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Re: C3g as driver for 300b

Posted by Thorsten on June 19, 2006 at 02:42:25:
Hi,
The C3g will work okay as driver for a 300B, it needs to be used wired as triode, this will give, when CCS or Choke loaded a gain of around 40. With 1.6V RMS input this translates into around 90V peak out. If you use the 300B at the standard 350/80mA Operation this leaves a little headroom, not greatly much but enough to get by.
If you use a resistive load you would need to use a vary high value so as not to losse too much swing.
If you look at the triode curves you note that you need to run the C3g with quite high anode voltage and low current (and high anode load) to get good swing and linearity.
With a anodic supply voltage of 225V, a 430 Ohm Cathode resistor (bypassed with 47uF MKT or MKP) and an anode choke of around 100H (or some form of CCS/Mufollower load) you get a gain of around 39 and an output impedance of around 3KOhm with a (peak) swing of 88V from a 1.6V RMS input and around 150V maximum peak swing.
If you where to use a 22K resistive anode load your gain drops to around 34 and the swing from 1.6V RMS to 77V. This might be marginal, as I have noted several 300B's that bias at more than 77V when used in the standard "WE" circuit with 430V +B and 880 Ohm Cathode resistor.
If you want to try anyway, use a 1K/22uF +B decoupling circuit, 22k Anode resistor, 430 Ohm Cathode resistor and 47uF cathode bypass capacitor. BTW, do not forget the gridstopper!!!
For the output stage with 300B you'd have a 1K 12W cathode resistor in parallel with 6K8 2W, a 47uF (160V) cathode bypass capacitor and ideally 12uF (630V) from the 300B Cathode to the +B. The Output Transformer should be 2K3....5K, +B 430V. Coupling Capacitor 0.22uF and 300B Gridleak resistor 240k and 100R Gridstopper. The +B Circuit can be used from any sort of "WE91 Copy" Amplifier, so around 430V under load of 90mA (per channel).
Anyway, that should make a working first cut Amplifier to see how you like the sound in general.
After that you can experiment to your hearts content with Constant Current Source loads (they replace the 22k Resistor), Choke Load (the 22uF +B decoupling capacitor gets shifted to the anode end of the 22k Resistor and the choke is inserted between that and the anode), Interstage Transformer couplung et al.
Finally, you can use the C3g with resistor anode load and your 1.6V RMS input if you can find a good 10K:40K (1:2) line input transformer (something from Stevens & Billington or Lundahl) or autoformer (eg Magnequest EXO-173?) you get an extra 6db gain and this will then make sure that 1.6V RMS can well overdrive the input stage, which goes into overload long after the output stage has given up.
I hope that helps.
Ciao T