Paul J.,
Another question about another configuration -- I'm assuming that the nickel tfa-2004 jr and the bcp-15 gapped for 50 mA would not work very well with the paramount built as a 300b, but with a 2.5v filament and a high-power 2a3? However, would it be possible to adjust the bias of the 2a3 down somewhat to get the plate current in the <50mA range, at the expense of a few watts?
Basically I'm wondering if the iron I have on order would be suitable for the higher power 2a3 config in the new paramounts.
Thanks,
Jim
Changing the plate current without changing the plate to cathode voltage would require a different transformer impedance for good performance.
You can do it, at the price of increased tube distortion. You would have to increase the cathode resistor from 1000 ohms to 1500 ohms, approximately, and the 300B distortion at full power would probably increase from 3% to 8% or so (reading between the lines of the WE table of operating points).
The stock cathode resistor is made of two 2K/25watt resistors, in parallel for 300B operation (1K) and in series for 2A3 direct coupled operation (4K). Replacing them with 3K resistors in parallel would get 1.5K. A few people have made this switch, using them in series for 6K and running a type 45 tube. This is still a substantial deviation for the original design. :^)
In my opinion, the better alternative would be to keep the stock plate choke for 300B operation, but use the TFA-2004Jr. Then swap in the BCP-15 for 2A3 operation. The optimum parafeed capacitor would be a bit smaller with the stock choke (5 to 8uF?) but close enough to use the same cap.
