At your risk.
Always use a suitable fuse in series with any battery for fire hazard or other protection.
Protect your ears.
The cathode of the 1H4 and other cables to be grounded have been disconnected from Vf (1.5V) minus and are now connected to plus. So cathode and other cables to be grounded are now connected to Vb(12V) minus.
Since now the potential at plate is less than 12V instead of more than 13V, anode current has dropped from aproximately 50μA to 36μA.
There is still switching off noise when disconnecting Vf.
Sound quality seems to be better but test is inconclusive yet as only the Electro-Voice 635a mic has been tried. The sound is even less bass heavy. Still the signal to noise ratio is excellent.
Signal path:
Reverberant room - male singing or speaking voice - 635a - Pleiades V6 1H4 with Altec input, output transformers - Realistic Disco Mixer connection to its mic in - Sennheiser HD 580 - listener's (singer's) ear, brain
In practice it was found that increasing the fader on the Disco mixer although making voice louder did not make the click noise while switching off as loud, perhaps because this second stage is being clipped.
Indirect heater electron tubes do not suffer from this effect.
But since the direct cathode filament has a tremendous advantage is less battery power consumption (50mA compared to (200-300)mA...
And since no electrolytic capacitor is derived to be used across filament....
(would a small film capacitor reduce the switch off noise?..,)
it is decided to live with this noise when front end amp is switched off.
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