Working on a WAD KEL84

[willbewill]

I've recently bought a WAD KEL34 which I'm very pleased with. It's in lovely condition and working order except it has a low level 'rustling' 'crackling' noise on right channel when it has warmed up. I've cleaned all the valve pins and swapped valves left to right one by one but problem still remains. So I'm assuming it is probably the ECC82 anode resistors that need changing? This is a known problem:

"The ECC82 anode resistors dissipate approx .7W. They are 1W carbon film which should be OK on paper but they may break down after a couple of years. This causes them to read high when hot and shows as a low level rustling sound. (They are particularly tricky to pin down in the early stages of failure as they sound like a dirty contact somewhere.) Simply changing them to 2W values fixes this. The original ohmic values can be used to construct them or it may be more convenient to buy just 2 ohmic values as follows, R7/20/24/19 anode R`s to 2W carbon film, 47k`s are RS 131 671 or similar and 36k`s can be made up from 47k again, paralleled with 150k RS 135 443."

Now the reason I bought the KEL34 is that as it was a kit it should therefore be easy to work on? I've built a lot of HiFi gear over the years but it has only been lower powered stuff with the mains transformer being my only concern, although I did attempt to repair a pair of Kerr McCosh EL84 monoblocks I had a number of years back but they were eventually rebuilt for me.

I am fully aware of the dangers of high voltage DC - I had a mobile disco back in the early 1970s and I built a xenon strobe unit which had a very large 500v capacitor that I forgot t discharge, I'm thankful it happened when I was young and wth only one hand involved - so am rather cautious about working on the KEL34.

So my questions are:

How do I go about making the KEL34 safe for working on?

Should I get myself some good electrical gloves and can I work in them easily?

What other tips can you guys give?

Many thanks in anticipation.

Here are pics of it

[Puffin]

I had one of these amps, but I never had the need to go inside it. At the time I had my KEL84, World Designs used to sell thick rubber gloves that were good to 800v if my memory is correct.

Something like this :-

https://www.amazon.co.uk/dp/B08NWNHX...eviews_uk_3-21

[Bonky]

Hi, I see you have also asked the question on the WD forum (http://www.world-designs.co.uk/forum...ead.php?t=8028).

A wise move; please let is know how you get on .(I built the excellent WD Kel84 amp).

One hand behind one's back is also one of the recommended safety ploys!

[willbewill]

Thanks guys, yes I've asked on WAD forum. What surprises me is that a DIY design didn't include bleed resistors and seemed to rely totally on the rubber gloves.

[Firebottle]

Hi Malcolm, I strongly recommend that you don't work on it with it powered up, and check that all the high voltage has been discharged. Once the high voltage is discharged then it is safe to work on, with the mains disconnected.

I am assuming that the 3W resistor between the two large electrolytics is at HT potential (CRC filter?). Make up an insulated lead with two insulated crocodile clips and include something like 100 ohms in the lead by incorporating a suitable resistor.

With power off you can discharge the main electrolytics by clipping onto one of the negative black speaker terminals then clip onto the 3W resistor.
If you use a multimeter on the same connections you should see the voltage dissipate or check that it has. Anything below a few volts is then safe to work on.

Give me a call if you need further assistance.


PS Bleed resistors aren't necessarily needed as when the amplifier is switched off a lot of the stored voltage will drop as the output valves continue to conduct for ashort period. However you can't rely on this to make the amplifier safe.

[willbewill]

Thanks Alan that was all really helpful, I didn't think of using the speaker ground doh! Just what I wanted and no I don't intend touching anything with it live.

I will definitely make a lead up with a probe on one end, Nigel on LH suggested using the 'a' pin on the el34s to discharge.

I'll probably get a pair of those special gloves as well to make doubly safe until everything is discharged.

Cheers my friend and the offer of further assistance is much appreciated.

[Bonky]

Any news?

R

[willbewill]

The probes to make discharge tool and the gloves arrived yesterday from Amazon and this morning the resistors from HiFi Collective - a huge shout ou to them as I only ordered them yesterday afternoon.

I made the discharge tool out of one probe lead and a 1k 10w resistor.



Then opened up bottom of amp, donned the gloves, attached discharge crocodile clip to one of the neutral speaker posts and took readings at 'a' pins on EL34s - only 38v but amp had been off for a couple of days. But I'd made the discharge tool and I was going to use it, a couple of seconds with it and 0v observed.

Decided to do as Nigel suggested and leave the circuit board in place to change resistors. They all unsoldered easily and solder sucker cleared holes although one solder pad got detached. Fitted the new resistors, bent to right gauge and legs trimmed to go through board but also leave a good height above board to allow cooling, then flowed solder to joints keeping iron in place a little while to hopefully flow solder through to other side of board especially in the case of the one with the missing pad.

Then checked resistors for continuity with rest of circuit, all good. Also tightened the toroid's fixing bolt as it had loosened in transit.

It's back in system and playing very nicely, it actually sounds better IMHO.

A great result and many thanks to those who gave good advice, especially Nigel and Alan.

Cheers.