Kat6550 vibration/hum

[Dave999]

Hi all

first time back here in a long time

I have a Kat6550 running KT88s from Shuguang, mullard small valves and russian Military winged C Diodes

it is bog standard, was built by WAD as a complete unit rather than DIY.

i purchased it off a chap who did or maybe does still, post here. about 10 years ago and it has been and is, great

it works well no valves are trying to give me a sun tan..

however recent close encounters with it indicate that it hums, and i don't think it used to, not through the speakers, but a physical vibration that causes the bottom plate to resonate.

its got a certain 1958 Dansette or Regentone TONE to it when you stand near it.

i'm reasonably technically adept but value amp power supplies are not a strong point.

is there anything that springs to mind. i.e a common ailment that might make bits of flat steel plate vibrate.
i'd suggest magnetism driven by the 50Hz mains is playing its part.

I'm the first on the street after the substation so i'd also suggest i get a bigger slice of mains voltage than some

do they all do this?

i was wondering if after 15 - 20 years maye it was time for some new capacitors in the power supply before i "take out" something wire wound and expensive.

any views gratefully accepted.

Ta

Dave

[Phil Y]

Have the screws on the base plate come loose?

[bob orbell]

Hi Dave, I had just the same problem with mine a long time ago, there are not enough screws on the bottom plate, I drilled and added four more self taping screws equal distance from each end, it worked. BOB

[Dave999]

excellent many thanks

i will certainly look into that.

nice and simple

certainly stops if i prop the base up on the handle of my screwdriver.

more structural integrity needed.


Dave

[bob orbell]

Yes, I hope it works for you Dave, all the WD case's are Aluminium, so no issues with magnetic vibration, and they have more fixings. BOB

[Greg.]

I have found sorbothane feet under the chassis can be a real cure for a buzzy amp. Cheap as chips on eBay.

https://www.ebay.co.uk/itm/4-x-SORBO...YAAOxycD9TWUJj

[John Caswell]

Hi all,
The 6550 was a WAD design and as such had steel chassis, so of course things are a bit different.
A bolt to stress the base between the top of the chassis and the base may help or even some "Dedshete" on the base may be better.
It might be vibration of the power tx/choke causing it or just plain magnetic attraction.

John

[Greg.]

Yes, going on from what John has said, on my WAD 300B PP, I tried tightening up all the mounting screws on the mains Tx which was the source of the buzz to little effect. No buzz through the system, therefore nothing through the speakers, but for sure the amp was physically noisy, sufficient to spoil and interfere when listening to quiete passages of music.

Originally isolated on spiked supports, I changed to the sorbothane options I have suggested. There was no loss of sound quality. Whilst I acknowledge I did not cure the original problem, the Sorbothane feet provided a solution, completely nullifying audible buzz. The Tx buzz was being amplified by the shelf the amp was spike mounted on. The introduction of Sorbothane feet sorted it. Regardless of whether you have gained improvement by anchoring your base plate more securely, I really would encourage you to have a go with These feet.

Actually, my complete solution was the introduction of an Airlink balanced mains PSU especially after I introduced a DC voltage blocking module before the balanced mains Tx. I am now completely hum/buzz free, regardless of time of day. The problem was previously worst when there was maximum demand on electricity locally. This is no longer an issue.

[Dave999]

Ah haaa.. may well invest in new feet for the unit
the originals are going to be rock hard having been compressed for so many years.

yes to confirm, the base of the unit is pressed mild steel sheet with louvres for cooling...

I will do a little more experimentation as wedging the base with a nylon cap off a USB stick is not exactly a permanent solution...

balanced mains for it is a nice idea, but i can't imagine what kind of excuse i'm going to have to come up with to get one of those units through the door.....

Many thanks for your input guys


Rgrds

Dave