Posted by Damir [ 213.202.98.84 ] on May 21, 2006 at 09:10:20:
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 <25k, like in other high-gm «candidates».
On Fig. 2 are pinout and connection – center of the loctal socket can be used like local (driver) «star ground» point, and from it one wire to the chasis ground. Negative pole of the last filter cap (C4 or C8 on the PS schematic) can be grounded on the loctal socket, too.
R6`=R6`` = 560 Ohms resistors, tube has two cathode connections, pins 5 & 7.
The sound test – C3g «driving» the rest of the amp and with my speakers gave a very clean sound, somehow «high-mids» accentuated a bit, dry, without too-much bass, and the bass is a little soft, the sound is detailed, very direct.
In comparison with E180F – C3g has a little cleaner sound, but in a «dry, in-your-face, sterile» way. E180F has more bass, maybe more natural sound balance, a little more punch, dinamic (maybe C3g has a bit more «compressed» sound) and more «heavy» notes…
But, C3g is so detailed, «fresh» and clean…impressive. In fact, the amp sounds very similar with both drivers, those differences I described are subtle. 
Those are the first impressions, and more experiments and listening tests are in order – I`ll post a «addition» in a week or so.
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