K5881 speaker hum.

[bikerhifinut]

Originally Posted by Ali Tait

When did you last replace the valves?

I think he has tried that one already Ali.
I would be inclined to check the PSU end too.
Is the hum 50Hz or 100Hz?
Does this amp have DC heaters on the first stage and phase inverter? Maybe the DC supply to the heaters has failed and you are getting an AC component induced via the heaters?
Also how is the ground reference derived for the output valves? is it a CT on the 6.3V winding or is it a pair of 47 ohm resistors? worth checking i reckon.

A.

[shadders44david]

Hi All,

Thankyou for all your suggestions as to the source of the hum.

Earlier today every single solder joint in the amp has been resoldered and guess what ? No more hum I`m very pleasantly surprised by this as I`d spent hours checking and rechecking the connections and could find nothing obvious, it just goes to prove perseverance usually wins in the end. It`s been good to know that there are helpful people out there who are prepared to spend time suggesting possible causes and solutions. Thanks Andy, it would seem I have experienced a similar frustrating fault to your O/C coupling capacitor connectionon in your WD KT88 amp which appeared ok.

In answer to your query on the cathode bypass capacitors John, yes they do seem to fail every few years or so. One of the original ones exploded after about 6 years and the next few sets have tended to reduce the amps dynamics after a few years of use (had one Nichicon fail completely) and once a new set is fitted all is well again. The presently installed Elna Silmic IIs have been in around 18 months and sound the best yet. The amp is also now on its 4th set of power valves (Svetlana 6L6GC - winged C) and has had various types of coupling caps fitted over the years and is presently using Jantzen Superiors which sound the best to my ears.

Now normal service has been resumed it`s time to enjoy a newly acquired pressing of Melody Gardot`s latest LP, forget about hums/buzzes and appreciate the unmatched magic of valves and vinyl once more.

Regards, David.

[John Caswell]

Hi David,
Excellent result, just goes to show that we smart asses often look for sophisticated faults when a simple soldering job cures the problemt

John

[bikerhifinut]

Brilliant!
Dry jointitis.
i remember chasing one for days.
Good listening.

A.