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Old 8th Nov 2018, 1:08 pm   #21
audion_1908
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Default Re: Unknown 1950's valve radio chassis?

I plan to test the radio with the existing valves minus the ECC83 that has to go, but its good to have some spares for when they do go, why do ECC85 not last long? I did not know about valve remarkers and it's interesting.
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Old 8th Nov 2018, 1:11 pm   #22
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Default Re: Unknown 1950's valve radio chassis?

Quote:
Originally Posted by audion_1908 View Post
I plan to test the radio with the existing valves minus the ECC83 that has to go, but its good to have some spares, why do ECC85 not last long?
They suffer from cathode poisoning they are often run for many years often with no HT applied because the set was being used in AM.

It's a common problem with the AM/FM sets of the period.

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Old 8th Nov 2018, 1:15 pm   #23
audion_1908
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Default Re: Unknown 1950's valve radio chassis?

I have read that's why speshal computer grade valves where made, would it be a senseble mod to the radio to tern the filement of the ECC85 off when not used or put some HT on it, maybe 0.1ma 10x less than it's max?
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Old 8th Nov 2018, 3:00 pm   #24
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Default Re: Unknown 1950's valve radio chassis?

You could turn the heater off but it might be better to arrange a low voltage to the anodes whilst on AM. Something like a 1M resistor across that particular section of the HT switch. The valve will pass a few milliamps of current but not enough to operate. I'm sure they do something like that on a Baird radio that I have.
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Old 8th Nov 2018, 3:14 pm   #25
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Default Re: Unknown 1950's valve radio chassis?

That sounds like a good idea and very easy to do, or reverse, much easier than fitting a relay with a suppression capacitor across the relays contacts and it's control switch to suppress the EMI

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Old 8th Nov 2018, 10:50 pm   #26
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Default Re: Unknown 1950's valve radio chassis?

I did remember correctly! Just checked the circuit of the Baird 301 that I have and they wire a 1 meg resistor across the HT contact of the wavechange switch so that a small current flows in the ECC85 when switched to AM. It would also have a secondary function of damping any spark as the switch contact opens.

There would NEVER have been an ECC83 in that position. It's primarily an audio valve plus the pins are wired differently and as you say, the ECC83 (and the 81 and 82) have a centre-tapped heater.
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Old 9th Nov 2018, 1:43 am   #27
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Default Re: Unknown 1950's valve radio chassis?

Heathkit used 220K for that purpose in the AFM1 tuner and it's a pity for owners, though not valve retailers, that most manufacturers saved the fraction of a penny by omitting it!

That chassis looks famiiiar and is in the Radio & Television Servicing books under HMV.
It is too late to dig them out tonight, but, while my brain is active just before going to sleep, I remember there were details of a modification to the mains transformer chassis connections (to reduce modulation hum? that may already be done, but probably not as I think it used thick braid ) and one version's circuit showed an EL85 output valve, but (some or all?) production versions used an EL84.
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