As mentioned in the first part of this series, this amplifier concept can be scaled down for a low cost version. Although this will impact the sound quality the scaled down version will still perform very nicely. There are three areas which we can look at to reduce cost: capacitors, interstage transformer and power supply. The basic concept of such an amp has already been shown in the first part.
Instead of an interstage transformer, the more common RC coupling can be used. The interstage transformer will be replaced by a plate load resistor and a coupling capacitor. A 50kOhm resistor will work nicely. This resistor will dissipate some power so it should have at least a 10W rating. Two 100k resistors can be used in parallel. If you want to use a single resistor, a 47kOhm will work just as well. The grid of the output tube will need a resistor to ground. The datasheet of the 6CB5A gives 500k max for this resistor. A coupling cap of 220nF or higher will provide a very low roll of frequency with this 500k. Obviously the quality of the coupling cap will have a large influence on the sound. A good MKP type is what should be used. Expensive 'boutique' caps should be avoided however. This is a cost down amp, so an expensive cap would not fit into the concept, we could rather stick with the interstage transformer. The voltage rating of the coupling cap should be chosen such that it can withstand the maximum voltage which can appear at start up or during fault conditions. It should be 450V or higher.
Here is the detailed schematic of the 'low cost' amp with all parts values and approximate voltages:
The power supply also offers some possibilities to reduce cost. A capacitor input filter will simplify the PSU since it requires less secondary voltage from the transformer. But we want to keep at least one choke in the supply. Going to a solid state diode bridge for the rectification will reduce complexity of the power transformer further, since it only needs one heater winding and the HV voltage does not need to be center tapped: