K5881

[robi56]

I have had this amp for 7 years and I am now my 4th set of valves has just gone.
It seems that every now and then the cathode resistor on one valve goes open circuit, often taking the bypass capacitor with it. I suspect that the valve is shorting internally, but it is too intermittent to catch. When I checked it out the capacitor popped after about 20 minutes (soon after I stopped monitoring the cathode voltage in fact).
Is there anything else that I should check? Is this rate of failure unusual?
Could anyone recommend fuses for the valves and where they would be best placed in the circuit? You used to be able to get fusible resistors for this type of thing.
The last valves I used were Svetlana 6L6GCs, I thought the higher power handling would give a better margin & thus working life. Is there a better choice?
I need advice as the wife gives me dirty looks every time I say I need new parts for this and I have to say I am getting a bit fed up with it too. I normally have to wait quite a time for the parts to come.

[John Caswell]

Hi Robi56
Well the question is how often do you use it. If it is on 10hrs a day then 4 sets of valves in 7 yrs is not bad, as O/P valves get a hammering.
I would however look at the power line voltage to see what it is, as if it is say 250 as opposed to 230 then 10% high will cause the amp to eat valves. Measure the heater voltages as well to see that they are around 6.3v AC/DC.
because you have fitted higher power handling valves doesn't mean they will last any longer, in this amp I would stick with the 5881s.
5881s seem to be very robust and from my experience do not seem to suffer catastrophic failure as often as other O/P types.

John aka Dr John

[Richard]

Robi

Ditto John's coments; OP pentodes such as these are only rated for approx 2000 hours by Svetlana themselves. That would be 3 hours a day for 2 years. When I ran my Kit88 all day, sure enough, I got through a set of valves in about 18 months and considered that fine.

A cynic might chip in that was part of the attraction of transistors running in class B...

However, life will depend on how hard the valves are driven. K5881 runs them at 24W plate dissipation in theory but this may vary with mains voltage. If you've not done it,
(check cathode voltage and divide by 560)
and multiply by
(the voltage you measure at the anodes less the cathode voltage)
to find plate dissipation.
If it's high change the cathode resistors to 680R.

The other thing to check (or change to new plastic ones) are the coupling caps C5/6 in case they are leaking elctrically and causing the valves to turn on more than they should.

Rich

[NealG]

I've used 6L6 svets and the Harma's on my 5881 and the plates glow a very dulll red, not easy to spot unless the room is really dark. The supplied 5881's never had this issue.

[robi56]

Thanks for the info.
I measured the mains and it is 244V, so probably that is the root of my problems. Where I lived when I made the amp had a lower voltage & I never checked it since.
I had already changed the cathode resistors to 600Ohms and when I follwed Rich's suggestion and mesured the working channel that gives me an annode disipation of 28.8W. I hate to think what it was with 560! No wonder 5881s didn't like it.
I changed the blown cap and put a spare pair of K5881 in the damaged channel. During power up I heard a faint pop in the speaker and the valve started to glow red - cathode bypass capacitor shorted (100V Black Gate).
Now (with no capacitor fitted) the cathode voltage on that valve just keeps going up and the valve gets red. Input coupling capacitor does not seem to be leaky, I have tried different valves. It goes steadily up to 70V where I turn off again. The other valve of the pair seems fine. I'm mystified by this, any ideas?

[Global]

Hi Rob

Does the problem move with the valve (=dead valve) or not (=probable damaged cathode resistor)?

I'm assuming it's a cathode bias amp as I don't have a diagram.

Mark

PS - Just read your note again. If you are getting 70v on the cathode that should put a cathode biased amp at -70v bias. This would get the valve fairly near cut-off which it surely isn't. Can you check the voltage on the control grid with the amp running (briefly)? It would seem possible that the coupling cap might be leaking at higher voltages which you may not see on a simple resistance check.

[John Caswell]

Hi all,
Before messing about with anything else replace the cathode bias capacitor with a suitable cap so that the amp is back to normal working condition, monitor the cathode volts of the "faulty' valve and switch on. Obviously if it rises dangerously switch off.
Because all the 5881 amp variants are AC coupled on the o/p stages you can remove the coupling cap of the offending valve(s) and run it safely. Once again monitor the cathode volts and switch off if dangerous.
I have found that some of these PP amps are prone to instability ('Scope needed) due to long anode/grid leads in the o/p stage being in proximity so they may need moving around with an insulated probe to see if it cures the problem.
Have a play and report back

John aka Dr John

[robi56]

OK. removed coupling capacitor and (reluctantly) fitted last remaining cathode bypass capacitor. Cathode volts rose steadily to 60 at which point I turned it off.
Other volts were about: anode 464V, screen 416V, grid 0V (though they don't get a chance to stabilise before I have to switch off).
I measured the output transformer primary resistances and there was a difference, but I checked on the working channel too and they were similar.
The problem does not follow the valve around.

[NealG]

Just a thought, have you measured the resistance to ground from the top of the cathode resistor and also from the bottom of the resistor? Is your meter OK, has happended before that a faulty one has been used...

[NickG]

Check to make sure there isn't a dry joint or anything that would prevent the grid being referenced to earth. I would solder a wire from the valve socket grid pin to earth and see if that makes a difference.

Also are you measuring the voltage across the resistor or from the cathode to ground?

If the end of the cathode resistor was not connected to ground you may see the voltage on the cathode do what you are seeing.