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#1 Valve amplifier service

Posted: Sat Apr 01, 2023 7:45 am
by bostod
Hi guys. I have a world audio design 5881 valve amplifier that has developed a noticeable hum coming out of both speakers. Not noticeable when music is playing but very noticeable with no music or between tracks.
I have no technical experience myself so I am looking for someone or a company to service, bias and rectify the problem.
I live in Doncaster and I am looking for someone in this area. My preference would be to get there within the hour so a 50 mile radius.
I would travel further if no one in my area.
Do you guys have any recommendations

#2 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 5:57 pm
by simon
I'm not really the best person for this (understatement!) but if you don't find anyone else I could have a look at it - I'm in Donny too.

I've a feeling the WAD5881 was PP? I have no experience of PP but might be able to navigate if someone has a circuit diagram?

#3 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 6:29 pm
by Nick
Yep I think so, I am not sure there is much in there to actually "service" other than looking for burnt bits. I think its auto bias. If both channels are getting hummy it may just be a case of needing new valves. How old are the valves currently in it?

#4 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 7:39 pm
by IslandPink
There were both PP and SE. I think the SE was a parallel SE.
http://world-designs.co.uk/forum/showthread.php?t=7445

#5 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 8:06 pm
by simon
Burnt bits was my thought too. Or possibly a blown cap though not sure why that would necessarily make it hummy unless it's a filter cap. Worn valves was my other thought as you say.

#6 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 8:19 pm
by simon
IslandPink wrote: Sun Apr 02, 2023 7:39 pm There were both PP and SE. I think the SE was a parallel SE.
http://world-designs.co.uk/forum/showthread.php?t=7445
I saw that, also that it was PSE though I wonder if that was mistaken I'd.

#7 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 8:22 pm
by IslandPink
Take your pick -
5881 I.jpeg
5881 I.jpeg (141.17 KiB) Viewed 23515 times
K5881 2.JPG
K5881 2.JPG (112.12 KiB) Viewed 23515 times
wadpse5881.jpg
wadpse5881.jpg (112.1 KiB) Viewed 23515 times

#8 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 8:27 pm
by simon
Ah right...

#9 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 9:49 pm
by andrew Ivimey
I was going to say what Mark said but I'm a bit busy and the schematics say all. Still the problem could be the same ( and fairly easy to get to the bottom...) but I dont live near Donkers!

#10 Re: Valve amplifier service

Posted: Sun Apr 02, 2023 10:18 pm
by pre65
I did something similar a few years ago for a chap from Norwich. I got the build sheets from Greg, and gleaned a few upgrades and mods from the World Designs forum.

Upgrading the wire wound resistors to a higher wattage was one, from memory.

#11 Re: Valve amplifier service

Posted: Mon Apr 03, 2023 10:14 am
by Greg
Yes, I can supply the full build instructions with schematics for all the three versions of the WAD5881 kits. Worth having as the diagrams posted by Mark above come from the DIY Supplement articles which were often based on the prototype build and sometimes changed when the kit was finally released.

#12 Re: Valve amplifier service

Posted: Mon Apr 03, 2023 10:24 am
by simon
Thanks Greg, good to know.Let's see what bostod comes back with.

#13 Re: Valve amplifier service

Posted: Fri Apr 07, 2023 10:35 am
by bostod
Here is an update based on the above advice and my skill level, or lack of it.
I have replaced all the valves both the small preamp and large power valves. This made no difference to the hum. To clarify the hum. When I switch on the amp it is very quiet. After about 15 seconds the hum gradually gets louder until it becomes consistent, that process taking a couple of seconds.
I've taken the bottom off and took a look. While I dont understand it at all, there was nothing obviously burnt out.
I have 2 sets of power valves. The new set that are in have had maybe 100 hours use so like new I guess

https://www.dropbox.com/s/nqkkyjo0izel5 ... 8.jpg?dl=0

https://www.dropbox.com/s/48c89rw4z0gvd ... 4.jpg?dl=0

#14 Re: Valve amplifier service

Posted: Fri Apr 07, 2023 11:25 am
by Greg
It would help,to identify which version of K5881 you have. Easiest way is to check the small valves.
Mk1 has Valve 1, EF86. Valve 2, ECC83.
Mk2 has V1, 7025. V2, ECC83.
PSE has V1, 7025. V2, 6922.

It looks like the kit has the standard WAD upgrade components of Audionote Paper in Oil caps and Blackgate caps. Considering the kit’s age, the Audionote caps have got to be suspect.

#15 Re: Valve amplifier service

Posted: Sat Apr 08, 2023 6:06 am
by bostod
Well mine currently has ecc83 in all 4 positions. Could that be the problem.