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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's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

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's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5263

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's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

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's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5263

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's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

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's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3711

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's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 725

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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