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:

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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. 

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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. 

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Source Follower PCB

I’ve been posting about the use of source followers in the circuits with some interesting results from testing. Some interesting posts to read, If you haven’t read them so far:

  1. Slew Rate, Slew Rate (Part 2), Slew Rate (Part 3) and Slew Rate (Part 4).
  2. 6SF5 driver for 300B/GM70/813 SE Amps
  3. DHT Phono Stage Test

After several tests over a variety of circuits, I finalised the prototype for a Source Follower PCB. The circuit is incredibly useful. Some examples of uses cases are:

  • Amplifier output stage grid drive
  • Screen drive amplifiers
  • Screen voltage stabiliser for pentode stages
  • HT voltage stabiliser for preamps
  • Buffer stage for high-mu/high-anode resistance stages – either triodes or pentodes (e.g. Phono)

Some key aspects of the board are:

  • The PCB has been designed to accommodate all sorts of power MOSFETs (both TO-220 and TO-247), in particular the high transconductance and low Crss ones which perform the best in this role.
  • The tail CCS is simple and leverage the option of using same MOSFETs.   
  • The board takes into account the use of any bipolar supply up to 450V diferential. You only need to change a resistor depending on the supply voltage levels and make sure there is a sufficiently big heatsink on the MOSFETs.
  • There is a current limiter circuit built in to protect screen or grid from excessive current. This is also very useful when the board is used as a voltage stabiliser for a preamp. You can limit the peak current and avoid destroying the MOSFETs when capacitors are charged or if accidentally the output is shorted. This circuit can be bypassed easily with a jumper.

Here is one of the boards submitted to the usual abuse during testing:

This is a very useful PCB in my view which can be used extensively in preamps, line stages and amplifiers.

If there is sufficient interest, I will run a batch of PCBs for the DIY audio community:

Gyrator PCB Update – Rev07

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.


4P1L (4П1Л) Siberian Gen4 – DHT Preamplifier

The return of the Siberian

After trying out so many DHTs and pre-amplifiers, I decided to wire up my 4P1L preamplifier Gen3 and fit the gyrator board to drive my 4P1L PSE Amplifier.  

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!

6Ж49П-ДР/6J49P-DR High Gain Stage

Time ago I wrote about this sterling Russian valve. It’s extremely linear in triode mode, sounds superb and isn’t microphonic. My friend Paul LeClerq has used it as first stage of his guitar amplifier and is delighted. A real dormer one. I hope it doesn’t disappear when valve hoarders go out and grab every big lot of valve that exists. Anyhow, I have more than I need myself, so I’m not worried.

The triode driver

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Custom transformer for Discrete DAC

Some 3 years ago I built the famous “Starlight” discrete DAC and CD transport, the DIY version from Sonic Illusions.  This is, in my opinion, one of the best DACs I’ve every listened to.

One of the disadvantages of the DAC output is the high (10kΩ) resistance and AC coupling requirement (unless you are prepared to balance DC, etc.) The traditional output configuration is an output capacitor feeding a step-down transformer to provide lower output impedance. The DAC can do only 1Vrms maximum due to the TTL output logic. With a 4:1 output I had to date, I wasn’t able to drive properly my VT25/10Y preamp into the 4P1L PSE amplifier

I decided to try a 1:1 output transformer but I had a clear challenge. The big primary inductance (Lp) needed demands a careful design and construction of the transformer to minimise the leakage capacitance and inductance which will impact the HF response. As I’m driving a long cable into the DHT preamp, this presents a demanding load of 47kΩ||400-600pF.  

I contacted DvB transformers to order a custom transformer to fit my needs:

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