Author: Ale Moglia

"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)

4P1L / 4П1Л Siberian Gen4 in Screen mode (part 2)

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:

How does it sound?

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.

 

UV-201a final version – finished!

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!

2P29L DHT preamp (again)

Well, after enjoying the 2P29L as part of the Mule preamp project, I decided it was worth building it so I can continue with my adventures on the DHT land.  The 2P29L sounds so good that is worth having it as a standing preamp so here is my new version in progress. Thanks DHTRob for your inspiration using the IKEA chopping boards in a slightly different way I used them so far:


Simple and neat. The 2P29L were stripped out of their aluminiun cans. Naked as when we come to life.  The gyrator boards are ready and tested. Just need to get the Rod Coleman regulators and complete the wiring. 

Happy DIY audio!

Ba DHT Preamp (Part II)

I’ve been posting not very frequently lately. This is mainly due to lack of time and the level of business travel which reduced to nearly none the time available for DIY audio. 

Nevertheless, the scarce time always pays off. It’s incredible how selective I have to be in order to prioritise which project I should work on. The list is long though. 

Last time I did a quick exercise on the Ba DHT based on the curves I traced and the LTSpice simulation. Well, you always need to build and test in order to check against simulations. The result is, that you may need to adjust and learn from your practical experiences. 

The Ba (like the Aa) are tricky to use. They pick up any electrostatic induced noise. You don’t need even to place your hand close, the mains noise is induced already in its plate. This force you to shield these valves if you want to use them. Am afraid, that is what it is. My friend Rob (DHTRob) warned me, thank you.

The circuit I posted here, had to be readjusted.  Distortion was way too high. The operating point wasn’t good enough. You’d normally get inclined to run the valve as hot as you can, but I was wrong here with this one.

Continue reading “Ba DHT Preamp (Part II)”

Ba German DHT Preamp, here we go…

Flying around

Travelling around Europe on business is paying its toll. I’m away from home every week and pretty exhausted now. I don’t have much time free and whatever is available I spend with my family. Hence, the lack of posts recently. I hope this will change in the future.

Anyway, what’s up in the DHT world? I listened the Aa/Ba valves long time ago but never played with them. Mainly due to their higher anode resistance. With the gyrator load and the source follower output, things take a different dimension.

German precision

I have a nice stash of Aa from Valvo (globe) and Ba from Siemens. Interesting to see that curves are not easy to find, so I submit them both to the mercy of the uTracer.  Nice to see the linear curves with high mu about 14 on the Ba to 30 in the Aa.  

Here is an example of the Ba loadline:

Continue reading “Ba German DHT Preamp, here we go…”

Stop that oscillation please!

I’ve been playing for some weeks the marvellous UV-201a stage.  The addition of the source follower brings a new dimension to this stage.  However, I experienced random oscillation on this setup. I knew it was the MOSFET, but I wanted to pin it down. 

I replaced the SiC version with the famous AOT1N60. That cure the issue, but I wanted to find out what was going on, of course blaming my selection of gate stoppers. 

Continue reading “Stop that oscillation please!”

4P1L / 4П1Л Siberian Gen4 in Screen mode

Some time ago, I did some initial experiments with the 4P1L (4П1Л) with the screen performing as anode instead. Some DIYers claim the improved sound of the mesh type anodes. Kees Brakenhoff kindly sent me some PL519 to test in screen mode. He has done multiple builds with this mode of operation with great results. Unfortunately I’ve not had the chance yet to build such an amp.

What I could do instead though, was to mod very quickly my 4П1Л preamp to screen mode. It was a very easy and fast modification. I kept the same heating wiring and just adjusted the screen (anode) current down to 10mA: Continue reading “4P1L / 4П1Л Siberian Gen4 in Screen mode”

UV-201a DHT Preamp Revisited


I previously implemented a preamp with the UV-201a. These are very old globe valves, somehow fragile and hard to get in good shape. Despite all this, it’s a superb valve. I have managed to acquire a decent set of them to pair the best valves to use in my preamp.

Recently I developed a prototype PCB for the source follower circuit. The source follower is ideal to place at the output of this preamp due to its low driving current. My 4P1L PSE amplifier will be pleased with more current to pump the Miller capacitance effectively. 

Continue reading “UV-201a DHT Preamp Revisited”