Back in 2012 I did some experiments with this Russian valve. The 2Ж27Л / 2Z27L is a fantastic valve. I discovered that if I run it hot, it’s extremely linear and non-microphonic. Bingo! With its highish μ of about 16, it’s ideal as preamp stage or driver.
Author: Ale Moglia
"A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp)
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
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:
6SF5 driver for 300B/GM70/813 SE Amps
Some years ago my friend Paolo brought up this lovely valve which looking at the curves seemed to be a great candidate for a driver which required large voltage swing. This indeed is needed for 300B, 211/GM70, 813 SE Amps. I build it and tested with the 300B, which is great.
The immediate challenge (and probably the reason why it hasn’t been used extensively in the past) is that has a high anode resistance as well as very low current capability. These 2 things are of course a killer for driving these demanding output valves. However, with a gyrator load and a follower stage, the reality is different
2K2M DHP Test
The 2K2M is an unknown directly heated pentode from Russia with has variable mu. I have several pieces of these, and was wondering how useful they could be. This is part of my on-going saga of the “Mule“.
DHT Phono Stage Test
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:
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:
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.
Gyrator PCB Hack: final Enhancement Mosfet design
I evolved my previous design here, thanks to the help of Rod Coleman and fruitful discussions with him.
There is an option to improve the design by bootstrapping the top MOSFET to avoid using a bias Zener and allow the bottom device to have a constant VDS. This can be achieved by double bootstrapping the FETs. Here it goes:
Similar design as before. Only difference is that R7 is used to create the bias of T3, and thanks to the bootstrap of C2, the bottom FET (T4) now operates freely regardless the swing. D1 is needed to protect T4. R7need to be adjusted considering the output voltage expected as well as the maximum VDS before D1 starts to conduct.
There is an stability challenge and it can be addressed as Rod Coleman clearly points it out, a “guard ring” :
The other pro trick is the guard ring: this will dramatically reduce problems of dc-drift, if the PCB surface gets contaminated, e.g. when soldered with some old or poor-quality solder. Or damp air, fumes etc. It’s a conductor (pcb trace) around the high-impedance network formed by the 10M resistors. A staggered-pinout version of the TO220 is needed to implement it, as the TO220 is the hotspot for leakage (B+ of drain to the 10MΩ-driven gate!).
If there is a leakage path, it leads only to the guard ring, which is only a few volts away from the intended bias – rather than if the leakage can reach ground or B+, which would drive the circuit crazy. Connect the guard to a low-Z source – the Output in this case.Anyway – I hope it is useful in some way!(Rod Coleman)
How well it performs? Here you can see – no guard ring here, just adapted standard PCB for testing purposes:
Not bad at all with 3MHz bandwidth. However, considering the circuit complexity, I much rather stick to the depletion version which performs much better in my view:
Nearly 5.7MHz under same conditions!
Cheers, Ale
The Mule Project (continues)
Well, the VT-25 preamp now is part of my main system. I modified the 01a preamp to turn it into VT-25. Now the mule is ready for further adventures. I can look into my list of other valves for exploration with the gyrator design
Watch this space, there is more to come soon…