Pye BB (PCL83)

[Ed_Dinning]

Hi Folks, this one was initially a bit of a puzzle.
Recapped and working fine, all volts to spec.
After about 1/2 hour the HT would fall and the cathode volts rise as well as a general increase in hum & noise.
Output valve grid leaks were 20% high so changed, no improvement.
Checked grid volts when fault was present, one side at +12 volts, the other at 20mV. Valve tested OK in HSVT, but a new valve cured the problem.

Anyone experience of this problem on a PCL83 ?


Ed

[Techman]

Yes, runaway of the valves like this is a known problem. I had it recently on one of those big old Russian Rigonda radiograms. They use the Russian equivalent to the EL84 in a couple of push pull output stages. The annoying thing was that this wasn't the fault that its owner asked me to look at and only showed up after the thing had been left on for several hours. A replacement valve cured the problem. Ideally it would have been a good idea to replace all four, but it was a case of doing a cheap job for the owner, and anyway you wouldn't notice any slight mismatch on a thing like that and the other three seemed to be all good - for the time being anyway!

[Techman]

I should add to the above that all grid coupling capacitors were good replacements and although the old circuit board was a bit butchered and bridged in places, with a good deal of scorching from heat over the years, it all checked out good and there was no leakage other than the valve itself.

[GrimJosef]

Originally Posted by Techman

Yes, runaway of the valves like this is a known problem ...

Runaway due to internal outgassing is also something that valve testers often won't reveal. They simply can't get the valve's anode+screen dissipation high enough to raise the overall temperature to the point where the gas becomes an issue. A power amp can though, although it's quite possible that it will take half an hour to do it.

Cheers,

GJ

[Hartley118]

Originally Posted by Ed_Dinning

Hi Folks, this one was initially a bit of a puzzle.
Recapped and working fine, all volts to spec.
After about 1/2 hour the HT would fall and the cathode volts rise as well as a general increase in hum & noise.
Output valve grid leaks were 20% high so changed, no improvement.
Checked grid volts when fault was present, one side at +12 volts, the other at 20mV. Valve tested OK in HSVT, but a new valve cured the problem.

Anyone experience of this problem on a PCL83 ?


Ed

When I serviced my PCL83 Black Box some years ago, I must admit to modifying the output stages by fitting individual cathode resistors and decoupling capacitors. Whilst the original common cathode resistor saves one resistor and two decouplers, its DC stability depends on having two matched valves. If one of the PCL83 pentodes goes weak and its anode current drops, that reduces the grid bias on both pentodes so that, if the other one of the pair has normal emission, it will start to draw excess anode current, further biasing off the weak pentode and completey destroying the balance. Whether that could cause these symptoms, it's difficult to see, but my BB has remained healthy since the mod.

Martin