The use of CCS in HT power supplies is well known, however generally misunderstood why it can be a good addition to some circuits. There is an excellent article from Gary Pimm which has been lost and luckily I found it on my archive. Here it is in case you haven’t read it. Worth it as a refresher or for anyone who is new to the subject:
Continue reading “CCS in power supplies”Tag: preamp
01a Low Gain DHT preamp
How many times I’ve been asked “how do I lower the gain of the DHT preamp?” many builders love the DHT sound and yet don’t really need that gain. Of course when you use a transformer load, step down is the obvious choice. And it works really well when you invest in a very good quality OPT.
However, when you use DHT with high rp, generally most of the low current ones, you face a tough challenge with regard to frequency response.
With the hybrid mu-follower topology (aka gyrator) we have the problem that the gain is fixed to nearly mu.
I’m in love with the DHT sound and in a preamp a 01a and the 801a/VT-25 DHT Preamp Update preamp cannot be beaten in my view. They all deliver a gain of 8 with a hybrid mu-follower topology.
This can be overcome in the following way: Continue reading “01a Low Gain DHT preamp”
Complete Gyrator Boards for Sale
I have 2 pairs of gyrator boards for sale. They are fully functional ( I tested all of them in a 6e5p driver at 200 Vpeak to peak) and stuffed with my best selection of components:
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:
UV4 PTFE sockets
Looking to build a new 01a Gen2 preamp shortly. This time will be for the rare UV-201a valves. They need a special socket UV4 rather than the usual UX4. See width difference of both filament pins. In the UV4 all pins have same size as below:
Weather they have brass or bakelite bases, the new UV4 sockets produced by Luciano Bandozzi (Jakeband) are of supreme quality. I highly recommend them:
I already reviewed these and you can see more info here. I hope to post some pictures of the new preamp shortly
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!
CX371a / 71a DHT Preamp
More than 4 years ago I ran a lovely 71a preamp which sounded amazing. I used it for some time and enjoy its sound up until I continued with my exploration around DHT preamps. Recently I was asked about how to implement this lovely valve again.
The CX371a / 71a valve is a great candidate for a line stage with its low mu and anode resistance. In my experience you have to run it above 20mA and over 100V to get the best out of this valve:
The implementation of this preamp is dead simple and a few components are needed on top of the gyrator PCB:
I haven’t starved the filaments as I found this valve not to be microphonic. If you have an 01a preamp you can modify it slightly. The interesting thing is that you can run it with just 180V. Even 150V should work and you need 25mA on each channel. A J310 or BF862 lower JFET device will work fine and you will need a heatsink for the top device (e.g. DN2540). Filament resistor is anything close to 50Ω. I used some 51Ω Russian NOS wire wound resistors, but any combination will be fine.
Enjoy
Ale