Kel34 Hum

[Bickers80]

Good afternoon chaps, hope everyone is doing well and enjoying plenty of music in lockdown.

I've looked through some of the previous threads on the El34 but I'm hoping someone may be able to help me out.
I've developed a hum on the right hand channel, I've disconnected all inputs and changed valves around with the same results.
I'd already ordered some new valves ,the current ones have been on for about ten years so I thought I'd treat myself to some new(they haven't arrived yet)! It was serviced around 10 years ago too with new caps and resisters by a chap in Leighton Buzzard although I'm struggling to find his details now.
I didn't do the original build and I'd sooner not start meddling too much. So what I'd like to know is , what are typical reasons on this amp for a channel hum and also can anyone recommend someone to look at it for me? I'm based in Northamptonshire.

Thanks

Matt

[John Caswell]

Hi Matt,
The causes of hum are legion, however as you have done all the simple things we would suggest here, the next items are a bit more esoteric.
As it is only on one channel it tends to rule out the main smoothing capacitors.
Measuring voltages, comparing good to bad is a good start, bearing in mind that ±10% is quite normal - see FAQ.
I would start to look at the cathode decoupling capacitors and cathode resistors on the EL34s and also on the first stages, as it is possible after about 10years they may have dried out. Substitution is the real answer.
The other thing to look at are the high wattage resistors around the 6AU6 and ECC82 which have a habit of changing value.
Beware of the PCB, go easy with the soldering iron as the PCBs were not all that brilliant. If you think a component is suspect then the best solution is to cut it out leaving long leads, hope that the builder hasn't folded the leads over, gently apply solder and heat and remove the stub lead
That is a good start then report back here.

John

[Bickers80]

Thanks for the advice John, strangely enough the guy that worked on it before was also a John but based in Leighton Buzzard. I can see from the invoice that he changed 9 resisters and 4 capacitors, I was hoping to find him to service it again as I really don't have knowledge to carry it out. It's been going strong with no issues for a long time.

Matt

[John Caswell]

Hi Matt,
Apologies I assumed, wrongly, that you were au fait with electronics, soldering etc.
So we need to get someone to look at this and solve the problem for you.
I am based near Reading, Berks so if you can get the unit to me I will fix it.
Best PM me with phone/ nr. email and we can go from there

Regards

John

[Richard]

Yes mine had similar with the ECC82 anode resistors failing, could be other things too, a bit of a fiddle to do, but John's the man indeed

[dhaen]

John from LB here! Would someone be kind enough to pm me or link me to a KEK34 schematic.. It will save some time and effort with one.
Thanks in advance.

[Bickers80]

Hi John

Thanks for your help, I managed to hunt down the chap who did the previous work so I've sent it back to him as he's closer to me. I've also treated myself to some new valves so looking forward to having it back.

Thanks

Matt