Many times I get emails from DIY Audio builders who embark on building a DHT preamp when they don’t need gain, but instead what they need is a simple line stage to drive their amplifiers and interconnect cables effectively. Then they come back asking: “can I reduce the gain of the 01a or 4P1L preamps?”
For those who don’t need the gain, here is an interesting idea which brings together several design decisions which makes the DHT sound to its best. The challenge with many of the best sounding DHTs of low-mu is that is very hard to implement with filament bias. I’ve done a driver with a 46 in filament bias which was a crazy idea. I could turn of the heating with the amp running! It was a nice experiment though. With exception of the 71a and some other few DHTs, if you’re looking for good anode current and low ra, you’re in trouble. The 300B, 45/46, 50 and some other variations can’t be used in filament bias.
Subject to your religious beliefs in audio, you may not want to add a capacitor in the cathode, like me. I won’t dive into this discussion which is pointless as is a personal decision. If you continue reading this is simply because you value the sound difference in the DHT without a capacitor bypass in the cathode. Keep reading then…
A simple and yet effective way of creating a bias similar to the filament bias arrangement is shown below. When the current requirements are low, you can derive it from the HT and you end up with the simplest arrangement. Unfortunately when you need +20V bias levels, you will not be able to do this. If you reduce the bias current over RFIL1, you need a higher value of RFIL1. This translates into a higher resistance reflected at the anode and a degenerated stage which reduces further the gain. A compromise point can be achieved between filament bias currents (>1A) and a high bias resistor.
The above circuit is very simple. A classic DHT stage in hybrid mu-follower mode with a gyrator PCB. the 300B is run very cool at 25mA which is fine for a line stage. The 27V bias is generated by a third power supply which feeds a CCS set at 100mA which in turns generates the 27V bias requirement. The filament is floating and run at DC regulated with a Rod Coleman reg. It can also be a Tentlabs reg. The same 60V/200mA supply can be shared as the crosstalk between channels is highly minimised thanks that the CCS PSR.
Thanks to the low anode resistance of the 300B, the bandwidth of this stage is amazing. You can get a flat response up to 2MHz. This forces us to be cautious with the necessary stoppers etc as the stage could oscillate with stray inductance and capacitances at stake.
I’m sure many builders out there will entertain this idea and adventure on building one.
Hi Ale, this looks like an innovative plan, can’t wait to hear your notes comparing this to your more conventional filament bias preamps. I recall reading that Bottlehead do a 300b preamp kit where they claim it uses a ‘form of’ filament bias… I wonder if they’re doing the same?
Is “real” filament bias totally out of the question for a 300b? I mean it would burn off 100w of power per tube, but then a lot of class-A amps do that anyway. I think my First Watt SIT2 amps burn about this much in a bias circuit, which made me think of trying it for a 300b one day!
Hi Paul,
I implemented it with the 46 and believe me, waste of power. I don’t think is a good tradeoff in order to avoid another supply. I think with the external supply you will get the same results. Providing the supply is quiet and stable enough (via the CCS) you will achieve the same results.
The 300B is a great valve, however many people fail with its implementation by using a wimpy driver (6SN7) design and then claim that it doesn’t sound good.
Cheers
Ale