The similar schematic, and similar characteristics like E180F, E280F, D3a drivers from part 9. Supressor grid, g3, can be connected to the anode, cathode, or the ground. It seems to me that anode connection has «warmer» sound, the specs listed triode connection with g3=0V or connected at cathode, and I grounded it (cathode connected on the schematic, I didn`t hear the difference between cathode and ground connected g3).
The value, type and quality of cathode bypass capacitor C2 is critical, too. I tried 470µF electrolitic, and the sound was «rolled off», warm, and mid-bass accentuated. Then I tried 4 x 22µF MKT caps in parallel (88µF total), and the sound balance changed a little. Bass «bloat» dissapeared, and highs «opened». This, 88µF cap gives the –3dB frequency about 6,5Hz – good thing to «balance» possible (grid choke) resonance on those «few Hertz» frequencies.
All four «Siemens» tubes I have tested very closely, but interestingly – I measured µ=50…51, that`s larger then specs (µ=40).
The usuall grid stoppers precautions and input pot
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