pre65 wrote:With the 720v toroids I could get 950v in cap input mode, but I am going to go choke input, so HT WILL BE 600v plus or minus a few volts.
At the moment the driver valve has a CCS in the anode, but that would not do if it were wired as a pentode.
Having just 600V will make the initial arrangement with the zeners easier.
As for the driver tube, you could rewire the socket for ECC81. This way you could use both halves as driver and CF, applying in full the RH813 schematics. I cannot advice a better solution than that.
Once you try it , you can always consider increasing B+ and applying the other necessary solutions. But first try my suggestion: it should be an eye opener.
I've been searching. Most articles seem to be about driver pentodes, but I found this.
"Effect of G2 on Pentode Operation.
On most designs you see g2 (screen grid) either connected to HT or with a resistor and a decoupling capacitor. As you increase the series resistor on g2, the voltage on g2 will vary inversely with Anode voltage due to inverse relationship of Ig2 and Ia below the knee. This is thus negative feedback and if the g2 is decoupled with a capacitor only affects DC. Without a capacitor the Audio or RF gain is reduced and signal is more linear also any hum or noise on supply rail has less effect on the load.
Up to about 1.5x the Anode load, the gain is indeed reduced and as a result the frequency response is better. Transient response is better. As Rg2HT is increased the gain of edges stays roughly the same as at the 1.5x load. But lower frequency gain continues to decrease. This means with transients there is severe "overshoot" About 50% when R on G2 to HT is about x3 or x4 the Anode load. So.. Add a decoupling capacitor? Well that gives terrible transient performance at larger signals! Maybe all pentodes behave like this? I dunno. Literature suggests up to x5 resistor for G2 load than Anode, but that doesn't work at all on these Russian tubes."
So, as the anode load is 5K, would a 15k - 20k load resistor be what I need ?
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