~ Beard Audio P35 active bias potential problem ~


This page is to answer a question I have received from several people about why I removed the active EL84 bias control from a Beard Audio P35 as shown on the schematic here ~ This appeared to upset someone who claimed they were the new owner of Beard and that I should pay £100 to buy their schematic and must not publish any modifications

If you open the pdfs in the link above you can see that I removed 6x voltage regulators which were configured as 42mA current sources [a bit high for push pull EL84s] and replaced them with conventional cathode bias simply by fitting 270Ω resistors to give a more suitable ≈36mA per valve ~ The fault with the P35 was transformer primary failure

Simple cathode bias as used in many valve amplifiers uses a low value cathode resistor to control the effective negative grid voltage such that an increase in cathode current generates more –ve grid volts and this negative feedback keeps the cathode current relatively constant but not precise enough for some people

With a constant current source [using voltage regulators or other active devices] each valve maintains its cathode current irrespective of its condition and grid voltage [which should be held at 0V] and where the grids in the P35 are paralleled this can cause problems on all 3 valves when one develops a fault and was likely the casue of the transformer failure

A better way if you must have solid state electronics in your valve amplifier would be to sense each cathode current with a low resistance and use the signal to control each grid but then each grid would require separate coupling to the driver which can cause instability which the P35 already had ~ hence the stopper resistors R16 . . .

If an amplifier is not using parallel valves for either push pull or single ended output then active cathode bias using circuits like LM7805 voltage regulator or LM317 etc. should be okay ~ but diodes and LEDs or batteries mentioned in some books are only for lower cathode currents and provide fixed bias not regulated cathode current

I would like to have made a page about Beard Audio amplifier modifications but I have only worked on 2 of them and I don't need the threats or stupid comments from DIYAudio moderators that followed my past statements about the 'design' ~ If you need a current to voltage convertor for any reason a good resistor is the perfect choice


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