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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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22nd Jul 2019, 1:10 pm | #1 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,195
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Pye BB (PCL83)
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 |
22nd Jul 2019, 1:31 pm | #2 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Pye BB (PCL83)
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!
Last edited by Techman; 22nd Jul 2019 at 1:44 pm. |
22nd Jul 2019, 1:39 pm | #3 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Pye BB (PCL83)
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.
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22nd Jul 2019, 2:13 pm | #4 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,311
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Re: Pye BB (PCL83)
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
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22nd Jul 2019, 4:20 pm | #5 | |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
Posts: 2,198
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Re: Pye BB (PCL83)
Quote:
Martin
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