Bottlehead Forum
General Category => Technical topics => Topic started by: vetmed on March 15, 2012, 09:05:37 AM
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The heater winding on the Eros PT is rated 1.3 amps. A full wave bridge is used to feed a cap which then feeds the regulator. So according to my understanding we are talking about a capacitively coupled fullwave bridge circuit that should have an output, in terms of current, of 0.62 X 1.3A = 0.8A. The heaters of all the tubes on the Eros draw about 1.3 amps. So my question is how is the regulator working to "restore" the necessary current? Just trying to understand how these things work. Thanks
Robert Lees
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The regulator is working pretty hard. Look at the heat sink Doc has on it. It regulates to 6.3V and delivers the current tubes draw. If that is 1.3A, I haven't added it up, then it delivers all 1.3A.
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I just want to confirm that the combined heaters draw 1.3+A. It's 2x200mA for the EF86 tubes, 1x600mA for the 12BH7A at 6.3V and the somewhere between 300mA and 365mA for the dual triode depending on the tube used there (6922, 7308, 6DJ8, etc.). I haven't checked into the transformer data yet. It is an interesting question, if the OP is correct about the transformer specs.
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BH sells the PT separately. Have a look in the parts section, I believe it is the PT-5.
Robert Lees
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Did you notice the "ADC" in the spec? "The heater secondary is rated at 9V 1.3ADC, having been designed for use with a regulator for a very quiet DC heater supply." ... from the product description. I suppose that the spec already takes the AC to DC conversion into account, in other word AAC would be correspondingly higher. The 9V vs 6.3V difference is there to have the required regulator headroom.
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I had not noticed that. Guess a good first step to really understanding might be to get the meter out to take some measurements. Thanks.
Robert Lees
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The design of the filament secondary is actually 9.64 volts open circuit, with an equivalent resistance (primary plus secondary losses) of 0.688 ohms. The maximum AC RMS current rating is 2.8 amps. A fullwave bridge rectifier into capacitor-input filter will usually draw an RMS current of 1.5 to 2.0 times the DC current supplied, depending on resistive losses and capacitance.
This is designed to maintain regulation even if the power line drops 10% below the nominal 120v, and the heat sink should handle the extra load even if the power line goes 10% high.