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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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8th Apr 2018, 3:49 pm
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#1 |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Murphy A372 heater chain.
The Murphy A372 and its very similar predecessor the A362 uses an autotransformer to feed a 0.1A heater chain. The transformer also has a tap to fed 6.3 volts at 0.45amps to an EABC80 and a further tap to slightly under-run a 6.3V 0.3A dial lamp.
The total voltage of the 0.1A valve heaters is 147volts but instead of feeding this chain directly from a 147 volt tap the transformer has a 166 volt tap and the heater chain is fed via two 390 ohm resistors in parallel. Details of this arrangement are in the attached PDF file. The two resistors concerned are R33 and R34 and are 2 watt carbon types and not wire wound types as one might have expected, although the power dissipation in each of these resistors is a fraction under 1 watt and so well within their power rating. A quick look though a number of circuits of autotransformer fed heater chains from Murphy and other manufacturers seems to suggest this is an unusual if not unique practice. In an attempt to explain the reason for this design decision I have come up with four possible answers. 1] To save costs Murphy used a transformer designed for a model with a 166 volt heater chain rather than manufacture a second transform design. (A search of other Murphy circuits does not support this idea). 2] Again to save costs Murphy purchased an "off the shelf" transformer from an outside source. 3] The resistors provide a surge limiting function designed to optimise long term reliability. 4] This chassis was originally designed to include an EM85 tuning indicator which has a 19 volt 0.1 amp heater. At a late stage the EM85 was dropped from the design to cut costs. Of my suggestions I favour number 4. What do you think? Perhaps you can suggest another explanation? |
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8th Apr 2018, 4:24 pm
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#2 |
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Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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Re: Murphy A372 heater chain.
Is there an export model of this chassis with an EM85?
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8th Apr 2018, 4:39 pm
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#3 |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy A372 heater chain.
If there is Sam, I wasn't able to find it.
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8th Apr 2018, 4:41 pm
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#4 |
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Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Murphy A372 heater chain.
UM85?
Lawrence. |
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8th Apr 2018, 5:22 pm
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#6 | |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy A372 heater chain.
Quote:
Data sheet below Lawrence. |
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8th Apr 2018, 5:28 pm
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#7 | |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy A372 heater chain.
Quote:
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8th Apr 2018, 5:41 pm
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#8 | |
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Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Murphy A372 heater chain.
Quote:
Lawrence. |
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8th Apr 2018, 6:01 pm
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#9 |
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Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,215
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Re: Murphy A372 heater chain.
Surely an E-anything has a 6.3V heater?
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8th Apr 2018, 6:09 pm
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#10 |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy A372 heater chain.
Sorry, Tony and Lawrence I hadn't spotted that I had put EM85 in my OP. That was of course in error.
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8th Apr 2018, 6:36 pm
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#11 |
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Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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Re: Murphy A372 heater chain.
Thank heavens, I thought my knowledge of E80 series was lacking, Yes UM85 would be the one to use, nice one Lawrence.
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8th Apr 2018, 6:48 pm
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#12 |
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Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Murphy A372 heater chain.
I'm thinking option 3 in Post#1
Lawrence. |
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8th Apr 2018, 11:49 pm
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#13 |
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Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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Re: Murphy A372 heater chain.
It could have been just a bit of future-proofing, allowing an additional small-signal valve (with up to a 1.9W heater) if ever needed in a later design using the same transformer.
Why did they use an EABC80 anyway in an otherwise 100mA heater set, as opposed to the more obvious UABC80?
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
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9th Apr 2018, 12:06 pm
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#14 |
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Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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Re: Murphy A372 heater chain.
Its a weird valve line up all round, 10P14 output, U404 half wave rectifier, 10F18 & 10C1. A very mixed bunch of valves and heater chains. The mains autotransformer seems very complicated for a simple radio. The UCC85 is mundane by comparison.
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9th Apr 2018, 9:26 pm
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#15 | |
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Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy A372 heater chain.
Quote:
Against this argument though: 1] The UABC80 has a 28 volt heater and not the 19 volts we are looking for. 2] Using a UABC80 makes the 6.3 volt tapping is redundant (note that the dial lamp does not use the 6.3 volt tap) - if you are adding a 6.3 volt tap in order to change from UABC80 to EABC80 why not change the main tap to fit the new 0.1A chain voltage? You see what I have done here; I have had an argument with myself and completely negated my own suggestion!! 
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Re: Murphy A372 heater chain.
