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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: 5176

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: 5176

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: 3616

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


 
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