Help with Kit88

[lovejoy]

Hello all,
I've been lurking on the forum for a little while having been interested in taking the plunge into valves for some years. In the last few weeks I managed to pick up a pre-built (but not from WAD) Kit88 from Ebay for what seemed like a pretty good price and having met the seller and made the transaction and got home with my purchase, I settled down for some music. All was well apart from a few odd little pops whilst the amp warmed up. Overall though it sounded excellent and the next few nights were spent rediscovering my vinyl and CD collection.

I realised that when I switched the amp on, one of the ECC82s was glowing brightly for the first few seconds before calming down, so I decided to swap the ECC82s around to see if the fault followed the valve or whether I had a problem internally. Sure enough, the fault followed the valve so I decided a replacement ECC82 would be all that was needed. But then 20 seconds later, the KT88 on the left channel (the one that now didn't have the overly-bright ECC82) started arcing bright purple. Off it went straight away and I was quite sure there was some smoke from the amp.

This was where the alarm bells started ringing as taking out two of the KT88s (the one that popped and its partner) it turned out that they didn't match. One is a Sovtek, the other unbranded. The alarm bells started to ring further having got in touch with the seller to ask if this had been a recurring problem and receiving no reply.

Tonight, I get the amp on the bench for a better look and find:
1. Rounded and non matching screws in the base, suggesting this amp has been opened on a good few occasions, unless someone's not very good with screwdrivers.
2. Some pretty poor soldering work, including lifted track and wires that have had their insulation melted through.
3. Having downloaded the original build documents, the mains fuse that's supposed to be 1.6A is only 630mA, which seems rather odd, and a bit worrying.
4. Some of the high wattage resistors are too close to the board and the board has blackened.

It looks as if R1 has fried, so this may be what burnt out when the valve went pop.

I might be worrying over nothing much more than a KT88 having gone pop and taking the resistors with it, but now having seen inside I'm a bit more concerned that other things are amiss.

Is there anyone here who knows their way around one of these things that could offer some advice, or would be willing to give it a looking over. Happy to travel and pay for your time.

Or does it sound like I've been had?

Rich

[John Caswell]

Hi Rich,
Yes I know a little about these amps.
1) ECC82 heater flaring - Quite normal do not worry about it very common with Mullard valves
2) KT88 arcing and failing - Yes they do this, not much you can do about it except replace it. It was called catastrophic failure for obvious reasons.
3) The KT88s should be same make and ideally matched pairs.
4) The mains fuse should be 1.6A T ( slo blo) although in some areas of very high mains voltage we raised it to 2A T.
5) I seem to remember that the build info said space the high wattage Rs about 10mm away from the PCB. I always space them 20-30mm away.
6) Careless soldering/ too much heat will lift the track very easily. I have always used a 50W iron for thermal mass and high speed soldering ( I learned very quickly back in the 50s-60s when PCB s first came in in TV sets).
7) Don't have the cct handy at present but I seem to remember that R1 is the input grid resistor so unlikely to burn.
8) There were several resistors that we thought were underrated around the ECC82 which ideally should be upped to 2W.
Swindon not too far away from where live (Wokingham) so am happy to look at it for you.

John

[Greg.]

Rich,

Being a previous owner of a WAD KiT/KaT88, I completely concur with all John has said. Wokingham is not that far from you and I wholly encourage you to take up John's offer. Many years ago I did just that when I was a novice in this game and it was very worthwhile. John is a little modest but actually he is one of our two resident experts on this forum and he's been building and repairing WAD and WD amps for years. Actually, we used to call him Dr. John in recognition of his knowledge, skills and abilities. He completely knows his way around your amp and if you visit, will astound you when you witness his speedy dexterity inside the powered up chassis

If memory serves me well, he and his good lady are partial to good quality red wine, although obviously, how your amp is may determine a more commercial agreement if necessary. I'm sure John will advise accordingly.

Go for it! You can't go wrong on this offer!

[lovejoy]

Many thanks both. It's great to see that there is still an area of audio that still has its real enthusiasts. I have already PM'd John and hope we can arrange something soon. I'll be sure to keep this thread up to date with how things go.

Rich

[colin.hepburn]

Yep what Greg says Dr John will sort it out

[lovejoy]

Well the amp is now in the excellent care of Dr John. An initial inspection showed that the amp has been through the mill a bit and there were signs of things having caught fire previously and tracks repaired quite badly with thin bits of what looked like wire-wrap cable.

So a bit of work to be done and a few mods to get it running more stable, but the good Dr assures me that it will be up and running nicely again, so my mind is at rest.

It was great to meet Dr John on Friday. He also has a workshop to die for .

I should have taken some pictures of the board for some before/after comparisons!