4P1L DHT Preamp Siberian http://www.bartola.co.uk/valves/2014/04 ... -finished/
Building a new version of the venerable 4P1L “Siberian” was very encouraging. This belated project finally came to life after some recent work on a new set of power supplies. So why 4P1L again? I always found the 4P1L sound to be unique. Great detail, overall tone and fantastic treble. What it makes it well suited for pre-amplifiers is not just its linearity (probably being the most linear valve out there) but the fact that it has a low anode resistance and current capability to ensure any challenging load can be handled effectively without any sound degradation. This can be heard particularly on the treble where the input capacitance of the amplifier is more evident and it is translated into treble loss. Other DHTs like 26, 01A, 30sp can only handle a few milliamperes of anode current and is not enough to charge and discharge the parasitic capacitance at high frequencies. More importantly, the 4P1L has filaments which aren’t demanding. This is a unique feature amongst DHTs that is rare and very useful. Having low-current filaments that can be either configured at 325mA or 650mA, low grid voltages and high transconductance in a valve is very useful. This mean that filament bias can be easily implemented without burning unnecessary power by swinging many volts to perform the desired level of amplification.
However, nothing comes for free. The 4P1L is a dancing lady. You have to tame her singing (i.e. microphonic noise). This may discourage you and has done so to many people before. However, it can be done. Many ways have been discussed out there, but what worked well for me is the use of silent blocks and silicone rubber. Also starving the filaments is a very effective way of reducing microphonic noise. I parallel the filaments to increase linearity and also to increase the filament current which helps keeping the filament resistor value low which doesn’t impact the output impedance much. Remember that any resistance in the cathode (e.g. the filament bias resistor) is reflected int the anode at (μ+1) times.