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:

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.

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 Build: UV-201a version – step 2

Adding filament bias and output caps

01a Preamp Build: UV-201a version – step 1

This is what I managed to do with a couple of hours at the workshop. Time is gold for me these days. Quick drilling and fitting the main components. You can’t get a smaller preamp than this one. It’s quite packed:

Planning the layout before drilling

Back panel with all connectors (supply and signal)

Inside the preamp

Close look at the gyrator boards

UV4 sockets with rubber dampers

Fitting a pair of brass base UV201a

Next, soldering. Yay, just looking forward to my favourite part of the build process

01a Preamp Build: UV-201a version – layout

Just playing with the layout a bit. The gyrator boards, the UV4 sockets and the Rod Coleman regulators. All in a tiny aluminium box:

26 DHT preamp Gen3

Using the Gyrator PCB board:

Building gyrator boards

I’ve been on some business travel so haven’t had much time to work on stuff, however I did get a set of gyrator boards for a friend and a customer:

BF862 configured for 4P1L preamp
2SK170 configured for 01a preamp

4P1L preamp with BF862 gyrator

Many have asked me about this preamp with gyrator load. Here is the latest implementation which I preferred most in terms of sound. The mu resistor is 470Ω which is a nice compromise between BF862 transconductance and distortion. I adjusted it on test. I use a 100nF for C1 so R6 is 10MΩ. R4 can be either 300KΩ, 330KΩ or even 390KΩ. Difference would be only on the voltage range for the CCS. I found running it at 25mA to be perfectly fine, some BF862 can even do J310. I prefer this SMD compared to the J310. It performs much better even at high frequency:

Gyrator load – test mule

I’ve done several tests using a simple gyrator PCB test mule. It was time to build a proper and flexible test mule for extreme abuse:

2 Boards for current flexibility
Board 1: BF862
Board 2: J310
Top FET is IXTP08N100D2 for 1000V operation
ZIF socket pins for CCS reference resistor and RMu. This will give the necessary flexibility to try any combination in the gyrator depending on the triode and or the power supply
External pots for ease regulation of anode voltage

The top MOSFETs are bolted on the aluminium case which will act as heatsink. For tests this should be sufficient.

The top plate of the case looks like this:

There are 4mm posts are for HT supply, GND, mu-output, Anode. There are also a pair of 2mm posts for current sensing per board.

Some further soldering to do and job done!

2P29L – Preamp and driver for 4P1L PSE Amp

A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:

The triode curves are really nice:

This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:

Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.

We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:

Slew Rate in Preamps

Introduction

You may probably already asked yourself: what is this “slew rate” business? Has this guy lost his mind? Probably I have, but not due to this interesting phenomenon. The second question that should have probably popped in your brain is: why? Well, writing about this stuff came up by sheer coincidence.

We tested my friend Tony’s 01a preamp which has an older gyrator board I made for him about 3 or 4 years ago. The preamp (as well as his system) sounds extremely good in my opinion and the 01a has brought a new clarity which is what you’d expect as a result of the introduction of a DHT stage. We decided to run some frequency response tests since Tony has made some interesting mods to his Push-Pull amp. We encountered an unexpected challenge as I didn’t have the right XLR connectors for my testing gear so we run the FR tests on the power amp with the 01a.

The 01a preamp measured as expected with a flat response up until 40kHz before the sound card rolloff kicked in – I’m aware of this and this is the limited bandwidth of my current portable measurement gear. The distortion was also really low (H2 predominantly ) with less than 0.02% for 2 Vrms output signal. This test was done with Pete Millett’s interface which has an input impedance of 100kΩ: Continue reading “Slew Rate in Preamps”