300b lovers

There is a simple and inexpensive means of dealing with heat due to amplification. Ducts in the ceiling above the amplifier(s) can be added, along with dryer style hose the connects via a squirrel fan to the outside. This can be installed for a few hundred dollars (mostly labor), is quieter and far less power draw than air conditioning. 

Do you use load a resistor after the interstage transformer? What kind resistors do you use?

@alexberger If any audio transformer is not loaded it can 'ring' (which is to say it will generate harmonic distortion, which can be quite profound). The correct loading will cause a state of 'critical damping' in the transformer, where when a square wave is put thru the device, it will have little or no 'overshoot'. Since the grid of a tube tends to be very high impedance (other than its capacitance) some form of resistive loading is a good idea to explore!

Since you sound like you are up to something with your own design, I recommend an empirical approach, which might be to drive the transformer with a square wave and have it drive in turn a power tube which is operating normally. A potentiometer and oscilloscope's probe across the output of the transformer would then allow you to vary the potentiometer and observe the result on the square wave. In this manner you can exactly dial in the correct loading value.

I agree with Atmasphere. Some things cannot be accurately modeled. Put it on a bench, attach a dummy load, light it up, and measure square waves. Tune for nice-looking leading (and trailing) edges. Measure both low-level (below 1 watt) and also just below clipping.

Any indication on target preamp pricing?

I just have the first production one running  in now in prep for Seattle show.  It will be at least $5000 as it is cost and labour intensive and all very high end parts.  

Although a glance at a schematic might lead you to think it is simpler than a classical (Golden Age) PP KT88, the parts are more expensive. Way more expensive, just as 300B’s (as a group) are more expensive than KT88’s (as a group).

Another factor is sensitivity to parts quality. An amp with 20 dB of feedback (which is nearly all Golden Age amps) tends to wash out differences in the sonics of different parts. This is exactly what feedback is meant to do ... 20 dB of feedback is a 10:1 reduction of all sources of coloration. A zero-feedback amp, by contrast, reveals the sonics of every single part, particularly at critical nodes in the circuit. This raises costs compared to the PP KT88 equivalent.

Depending how you feel about the sonics of solid-state and feedback, you can travel a continuum between modern Class D, with sophisticated and complex feedback, to Class AB transistor or MOSFET with 20 to 40 dB of feedback, to Class AB push-pull pentode with 20 dB, to Class A with zero feedback. Each type sounds different and has different distortion spectra.

Comparisons between modern Class D and all-triode Class A are not absurd, despite radically different technologies. Class D and Class A both skip over the many difficulties with Class AB device switching, whether bipolar transistor, MOSFET, or pentode (each device type has different artifacts associated with the AB transition). The difference is Class D switches at 100 kHz or higher, with pulse width translating to signal level, while Class A is non-switching and like a preamp, fully analog from start to finish.