Hot and cool valves

[bob orbell]

Hi chap's, I have question if any one can answer it, at the moment I am using a WA88 power amp. it has been performing good for 4 or more years, it still is performing good but this morning I gave the thing a dust, it had been on for about one hour, after switching off I let it cool for 30 seconds before removing the valves, now hear is the interesting bit, looking at the amp from the front at the four power valves (KT88)s from left to right no1 valve cool to the touch, no 2 valve very hot, no 3 valve cool, no 4valve very hot, suspecting valve problems I reversed the valves switched on and left for ten minits, same thing, one valve on each chanel very hot the other two I can bear my hand on them even after one hour or more, the other two will take my skin off, I have checked heater voltage and that is spot on, will do more voltage checks in the morrning, any thought's. BOB

[Global]

Hi Bob

Check voltages at control grids which should all be virtually identical and voltages across cathode resistors. Any differences will denote differing extent of the valves' conduction. Use ohm's law to determine the current through the valve from cathode volts / cathode R. Going from memory it should be in the 50mA ballpark.

Differences will probably be down to valve age.

Mark

[Richard]

Hi Bob,

You don't say how you reversed the valves and whether the result followed/moved with the valves or stayed with the position.

If it's the valves then I'd go with Mark's last sentance; old or mismatched valves, can you try another set?

[bob orbell]

Thank's for your help gent's, now sorted, two cathode risistors open curcuit (100 ohm 3w) dont know for how long they have been like that, or why, thing is it still sounded good, mind you it sounds cleaner now and all valves are the same tempreture voltages also good. BOB

[John Caswell]

100R 3W in the cathodes? Don't remember either of the WAD/WD88s having that value there.

John

[Richard]

That's good Bob

They are the g2 resistors between OPTX UL tap and pin 4 g2. They shouldn't go in normal use so it would be worth checking their rating. They can blow if a valve fails and they receive too much current. I don't know if that's happened to you or a previous owner if you didn't build it.

The other (maybe obscure) thing that comes to mind is that the drawing is wrong and it's possible the builder has linked pin 4 and pin 8 as shown, on some or all of the valve bases, so it's worth checking.

What it should show is that link, which is internal to the valve, between g3 beam plates and cathode, not g2 and cathode. It may have been interpreted as an external wire link.

If that has happened, possibly on just 2 valves, the 100R resistors may have blown a while back and the problem "sort of" corrected itself.

Bear in mind also that these class A pp amps can run on one valve to quite a suprising volume level without anyone knowing about it if they do not test them. I would get it checked through with a scope etc.

[pre65]

R19 & R20 are 100R 3W and go to the OPT, are they UL taps ?

[bob orbell]

Thank's for all the responce chap's, John, sorry my mistake the resistors are in the UL taps not the cathode, I don't have the curcuit diagram any more so get confused, I did build the amp some four years ago and every thing is to the instructions Richard, all voltages are very good too, the only thing I can think that hapened is I lent it to a friend for a month, could he have over driven it? I know his speakers are not very efficient, anyway thank's again. BOB

[Richard]

Hi Bob, not really, a bit of a mystery then. The amp is all class A so the valves are running flat out when idling and have to convert all the energy to heat. When playing music they can dissipate some energy to the speakers so your friend may have given them an easier time. A possibility might be that it's been operated without speakers attached. Anyway, I'd get it checked out fully if poss to be sure it's giving full power and is stable.

[bob orbell]

Thank's for the advise Richard, I have been in touch with Chris Fidemore this morrning, ( ex. WD ) and he will check it with his scope for me some time next week, I will let you know the outcome. BOB