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.


01a Preamp Gen2: New Build

Yet, another 01a preamp. This time with a beautiful machined top plate and wooden frame:

Will try the Russian filament resistor which we tested at Andy Evans’ and sounded amazing. Just need to build a new pair of Rod Coleman Boards and do the wiring:

4P1L PSE Amp: assembling top plates

The additional machined top plates arrived yesterday. I assembled them this morning. Just need to assemble the chassis now before soldering!

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 🙂


4P1L: pump up the current!


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:

  BF862 BSH111BK MMBFJ310L 
Ptot  (W) 0.3 0.3 0.225
VDSmax (V) 20 55 25
VGS off (V) -1.2   -4
IDSS (mA) 25 210 60
Gfs (mS) 45 640 18
Ciss (pF) 10 19.1 5
Crss (pF) 1.9 1.5 2.5
Coss (pF)   2.7  

Continue reading “4P1L: pump up the current!”

3B7 DHT Preamplifier

An enthusiast blog reader (Paul Prinz) implemented a nice version of the DHT preamplifier using the gyrator PCB but for the 3B7 triode. The 3B7 triode has a pair of DHT triodes on the same bottle. It has a high mu for a DHT (about 20) but with that it comes the higher anode resistance. This was a drawback when implementing a choke or transformer loaded stage due to the high anode resistance (there is no free lunch am afraid). However, with the mu-stage, this doesn’t become an issue and we can get the most out of this valve using the gyrator load. 

Although I tried the 3B7 in the past, I proceeded to get it out from my valve stash and trace it again. Here is a nice set of a Sylvania military NOS one:

Continue reading “3B7 DHT Preamplifier”

4P1L PSE project started

 A new amplifier build has begun. The 4P1L PSE is coming to life. First top plate developed and arrived this morning. A succesful layout for being the first one. Need to submit for machining the other 2 panels. It will be modular so I can make changes and experiment with different output transformers I have at hand (Monolith Magnetics, Lundahls, etc.). The HT power supply is ready and I’ve been fitting the filament supplies in child-proof boxes over the Christmas holidays. It will take me some time though to get this one up and running. Not much time to work on it am afraid. 

01a Gen2 Preamp Build from Barry French

Barry French has recently build his version of the 01a Preamp Gen2. Here are a couple of pictures: 

And Barry’s impressions:

“The 01a Amplifier is a stunner, personally I feel it leaves the 26 out in the cold, better top & bottom by a Country Mile, this was built using the Russian FT-3 Caps on the Output, Russian PIO Caps on the Boards with Jupiter 0.1 μf Wax/Oil Caps from B+ to Ground, the Power Supplies for both Filaments & B+ are from my original 26.”

01a Preamp (Gen2) Universal HT Supply


I received many requests throughout this year of folks building the 01a preamp (Gen2) asking for advice on the HT supply design.  Well, I have my own incarnation which serves multiple purposes as is a shared supply, therefore isn’t useful to anyone. 

My friend Andy Evans came up with a supply using available components. This is exactly what you are looking for the HT supply.  I made some tweaks to Andy’s design, so all credit should go to him.


The HT supply design is very simple. It’s mainly a choke-input valve rectifier supply. It has an additional LC smoothing stage. Here is the high-level circuit, see some notes

The transformer is a 250-0-250V / 50VA with an output current of 60mA. It has two windings for 6.3V AC heaters, but you only need one. Here is a great choice from a recommended seller. 

The valve rectifier is a double-diode damper. Of course you can use some other options, but I like the sound of them. Here are some you might want to consider in your build:

  • 6BY5G: a nice double-diode damper with lower filament requirements than the commonly used.  This is my first choice
  • EZ-80: the famous rectifier. This is what Andy used.
  • EY-91: you will need 2 valves as there is one diode per bottle.
  • 6C4P-EV:  a nice Russian small double kenotron rectifier. Very cheap alternative for European builders 
  • AZ1 / AZ11 mesh valves. Their sound is unique, however they are very expensive these days. You will have to add a pair of voltage dropping resistors to accommodate the lower filament voltage requirement. 

For the indirectly heated diodes, it will be better to connect one end of the filaments to the cathode. 

The chokes are commonly available. These are from Hammond (155J) and have 15H @ 30mA. The downside is the high resistance, over 1kΩ. This isn’t an issue here as the current consumption is low so the voltage drop is minimal.  With a choke input supply you need a minimum current to operate. In this case is about 15-16mA so a bleeder resistor is needed (29kΩ 5W wire wound) . 

C1 serves to equalise the output voltage. It pushes the supply to operate a bit more like cap-input (hybrid) by increasing the output voltage. I use it to tune the output voltage to 200V. 

C2 and C3 are classic motor run capacitors. I personally use ASC Oil ones 450VAC rated.  You can choose what you can get hold. You can use any good quality film capacitors. I like the WIMA DC-LINK ones, they are great.

The output ripple is about 7mV. This isn’t a problem as the gyrator load has a very high supply rejection (PSRR) so no need to go crazy on this. If you 

The design is so simple that anyone should be able to build this easily. 

Hope it works for you.

Merry Christmas!