If you use a choke the DC resistance is very low not for AC but the grid DC potential can't move much so it's ok.
A cathode follower provides a sort of stabilised voltage for the grid.As long as the leaking current is lower then the bias current of the follower the voltage doesn't change much.
Mona
Isn't it typical for any grid bias that the leak resistor refers to the negative bias voltage?
Best regards!
Sure, but with a fix bias voltage the potential is the same on both ends of the grid resistor, giving the grid freedom to move. With a follower/driver the DC-bias is presented to the grid without a serial resistor. It's kind of a stabilised DC grid supply with the AC modulated on the DC. Not much freedom to run away.
Mona
Mona
re: #37
The "15k" max is to ensure "small" grid-shift with a simple passive gridleak.
Bias is 50V so "small" could be 5V. 5V in 15K implies 0.3mA possible grid current.
The cathode follower is working at 250V/59k= 4mA. The cathode impedance is under 1k. Whichever way any 0.3mA grid current flows, it is swamped by the 4mA of the cathode follower. 0.3mA in a <1k impedance is 0.3V grid drift, which is indeed "small" compared to a 50V bias.
The decoupling at 12k+47k slightly affects this but probably no matter.
follow minimum reactivation to free thems from gasUnfortunately I have to report, that a newly installed Philips QE08/200 tube in mint condition (so the seller told me), blew up!
Radio tube reconditioning
Thank you all very much indeed for the constructive discussion!
I already managed to get these particular problems under control with your help.
But the challenge for technical improvement is still going on, my setup is just a workaround at the moment.
With appropriate bleeder resistors on G1 and G2 of the QE08/200 tubes, for the first time the circuit is running absolutely stable at last!
But the stability is dearly bought, the performance is suffering.
My 6SN7GT preamp/phase inverter stage isn´t able to drive such a low resistive load, causing loss half of the output power and increasing distortion.
But it is an important insight for me to see the particular characteristics of the QE08/200 at work.
Here another video, quite some days ago. Just without these the improvements, and the strange behaviour misleading me, the tubes taking more and more current.
The reason why the plates glow dark red in the video. But only for the very sensitive camera that bright, for your eyes it feels ok.
YouTube
I already managed to get these particular problems under control with your help.
But the challenge for technical improvement is still going on, my setup is just a workaround at the moment.
With appropriate bleeder resistors on G1 and G2 of the QE08/200 tubes, for the first time the circuit is running absolutely stable at last!
But the stability is dearly bought, the performance is suffering.
My 6SN7GT preamp/phase inverter stage isn´t able to drive such a low resistive load, causing loss half of the output power and increasing distortion.
But it is an important insight for me to see the particular characteristics of the QE08/200 at work.
Here another video, quite some days ago. Just without these the improvements, and the strange behaviour misleading me, the tubes taking more and more current.
The reason why the plates glow dark red in the video. But only for the very sensitive camera that bright, for your eyes it feels ok.
YouTube
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