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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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23rd Aug 2017, 1:25 pm | #21 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,863
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Re: PX25 fault?
I'd probably go for 807/6L6/KT66 triode strapped. Their running voltage/current characteristics will be a reasonable fit to the load line imposed for the PX25s. You can still find NOS 807 and 6L6 at friendly prices, and if you want KT66 there are new ones out of China.
An add-on subchassis with a filament transformer (or auto transformer as Ed says) and a couple of valve holders will do the job without ruling out future restoration if you ever find another PX25. David
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23rd Aug 2017, 11:24 pm | #22 |
Tetrode
Join Date: Mar 2007
Location: Bath, Somerset, UK.
Posts: 69
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Re: PX25 fault?
Thanks for all your esteemed comments. I have now obtained a replacement valve which involved a long drive. The postal service in this country is designed for EF50s not PX25s.
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23rd Aug 2017, 11:43 pm | #23 |
Tetrode
Join Date: Mar 2007
Location: Bath, Somerset, UK.
Posts: 69
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Re: PX25 fault?
A couple of pictures of the new valve in place.
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24th Aug 2017, 9:34 am | #24 |
Dekatron
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,601
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Re: PX25 fault?
I would put that in a glass case if I were you.
Peter |
24th Aug 2017, 11:40 am | #25 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: PX25 fault?
Well, at least that's one valve that's been saved from being sent across the water into 'audiophoolery stockpiling' never to be seen (or indeed used) again! Well done for putting one of these valves to use in a genuine vintage repair.
When the heaters go like the one you had it often causes hum due to the imbalance with them being directly heated. Often it's a high resistance connection within the base pins, but in your case with such bright filament runs in the centre with nothing on the outer ones it seems like a genuine internal failure. |
24th Aug 2017, 12:05 pm | #26 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: PX25 fault?
The half dead PX25 may still be of interest/value to someone out there.......
For an application where lower output doesn't matter, perhaps, or a single ended use.
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24th Aug 2017, 1:30 pm | #27 |
Tetrode
Join Date: Mar 2007
Location: Bath, Somerset, UK.
Posts: 69
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Re: PX25 fault?
Indeed the valve in question was in fine fettle before losing half its burner. I have seen an example with much lower emission from an intact filament so it's certainly mightily useful. I don't suppose anyone ever designed equipment using a single PX25 unlike PX4; did they?
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24th Aug 2017, 1:49 pm | #28 |
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: PX25 fault?
You've got to wonder about non-invasive repair like laser welding?
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
24th Aug 2017, 9:11 pm | #29 | |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: PX25 fault?
Quote:
Lowther mostly used triode outputs (PX4 and PX25) until 1956, when it swung over to "Lowther Linear", its own development of the Hafler & Keroes ultralinear circuit. (It did later use conventional UL with output valves that did not have accessible suppressor grids.) Cheers, |
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