On my previous post, I covered my initial build work on the gyrator test mule using the gyrator PCB. I did all the lovely soldering work (which I do enjoy not like milling or drilling) and proceeded to do several tests.

Some interesting observations based on my abuse of the gyrator which yielded on several MOSFETs and JFET damaged as a result:

• CCS reference: I used an external multi turn 5KΩ potentiometer via lead cables. I wired it incorrectly and that contributed to one of the initial faults. Be sure you look carefully on this if you use an external pot. if you use the on-board trimpot, this is not an issue.
• JFET: this is the interesting one. If you want to run the lower JFET at very low biasing current for a larger jFET (e.g. J310) you will find that the JFET needs to operate close to cut-off voltage (somewhere between -2 and -6V). This VGS required will definitely forward bias the Zener protection diode D1 and prevent from reaching lower bias current (I found it about 10mA for J310). To resolve this you just need to add a back to back zener as shown below. This isn’t a problem for an BF862 or a 2SK170 as their cut-off voltages are quite small.
• Failure: if you abuse the FETs, they will die. And if they die you will get a nice short across them and you will measure nearly HT at the mu output. Just replace the MOSFET and JFET (probably both are damaged)

Here is the implemented J310 gyrator:

 

Final Build

The build itself is quite packed. Thanks to the 2mm connectors I can easily service the boards. The fiddling part is replacement of FET and the MOSFET. My word of advice here is to cut the device leads and leave the leads soldered in the PCB. You then solder the new FET/MOSFET on top of the old leads. It’s not neat, but it’s much easier than removing the previous solder and re-soldering the components.

Some interesting tests

First test I’d like to share is the famous 4P1L. As a driver it’s superbly linear, however due to its low mu (8-9) we need a step up transformer or another stage. Anyhow, here is a simple test with filament bias. Without being pretentious, the operating point isn’t optimal and it can be improved should you need 200Vpp or more:

The harmonic profile is very nice and 0.12% @50Vrms (140Vpp) is very good.

Now let’s look at the secret valve which has been talked about in the forums for some time. The 12HL7 frame-grid pentode (Va=400V, Vs=330V – Pa=10W). This is a beauty of a pentode which is very linear. It performs as well as the 6e6p-e, 4P1L in terms of linearity but has a nice mu of about 30 which makes it an attractive option for a driver:

As you can see on the above, the 12HL7 is biased at about 30mA, 200V at the anode and the distortion is very low (0.2%), with predominant H2 profile.

Now I can do some more interesting tests quicker with this mule, including Parafeed drive. I hope the mule doesn’t give up and die eventually 🙂