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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2P29L DHT Preamp

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:

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3B7 DHT Preamp

The VT-25 DHT Preamp is now part of my system as I do like it a lot. Therefore the Mule was vacant for a new experiment. It was the time for the 3B7 valve. This was introduced to me by my friend Paul, who has implemented it using the gyrator and is very happy with it. The 3B7 has a pair of triodes in the same bottle. I wired them in parallel as well as the filaments, which have same current specifications as the 01a. 

Most of the mid-mu DHTs tend to be microphonic. This one is an exception, although some singing comes out of them, its very minor and not impacting the stage. 

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Gyrator PCB board updated (Rev06)

After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:

  1. Protection Zener (D3) between drain and source (through-hole)
  2. Back to back protection Zeners (D1 and D2) between gate and source to ensure positive gate bias for higher currents on jFETs and use of enhancement MOSFET

Layout was carefully adapted to ensure track separation due to HV in place. Result is that the new gyrator board provides all protection needed on the lower device and simplifies the build process

 

 

Here is an example of a completed board tested:

Gyrator Board Rev06

 

01a Preamp Gen2: Build Complete

Yesterday I started with the build of a new 01a preamp Gen2. I made some component changes during the build process thanks to Andy Evans who reminded me of the Russian FT-2 teflon capacitors.  I had a pair left of 220nF FT-3 caps in stock!

The circuit is the same as the original preamp but with some component changes:

  1. C1 is 100nF/630V ClarityCap polypropylene 
  2. MOSFET is DN2540 and jFET is BF862
  3. Rmu is 330Ω Kiwame
  4. Filament resistors are NOS Russian wirewound 51Ω/20W. I use a pair of them in parallel. Bias is about 5V. 
  5. Filament bias using Rod Coleman v7 regulators. Set starved to 200mA
  6. The output caps are Russian NOS teflon FT-3 220nF / 600V. You can use a pair of FT-2 100nF alternatively.

The bias point is changed slightly up to 5V so the anode voltage is increased to 115V to get the 3mA of anode current. This time I’m using the BF862 which can be soldered in the gyrator PCB instead of the 2SK170. I preferred the sound and higher bandwidth as well as lower output impedance. The BF862 is a real winner as lower FET. 

Here is a view of the preamp inside:

The heavy FT-3 caps are mounted on top of the gyrator PCB boards. The top anodised plate is 4mm thick and anodised. The teflon UX-4 sockets from Luciano Bandozzi (Jakeband) are mounted with silent blocks and Rod’s regulators are bolted to the top plate. you don’t have to as they dissipate very little power in this case. 

How does it sound? Well, just played it for a couple of hours and I’m amazed with the subtle differences that the Russian wirewound resistors and output cap + BF862 can bring to this preamp. We did some listening tests recently with Andy Evans comparing filament resistors and these ones were real winners for both of us.

I hope it improves with time after breaks in a bit more.

 

DHT preamp “The Mule”

The birth of the Mule

The name I guess says it all. This is yet another DHT preamp with the gyrator PCB. So what’s different? Simply, a breadboard DHT preamp module ready to be abused.  I’m planning to mod this to death and try a long list of other DHTs with the gyrator load. 

I will only need to change the valve sockets (or build an adaptor) as well as the filament resistors and Rod Coleman filament regulators. Simple changes which can be done fast, will open the door to quick tests on my system.

In order to make this simple and a rapid build, I opted to use an IKEA chopping board. These are made of a laminated hardwood and are dirt cheap. A couple of hours are required to drill all the board like this:

Job done. You only need to do this once. Here is another look at the half-build Mule:

The initial sockets are NOS short pin UX-4/UV-4. I will play around with the 01a before I move to other DHTs. I still need to add the tag strips for filament resistors, output capacitors and the filament regulators. 

Wiring will take a couple of hours and we should have another DHT amp to play with 🙂