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AWA C79 Chassis transformer replacement with a transformer with no 2.5V tapping.
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Return to top of page · Post #: 1 · Written at 4:25:18 PM on 10 September 2019.
	Tippy Location: Mount Cotton, QLD Member since 20 February 2018 Member #: 2214 Postcount: 134
Hi all, I am restoring a AWA C79 chassis from 1931. The power transformer is kaput, it has tappings for the centre tapped secondary a 5 Volt tapping for the rectifier and a 2.5 volt tapping for the filaments of the other 4 valves. It uses an 80 rectifier, 3 X UY224 valves and a 45 output valve. I have a transformer from another set with only the normal centre tapped Secondary plate winding and a 5 Volt winding for the filaments and rectifier but no 2.5 Volt for the other valve filaments. If I use this transformer what is the best way to get the 2.5 Volts for the filaments from the 5 Volts? Thanks for any help, David

Return to top of page · Post #: 2 · Written at 5:59:44 PM on 10 September 2019.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6828
If I use this transformer what is the best way to get the 2.5 Volts for the filaments from the 5 Volts? The best (most efficient) way is with a transformer that has a 2.5V winding. If you want to use a 5V winding then you're going to have use a resistor to drop 2.5 volts at whatever the total filament current is, with the wattage required being the current squared multiplied by 2.5V.

Return to top of page · Post #: 3 · Written at 6:28:54 PM on 10 September 2019.
	Tippy Location: Mount Cotton, QLD Member since 20 February 2018 Member #: 2214 Postcount: 134
Thanks GTC, I have just realised the transformer I thought was alright is unsuitable. So my question is now invalid. If anyone has a transformer for a 1931 AWA C79 or a suitable alternative that they wish to sell I would be interested. Thanks, David

Return to top of page · Post #: 4 · Written at 7:19:22 PM on 10 September 2019.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6828
You may well be lucky, but my guess is that it will be difficult to find a good one of those. Maybe consider getting what you have rewound: http://www.transformerrewind.com.au/

Return to top of page · Post #: 5 · Written at 8:20:29 PM on 10 September 2019.
	Tippy Location: Mount Cotton, QLD Member since 20 February 2018 Member #: 2214 Postcount: 134
I'll check them out tomorrow, Thanks GTC

Return to top of page · Post #: 6 · Written at 11:17:26 AM on 12 September 2019.
	Marcc Location: Wangaratta, VIC Member since 21 February 2009 Member #: 438 Postcount: 5492
Do note that you cannot use the 5V, winding of the rectifier for the filaments: That winding has to float as it has B+ on it. It's a bit more complex than you think #24 Tetrode is a heater valve and #45 is a filament valve like #80 and will need a separate winding of its own. I may have a circuit of it? Marc

Return to top of page · Post #: 7 · Written at 1:21:57 PM on 27 September 2019.
	Wa2ise Location: Oradell, US Member since 2 April 2010 Member #: 643 Postcount: 833
To use a 5V winding on 2.5V heater tubes, and IF the heaters are rated for the same current, and IF all are indirectly heated cathodes, you could series connect them. And I'd connect the midpoint (between the pair of tubes) to ground. More pairs of 2.5V tubes could also be connected like this. and any imbalance taken care of with a power resistor. As mentioned above, this assumes that the rectifier tube has a separate heater winding from the transformer.

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