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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 18th May 2022, 8:52 pm   #1
Ed_Dinning
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Default Pye BB (PCL83) instability, possible cure

Hi Folks, I have repaired many of these units over the years. The newer PCL83 valved type is certainly much more of a problem than the earlier EL42 based version.
Normally, despite replacing many components and checking others are all in tolerance the amp is generally unstable, with both howling in the audio band and HF oscillations at 200KHz.
This is usually accompanied by a Vk of well over 14v on the output bias resistor that leads to runaway and sometimes the A to HT snubber resistors overheating and burning out; followed by one or both transformers. Not then a very economic repair !

The unstable loop comprises most of the amp, from the output transformer to the anode of the first triode, excluding the volume control.
The treble control also seems to kick things off by developing a bad spot at one end of the track. This can be cured by adding a 100K resistor in series with the hot end of the treble control.
The main cure is the use of an inductive grid stopper of 0.22mH in series with the grid connection to the second triode. Resistors do not seem to work, this was one of those small resistor like chokes.

This appears to work well, with no noticeable change in tone range of the amp.

I also up rared the snubber resistors to 1W types and reduced their value from 3K9 to 2K7, in an attempt to prevent burn-up if it drifts into further instability in the future

Ed
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Old 25th Jun 2022, 12:08 am   #2
nicam49
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Default Re: Pye BB (PCL83) instability, possible cure

Thanks for that invaluable advice Ed!.. I'll add it to my BB file for future reference. Much obliged.
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Old 25th Jun 2022, 6:42 am   #3
Radio Wrangler
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Default Re: Pye BB (PCL83) instability, possible cure

Those chokes are fairly low Q, so it may be the combination of the lossiness with the low-ish resistance path through the copper. So it acts as an R-L snubber in one package. I think this is one where you'll need to stay close to the version of choke Ed's used.

David
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