Flexible HT Power Supply (Part V)

I’m now back in business. Building a new 4P1L PSE output stage so will reuse the 300B/4P1L Flexible PSU. I never managed to post an update on the troubleshooting I had to do to get this HT PSU to its optimal state.

The output voltage was lower than expected and the 50Hz component extremely high. Something was wrong. So I traced the issue down. I found a bad solder in one of the rectifier’s cathode. The supply was operating in half-wave mode.

The supply is choke-input with 6AU7 rectifiers (hybrid bridge with FRED rectifiers). The transformer is custom made and has multi-taps for 300-400 and 500V. The tuning capacitor for the choke input is 470μF, then choke is 2.5H into 50uF oil cap. The filtering stage per channel is 20H + 100μF Oil caps;

Here’s a test of the supply at 330V/60mA per channel. It’s very rewarding now to see no 50Hz component and that the ripple at 100Hz is just 4mV (ignore the mA typo on the image):

SiC MOSFET Follower Driver

How many more times

Led Zeppelin wrote a fantastic song on their first album: how many more times. You may not be a rock fan, but hey: what a great song. How many more times do I want to get back to this “slew rate” theme? I don’t know, as much as I have to. Plenty of comments out there of bad designs with wimpy drivers attempting to take the 300B/2A3 or even 45 valves to full tilt with disappointing results. Either way, they always blame the valves.

I came back to revisit the driving of capacitive loads effectively as I’m working on a new 4P1L PSE amplifier. Slowly, but getting there. Previously I looked at adding a buffer to the 01a preamp as a result of slew rate limitations found in Tony’s implementation of this preamp.

The circuit design

Continue reading “SiC MOSFET Follower Driver”

2P29L – Preamp and driver for 4P1L PSE Amp

A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:

The triode curves are really nice:

This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:

Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.

We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:

Gyrator Test Mule: 4P1L Preamp

It’s always great to come back and revisit a great design. The 4P1L preamp performs flawlessly so I tweaked the gyrator board to see how it worked with the BF862 FET. The result is great, it sounds as good as it measures:

The 4P1L is biased to 150V/25mA which is the maximum current that the BF862 can do (IDSS max). You can see that the frequency response is flat up to 1.5MHz. The LF response of my test mule is affected by the AC coupling of the measuring gear. However it should be around 5-10Hz.

The distortion of low-level signals is really good:

Predominantly H2, it’s very nice to see THD<0.015% for a 4Vrms output. The load is 100KΩ which is the typical input impedance of an amplifier (with exception of solid state gear)

This low distortion manifests across the entire audio band (ignore the THD below 20Hz which is a byproduct of my testing gear):

The nice thing to see also, it’s how well the 4P1L can drive larger voltage swings:

We can see H4 popping up, however odd harmonics are lower (H5 in fact is higher than H3). THD at 10Vrms is still below 0.03%!

Rolling valves

I’m not a big fan of rolling valves. Perhaps it’s likely to do with the fact that I don’t have too much free time these days. However, I do look into burning in the valves. A noticeable change found on my 4P1L preamp after 350-400 hours use. The microphonic noise reduced to a minimum whilst the sound became more rounded. I added an electronic clock (LCD module) to the HT supply to monitor the exact number of hours between any changes I made to the system. It’s very handy. I’ve been running now the same pair of 4P1L valves for over 850 hours and they sound better than before. No further mechanical expansion noise is heard during warm up, something which was noticeable at the early days of use.

The second aspect I’m also in constant monitoring is the impact of the filament starvation in the 01a. Running them at 20% less current than expected is not recommended as the filaments will not operate at expected target temperature. However, after some years of joy, I’ve not seen any issues. I may remove and trace the pair of 01a in use currently to check their health.

Have fun!

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.

Continue reading “Russian PSE in steroids (4P1L into 4P1L)”

Flexible HT Power Supply (Part III)

After some further efforts, I managed to complete the power supply. And it works, which is always rewarding when you get it right first time:

Continue reading “Flexible HT Power Supply (Part III)”

Flexible HT Power Supply (Part II)

Here are some picture of my slow progress on this flexible power supply. I started with the layout of the parts before any marks were made on the top plates:

Working out the layout of the top plates

Once I was happy with the layout, I submitted myself to the mercy of the most dreadful job on earth which is metalwork!

Continue reading “Flexible HT Power Supply (Part II)”