KT88 construction problem

[Greg.]

Great result, Alasdair. Well done! Likewise to John, Richard and Bob.

This thread is an excellent example of a punter with a problem and the knowledgable offering generous advise and for once, the punter follows properly the guidance of the knowledgable, coupled with a bit of personal intuition that leads to a positive and intended outcome.

So often in the past John, Richard and Bob have responded to requests for help and the person in need simply fails to follower their guidance with a result of no positive outcome, or at least, an agonising process to get there eventually. In this case, Alasdair took on board everything suggested and the end result is a properly working amp with not too much agony in reaching the proper outcome. Many could learn from the lesson spelt out in this thread.

[A.N. Beal]

Dear Everyone,
Thanks for all your help.
In answer to Richard's questions, it's not my first kit - I haven't done lots but I built a KEL84XL previously and a Graham Slee Genera phono preamp. Years ago I built some Heathkit transistor stuff - and my earliest introduction was 'helping' my dad to build a Heathkit valve amplifier when I was 4 or 5 years old. The amplifier I have just completed is a WD88XL integrated. It sounds lovely already, I suspect it may sound even better after running in and I'm not in any rush to fiddle about with it and change things. (Funnily, although I found the white Soniq signal cable too bright in the KEL84XL, it sounds fine in the WD88XL.)
Well done all,
Alasdair

[A.N. Beal]

Postscript
Further investigations by Matthew Snell found that some of the latest supplies of output transformers have the output connections reversed by mistake, making this the likely cause of my problem. He supplied me with revised labels to put on the transformers and I have rewired the relevant connections as they should be taking this into account. It is nice to be able to pin down the cause of a problem and put it right - thanks to all concerned.

[A.N. Beal]

Help! More problems! As previously reported, after switching wires as recommended by John C. the amplifier worked perfectly. Matthew then confirmed that the probable fault lay in reversed transformer outputs and helpfully provided corrected labels. I decided to rearrange the wiring accordingly and everything worked perfectly ... for a few days. Then one day I switched on the amplifier and there was a 'tap tap tap' sound on the right channel. Tapping the case produced crackles from the right hand loudspeaker, then a hum developed on the right channel plus some swishing noises. I switched off and the swishing noises continued for a while before dying away.
Assuming that there was a problem in my soldering when rearranging wires, I resoldered all the joints I had done in the process, switched on and all was perfect. I then enjoyed over a week of lovely music ... and then tonight I switched it on and was greeted with 'tap tap tap' on the right channel, crackles when I tapped the amplifier case and then a hum and swishing noises on the right channel, which continued for a while after it was switched off. When I switched on again there was a hum on the right channel (volume independent) but no crackles when tapping the casework. I switched off and swapped the left and right valves - no change, still the same problem on the right channel. I then resoldered the joints associated with the wiring change, switched on again but this time the resoldering hasn't solved the problem. Grrr! (The left channel has been fine throughout.)
I have inspected all my soldered joints but see nothing obviously amiss. If the worst comes to the worst I suppose I may have to resolder every joint on the right hand channel but that is a task I would like to avoid having to do if possible. Can anyone help me narrow down the search by suggesting the most likely cause and location of the problem?
Yours with fingers crossed,
Alasdair

[bob orbell]

Hi Alasdair, I think this is possibly a problem with the DC from the power supply to the right channel, somewhere, try and follow all the high voltage connections and give the cables a small tug to see if anything is loose, WITH THE POWER OFF, pay particular attention to the power supply capacitors C9, C12 & 13 will not be the problem because the problem is only on one channel, also make sure that all 0 volt connections are good. That's as far as my thinking goes. BOB

[A.N. Beal]

Dear Bob,
Thanks for the suggestions. I've checked and found a couple of dry joints, including one on the earth side of C9, which has got rid of the 'tap tap tap' sound, swishing noise and crackles at switch-on. However as it warms up a hum appears on the right channel, which is independent of volume setting. There is sound on both channels. I'm struggling to identify the source of the hum. I have checked all test voltages and they are all as they should be. I shall try some more wiring checking but any more helpful suggestions of particular places to look would be appreciated. What perplexes me is that the amplifier worked perfectly for a week before this.
Regards,
Alasdair

[A.N. Beal]

I found another dry joint and I think this has fixed it.
Alasdair

[bob orbell]

Alasdair, it seems that your soldering may not be the best, please don't take this as an insult, soldering is quite an art to get a good electrical joint and neatness, are you just using fluxed solder? or do you use electrical flux past?, I use both, do you clean component joints? { a fibre pencil is perfect for this } how much heat dose your iron produce? is it a variable temperature iron? I find that using a hot clean iron with flux past around the joint gets the solder flowing quickly, but not too much solder. I hope this is of some help to you. BOB

[A.N. Beal]

Dear Bob,
Thanks for your email. I think that the couple of dry joints were just caused by not being patient enough and rushing a bit - I've been soldering (on and off) for a long time and don't usually have a lot of problems OK. However I do find that silver solder is a bit more fussy to work with than the old lead solder. I use a 15w Antex soldering iron generally but borrowed a larger iron for soldering the bus bar on the KT88. The solder has a flux core. I haven't done things like special cleaning of joints or separate paste flux.
PS The KT88 is now working perfectly and sounds splendid. was interesting to go back to the KEL84XL for a while when the KT88 was having trouble. Initial impressions and at modest volumes there is very little between them. However as listening proceeds it becomes noticeable that the KT88 does do some things clearly better: (i) more power, (ii) more low level detail, (iii) better handling of high frequency nasties, so CD sounds more like analogue and analogue sounds ... great, as if pickup tracking has improved (which is, of course, impossible) and (iv) some instruments like bowed double bass or cello are startingly realistic.

[Greg.]

Originally Posted by A.N. Beal

However I do find that silver solder is a bit more fussy to work with than the old lead solder. I use a 15w Antex soldering iron generally but borrowed a larger iron for soldering the bus bar on the KT88. The solder has a flux core. I haven't done things like special cleaning of joints or separate paste flux.

OK, I suspect, from what I have read that there is a lot of urban myth around the need for silver loaded solder when connecting silver plated cables. However, a good low melting point solder that includes silver will be a nice option in your circumstances so can I recommend this: http://uk.rs-online.com/web/p/solder...35312D36373126

This stuff is big on initial financial outlay but will last you probably the rest of your life so therefore, very good value. It's a great product and contains the silver you desire. Very easy to use owing to it's low melting point and I believe used and valued by very many DIYers indluding several significant members on this forum. Certainly I am a fan.