12AX7 cathode resistors R5 and R6 1k vs. 1.3k

for the DIY ST35, the Dynakit and every other PP EL84

12AX7 cathode resistors R5 and R6 1k vs. 1.3k

Postby tomlang » Tue Aug 18, 2015 9:20 am

The rev. E specifies 1k cathode resistor for R5 and R6 but earlier versions and the original ST35 specify 1k. Why?
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Re: 12AX7 cathode resistors R5 and R6 1k vs. 1.3k

Postby paart » Thu Aug 20, 2015 9:28 pm

I can’t answer why the change was made, but the original Dynaco units used a 1.35K precision resistor in most of the production units. In the very late amps, it was changed to 1.65K. Why? Who knows? I could only guess.
The actual cathode resistance will always be slightly lower than the actual resistor value, since it’s in parallel with the (R10) feedback resistor. The feedback resistor is connected to DC ground through the secondary of the output transformer.
The exact value of this cathode resistor isn’t critical.
Increasing the value of the cathode resistor will increase the “headroom” of the input stage, and decreasing it might cause the tube to operate in a more linear region, more of the time.
But then, a change in it’s value will also have a slight effect on both the positive and negative feedback loops…………..
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