@mjmajilton the 5840 is strapped as a triode in this circuit (2nd grid grounded) and for the the dual triodes I only wire one side of the tube. There are 3 reasons I chose the 694X tubes over the 5840. First is R-PK, only 2.3k for the 6948 at full heater voltage and around 3k for the 5840 at 4.5v, this make a dramatic difference with the BV11R (less so with the classic) due to its higher impedance. The 6947 has very high transconductance as well, the 6948 less so. The other reason for choosing these is lower grid capacitance which should result in lower miller effect at 100M input. The internals are also more robust in the 694X tubes than what I have seen in the 5840 (Sylvania and Raytheon).
The heater voltage thing is kind of a hack. Lowering the heater voltage can reduce RP which will also reduce R-PK but at the expense of a smaller operating envelope for the palte->grid relationship. I have hooked up a pot to R3 and fiddled around with lower resistance and different heater voltages using the internal trim pot in the PSU to taste. I found that around 2.7k R3 value and 4.5 - 5v resulted in the best rise curves on the oscilloscope for the 5840. The 694X I run at full voltage so this not an issue there.
As to the output volume. The 6948 is a full 6db louder at full freq spectrum than the 5840. The 6947 was almost this loud but not quite. Im pretty sure most JAN triodes selected for Guided Missile use will have similar specs so maybe try a couple variations.
I recently designed PCBs to replace the top 2 acrylic decks in the TAB kit. This allows me to isolate the grid and cathode circuits in between poly-pour layers. Hope is this will quiet the mic down considerably. I also added a trim pot for R3 so I can easily adjust bias if needed.
The heater voltage thing is kind of a hack. Lowering the heater voltage can reduce RP which will also reduce R-PK but at the expense of a smaller operating envelope for the palte->grid relationship. I have hooked up a pot to R3 and fiddled around with lower resistance and different heater voltages using the internal trim pot in the PSU to taste. I found that around 2.7k R3 value and 4.5 - 5v resulted in the best rise curves on the oscilloscope for the 5840. The 694X I run at full voltage so this not an issue there.
As to the output volume. The 6948 is a full 6db louder at full freq spectrum than the 5840. The 6947 was almost this loud but not quite. Im pretty sure most JAN triodes selected for Guided Missile use will have similar specs so maybe try a couple variations.
I recently designed PCBs to replace the top 2 acrylic decks in the TAB kit. This allows me to isolate the grid and cathode circuits in between poly-pour layers. Hope is this will quiet the mic down considerably. I also added a trim pot for R3 so I can easily adjust bias if needed.
