Kerr McCosh Monoblock Problem

[willbewill]

As you may know I recently bought a Kerr McCosh Passive Preamp and a Pair of Kerr McCosh valve monoblocks from woody:
http://www.lencoheaven.net/forum/index.php?topic=106.0
I'm generally very pleased with them. Scott did a beautiful job on the passive and woody had got the monoblocks cleaned up and working prior to the sale.

However one of the monoblocks has a fault. When it has been on for a while 20-30 mins it starts to make noise through speaker - a whilstling sort of pink noise sound and occasional pops and squeaks/squeals, distorting the sound in the process. It then may 'pop' and be ok for a few minutes before starting again with the noise.

I have changed the valves supplied (The amps came with a mixed bag of vales: mullard EZ81 the ECC81 and EL84 have little or no markings but are not all the same - one el84 was a mullard in the faulty amp but became very noisy - this was the first time I heard this sort of noise ) to Pinnacle ecc81, general el84 (thanks to crag) and tesla ez81.

On Scott's advice I've swapped the valves from one amp to the other but the problem remains, I've also contact cleaned the sockets.

Is it likely to be a valve problem or do I have to delve further? Bearing in mind I only know very basic electronics but am willing to learn and would like to sort myself. Woody couldn't find a schematic anywhere and neither can I.

HELP!

[ian]

Hi Malcolm,
OK, this is like the blind leading the partially sighted but the symptoms you describe sound similar to the ones I had with my Croft power amp.  This was down to knackered paper in oil capacitors as diagnosed by Glen Croft.  Their replacement cured the problem.

If you don't get anywhere via the DIY route it might be worth contacting Glen.  I understand he does service other vintage makes so he may have seen yours before.

Cheers, Ian

[willbewill]

Thanks Ian

Some of the caps have already been replaced but it doesn't look like all so that may be the route.

[willbewill]

Just put ALL the valves that came with amps back including the mullard el84 I thought was faulty (in the other monoblock) and they are both working - now will I get a problem in 20 mins?

[brian]

Maybe a dirty valve socket? Usually pulling and replacing valves a few times does the trick.

[willbewill]

They've been on for about an hour - there is now faint 'interference' type noise on the one monoblock (yes still the same one)

[willbewill]

2 cds on (yeah cds - while I'm cleaning house) and still only very faint 'interference' - hopefully it only was a dirty socket. Gotta go out now so will leave on and see what's like when I return

Don't know whether it's relevant but the 'good' amp gets a lot hotter than the 'bad' amp - mains transformer is pretty hot and output is warm whereas on other mains is warm and output cool. Also the valves glow brighter in the 'good' one

[willbewill]

On for 4 hours or so and still only faint interference and that not all the time - looks like it must be something to do with the other valves or was a dirty socket. Next I'll have to try swapping other valves in...but that's another day.

[woody]

Malcolm have you checked the Caps underneath ? theres a couple of wax/stuffed caps  can you see any wax/damage or ? if you have any contact cleaner take all the valves out spray pins and insert the valves a few times  to clean any debris from the socket , are all the sockets intact not cracked in any way ?

if you want to give me a phone ?

[willbewill]

Thanks Paul - I've cleaned the sockets, will check caps

[GP49]

Check the voltage on the grids of the tubes that are running hot, and compare to the same grids on the other channel.  If one is wrong, suspect a leaky coupling capacitor to that grid.

[Jolyon]

I agree.

I had a pair of Leak TL50+ valve monoblocks and one of them had leaking capacitors on the grids to the KT88s and they used to glow orange until I changed them.  You could fry an egg on the mains transformer as well, but despte that the wax hadn't melted in it.  However, It's a known failure mode for old cathode bias amplifiers as the valves will conduct more and therefore place a higher demand on the mains transformer. On Stereo 20s Leak put a 2W resistor in the power cct which was desiged to run hot in normal use.  If a fault occurred the resistor would run really hot and desolder itself and fall out cutting the power.

J.

[woody]

Malcolm

 there are 2 black with red end caps underneath the amp about 2-3 cm long please check them ? are they ok ?

[willbewill]

OK so now I am confused - so the hot quiet amp is the faulty one, not the cool noisy one

[willbewill]

Check the voltage on the grids of the tubes that are running hot, and compare to the same grids on the other channel.  If one is wrong, suspect a leaky coupling capacitor to that grid.

For that I will need help - I'll try to post some pics of the inside