All about electronic valves and hi-fi
If you have a small room and high-efficiency speakers, then keep reading. If not, you can still enjoy reading about probably the best sounding output valve in my view. The 45. I wrote about it few times and have to say, it’s still one of the best. Better than a 300B but unfortunately can do only 2W. You can use it in Push-Pull and is superb. Have a look at this.
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?
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…
Recently I revisited a beloved amp, the SE 45. This time I will share a more orthodox design without sand in play. Surprised? Well, I love lots of iron as well and here is a design I’ve been playing around for some time as I have all the components at hand.
IntroductionMore than 5 years ago, I built a fantastic single-ended amp with the unique 45. The 45 has a distinguished tone and personality despite its mere 2W of output power. If you have high efficiency speakers, then it’s a great amplifier to build. With 2W you can enjoy music in a mid-sized room. You don’t need more, this amplifier performs at its best at low output levels and in particular when playing jazz or classical music.
There are plenty of design circuits out there. I settled for a simple triode driver using a gyrator load. The choice was down to the 6J5 and 7193 (a military version of the 2C22). Both triodes are medium mu and sound really nice. Depending your needs, you may opt for a different driver (even pentode). However, they need to be able to drive the large voltage swing required by the 45. I’d go for a 6J52P, 6e5P, 6e6P, D3a or C3g these days. It all depends on your needs and available valves. The driver is biased at 7mA to provide enough grid current to avoid slew rate issues. An improved version would be to add a MOSFET follower to provide better performance under grid current. An example of a follower implementation can be found here.
The 45 is biased hot at 34mA/300V. The anode can handle 10W and this operating point provided best sound in my view. The OT is crucial, so invest as much money as you can afford. Rod Coleman regulators are needed to implement this amp without hum and the unwanted inter-modulation effects.
I carried out several tests on the driver to find the sweet spot for minimum distortion and full swing. The driver is a hybrid mu-follower composed by the gyrator and the 6J5/7193 triode. The valve is biased by a set of 5 red LEDs to about 8V. I think I had a combination of a white LED and LED to provide 8V in my implementation. The dynamic resistance is minimum and won’t impact the performance of the stage.
I used the Sylvania metal-base 6J5 but then settled for the 7193 valves. They sounded better and I was quite pleased with the overall performance of the amplifier.
The amplifier design is very simple. With the gyrator PCB you can simplify the driver build and also use different valves to experiment with them. I originally didn’t have a PCB so I built my gyrator in a prototype board.
I’d highly recommend you building this amplifier. If you want to experience the single-ended sound, then this is one of the amps to build. Of course you can go for higher power levels with a 2A3 or 300B, however, the sound of the 45 is unique. Worth trying
We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.
When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO) to fudge the real power of a solid state amplifier.
Last week I looked at optimising the 45 loadline in A2. Clearly we shouldn’t be attempting to get more than 2W from this valve without a significant level of distortion. However, having about of 3W would be attractive for the transient response of this amp.
So how will this circuit perform in a simulation? Let’s see what the spice results are:
The THD is significantly better due to the harmonic cancellation between the two stages. The driver distortion is 0.3% at full tilt (150vpp) and this could be improved. I guess the 6e5p could do better, but interesting to see how the cancellation of harmonics may play around. The new operating point and the stacked supplies will demand different MOSFET parts of 1kV for sure…
What about driving it at full tilt in A2?:
The driving signal has to be about 160Vpp to provide maximum swing, so power is:
H2 distortion:
Which yields a whooping 6%!
Ok, the previous operating point wasn’t optimal from a musicality perspective. As suggested by 45, I reworked the load line for 36mA/275V and anode load of 5.6KΩ (which is what I could get with my OT):
The driver should now provide 
.
The output power will be around:
Efficiency would be around 36% in theory. Happy if around 3W can be obtained from this valve here with a reasonable distortion…
On the previous post I explored how more power could be extracted from the 45 in class A2. Here is a first draft circuit using the 4P1L as the driver stage with a mu-follower gyrator load to provide current with low output impedance:
3.5W can be obtained with 13Vpp input signal. Clearly a pre-amp is required, but that is the output level I’m currently getting from my preamp so should be fine. The LND150 CCS reference can be optimised and derived from the stable 280V.
Not a bad idea, just an additional power supply to avoid any nasty cap in the path.