CX301a DHT preamp
Here is my latest incarnation of the DHT pre-amplifier:
Many claim that the 26 is the best sounding DHT valve for a pre-amp. I will agree to a certain extent, however I personally found the thoriated-tungsten filament sound a bit more rewarding to my ears. A more clear and defined treble in my opinion.
Since I plugged in my CX301a incarnation of my breadboarded preamp, I just left it there as I loved its sound. Certainly there are things to be improved to enhance the dampening of microphony, albeit I have to confess it hasn’t been a problem to me. Have heard some valves to howl, and this is not one of those. Clearly suspending the valve socket or adding the rubber dampers to the valve holding plate or socket will help massively.
Filament bias is a must in my DHT designs. Since discovered it, can’t avoid not removing most capacitors that I can from the signal path. In this case the filament resistor R9 will increase anode resistance by R9 times (μ+1). This will also impact the stage gain, but here all this is not a problem. You may find this is way too much gain in your system. Rod Coleman’s filament DC regulators are crucial to provide a hum-free stage. Attempting AC or other DC regulator is likely to bring frustration to your design. Believe me, I’ve been there before…
Now turning our attention to the anode load I will not open a debate here (or a can of worms!). You can make your choice of using a superior quality output transformer (and by superior means a lot of money!) or you can look at various options. A choke is a great idea, but special care needs to be taken to ensure choke is not picking up any hum from the remaining parts of the circuit – specially the supply transformers, etc. I have experimented for some time various types of CCS or gyrators as sandy loads for the valves with excellent results. If you are one of those that feels that sand is a sacrilege, then I suggest you stop reading this post now.
Gyrators are superb. They can simulate the AC response of an inductor of 300H (but without storing energy as a real inductor) or above very easily at 1/100 of its cost. You can easily adjust the valve operating point ensuring this is maintained despite the ageing impact of the valve or the eventual replacement of it. The anode voltage will be fixed by the gyrator, the current not. Cascoded MOSFET gyrators provide better supply ripple rejection and isolation. Using Q3 as a CCS instead of a high resistance potentiometer to set the anode voltage is better as it helps providing a better frequency response as impedance on this node is increased. A higher value of R10 will help reducing the size of the gyrator capacitor and the smaller the better it will sound in my experience.
M1 and M2 can be your depletion FET of choice. M1 should be a 250V rated one at least. Depending where you live, you will be inclined for using BSP129, LND150 or DN2540.
Previously I mentioned in some other posts that the mu-follower setup of the gyrator here provides a better output impedance and improves the performance of this valve significantly given its high anode resistance compared to other more suitable DHTs for this purpose such as 4P1L, 46 or 71a.
I’m not going to cover the HT supply here, but using a shunt regulator such as Salas, is one of the best choices here.
With Russian PIO capacitors you will get a fantastic result here, no need to start burning serious money on the capacitors until you are happy with the end to end build and you can then start looking at how to improve the sound of it by replacing some bits with better (or preferred) quality components
With an operating point of Ia=3mA you can get THD=0.08% at Vo=10Vpp. This will be subject of the quality of your CX301a. Some older globe 01a’s have a great sound, but they are not that linear. Hard to pick and chose your precious ladies here without testing them for linearity.