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.
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
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:
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.
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)
Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)
I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.
Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.
The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)
Here is the design:
The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V. As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place.
The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.
There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C.
So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:
However, my prefered stage can do 3.4Mhz under same conditions!
I’m a firm believer than sharing knowledge and experience is the best way forward to continue learning yourself. It always pay pack at some point. This time Paul Prinz, a fellow implementer of the 3B7 DHT Preamp using the gyrator PCB, came back with a great suggestion. He found a MOSFET which could do high drain currents, it has high transconductance and most importantly the parasitic capacitances were low even close to the BF862. Hooray, I thought. We may have a great solution here to use the gyrator load for currents above 25mA and with similar performance to the great BF862. There are some other depletion MOSFETs that can do high currents, however they all have relatively high capacitances and low transconductances when VDS is low, like in the cascoded gyrator circuit.
The BSH111BK is an enhancement MOSFET, so doesn’t have a “depletion” behaviour like the jFETs. This isn’t a problem as the bias voltage can be set by the reference CCS.
For comparison, here is a brief summary of the key characteristics of these three devices:
|VGS off (V)||-1.2||-4|