I received the first batch of PCBs after several tweaks during the prototype phase. I’m very pleased with the result:
Tracing valves: an obsession
Since my early days of valves and DIY audio, I developed an obsession around testing and tracing valves. This led me to design and build my analogue curve tracer which I used for many years successfully until I build my uTracer, which was a great innovation in curve tracing. I do have many valve testers (some which I made myself) so why building another one?
Well, Chris Chang from Essues Technologies developed a fantastic new digital curve tracer for valves, the eTracer. There are a few things which will grab anyone’s attention on this curve tracer. Firstly, the power supplies can accommodate a large range of valves which the uTracer can’t. HT can go as high as 750V @ 300mA and the grid supply down to -170V! This is exactly what you need to test your transmitting valves or even a 300B. Secondly, the tracing speed is surprisingly fast. This is a nice feature, specially when you want to trace pentodes at various screen voltages to develop a Spice model for example.
The gyrator PCB has been updated to fit now a wider variety of lower enhancement MOSFETs with low capacitance and high transconductance. The best examples are the BSH111BK and BSN20BK which are great options for currents above 25mA:
The board offers now all the flexibility needed in terms of different TO-92 and SOT-23 package pin-outs to use whatever FET you want.
Some years ago my friend Paolo brought up this lovely valve which looking at the curves seemed to be a great candidate for a driver which required large voltage swing. This indeed is needed for 300B, 211/GM70, 813 SE Amps. I build it and tested with the 300B, which is great.
The immediate challenge (and probably the reason why it hasn’t been used extensively in the past) is that has a high anode resistance as well as very low current capability. These 2 things are of course a killer for driving these demanding output valves. However, with a gyrator load and a follower stage, the reality is different
The VT-25 DHT Preamp is now part of my system as I do like it a lot. Therefore the Mule was vacant for a new experiment. It was the time for the 3B7 valve. This was introduced to me by my friend Paul, who has implemented it using the gyrator and is very happy with it. The 3B7 has a pair of triodes in the same bottle. I wired them in parallel as well as the filaments, which have same current specifications as the 01a.
Happy Easter to all! (whatever you celebrate, doesn’t matter, it’s always good to have some days off)
I have my preferred gyrator setup which includes a top (depletion) MOSFET IXTP08N100D, which has a unique high VGS(th) which helps improving the performance of the bottom FET, in my case the BSH111BK. The combination of both is superb and they do measure (and sound) superb. The frequency response is flat until 3.4Mhz (-3dB). Yes, a high bandwidth amplifier, so you need to be mindful of this when using high gm/gain valves. I read somewhere people complaining that gyrator “oscillate”. Well they don’t, however they create a high bandwidth amplifier which is therefore prone to oscillate if you don’t take the right measures. If you don’t know what you’re doing, it will oscillate for sure, you have been warned.
Ok, if you can’t get hold of (any) depletion MOSFET as the top device, there is an option, a la Gary Pimms.
The circuit can be tweaked slightly, as can be hacked the PCB (I can show you how if you’re intending to use this circuit)
Here is the design:
The main difference is that D4 provides a stable reference voltage (18V) which ones you subtract the VGS(th) of the top MOSFET (typically 2-5V) then will give you enough headroom to allow the bottom FET to operate under low output capacitance due to higher VDS. This is the common limitation of the cascoded pair of depletion devices. You can’t get more than 2-3V. As the top device forms a “cascode” with the bottom, it also limits the maximum voltage possible to the drain of the bottom device. The protection zener of the bottom device can be removed to ensure maximum swing. This stage can do 20Vpp easily. C5 provides some filtering to the zener noise, which is very low. I can’t see an issue at the driving levels in place.
The protection zener (D2) for the top device is needed unless the MOSFET comes with a pair of back to back as some do.
There are multiple options for the top MOSFET. I like the (nearly EOL) STP3NK60ZFP which is a FP TO-220 device, very handy for heatsinks and high voltage and comes with the bonus of the protection zeners. The best option is the AOT1N60 and also the easier to get hold off FQPF2N60C.
So, the performance is great. You can get flat response up to 2.1 Mhz. Here is a snapshot with my buffer which limits to 1.5Mhz:
However, my prefered stage can do 3.4Mhz under same conditions!
If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:
The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR).
The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please.
This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.
Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.
As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.
The start of a different DHT experience with the Mule
I built the “Mule” to provide enough flexibility to test other DHTs as pre-amplifier / line stage. Using the gyrator board, the flexibility is fantastic. Can share same HT and dial the right anode voltage. The LT supply can also be shared amongst many DHTs and Rod Coleman provided me with a set of different resistors to test the list of 9 or 10 DHTs I have in mind which haven’t listed carefully on this design.
After some further testing and prototyping, I’ve updated the gyrator board PCB to provide additional protection to the lower FET device with:
- Protection Zener (D3) between drain and source (through-hole)
- Back to back protection Zeners (D1 and D2) between gate and source to ensure positive gate bias for higher currents on jFETs and use of enhancement MOSFET
Layout was carefully adapted to ensure track separation due to HV in place. Result is that the new gyrator board provides all protection needed on the lower device and simplifies the build process
Here is an example of a completed board tested: