it be wise to replace these caps and resistors on the other valves or is it not an age related thing? there does seem to be a faint bluish tinge to the tag board connectors but they always ran hot.

Thanks

[John Caswell]

Hi
The fact the capacitor has disintegrated means it has been subject to over voltage.
Usually the associated valve has gone s/circuit in some way or other, so clean up all the mess - replace the components and valve and also check the coupling capacitor to the grid of that valve for leaks.
Simplest way to do that is remove the valve(s) on the offending side and switch on monitoring the grid voltage (KT88 = Pin5). If the resultant voltage is anywhere above a few millivolts change the capacitor.
Ideally if one KT88 has failed it is best to replace with a matched pair but you can often get away with replacing just the one.
Let us know how you get on.

John

[Bj123]

Thanks John
I'll let you know how I get on.

This may sound like a daft question but I've never bought these type of items separately before, Should I get the components off WD or are they to trifling to bother them with & go to RS for instance?

[John Caswell]

Hi,
You can get them from where you wish, but initially I would call Matthew at WD as RS, Farnell, and CPC have minimum quantities and/or costs.
Maplin is also another good source.
If you get stuck pm me as I keep the odd bits in stock

John

[Bj123]

Well, I finally got around to fixing it. Cleaned out the remains of capacitor & resistor with the vacuum, removed and replaced them, plugged in and checked the volts.

Voltages where all to the specs. in the original instructions if at the top end of the tolerance and no voltage leaking past C5 or C6.

I have changed the valves from KT88's to 6550C's. I put some music on and if anything it sounds a bit better although if the old valves where deteriorating I might have got a false impression.

I'm just a bit wary of leaving it on unattended now and probably could do with a basic solid state amp for when I am away and others are here.

Anyway thanks are due to JC for advice and Mathew for supplying parts promptly.

cheers
John

[bikerhifinut]

Congratulations on getting your amp running again.
I think I should maybe check the voltages on my 88VA as I built it in 2007 and has had no critical component changes since the end of that year when i merely replaced the coupling caps for soniqs polyprops.
I Bought 2 matched pairs of GEC NOS 6550A this autumn at a swapmeet and have plugged them in. In some respects they are to my ears a cleaner faster sounding valve and I think the amp does sound good with them. In other respects I prefer the Svetlana KT88's especially when playing at highish volumes. I'd expect that as the 6550 is said to overload more steeply than the beam tetrode KT88.
All the same I do like the presentation of the 6550 with accoustic music.

Its all a matter of taste I guess.

[Bj123]

Thanks
I was wondering what the difference was between the two valve types. As I don't get much opportunity to play it at high volume I guess it won't make much difference to me.
Acoustic music does sound incredibly good on it at the moment though, at least it does to my ears but I'm no expert on these things.

[bob orbell]

If you are happy with the sound you hear, then that is all that matters, you could end up spending a lot of money, and not liking what you hear, I know, iv'e done it . For two and a half years I have lived with a pair of Monitor Audio PL 100 speakers, £2300.00 as time went by, the HF sting became intolerable, I changed valves and cables, but it made no difrence. Last year I picked up a copy of Hi-Fi World to read about the WA 88, which I use, on the next page was a report on a pair of Wharfdale Denton 80th anniversary speakers, the report was very good so I picked up a pair from Classic Hi-Fi, the rest is history, they cost me £450.00 and to my ears are so much better than the PL 100's. BOB

[david]

Hi John,
I use a Roksan Kandy alongside my 300b push pull clone. It's great for day to day use rather than serious listening, or when you are dashing in and out, and means I restrict my 300b for evenings etc when we sit down for a listen.
David

[Bj123]

Hmm, nice amp but costs more than my WD88 did or are you referring to an older one?