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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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7th Mar 2019, 11:46 pm | #1 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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UL41 leakage
I suspect thus has been discussed to death, but I can't find any hard information about what positive grid voltage might be "low enough" in a set with a new film 630V "that" capacitor.
I've got a DAC10 on the bench- its original valve is running around +700mV on the grid and the only replacement I have, around +300mV. Does anyone have a "feel" for whether either of these valves would do for a few more years use in an OAP's bedside radio? I suspect that £20 odd for a new one would be a show stopper. I could reduce the grid resistor somewhat and/or increase the cathode resistor to tame anode current for now......
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8th Mar 2019, 3:38 am | #2 |
Heptode
Join Date: Jul 2012
Location: Derby DE1, Derbyshire, UK.
Posts: 626
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Re: UL41 leakage
Hi Chris, I'm sure it would be OK for a few more years, as long as "that" capacitor is up to scratch, and as long as the set isn't going to be run to death.
I can let you have a UL41 valve or two FOC that are reasonably new if it helps. But it might take me a few minutes to find them, as i've not unpacked them yet since i moved PM me if you would like to take up the offer, if it helps. Paul. |
8th Mar 2019, 4:01 pm | #3 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: UL41 leakage
My rule of thumb puts the limit at about 0.5V on the grid. You should certainly start worrying if you get up to a volt, because gassy valves generally only get worse - unless they are gassy due to not being used for 50 years.
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8th Mar 2019, 4:19 pm | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,788
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Re: UL41 leakage
It'll certainly be OK to be going on with, but there's no knowing how long it will last. It could soldier on for decades like that.
Bumping up the cathode resistor is a good idea. You hardly need 3W of output from a bedside radio. You can get grid voltage because the valve is leaky rather than gassy. Have you tried zapping the leakage? |
8th Mar 2019, 4:31 pm | #5 |
Dekatron
Join Date: Sep 2003
Location: Near Swindon, North Wiltshire, UK.
Posts: 3,595
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Re: UL41 leakage
Have you read these relevant threads:
https://vintage-radio.net/forum/showthread.php?t=45446 https://www.vintage-radio.net/forum/...highlight=ul41 |
8th Mar 2019, 10:50 pm | #6 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,550
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Re: UL41 leakage
Remember that you should read the grid voltage with respect to the cathode....the cathode resistor is actually providing grid bias. You should find around 7 to 10 volts negative on the grid so 700mV positive (with respect to chassis) is not going to make a lot of difference. What you need to do is monitor the voltage (with respect to chassis) as the valve warms up to see that it doesn't increase significantly. If the 700mV starts to increase towards a volt or more it indicates either a gassy valve or a leaky valve.
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10th Mar 2019, 8:09 pm | #7 |
Heptode
Join Date: Jul 2012
Location: Derby DE1, Derbyshire, UK.
Posts: 626
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Re: UL41 leakage
Hi Chris,
Thank you for the PM, the UL41 valves are parcelled up for you and are ready to be posted off tomorow. Paul. |
12th Mar 2019, 8:38 pm | #8 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: UL41 leakage
Just to close the loop, Paul's generous provision of a free UL41 has cleared the last problem with the DAC10. Now less than 100mV on UL41 grid.
Other parts tally was 3A fuse to replace one that came with the set which had been eaten by the RF suppression cap and 10 assorted leaky waxies. There were 11 altogether but the V1 cathode RF bypass was left since it will have to drop to less than a K to have any noticeable effect on the 220R it's decoupling. The owner seems dead chuffed with a total repair cost of less than a tenner and is due to pick it up on Thursday.
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....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |