Chasing that sound
The 45 DHT is probably one of the best sounding valves out there. In fact, I have struggled to get a similar level of detail and timbre in a 300B or 4P1L output stage. Even my 814 SE Amplifier (which was class A2 and had thoriated-tungsten filaments) couldn’t replicate that sound. I posted time ago my incarnation of the 45 single-ended amplifier here. The main challenge with this valve is that it can only put out there nearly 2W, not more. With its 10W anode dissipation, you will struggle to get more juice from it in class A at a low distortion level.
However, if we look at a push-pull amplifier with the 45, we can hopefully retain the timbre characteristic of the valve, despite it won’t be a single-ended one. Well, I love good PP amps, so why not?
If you read carefully the datasheet, you will find that you can extract a lot of power from this valve in A2 operation. Precisely, AB2 in PP mode. Up to 19W from a pair! Wow, that’s impressive. I’d be happy with 10W on my Alpairs. Having enough headroom is good from a dynamic response point of view. You won’t probably need more than 3-5W on average.
Tracing the 45
I had at hand a pair of Sylvania 45 which I submitted to the mercy of the eTracer. The one shown below measure well above 80%, so it’s a good reference. Nevertherless, I’m aiming on a fixed-bias design, so providing the pairs are matched and measure decently well, then it should be fine.
Here are the lovely curves of the 45:
Creating the Spice model was easy. You can download it from here: 45 DHT Sylvania model.
The output stage
The suggested operating points from the datasheet are pretty obvious. At the end it will come up to what OPT you have at hand. Somewhere around 3.2KΩ – 4KΩ Zaa is preferred. Below is a snapshot of how much we could get out of the valves with a 4KΩ transformer:
Of course it doesn’t match the datasheet recommended point, it’s fairly close. With 240V/28mA on the anodes you will need a bias of about -46V. The grid needs to be driven pretty hard up to close 200Vpp to achieve full power. The grid will also sink about 20mA of grid current during A2 operation. Please ignore the THD estimate from the tool as it’s wrong.
Simulating this circuit in LTSpice with an LL1682PP (which is 8.8KΩ Zaa), can deliver 10W at 1.1% THD, mainly H3.
Below is the circuit I’ve been toying with for some time:
The circuit is very simple. The output stage has a DC filament regulators (e.g. Rod Coleman) and a mix of fixed bias with filament bias for better stability. As we need to provide the A2 grid current, a source follower is mandatory here. The grid bias is generated from a Swenson Regulator PCB (or a Rod Coleman fixed bias board) and adjusted independently per board to ensure there is balanced anode currents.
The grid voltage can peak to 60V so at least 80V are needed on the top MOSFET of the SF PCB. Likewise, the grid can go as negative as -140V so -160V is needed as the negative supply.
Building the above in separate chassis provide some flexibility in terms of the drivers used. I was going to use a differential pair of pentode valves in a hybrid mu-follower configuration. That is with a gyrator PCB and a tail CCS. very simple and yet effective. It can deliver sterling gain and linearity for 200Vpp operation. Similarly, triode-strapped pentodes like D3a, 6J52P or similar would work very well here.
Lots to work on, just wanted to share initial ideas on this design. Probably will inspire others.