Posted by Damir [ 213.202.97.151 ] on July 11, 2006 at 11:08:51:
In Reply to: Re: SE 300B Project, Part 13 - The finished amp posted by horny on July 11, 2006 at 10:23:20:
Hey, I tried various resistors "across" the grid choke, from 100k-1Meg. It can help a little to damp RCL resonance, and/or to "kill" RF oscillations problems... Sometimes I thought that I hear not much of the difference, but in the end my conclusion was that it somehow "kills" that 3D "sound" and sounds somehow "flatter" and 2D...huh, too much "audiophile" language... 
But, I noticed that the grid choke can be a giant pick-up on the input grid, oscillations and unstable work/sound are possible... You must mount it very close to the 300B socket, at least 1-2cm from the chasis, it`s orientation in respect with other magnetic components (OPT) is important... and check for oscillations. Lower values of parallel resistor "defeat" any benefit of the grid choke, IMO ... your 10k in parallel with high impedance of the choke is 10k...but also in the parallel with unecessary choke winding capacitance. And not just that - this grid resistor of 10k is actually in parallel with triode`s load (anode resistor, or CCS when you use anode out), and this resultant driver load is too low.
In other words, your driver "sees" just the 10k as a load, parallel with Cw... not too good, IMO.
Large input capacitance of EC8010 means that you must be carefull not to use large series resistance (pot) before the tube - HF "roll-off".
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