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In Reply to: C3g as driver for 300b posted by horny on June 18, 2006 at 14:37:49:
Hello,I used it as a driver for a 45. Works great.
berlin
Follow Ups:
hello
do you have a schematic for me?
how do you use the c3g in triode or pentode ?best Robert
Friend of mine very much recomends it as a high transconductance pentode driver. His view is that for pentode drive you get best results from valves of the sort you are talking about.I haven't studied the valve, but for pentode operation you should ideally use a vr tube (paralleled with a series rc network to fix noise) as the stable screen voltage mechanism, plus a small cap screen to cathode (not ground).
For single ended (as opposed to differential) pentode operation you must use resistive load of very low value. Lowest you can get away with sufficient for enough voltage gain. Output impedance is effectively the value of this resistor, gain is increast the greater it's value, linearity particularly at high current low voltage end of the swing is improved the lower the value. You balance the situation for yourself.
Never use an inductive load or a ccs.
Paul,
You know I was using the 12GN7 with a 9.5K load as a driver for the VV32 amp ? So you don't need to use a really low anode load ; and you need something on the anode to 'use up' some of the high B+ voltage with something like the C3g . Ideally a C3g would need about a 350V supply for a driver in pentode mode - then you could use about 8K load and get a reasonable anode value ( say 230 v ) with 15mA and so keep the anode dissipation sensible . This would be a great driver with over 100 gain . The only problem with the C3g in 300B application is the limited dissipation of less than 4 watts . The C3m is a little better in this respect . I'm surprised T has said nothing about using it as a pentode yet .
Mark
Hi,> I'm surprised T has said nothing about using it as a pentode yet .
Look at the curves and try to get reasonable swing (200V PP) into 10K from it (or any sensible load, actually).... Virtually impossible. Compare the C3m.
High Mu Pentodes just cannot swing enough voltage on the anode for driver stages. They are excellent choices however to produce a lot of gain into low impedance loads, making them ideal for low level high gain stages (Phono- or Microphonepreamp comes to mind).
Ciao T
Sorry Mark your splitting hairs.It is well known you get the best from pentodes by looking at the curves on their side, and using low load resistor to keep the loadline away from the knee. That's what I was refering to, I knew this long before you built your first amp. Don't get too big for your boots.
.. to ground or to anode?I like pentode operation driver, based on EF80 from "Revox" or E83F from "Telefunken" tube gear.
Nice to read you here Paul. Be frequently here on asylum.
Thanks.
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