Posted by Damir [ 213.202.86.38 ] on December 20, 2005 at 12:21:50:
In Reply to: what tube are you using? posted by MQracing on December 20, 2005 at 09:13:48:
High-gm pentodes (say, D3a, E280F, E810F, EF184, 12BY7...) can be usable for single stage 300B driver with relative small anode load, say 8,2k-15kOhms. A=Ra*gm, and with Ra=10k and say, gm=5mA/V (possible with unbypassed Rk), we have A=50. But, internal anode resistance rp of the pentode is high, much larger then Ra - then our Rout = Ra // rp, and Rout ~ Ra - or 10 kOhms in this example.
We can observe cascode like "quasy-pentode" without g2 current and less noise, but the same "problem" is present - large internal resistance. I tried 5687 tube here, see the "report" down low... 
For long time I experimented with various drivers, and best subjective performance/sound would be finally soldered in a nice box... Didn`t decide yet...
P.S. Interesting comparison of various pentodes (and some triodes) can be found here:
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