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 zxx123 19th January 2008 09:30 PM

Power Transformer for 6V6 SE Amp Question

I bought used one from ebay for my 6v6 SE Amp Project.
The seller said it would be perfect one.

When I got and measured secondary taps without rectifier, the tester showed 330V. You know it is too high for 6V6 SE Amp. I claimed to the seller, but he said "the voltage will be ok when you connect it to tubes."

Well, I cannot understand his reply. For instance its heater taps is measured as exactly 6.3V. According to his logic, the heater voltage will also go down, when I attached it to 6V6 SE circuit.

Thanks.

 BudP 19th January 2008 10:01 PM

We would need to know what type of rectifier is used for the B+, SS or Tube and what rectifier circuit is used, full wave bridge, full wave center tap, or some hybrid. Do you have a copy of PSUD2 you can model the power circuit with? By entering the measured no load values and what your power supply is built like you will bet a very accurate number for DC on the B+.

Typical cheap power transformer will show from 10 to 15 % loss of voltage under load for B+, but usually not more than 5 to 7% for the heater voltages, so you will probably be close to needs for both, just as a general note.

Bud

 ThSpeakerDude88 23rd January 2008 05:16 PM

330v is probably no load voltage, if your windings resistance is high ( IE the transformer cannot supply very high current) than it will most certainly drop when a load is attached.

For instance: I have a SMALL power transformer from an old organ. Its about the size of a 1-2 amp 12 transformer. Its outputs measure at 350-0-350vac with 123v input, but with a load that drops to around 270vac on each leg. My no load output voltage from the 5Y3 rectifier is around 380vdc, but when the amp is switched on that drops to a more comfortable 340vdc, which the 6V6 CAN and WILL handle just fine. Just watch your bias current.

Try measuring your secondary resistances to the CT. 150-250 ohms means you have a relativley low current transformer, 50-100 means it can supply quite a bit of current. The less winding resistance the more current it can pass, the higher the winding resistance the less current it can pass and the more a load will affect how much voltage is put out.

 zxx123 25th January 2008 02:06 PM

...

Quote:
 Originally posted by ThSpeakerDude88 330v is probably no load voltage, if your windings resistance is high ( IE the transformer cannot supply very high current) than it will most certainly drop when a load is attached. For instance: I have a SMALL power transformer from an old organ. Its about the size of a 1-2 amp 12 transformer. Its outputs measure at 350-0-350vac with 123v input, but with a load that drops to around 270vac on each leg. My no load output voltage from the 5Y3 rectifier is around 380vdc, but when the amp is switched on that drops to a more comfortable 340vdc, which the 6V6 CAN and WILL handle just fine. Just watch your bias current. Try measuring your secondary resistances to the CT. 150-250 ohms means you have a relativley low current transformer, 50-100 means it can supply quite a bit of current. The less winding resistance the more current it can pass, the higher the winding resistance the less current it can pass and the more a load will affect how much voltage is put out.