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Subject: Simple amplifier questions
I am interested in building the amp in the September 1999 issue Volume 1 Number 7. I have a transformer a secondary rated at 320v-0-320v @ 300 mA which after rectification with a 5R4GA rectifier tube I wind up with approximately 460v DC. I have 3 questions:
1) how to bring the plate voltage down to 360 vdc as described in the article without slowing down the capacitor recharge time?
2) isn't this plate voltage too high? The manual for the 6BQ5 recommends 250v operated in SE triode mode?
3) can you send me more exact information on how to wire the 6BQ5 in triode mode?
Bill
The 360 volt power supply was arbitrarily chosen to match what a 115-to-240 volts step-up transformer yield when rectified with an input wall voltage of 121vac. My guess is that your power supply will naturally lose 20 to 40 volts when loaded by a stereo amplifier, which will move the B+ closer to the 360 volts.
1) The power supply in an SE amplifier sees a fairly constant current draw, as the output transformer's inductance buffers the power supply from the power amplifiers current variations as it traces the output signal. Consequently, adding a dropping resistor and a filtering capacitor will not harm the amplifier. In fact, it will decrease the power supply noise at the output.
2) The 6BQ5/7189A/EL84 is a tough little tube. I have seen them work handsomely in Dynaco ST-35 and SCA-35 amplifiers (with 380 volts power
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supplies and 35 mA cathode currents) for decades. The Philips ECG tube manual gives a maximum plate voltage of 300 volts for the 6BQ5 and 440 volts for the 7189A, which in actual practice may prove to be a distinction without a difference. My guess is that poorly made American tubes were labeled "6BQ5" and well made European tubes were labeled "7189A."
The problem with tube maximum ratings is that they are not absolute in the same way solid-state maximum ratings are. If a transistor sees twice the maximum collector-to-emitter voltage, it burns out. If a tube sees twice its maximum plate voltage rating, no damage may result if there is no arcing and the over-voltage is short-lived. (In a transformer coupled amplifier the plate voltage will nearly double with every half cycle.) With solid-state devices, the maximum ratings are like having the highway patrol bazooka any car that exceeds the speed limit. With tubes, the maximum ratings are like driving your car at a hundred miles per hour; your car will not last as long as it would at 55 MPH, but it will not instantly breakdown.
3) Shown below is the bottom view of the tube.
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