12V AC for 5Y3GT filament? - diyAudio
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Old 23rd August 2012, 04:58 AM   #1
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Default 12V AC for 5Y3GT filament?

Hello all, I have an old General Electric amplifier I pulled from a console, and during an attempt to repair it, I found that the power transformer was bad. I found a replacement in a radio I had laying around, only to find that the replacement doesn't have a 5V winding for the 5Y3GT on the amplifier. I have a few 12V transformers lying around and was wondering if it would be safe to use one in place of a 5V transformer? (I cannot seem to find one online and/or I cannot afford one at this time).
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Old 23rd August 2012, 06:23 PM   #2
DF96 is offline DF96  England
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You would need to be sure that the 12V winding has sufficient insulation to handle the full rectifier output voltage. Modern double-bobbin transformers will probably be OK. Use a resistor to drop the excess 7V. Be aware that the 5Y3 is a directly heated rectifier, so the resistor will be at full rail voltage.
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Old 23rd August 2012, 07:10 PM   #3
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Knowing about the direct heating of the 5Y3 and the fact that the full B+ would be on the winding, that's why I was concerned with using the 6.3V on the transformer (that and most likely insufficient current output for the trannie). What size resistor would you recommend for the 12V transformer? (I'm fairly new to the mathematics of tube circuitry, still trying to understand the circuit design)
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Old 23rd August 2012, 07:24 PM   #4
DF96 is offline DF96  England
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5Y3GT filament is 5V at 2A, so for a 12V winding you would need 3.5ohms, but 3.3 is probably the nearest you could buy. It will dissipate 14W so maybe a 25W wirewound?

Much better to find a more suitable transformer.
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Old 23rd August 2012, 07:27 PM   #5
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I'll be keeping an eye out for a suitable transformer, never know when ebay has one. Until then I'll probably try the 12V and resistor scheme, whichever comes first. Thank you so much for the reply, greatly appreciated
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