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.
After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:
- Protection Zener (D3) between drain and source (through-hole)
- 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:
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:
- C1 is 100nF/630V ClarityCap polypropylene
- MOSFET is DN2540 and jFET is BF862
- Rmu is 330Ω Kiwame
- Filament resistors are NOS Russian wirewound 51Ω/20W. I use a pair of them in parallel. Bias is about 5V.
- Filament bias using Rod Coleman v7 regulators. Set starved to 200mA
- 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.
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 🙂
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|
I have just completed my 01a preamp with STP3NK60ZFP output follower. The preamp sounded great !!
Well done Cheah!