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Old 3rd Feb 2020, 7:56 pm   #1
Michael Maurice
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Default Changing R and C in a Murphy radio.

I am repairing a Murphy radiogram, an A242R.

There is a can capacitor which has two sections in one can, one is 16uF the other at 4uF which decouples the screen grid of a pentode.

because of the way the chassis is laid out its not exactly easy to fit two separate capacitors but I could put one 16uF + 16uF 500V.

What concerns me is the time it takes for C82 to charge up. with a 4uF it would take approx 3.3 seconds to charge whereas with 16uF it would take around 13 seconds.

Could this slow charging damage the valve?

What if I reduced the value of R43 to say 390K or 470K, that may increase the screen grid voltage but would it have any serious impact on the circuit.

What I dont want is for the valve to have warmed up whilst its screen grids voltage is rising. With a 470K the time taken to charge would be 7.5 seconds and with 390K 6.2 seconds.

Your input (excuse the pun) would be most helpful
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Old 3rd Feb 2020, 8:33 pm   #2
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Default Re: Changing R and C in a Murphy radio.

If the screen voltage is low it will reduce the cathode current until the voltage is correct. Can’t see that damaging the valve.
In fact many protection circuits in class C amps remove the screen voltage if drive is lost to protect the valve.
See what other members have to say though.
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Old 4th Feb 2020, 10:25 am   #3
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Default Re: Changing R and C in a Murphy radio.

Would re stuffing the can with a 4uF and 16uF be an option?
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Old 4th Feb 2020, 11:26 am   #4
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Default Re: Changing R and C in a Murphy radio.

Surely it would not make any difference as the valve would still be cold. So for the first 10-20 seconds the valve would still be cold and not conducting.
My experience with valves means that if on a Pentode the screen grid is low or missing the valve simply does not conduct and will sit there running cool. You can get problems if the anode loses its supply the valve then draws more current through its screen grid which can damage the valve.
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Old 4th Feb 2020, 11:44 am   #5
Michael Maurice
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Default Re: Changing R and C in a Murphy radio.

Thanks for your replies, I'm not good with restuffing cans, though I could try.

I think I'll go for the 16+16 and see what happens. In any case the output valves usually take 20-30 seconds to warm up.
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Old 4th Feb 2020, 12:19 pm   #6
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Default Re: Changing R and C in a Murphy radio.

The time-contant with 16uf may work out to c 14 seconds, but that screen grid is shown with only 20V or so. So, the time to charge to that voltage will be much shorter, more like 2 seconds.

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Old 6th Feb 2020, 7:03 pm   #7
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Default Re: Changing R and C in a Murphy radio.

Maybe not if the 20V or so was measured with the usual AVO. But anyway as others have said, no real problem with higher C but there might be with higher screen volts!
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