KEL34 making safe to work on

[willslenco]

I've recently bought a WAD KEL34, see my introduction here:

http://www.world-designs.co.uk/forum...2818#post92818

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?

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.

[bob orbell]

Hi Will, there is only one sensible answer to working on live HT, and that is be very careful , a thick rubber matt to stand on may give you some self satisfaction as will HV electric gloves, but I don't know any one on here that uses gloves. Bob

[willslenco]

Originally Posted by bob orbell

Hi Will, there is only one sensible answer to working on live HT, and that is be very careful , a thick rubber matt to stand on may give you some self satisfaction as will HV electric gloves, but I don't know any one on here that uses gloves. Bob

Thanks Bob, I don't really want to work on it live but to render it harmless for me to remove the offending resistors and replace with new.

So it's really about the big caps being discharged, I'm assuming there are no bleed resistors on KEL34?

[bob orbell]

If they are present, they will be directly across the capacitor terminals, about 220K, if not use a 100 ohm resistor with two lengths of insulated wire soldered to the resistor and short the capacitors for a few seconds, there may be a small crack as you make contact, but there may be no charge at all in them, you can check with your meter for voltage, anything lower than 50 volts will not be felt. Bob

[willslenco]

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.

[willslenco]

OK guys, looks like I have couple more problems:

1. It only affects the right channel and displays itself as intermittent low level crackle through speaker. It is worse on start up and slowly settles down over the course of about half an hour, it is much less frequent and obtrusive after this and only really noticeable with no music playing (I have left amp on with no music for a prolonged period to ascertain this). After an hour it's pretty much silent.

2. Getting a crazy buzz, and other weird noises, through the speakers at about 1/2 to 3/4 volume, both channels, with or without inputs connected, on all sources. It's not hum but a definite buzz which appears around that volume and disappears again after it. I don't normally play above 1/2 volume so it may have been there a little while. I've tried different speakers but same result. Also swapped a couple of EL34s in/out and fitted new 6AU6s and ECC82s but no change.

Where should I look next?

[bob orbell]

Your crackle is most likely a poor solder joint, may be one of the valve bases, run a iron over all on the bad channel, check with your meter to see if the body of the potentiometer is in contact with the chassis. Bob

[Richard]

Originally Posted by willslenco

OK guys, looks like I have couple more problems:

1. It only affects the right channel and displays itself as intermittent low level crackle through speaker. It is worse on start up and slowly settles down over the course of about half an hour, it is much less frequent and obtrusive after this and only really noticeable with no music playing (I have left amp on with no music for a prolonged period to ascertain this). After an hour it's pretty much silent.

2. Getting a crazy buzz, and other weird noises, through the speakers at about 1/2 to 3/4 volume, both channels, with or without inputs connected, on all sources. It's not hum but a definite buzz which appears around that volume and disappears again after it. I don't normally play above 1/2 volume so it may have been there a little while. I've tried different speakers but same result. Also swapped a couple of EL34s in/out and fitted new 6AU6s and ECC82s but no change.

Where should I look next?

Hi Will, yes as Bob says, and my thoughts are,


1. go to http://www.world-designs.co.uk/forum...read.php?t=135 and scroll down to the ECC82 anode resistors. I had KEL34 and Kit88 amps and wrote up that FAQ from first hand experience.


2. can you outline your chain of connections; source>pre>amp>speakers and if there is any other attenuation before KEL34 vol pot. Sometimes what you describe can be an impedance mis-match from 2 pots on the input for example. Otherwise as Bob says check for joints and connectors and swap out what may simply be a noisy pot, it may have been used a lot around that setting by the previous owner.

[willslenco]

Originally Posted by bob orbell

Your crackle is most likely a poor solder joint, may be one of the valve bases, run a iron over all on the bad channel, check with your meter to see if the body of the potentiometer is in contact with the chassis. Bob

Many thanks Bob, you seem to have hit the nail on the head with that. Took the volume pot out and saw that there is an earthing washer connection between it and the chassis, as detailed in build instructions I now see. It all looked quite oxidised and the solder on washer was also interfering with the connection, so filed it flat and got some emery cloth on all the mating faces and screwed it back in tight. Checked continuity which is now good.

Have had it back in system playing for about an hour and all appears well, in fact it seems to have solved both problems.

[willslenco]

Originally Posted by Richard

Hi Will, yes as Bob says, and my thoughts are,


1. go to http://www.world-designs.co.uk/forum...read.php?t=135 and scroll down to the ECC82 anode resistors. I had KEL34 and Kit88 amps and wrote up that FAQ from first hand experience.


2. can you outline your chain of connections; source>pre>amp>speakers and if there is any other attenuation before KEL34 vol pot. Sometimes what you describe can be an impedance mis-match from 2 pots on the input for example. Otherwise as Bob says check for joints and connectors and swap out what may simply be a noisy pot, it may have been used a lot around that setting by the previous owner.

Thanks Richard, yes I changed the anode resisters as soon as I got the amp and it seems the volume pot earth to chassis was the cause of the problems.

Great diagnosis guys, much appreciated.