Comparing ECC83 with ECC808
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ECC808 info from tubemuseum.org
"The ECC808 is a low hum, low microphony and low crosstalk dual triode for high sensitivity AF amplifiers, especially for two channel stereo circuits. The static values of the ECC808 are practically identical to those of the ECC83. Unlike the ECC83, however, the ECC808 is only intended for parallel heating (6.3 V / 0.34 A).
In combination with a more practical socket circuit by improved shielding and better cathode isolation, a particularly high freedom from hum could be achieved. (Ripple voltage at G1 approx. 4 μV with pin 4 at ground.) The microphony resistance could also be ...