All about electronic valves and hi-fi
Recently I’ve spent some time designing some PCBs for my own use and why not to share them as well to the DIY audio community. Currently I have a flexible CCS board for anode loads and a tail CCS, which is the one I will write about on this post.
I came up with a 3cm x 3cm PCB board which provides the maximum flexibility for a tail CCS circuit. Below is the diagram:
I’ve been using my choke-input DHT filament supplies for many years. I’ve got many of them as you’d expect. Something I do really hate is to build power supplies. It’s dull and boring, but they’re a necessary evil. I’m afraid I have to admit.
Recently I experienced the dreaded smoke of capacitors blowing up. It’s actually funny in hindsight, however at the moment of the fireworks you don’t laugh. I killed one Coleman regulator as well with my experiments on a flexible DHT supply. That made me revise the design and the stress put on the components, in particular when you’re using filament bias in anger, as I do.
Anyhow, over the last 3 months I’ve been playing with SMPS supplies to try to get the commercial available ones quiet enough to be used with a DHT preamp. I started with classic filter stages like CLCLC, morphed to gyrator filtering (which wasted a lot of heat) and then resisted using LDO regulators which I knew it would do the trick. However, getting rid of the HF noise is a daunting task.
Actually, I have a variable HT SMPS supply built which sounds really good and is extremely quiet. It can deliver 2 channels of 600V/100mA. For a filament supply, the SMPS challenge is of a major league game. You can get the noise to about 1mVrms but the harmonics spread well over the mid and high range. Big chokes have also a big leakage capacitance which makes the choke not that effective at filtering off this HF noise. 1mVrms in 600V is fine, however 1mV in a 16V supply which si feeding the filament bias resistor is a problem.
After giving up, my patience these days isn’t the best I have to admit. In particular when time available for DIY audio is very limited. So I said to myself: “sod the SMPS, I will get a nice pair of custom made transformers with multiple taps and job done”. And that is what I precisely did. JMS transformers in the UK provide an amazing service. I’ve mentioned them in the past several times. I get all my power transformers from them these days. I ordered a pair of split bobbin, with outer copper screens and multiple taps to cater for all the voltage ranges needed for my output stages. From a 4P1L to a 300B.
As I always use choke input supplies for filaments, I used the LL2733s I have in stock and carelessly wired it on 400mH (series bobbins) which provides a huge voltage peak output when the minimum choke input current isn’t in place. This happens at start up, the voltage will raise to the level of cap input supply when the filament is just starting up thanks to the gentle raise of the Coleman regulator. The result is the high voltage peak output which can damage the capacitors.
The solution was to wind the input choke with the two bobbins in parallel to get inductance down to 100mH. Also the resistance is reduced significantly which avoid wasting too much voltage drop on the choke. Also adding a tuning cap (C3) to make the supply to operate in a hybrid mode between choke and cap input is great to dial in the output voltage.
Here is my supply which I use for the VT-25 and the 4P1L preamps:
Since my early days of valves and DIY audio, I developed an obsession around testing and tracing valves. This led me to design and build my analogue curve tracer which I used for many years successfully until I build my uTracer, which was a great innovation in curve tracing. I do have many valve testers (some which I made myself) so why building another one?
Well, Chris Chang from Essues Technologies developed a fantastic new digital curve tracer for valves, the eTracer. There are a few things which will grab anyone’s attention on this curve tracer. Firstly, the power supplies can accommodate a large range of valves which the uTracer can’t. HT can go as high as 750V @ 300mA and the grid supply down to -170V! This is exactly what you need to test your transmitting valves or even a 300B. Secondly, the tracing speed is surprisingly fast. This is a nice feature, specially when you want to trace pentodes at various screen voltages to develop a Spice model for example.
I’ve been prototyping a flexible CCS PCB. The intent is to provide a cascoded FET pair with some interesting features:
The board is very flexible and can be used for multiple purposes:
I’ve been running some tests with excellent results.
If there is interest, I will run a batch of PCB to offer to the DIY community.
Cheers
Ale
Here’s an excellent build of the 01a Preamp with the gyrator PCBs and the Source Follower PCBs by Paul. I will leave to him to write down his impressions of this preamp and the build experience.
If you like your VR valve glowing in the dark but you’re concerned about how quiet it will be, then you may be interested in this post.
I like the VR, I used them a lot. They are quieter than most people claim. However, they’re not the quietest if you’re looking for the lowest noise level.
Below is a very simple circuit which you can implement easily and use your lovely VR valves glowing in the dark! The VR (U1) is fed from the HT source via R1 .R1 should be sized to at least provide 10mA to the VR valve. C2 is the maximum allowed cap. R2 and C3 form the cap multiplier section. R2 isolates U1 from C3 to ensures it doesn’t oscillate. C3 to 10uF can provide about 50dB reduction at 100Hz, so great to smash out any remaining noise from the VR. Q1 provides current limitation in conjunction with R4. This will protect M1. The output is about 5V lower than the VR level. Well, that’s what you pay with a follower or cap multiplier.
The PSRR of the cap multiplier below is about 110dB @100Hz. Great performance:
The source follower PCB can be twicked easily to use for this purpose. It can also be used as an electronic choke (aka gyrator) and or a simple cap multiplier.
Q1 can be a BC547 thanks to the protection diodes D1 and D2 (15V) which will prevent from exceeding VCEO levels.
I’d use this circuit in many configurations, not just the screen supply.
Finally I managed to build a stable version of the 2P29L preamp. The wooden modular version inspired on DHTRob designs paid off. Here it is:
The result is fantastic. The 2P29L is a superb DHT, quiet and it sounds amazing. The circuit is similar to the original one but with a slightly different operating point due to the valves themselves:
I’m still running them at 20mA. This time the anode voltage has to be increased to 150V to achieve this. Consistent across the 2 valves used.
C2 was modified to fit a gift I received from my friend Vyacheslav. Instead of the usual 220nF, I fit a bigger 470nF FT-3 version which improves the LF response.
The frequency response is very good, as I published before. For the curious ones, here are some further pictures of the build:
I can’t say more than what I said before. This is a fantastic DHT, it provides a great detailed sound and is quiet. No microphonic noise and the dynamics are great. One of my favourite clearly.
Last time I wrote about the 4P1L in screen mode. It was great to see some DIYAudio member (Blitz) to post about his work on the 4P1L with screen as anode. I call it screen mode but probably is incorrect.
His post about G3 structure remind me to post this, I have tested it but never blogged about it. Yes having G3 as part of the anode structure will increase conductance and will form a nice “mesh” anode. Here it is how I implemented:
The pin 4 (G3) is now connected to 3 (G2) to form the anode. I reduced the anode voltage down to 110V to get 10mA. It could be increased, sure within the Pd limits.
The response is very good:
Here you have the distortion at 1kHz:
Well, I wrote about it before. The 4P1L is one of my favourite valves. In this mode it sounds great, with a particular clear detail in the treble. I like this valve and will play it for some time to get further impressions.
After doing all the soldering part (which I enjoy much), the preamp is now finished. It sounds as good as the original breadboard:
For the curious ones, here you have a picture of the inside:
The teflon sockets are bolted straight into the 4mm top aluminium plate. No microphonic noise this way. Rod Coleman V7 regulators set to 200mA. A pair of Russian Military NOS wire-wound resistors in parallel provides the filament bias. The gyrator PCB is set as per original circuit and each valve at 3mA. These are DC coupled to the MOSFET follower PCB set at 10mA each. The output is then taken out from a pair of FT-3 teflon caps.
Now to enjoy this beauty!
Top plate arrived as well, so we can put the UV-201a DHT preamp in its final frame. Now it will be time for soldering, yay!
