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In Reply to: RE: The 300b is one of the most linear amplifying devices ever invented posted by Caucasian Blackplate on June 29, 2008 at 12:44:33
Hi,
I'm using a tapped OPT that allows me to try 4K, 5K, and 6K (roughly) per 300b anode (400V a-k) in push-pull. So far, driving box speakers, I very much prefer the higher damping factor (DF~=8) I get running at the 5-6K load per anode end of the scale. I have yet to try it on horns or my in-progress dipole test rig.
I have tried standing current from 40-50mA per anode (up to 60 mA a couple of times) and I find that the only noticeable degradation was 40 mA with 4K/anode. It seemed top lose headroom, and increasing the current helped some but not the same as increasing the DF. I like 6K and 45mA best with my JBL 4312s. Even though the power output is theoretically less than at the higher current and load OPs, the amp sounds more dynamic. Again a caveat is I'm driving JBL 4312s. Different speakers might respond differently to the damping factor but to me it's a very important factor in load line selection.
I'm using direct coupling with a differential anode choke on the balanced input ECC99 drivers at about 15mA standing current. This setup will only drive the grid a couple of volts positive. I'm going to try a MOSFET follower assist here to see what happens going more into A2. I expect more headroom. Since the low-flat load line is not going anywhere near cutoff I have room for at least 10 volts more grid swing at each end, with more contribution to output, without going into AB.
Michael
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