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davew 23rd January 2019 09:12 PM

Problems with K5881 MK2

I've been running my K5881 MK2 for over 10 years now. it's a great little amp - The only problems I've had are the 2 O/P transformers failing - (I got 2 replacements from Geoff at for 200)

The problem that I have now that after running for a while the amp makes a very loud physical buzzing - not hum.
I'm assuming that this may be due to heavy load on the mains transformer although I can't really locate the source of the buzz.
I ran the amp with only one set of valves on each channel in turn and it seems OK so possibly its the extra load on the PSU causing a failure somewhere.
There's only a bridge rectifier, smoothers and choke - I'm hoping it's not the mains transformer this time! - Anyone seen anything similar before I do further testing?



snowman_al 24th January 2019 07:54 AM

Re: Problems with K5881 MK2
Hi Dave,
Two things to look at then.
First is the amplifier drawing more current than it should be? Have you checked the voltages on the 5881 cathodes (pin 8)? You can work out the current through each valve then using ohms law. The circuit has 56 to 60mA per valve.

Second is the mounting of the transformer and the end bell.
Are the red washers there, is the transformer mounted directly to the chassis or on rubber grommets etc.? If you slacken or tighten the bolts does it improve or worsen the buzz etc.?
You might have to rework the mounting.


davew 24th January 2019 10:42 AM

Re: Problems with K5881 MK2
Thanks Alan,

I'll check the cathode voltages as you suggest
Just to add a bit more detail, the amp is silent after switch on and the buzzing will suddenly start sometime later - Also you do hear a click through the speakers so I don't think it's a physical mounting problem although I will check this.

I'll let you know how I get on



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