World-Designs-Forum  

Go Back   World-Designs-Forum > WAD > WAD Problems
Register FAQ Members List Calendar Gallery Today's Posts Mark Forums Read

WAD Problems For questions and answers re older World Audio Design Projects

Reply
 
Thread Tools Display Modes
  #1  
Old 7th June 2021, 11:08 AM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default KEL34 making safe to work on

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.
Reply With Quote
  #2  
Old 7th June 2021, 12:03 PM
bob orbell bob orbell is offline
Senior Member
 
Join Date: Nov 2009
Location: kettering northants.
Posts: 2,313
Default Re: KEL34 making safe to work on

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
Reply With Quote
  #3  
Old 7th June 2021, 12:12 PM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default Re: KEL34 making safe to work on

Quote:
Originally Posted by bob orbell View Post
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?
Reply With Quote
  #4  
Old 7th June 2021, 01:40 PM
bob orbell bob orbell is offline
Senior Member
 
Join Date: Nov 2009
Location: kettering northants.
Posts: 2,313
Default Re: KEL34 making safe to work on

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
Reply With Quote
  #5  
Old 9th June 2021, 03:49 PM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default Re: KEL34 making safe to work on

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.
Reply With Quote
  #6  
Old 15th July 2021, 11:36 AM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default Re: KEL34 making safe to work on

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?
Reply With Quote
  #7  
Old 15th July 2021, 02:22 PM
bob orbell bob orbell is offline
Senior Member
 
Join Date: Nov 2009
Location: kettering northants.
Posts: 2,313
Default Re: KEL34 making safe to work on

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
Reply With Quote
  #8  
Old 16th July 2021, 07:14 AM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 5,120
Default Re: KEL34 making safe to work on

Quote:
Originally Posted by willslenco View Post
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.
Reply With Quote
  #9  
Old 16th July 2021, 12:27 PM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default Re: KEL34 making safe to work on

Quote:
Originally Posted by bob orbell View Post
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.
Reply With Quote
  #10  
Old 16th July 2021, 12:30 PM
willslenco willslenco is offline
Member
 
Join Date: Jun 2021
Location: Swansea
Posts: 43
Default Re: KEL34 making safe to work on

Quote:
Originally Posted by Richard View Post
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.
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 03:05 AM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Copyright World Designs