Saturday, 24 September 2016

Fixing a serious error

I never said I was perfect at this stuff....

Reviewing the circuit plan and also others' circuit plans, I determined the design for my fixed bias circuit for the KT88 output valves had rather a dangerous flaw. The nature of these valves is that they require a constant negative voltage on the grid or else they'll go into full conduction, followed very shortly by the anodes getting red hot and then shortly thereafter by pyrotechnics.

The negative current is supplied by the bias supply, which in my design runs at around -80V and is adjusted by potentiometers until you're getting 0.5V across the cathode resistors (in other words, 50mA quiescent current, regulated by the bias voltage).

The potentiometers used to make this adjustment are a vital part of the circuit and if one of them should happen to suffer a failure – such as a wiper lifting – then in the current design, the bias voltage completely disappears. Followed closely thereafter by the output valve and potentially other parts as well.

Fortunately this is quite easy to correct. I've re-drawn the circuit to a fail-SAFE mode, rather than fail UNsafe as it was previously. If a pot wiper should lift now, it will send the bias voltage more negative, rather than cutting it off.

This is the relevant section of the circuit now:

The drawback with this design is that it requires a little more current. Now, Each side of the bias circuit will be passing between 2.6 and 4 mA - so total potential around 8mA ... the bias winding on my transformer is rated at 200mA so I think we're safe.

The values of the resistors may need a little tweaking by experimentation to get the exact bias currents needed through the valves. I'll find that out when it comes time to build the output stages.

But for now, I think this is a worthwhile improvement to the design.

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