A full write up of this preamp design and build can be found here
Tag: dht
DHT Push-Pull filament regulators
A new article is available regarding filament regulators in push-pull designs
01a Preamp Gen2: Build (Part 1)
The first step in the building process of this preamp has been to get the active load boards ready. A reasonably quick assembly process this afternoon:
HY1269 curves
I’m a big fan of the thoriated-tungsten filaments and I value them not just because they do look very cool, but fundamentally due to how they sound in a single-ended stage. I hope to build a nice push-pull amp with these type of valves soon.
I found a pair of HY1269 valves recently. This valve is not well-known amongst the used ones out there but they do have some interesting characteristics as a directly heated tetrode that would be interesting to consider it for a plate-to-grid (Schade) feedback configuration. With its 30W of anode dissipation capability, it’s a good candidate for an output stage. However, it’s quite likely that you will have to drive it in A2 to get the most out of this valve. Like designs using 811a and similar transmitting valves, the HY1269 can be operated in class A2 even with no signal on the grid.
As per my “Robustiano” design using the 6P21S, it would be nice to see extracting 6W or more out of this transmitting valve. I’m sure that the thoriated-tungsten touch will provide a lovely sound if properly implemented.
4P1L DHT Preamp Siberian (Gen3) finished!
Introduction
Building a new version of the venerable 4P1L “Siberian” was very encouraging. This belated project finally came to life after some recent work on a new set of power supplies. So why 4P1L again? I always found the 4P1L sound to be unique. Great detail, overall tone and fantastic treble. What it makes it well suited for pre-amplifiers is not just its linearity (probably being the most linear valve out there) but the fact that it has a low anode resistance and current capability to ensure any challenging load can be handled effectively without any sound degradation. This can be heard particularly on the treble where the input capacitance of the amplifier is more evident and it is translated into treble loss. Other DHTs like 26, 01A, 30sp can only handle a few milliamperes of anode current and is not enough to charge and discharge the parasitic capacitance at high frequencies. More importantly, the 4P1L has filaments which aren’t demanding. This is a unique feature amongst DHTs that is rare and very useful. Having low-current filaments that can be either configured at 325mA or 650mA, low grid voltages and high transconductance in a valve is very useful. This mean that filament bias can be easily implemented without burning unnecessary power by swinging many volts to perform the desired level of amplification.
Continue reading “4P1L DHT Preamp Siberian (Gen3) finished!”
Siberian DHT Preamp Gen3: filament supply
Time to upgrade my pre-amplifier (again) and is perfect timing to go back to 4P1L. The Siberian preamp had a fantastic bass response and detail.
Here is the new filament raw supply. It has split bobbin transformers, schottky rectifier bridge and input choke LC filter stage. It also has a CM choke and EPCOS electrolytic capacitors:
Power supply ready
Input chokes
Chokes and capacitor bank
Schottky Rectifier Bridge plus snubber
Building the power supply
Dead quiet at 16V output and 550mA which is the load used by 4P1L starved filaments in parallel with filament bias.
Soon to build the preamp!
A collection of latest images
After upgrading my 26 preamp, turntable and adjusted the system to a great sounding state. Looking forward to play many records now…
HT power supplies
Latest System
26 preamp and 4-65a SE Amplifier
DHT filament supplies (and many of them)
26 and bigger brother 46
Reddish 4-65a
26 preamp Gen3
Lenco turntable
Setting up to play in the evening
307a with schade feedback (Part II)
Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.
The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!
With 15.5% feedback and 125V screen voltage I got the best curves as shown below:
Nice to see the anode resistance coming down to 1kΩ.I tried matching a triode model to these curves with a good result
4-65a SE Amplifier: testing filament regulators
The Rod Coleman Filament Regulators were already tested in the New Year. It seems a long time indeed. Here is a selection of pictures after fitting the regulators in their heat-sinks and doing a final test before assembly:
4-65a
Testing the 4-65a filament regulators
4-65a filaments are lovely
46 filament bias and the regulator
46 Driver regulator
46 DHT driver final tests
Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.
The following tests conditions were used:
- 4P1L first stage:
- DN2540 gyrator in mu follower output
- 220nF/450V Capacitor coupled into 46 driver
- Filament bias: 15 ohms, Vgk=-10V
- Vsupply=355V and Va0=210V
- Output set to about 30-32Vpp to drive 46 at 200Vpp
- 46 driver stage:
- IXYS 01N100 gyrator in mu follower output
- Load impedance is 100K (Pete Millett’s interface)
- Filament bias: 10 ohm / 100W Vgk=-17V
- Vsupply=355V and Va0=204-208V
- Output set to 200Vpp
I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:
Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…