Friday, 30 September 2016

Substituting the output stage load

So far I've built the gain, splitter and driver stages on my test rig. While I am well pleased with the results, this has been with a higher B+ voltage than intended. As a consequence, I've had to use higher resistors than planned to get my HT+ voltages right for the valves.

Part of the reason for my high voltage is that I don't as yet have the output stages - each of the output valves will be biased to a quiescent current of around 50mA... representing a 200mA load on the B+ which I would expect to drop the voltage by some amount. 

Question is.... will the extent of that voltage drop require me to change the values of my dropping resistors?

Without the output stage present, my thoughts turned to constructing a dummy load, something that I could sink around 200mA at 550V DC into.

A few calculations later and I concluded that three 60W lightbulbs wired in Series would do the trick. So construction of the world's ugliest dummy load was commenced... this is the result:

We have tamed the B+ to 565V which is right where I want it.


In place and power applied. Some nice voltages flying around on my bench today

Just to make sure my calculations weren't completely wrong, I wired the bulbs in series with a 10ohm resistor. Which is dropping 2.25V across it, meaning that the lights are drawing 225mA

So about 25mA more than I'd planned... I can live with that. Valve electronics is not always an exact science.

So what did this do to my voltage? It took around 15-20V out of the supply. Perfect.

I changed my supply resistor to the 12AX7 from 270K to 220K and I will change the 12AU7 supply resistor to 27K from 30K as soon as I have a 27K 5W resistor... yep... back to RS for more parts!

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