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In Reply to: Loftin White 2A3 voltage high posted by Gerado on January 30, 2007 at 00:57:52:
You need to add a B+ drop resistor to get it down,bring down the K-A (cathode to Anode) of 2A3 back to 250V (per RCA spec. of 250V @60mA).To calculate this is to take a reading on your B+ end with the total current consumption, and calculated the proper voltage drop needed in order to bring your B+ down using Ohm's law (V=IR). 100R may not be enough in your case (you need to raise it proportionately to get the appropriate voltage drop.
Another alternative ways is to get a different mains transformer that suits your need. I've been asking ISO-Tango to get custom-wound mains transformer for my LW2A3 amp projects for years and it's been very satisfying so far.
Follow Ups:
I have two suggestions that will avoid adding additional resistance to the supply.First, it sounds to me like you are not using a high enough wattage cathode resistor, which is causing it to fail. You should aim for a power rating of 3 to 4 times the actual power drop through the resistor.
Second, you can add resistance to the cathode of the 2A3, and rebalance the voltage divider to the driver so that it is at the correct bias voltage. It will change the driver operation point, but it may allow you to experiment with drivers.
Alternatevly, use modern high power dissipation monoplate 2A3s like the Sovtek, Shughang 2A3c , or JJ that can eat the voltage up. Just make sure adjust the value of the cathode resistor to suit.
Thank you David,
From a beginers point, I understand your first point and is easy to implement , though I thought that that part of the circuit should not be altered too much if one had the choice because of the effects it has on the 2A3 operating range.Your second point? I think I get what you are saying. Could you elaborate a little more?
The first point is one where the selection of the power rating of the resistor is a problem, not necessarily the value. You can safely use a larger power rating without changing the circuit. Of course, you seem to have a higher B+ voltage than the circuit is designed for, which throws the entire circuit balance off, i.e. the 2a3 biasing is off, creating a situation where the 2A3 is conducting more than the circuit is designed for, and the higher current stressing the power resistors in the cathode.To bring it back into balance, you need to simultaneously raise the offset voltage on the cathode of the 2A3, and the voltage at the plate of the driver tube so that the voltage differential between the cathode of the 2A3 and the output of the driver is equal to the bias voltage needed for, say, 60mA through the 2A3. You do this by adjusting the voltage divider for the driver stage, as well as adding additional resistance to the cathode resistor of the 2A3. You will have to look at tube characteristics of both tubes and use a bit of Ohm's law to determine how to do this, which can be a daunting task for a beginner and any further assistance would require detailed information about the circuit you are working with.
Thank you again david
I am quite sure I am ready to tackle it,
the learning curve has been exponential for me, but its like magic how simple the conceps are and how great the results are when you balance everything just right.I get the impression from the flavour of what others have said that reducing the B+ by adding a resistor (+ or - a capacitor) at the rectifier level is not favoured , in order to bring all the voltages down to original spec. Am I missing something with this train of thought?
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