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:

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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.”

CX-301a DHT Pre-amp Build from Malaysia

A fantastic build of the 01a preamp using the gyrator PCB from Cheah:

I have just completed my 01a preamp with STP3NK60ZFP output follower. The preamp sounded great !!

A great looking preamp using Rod Coleman’s regulators, output follower and V-Cap capacitors. The jFET is BF862.

Well done Cheah!

UV-201a preamp test

First listening test with these beauties, it sounds really nice!

And here is the look inside for the curious ones who always ask me to provide more details about the construction. Here is a good ground wiring example. The preamp is dead quiet:

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:

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:

Tony’s 01a Preamp

I went to see my friend Tony today and helped him to fix his 01a preamp implementation. Time ago Tony used a prototype version of my gyrator PCB to build the Gen2 preamp with the addition of an output follower to address the slew rate limitations he had on his system due to the larger capacitive load.

Luckily we found the fault easily and it was a bad solder in one of the smoothing HT chokes. Once fault was rectified, we proceeded to take some measurements of this preamp.

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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!