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Old 17th June 2010, 07:29 PM   #1
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Default filament voltage question

i measure ac voltage from transformer secondary and it is 7.2 volts
i rectify ac voltage with some diodes and voltage drops to 6.2 volts
i hook up leads to a pins 4 and 5 of a 6922 tube and the voltage drop across it is 4.5 volts.

my question is shouldnt the voltage drop across the load equal the supply voltage i.e. 6.2 volts?

thanks
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Old 17th June 2010, 08:49 PM   #2
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Quote:
Originally Posted by chopchip View Post
i measure ac voltage from transformer secondary and it is 7.2 volts
i rectify ac voltage with some diodes and voltage drops to 6.2 volts
i hook up leads to a pins 4 and 5 of a 6922 tube and the voltage drop across it is 4.5 volts.

my question is shouldnt the voltage drop across the load equal the supply voltage i.e. 6.2 volts?

thanks
AC sine wave you measure on 'AC' setting on a multimeter.
DC you measure with 'DC' setting on multimeter

Anything not DC and not pure AC is pot luck - including rectified AC without smoothing! Unless you have a fancy RMS meter of course

Also you get voltage drop across the diodes and voltage drop due to load variations..
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Old 17th June 2010, 09:52 PM   #3
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7.2 volts AC measured on an unloaded transformer WILL drop as load goes on. 15% is not unusual ie your 7.2VAC becomes 6.1VAC under full design load.

What is the point of your question?
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Old 17th June 2010, 10:14 PM   #4
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Originally Posted by aardvarkash10 View Post
7.2 volts AC measured on an unloaded transformer WILL drop as load goes on. 15% is not unusual ie your 7.2VAC becomes 6.1VAC under full design load.

What is the point of your question?
. im just concerned that the tubes are getting too much heater voltage. there are several secondary windings on the transformer so as the load goes on to one winding will that affect the output of other windings too?
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Old 18th June 2010, 12:33 AM   #5
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yes - the transformer has a maximum TOTAL capability (typically expressed in VA) so the rated voltages are assuming that you are running all the windings at rating. If you under-utilise any winding, that makes slightly more capacity available to the others, so they output slightly more. The effects can be large or small depending on the transformer, its rating and the amount of load you have on it. Typically the voltages don't shift by much more than 15%, sometimes less. You really need to load the windings (even with a dummy load) to check the real working voltages.
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