KAT 6550 Humming and vibrating - help needed

[LSfarzo]

Sirs,

after having received the copy of the lost KAT 6550 manual (you all have been very kind to me!) I coudl make my check; here it comes.

The problem:
The KAT hums a lot, it is clearly audible and very loud; it vibrates a lot, too. Sometimes it hums 5/10 seconds after switching it on, other times the hums come and goes as a long period sinusoidal noise. The last phenomenon happens very rarely and during the "not humming" period the voltages I measure rises at an acceptable level.
The KAT, during a lot of years of great work, did never hum or vibrate at all. It was silent, good sounding and I am very proud of it.


What I see:
One of the rectifiers (5U4G), after switch on has is top red, as usual, the other seems to be dead. No red glows on top. But... The red one is warm while the "no red on top" one is really really too hot.

What I measure:
I attached a pdf with my measurements. Voltages at all checkpoints are definitely too low.
Moreover I followed a suggestion given in another thread: I removed 5U4Gs, switched on and measured the voltages at pins 6 and 4 of the 5U4G sockets. So I did and I think (please, could you confirm?) that voltages seems appropriate (phew...).


What I could conclude:
1) I need help to be sure of the problem !
2) Maybe one of the two rectifiers has gone

So,please, would you be so kind to give me some hint. I would not start buying around items (valves, resistors, capacitors) trying to find the bug without having completely realized where the problem would lie.

Then, if you think that the rectifiers are the culprits, please, woudl you be so kind to tell me where I could buy (on the internet) a pair of them without paying 200 euros, and which brand could improve the amp? I looked around and found prices a little too overrated , at least in my opinion.

Lastly, do you think 5U4GBs will fit the same (I think so).

I know, there are a lot of questions. I apologize but I really have very few time for me and I miss my music so much!

Thank you all for yuour attention and help

Luca
Italy

[John Caswell]

Hi Luca,
I would suspect the 5U4s here Luca, if one is getting hot and the other isn't, it would seem to me that the one not getting hot has an open circuit heater.
So first thing is measure between pins 2 and 8 of the 5U4 valves when they are not in the amplifier. I suspect one will be open circuit.
If they both measure OK then measure between pins 2 and 8 on the 5U4 valve base this should be a very low ohms reading 1 ohm or less. Then measure between chassis and pins 4/6 of the 5U4 valve bases they should both read approx the same about 28 ohms
Then with all the valves removed, turn the amplifier on and measure between earth/chassis and pins 4/6 of the 5U4s. This reading should be approx 425Vac. Also measure between pins 2 and 8 of the 5U4G bases, this reading should be approx 5Vac. I would guess that all these readings will be correct correct.
The report back here with your findings

John

[Richard]

Hi Luca,
You have some voltages which don't really agree with each other. On the ECF80 the 2 cathodes are correct but the anodes are not. As one will affect the other this is unusual. They are usually both correct or both wrong. I would try new ones.
One of the cathodes of the 6550 shows 51V which means it is drawing too much current. If one of a pair draws more than the other it will get hot anf fail. It may also hum.
How long is it since the valves were replaced?
I would buy all new valves. Even if you later find you do not need some of them they will be good for spares and you will need them at some time.
For rectifiers I like Russian 5U3 which is the same as 5U4 and very cheap off eBay,
http://www.ebay.co.uk/itm/x2-5U4G-5U...item2c6552bdbf
Svetlana 6550C are excellent,
http://www.ebay.co.uk/itm/6550-SVETL...item4845267aaf
NOS valves are a bit of a risk but I would try these ECF80 or similar,
http://www.ebay.co.uk/itm/VINTAGE-PA...item337885b963

[LSfarzo]

Dear all,

I finally could test the valve pins.
One of the two valve has pin 2 and 8 opened. This is not ok.
The other one has a reading of about 56 hom. Too much.

I made up my mind to change the two rectifiers. I bought a couple of new electro-harmonix 5u4gb. They are equivalent and I could easily find them in Italy for about 25 euros (both).

Now the amp is OK again

Anyway, after about ten years of great work I think it is time to follow Richard's suggestion: I will check all the valves again to verify if voltages are within the expected ranges.

Another question I have in "background" is: why the valve did break? As far ad I know, it should have lasted for a quite longer time....

I would sincerely tell you all guys, thank you for your kind and effective help!

Luca

[Richard]

Hi Luca,

The active part of valves burns away. They cease to emit or the filament breaks.

Power valves such as 6550 and EL34 may only last 2-3000 hours = 3 or 4 months of continuous use. Small signal valves should last 10x that,
http://www.world-designs.co.uk/forum...read.php?t=127

Rectifiers will depend a lot on how hard they are run. This is quite hard in your amp. Additionally they are wired as parallel diodes and should have balancing resistors fitted to the anodes. This is in the FAQ, http://www.world-designs.co.uk/forum...ead.php?t=3859

So 10 years is a very long time for a valve amp. Anyone running one should have a complete set of spare valves and swap them in to see if a problem is caused by old valves.

I would now check voltages carefully and in particular the 6550 cathodes where one appears to be drawing too much current which indicates they are getting old.

Originally Posted by LSfarzo

Dear all,

I finally could test the valve pins.
One of the two valve has pin 2 and 8 opened. This is not ok.
The other one has a reading of about 56 hom. Too much.

I made up my mind to change the two rectifiers. I bought a couple of new electro-harmonix 5u4gb. They are equivalent and I could easily find them in Italy for about 25 euros (both).

Now the amp is OK again

Anyway, after about ten years of great work I think it is time to follow Richard's suggestion: I will check all the valves again to verify if voltages are within the expected ranges.

Another question I have in "background" is: why the valve did break? As far ad I know, it should have lasted for a quite longer time....

I would sincerely tell you all guys, thank you for your kind and effective help!

Luca