KEL34 making safe to work on

[willslenco]

Just bought another WAD KEL34, as yet to arrive though.









Advert said:

World Audio Design as advertised as a prototype and was one of the first EI transformer versions (previously toroidal version) around 1999

It was part-built so I had to complete the build and took the opportunity to include some upgrades recommended, at the time such as:-

BR1 STTA512D Ultra-Fast HT diodes

BR2 Bias to MBR160 x 4 Schottky diodes

BR3 6AU6 DC Heater *IN5820RL x 4 Schottky diode and PI-filter

Elna Stargets cathode bypass caps

Alps volume potentiometer

And the bonus is it comes with a brand new unused PCB

PS is the KEL34 Upgrade Instruction Manual available?

[willslenco]

It's arrived and sounds very nice, except after a couple of hours I'm getting an intermittent 'clicking' noise.

I'll try different valves, but maybe it's the anode resistors?

Edit

No difference with different valves, noise was there on switch on

[willslenco]

So went looking for more obvious solutions... switched my media server off and clicking has gone

[willslenco]

Well that was a red herring - clicking is back

sounds like morse code, the same on both channels

[willslenco]

I'm assuming a power supply problem - dodgy rectification?

[A Stuart]

Morse for "Phone Home, ET"?

Seem to recall seeing earthing of (output?) transformer laminations as a solution for some type of clicking.

Perhaps elaborate on the nature of the clicking. Volume control dependent? Both channels? - sorry, just saw you did say both channels.

[willslenco]

Originally Posted by A Stuart

Morse for "Phone Home, ET"?

Seem to recall seeing earthing of (output?) transformer laminations as a solution for some type of clicking.

Perhaps elaborate on the nature of the clicking. Volume control dependent? Both channels?

Thanks

Yes gets louder with volume

Yes both channels

No sources connected

same on different mains outlets/cables

[Richard]

Very nice looking amp Will, I've often thought about another of those.

Build sheets were the same afaik. The early toroid TX ones also had 6CA7 op valves (mine did) but same circuit. Nice to see yours has the XLR speaker sockets replaced and generally looks tidy.

If you decide to keep it those psu caps C5 and C6 look very old and may well be causing the noise. The bigger C1 and C2 look like replacement Rifas which were a popular mod.

Next job for you really is a complete replacement of all the elctrolytics and those ECC82 anode resistors as it's not worth taking the PCB out just for one or two.

I did check mine with a temp probe back in the day and it ran at 48C so after the 24 years on those date stamps they will be well-ready

[bob orbell]

A suggestion for finding the click, remove bottom cover turn on its side connect up in a very dark room, switch on and see if you can see a spark when it clicks, it does sound like something arcing. Bob

[willslenco]

Thanks guys, helpful pointers

I've opened it up and these are what I've noted so far:

1. There was no residual charge left in PSU caps (my other KEL34 still has about 38v a day after switching off)

2. Measured resisters, all pretty good except R28/R29 (1M 0.66w) these show no resistance i.e. short circuit

3. C5/C6 tops are bulging (470uF/450v)

4. C1/C2 also slightly bulging (100uF/450v)

5. C7/C8, C15/C16 (0.47uF/400v) are discoloured)

6. There is a suppressor cap on IEC socket (could this cause the problem?)

7. No obvious signs of arcing, but not dark enough to try Bob's trick

Here's what I plan:

a. Change the ECC82 anode resisters to 2w

b. Replace C1/C2, C5/C6

c. Replace C7/C8, C15/C16, C13/C14

d. Replace R28/R29

I'm tempted to retain the Stargets (C9-C12) and other electrolytic caps for now as they were all part of the upgrade and hopefully are in good shape?