Lundahl LL2745 based OT DHT pre-amplifier

Before breadboarding a pre-amp with these nice OT provided by Thomas Mayer, I decided to simulate some options in LT Spice to see what results I got.

CX301a DHT pre-amp

First one is my preferred CX301a thoriated-tungsten DHT. I wired the LL2745 in 5.6:1 step-down configuration.  This should provide a low output impedance which is what we want in this configuration where we need to drive the cables to the amplifier with sufficient capability:

Looks very promising. Of course gain will be much lower than a gyrator-based pre-amp, the 01a anode load is optimised providing very low distortion: 0.015% based on my SPICE model with curves taken from real CX301a. Gain is low at 3.1dB, but we don’t want loads of gain in this pre-amp. Sound is what we are after…

26 DHT pre-amp

Now it’s time for the revered 26. I used Dmitry’s model based on the RCA manual curves. I’d like to simulate this again using a model based on starved filament curves with a real 26.

Originally planned to bias the 26 in a different operating point based on feedback from Andy Evans, however after playing a bit with the OP I found that a more linear point was around Ia=5mA and Va=114V @ Vgk=-6.85V

26 looks more interesting in principle as the output impedance with this model is lower than the CX301a thanks to having a lower Ra (7KΩ against 11KΩ) so LF response will be slightly better in a side by side comparison.

Will be trying these two with filament bias and Rod Coleman’s filament regulators.  The HT will be provided through a Salas HV shunt regulator.

Stay tuned…