Long time ago I breadboarded the 1624 valve and liked the sound. The 807 is famous and I never ended up with a permanent system with this valve. Yet, I like it. And if you have them at hand, use them. Even for guitar amps are amazing.
Tag: Single Ended Amplifier
300B SE Amplifier write-up
Finally I managed to get the time to write this up. Hope you enjoy it, you can find it here.
300B SE Amplifier Finished!
When everything was going to plan…
This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.
300B Amps: an early breadboard
Browsing my schematic archive I found this early implementation from some years ago on the 300B:
The amp design is straight forward. Let’s start from the output stage. The 300B is run hot at 33W (376V/90mA) with a fixed bias of about -78V. I used a pair of LL1623/90mA OPTs which I had wired on 3K:8 mode.
45 SE Amplifier
Introduction
More 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.
The 45 Amp design
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
Russian PSE in steroids (4P1L into 4P1L)
Introduction: DHT madness
I’m not going to dwell on DHT sound. I’d rather say that if you’re looking for a stellar DHT candidate, the 4P1L beats them all. It’s dirty cheap, reliable and sounds amazing. You can go any route you like, it’s your own decision of course. However if you’re looking for a 100% DHT amp to build, here is an interesting example for your consideration.
I’ve tried 4P1L in many topologies. The advantage of its low filament requirements is that you can implement it in filament bias and simplify the circuit significantly.
The gyrator driver using the PCB I designed recently, can be used to avoid iron and have an excellent first stage and make this Russian Amp in steroids: 4P1L driving 4P1L.
One minor caveat around 4P1L in excess. I have found (as well as many others) that if you use too many 4P1L stages (e.g. 4P1L line stage driving a 4P1L-4P1L amp) then it will sound a bit harsh in the treble. i suspect this may be explained due to the H3 component level when triode-strapped. I’d rather limit the number of 4P1L stages to two. You’ve been warned.
The other great thing about the 4P1L is that is quite consistent between samples and easy to match pairs. Also in PSE mode you can drive it to full tilt with only 20Vrms and achieve up to 5W in class A1 with a pair of valves.