My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.
Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:
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!
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.
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.
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:
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”
The gyrator PCB has been updated to fit now a wider variety of lower enhancement MOSFETs with low capacitance and high transconductance. The best examples are the BSH111BK and BSN20BK which are great options for currents above 25mA:
The board offers now all the flexibility needed in terms of different TO-92 and SOT-23 package pin-outs to use whatever FET you want.
The return of the Siberian
I have a pair of 4P1L/4П1Л dated 1968 which are properly burnt in. I’ve used them lately in my previous preamp incarnation with great results.
The circuit doesn’t need explanation, I think I’ve covered this repeatedly for a long time. I will only point out the differences:
The main change was fitting a pair of Russian wirewound 27Ω resistors in parallel to get closer to the 15Ω used in this position. I found these Russian wirewound resistors to sound extremely well as filament bias resistors. I tend to be skeptical about the “sound” of some components in circuits, however, they do make a big impact in the cathode of a filament bias arrangement.
The gyrator has my preferred combination: IXTP08N100D and BSH111BK. I have now an upgraded PCB Rev07 which fits the BSH111BK and similar FET and I will offer them shortly.
The latter benefits from the 30mA idle current. The result is lower output impedance whilst providing a great frequency response overall.
M3 needs a proper heatsink, it does get hot with about 2W of dissipation.
How does it perform?
Well, this valve has the reputation of amazing performance and low distortion. The gyrator setup provides the best out of this valve in my view. You can get a flat response as well as great bandwidht from 10Hz up to 3MHz loaded with 100kΩ:
The distortion is very low and is lower than 0.05% below 10Vrms. Dominant H2 with a lovely harmonic profile characteristic of this valve.
How does it sounds?
i’ve been listening and using this valve extensively since 2011. I have to say that it sounds amazing. I never get tired of its sounds. Before I listened to a 4P1L-4P1L system and found a slight edge on the sound (probably due to its H3 component) which I don’t hear on my system. The drive, clarity and tone is amazing. It can drive the 4P1L PSE perfectly well and you get a strong and clear bass. Very powerful. My +600 hours 4P1L are very quiet in this setup, no microphonic noise. I don’t have even dampers in the 4P1L sockets!
Anyway, if you need 19dB (x9) gain in your system or you need a driver for your SE amp, then this is the valve to go. I Still can be found cheaply and is a great contender to the thoriated tungsten filament DHTs like 01a and VT-25.
Build this one and enjoy!
High gain stage with DHT
Some time ago a colleague (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.
Tracing the curves, the first step
The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:
The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:
The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well: