I love the 46 DHT. To the extent that I went on the crazy idea to implement it in filament bias. It was my winter heating indeed. Stupid idea, but sounded brilliant.
I had several questions about the 46 lately. As I’m rebuilding my 300B amp, I’d love to play with the 46 again. Here is how I’d implement it today:
The gain of the stage is about 40. The 46 has a mu of 5 in triode mode. Instead of burning loads of heat in the filament bias arrangement, I use a degenerative cathode resistor (unbypassed). The input set up transformer is the brilliant LL7903 wired in 1:8. I have a PCB made for this which it’s very handy. A zobel arrangement (C1+R2) helps taming down the frequency response at HF. Running the valve at 30-33mA is ideal. 200V at the anode is ok and the HT may be adjusted depending on the swing you need here. The stage has all the bearings to drive whatever output stage. I will add a source follower prior to the 300B as I run the 300B in fixed bias mode.
Hope this helps
Ale
May I ask what is the trick here that you can use an unbypassed resistor with a DHT ? I beloeve normally we have tremendoius noise or buzz…no ?
Can you explain what is the trick here that you can use an unbypassed resistor at the cathode of a DHT ? Normally we have a buzz or noise when you do that, no ?
An unbypassed cathode resistor adds negative feedback which helps linearising the triode (even more). The negative side of it is that the reflected impedance in the anode is mu-times bigger which makes it not fit for purpose for a transformer or choke anode loads.
The circuit here isn’t a common cathode amplifier stage with a silicon load. It’s a hybrid mu-follower and the output impedance has a different relation to the reflected cathode impedance in the anode. If you’re interested you can read the ETF.18 lecture I posted on this blog which gives you more details and formulae.
Ale
Hello Ale,
Since you are not using filament bias with the 46 this time, have you experimented with connecting the positive output terminal of the Rod Coleman Filament Regulator to the Cathode Resistor per the recommendation in the latest integration guide (AN-DHT-RC-V8.pdf) on Rod’s website? Having tried this experiment recently, I found that there was a nice improvement having the positive OUT terminal of the filament regulator connected to the Cathode Resistor end of the filament when using Cathode bias.
Best regards,
Paul
Hi Paul
Thanks for pointing that out, completely missed it a I reused a filament bias circuit diagram for this and forgot to invert the cathode resistor terminal. Good point! I tried it as well and noticed a slight improvement on the overall sound detail. Haven’t experimented much as I mainly work with filament bias where possible (either resistor or SiC diode arrangement)
Cheers
Ale