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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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24th Feb 2015, 3:47 pm
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#1 |
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Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,326
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Thermistor VA1080
Has anyone any info or has one to hand to, measure the resistance of a VA1080 thermistor?
It's used in the audio output of a Mk1 Hacker Sovereign RP18. I'm sure a fixed resistor may suffice, but any info would be helpful. I've searched the web and this forum. The forum has a brief thread from April 2013, but it doesn't have any data. Maybe second time lucky...? Thanks, Ian |
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24th Feb 2015, 4:08 pm
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#2 |
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Octode
Join Date: Nov 2010
Location: Cullompton, Devon, UK.
Posts: 1,435
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Re: Thermistor VA1080
Do not know if this is any help.
John |
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24th Feb 2015, 6:10 pm
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#3 |
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Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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Re: Thermistor VA1080
I think that info is for varistors not thermistors..... the VA1000 bit is coincidental.
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24th Feb 2015, 6:30 pm
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#4 |
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Heptode
Join Date: Mar 2011
Location: Dorset, UK.
Posts: 522
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Re: Thermistor VA1080
Hi Ian,
Having a trawl through all the Mullard/Philips books I have has drawn a blank for VA1080. However in the Mullard Passive Components - Quick reference guide dated 1973 - 74 they list a VA1086 and brief data as follows... Resistance at 25°C = 2.2 Ω B Value = 2650 K Max Power Dissipation = 1W Resistance at Max Power = 0.29 Ω Maximum current = 1.8A Dissipation Factor = 10mW/°C Diameter = 9.4mm Thickness = 5mm Is VA1080 used as an emitter ‘resistor’ or part of the bias network? Rich
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To an optimist a glass is half full; a pessimist half empty; an engineer twice as big as need be! |
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24th Feb 2015, 7:04 pm
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#5 |
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Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,326
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Re: Thermistor VA1080
Thanks Rich. That should be quite helpful.
I'm helping a friend who has a Mk1 Hacker Sovereign RP18 with a missing VA1080. It's in the biasing network - R22. Apparently earlier versions of the set didn't have one, so I guess he could slightly re-jig the circuit to match. Relevant part of circuit below. Ian |
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24th Feb 2015, 8:20 pm
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#6 |
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Heptode
Join Date: Mar 2011
Location: Dorset, UK.
Posts: 522
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Re: Thermistor VA1080
Ian, thanks for circuit snippet. Other sets in the mid sixties, such as GEC G818 with the same OC81/AC127 transistor configuration use VA1040 in parallel with a fixed resistor of 36 to 39 ohms between each base. While others use a single AA119 or BA114 diode.
Might be worth hunting down a VA1040 and possibly no other changes will be needed: Resistance at 25°C = 130 Ω B Value = 4600 K Max Power Dissipation = 1W Resistance at Max Power = 2.6 Ω Maximum current = 0.6A Dissipation Factor = 10mW/°C Diameter = 9.4mm Thickness = 5mm Rich
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To an optimist a glass is half full; a pessimist half empty; an engineer twice as big as need be! |
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3rd Mar 2015, 6:59 pm
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#7 |
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Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,783
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Re: Thermistor VA1080
I looked into this recently, and took apart an RP18 amplifier to reverse-engineer the output stage.
I think the VA1080 must have been a "special" as no information appears in any Mulllard data-books which I can locate. My sample measured about 130 ohms at 21C, and and 85 ohms at c. 35C. I've found an NTC thermistor* stocked by CPC which is close tolerance, and yields c. 123 ohms @ 20C and 67 ohms at 35C. If we add a series 12 ohm resistor the result is close enough to substitute for the original part. John * EPCOS B57164K0101J |
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