Posted by Damir [ 213.202.104.71 ] on July 10, 2006 at 08:08:46:
In Reply to: Re: SE 300B Project, Part 13 - The finished amp posted by horny on July 09, 2006 at 14:26:23:
Hi, thanks, and welcome to the forum!
We need about 50Vrms (70Vp) to "drive" 300B to the full power. With CCS-ed driver triode (and high impedance 300B grid choke as a load), we have the "driver" amplification practically equal to the µ, or theoretical amplification factor (very close).
In the case of E180F in triode - A = µ ~ 50, or input sensitivity for the full power is 1 Vrms. Although C3g specs said µ~40, my "Siemens" tubes have µ~50, again - input sens. is 1Vrms. (If we`d have a tube with "right" µ=40 spec, then sens. would be 50/40= 1,25Vrms).
D3a has larger amplification, A=µ~75, and then input sensitivity is about 0,7Vrms.
Unfortunately, I have no EC8010, and didn`t try them. But, (based on its data) I expect it to be the similar like other high-gm "candidates" above, and others I tried (6C45Pi, E280F trioded).
If you have some EC8010 on hands, the best you can do is to try it for yourself. Use CCS, Ia~10mA, Rk=220-250 Ohms bypassed with 220µF (MKP of 100µF at least reccomended) - like your starting point. Beware that this tube has g1 connected on five pins... I`d use grid-stop resistor (say 100...220 Ohms, CC) on every input pin. Based on the specs, you can expect A=µ=60 times, or input sensitivity of 50/60 = 0,83Vrms. Adjust Ia (or Rk if necessary) to have Ua about 200V. Seriously, only you`d can say about "sound" in your system, and like/dislike factors :-).
You can expect rel. large input capacitance, unfortunately.
Do you have some parts (tubes, transformers, etc.), or you are still in "various schematics" phase? 
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