Intro

[Mikeg]

Hi All,

I have been building electronics for HiFi and R/C models since the mid 60s. I built my WAD KAT6550 around 2002 when I lived and worked in Seattle. It should have been a KIT6550 but WAD sent the wrong kit! It worked first time and I ran it with a Cyrus preamp. On return to the UK it was left boxed up until last year when I revived it and fitted the upgrade kit that had been parked since it was bought originally. Recently it started to hum and I joined this forum to, hopefully, find a cure which seems to be to replace the PSU caps.

Thanks for the help! Mike

[Greg.]

Welcome to the forum!

I’m sure we’ll be able to help you restore your KaT6550 to hum free status.

[Richard]

Hi Mike and welcome,

Yes old PSU smoothing caps lose capacity and their esr increases so they can no longer smooth the ripple and the result is hum. If the hum is on both channels and has increased gradually this may be the reason.

Do check all amp voltages though in particular the 6550 cathodes for approx 40V. The upgrade kit includes Jensen paper in oil capacitors which may leak electrically and turn a valve “on”. Check the amp in a darkened room for excess redness of the 6550 plates and make a note of what value cathode resistors are fitted as they varied.

The good news is old age in the amp is usually confined to the electrolytic capacitors and replacing them all (PSU and signal especially cathode bypass) would be a good move after nearly 20 years.