Someone had to invest and sacrifice some gyrator boards to test various lower FETs (either depletion or enhancement devices as well as TO-92 or SMD options). That was me.
Why? Because I want to push this circuit further and find the best options as well as provide to the builders out there some other device alternatives when they can’t solder SMD components.
So let me present you the abused test mule and the various boards under the mercy of my tests:
So What FETs where analysed? Here is the list:
- BSSH111BK: N-Channel, Enhancement, 55V, 335mA, SMD device (needs to be mounted in 45 degrees or with an adaptor)
- 2N7002: N-Channel, Enhancement, 60V, 115mA, SMD device (needs to be mounted in 45 degrees or with an adaptor)
- 2N7000: N-Channel, Enhancement, 60V, 200mA, TO-92 – fits perfectly on the PCB board.
- BS107A: N-Channel, Enhancement, 250V, 250mA, TO-92 – fits perfectly on the PCB board.
- BSS123: N-Channel, Depletion, 100V, 150mA, SMD device (needs to be mounted in 45 degrees or with an adaptor)
The test circuit
The test circuit is very simple. The classic DHT preamp stage with a 4P1L in filament bias. The HT supply is regulated 300V with a decoupling 100μF ASC Oil cap with a 100nF film cap in parallel. To separate FR impact from the load, the load is the test set in parallel with the 510kΩ R8. The following components differ between boards as am planning to use them in different circuit from the 4P1L but shouldn’t affect the results of the below tests:
- R4: Either 300kΩ or 390kΩ (preferred for 4P1L)
- C1:100nF and R6 10MΩ
- R7: 300Ω or 470Ω
- Rf: 15Ω
All tests were conducted with If=650mA and the 4P1L biased to Ia=30mA
Below are the tests conducted.
So in summary, here’s my doggy bag take:
- BSSH111BK will give you the best performance. However, there’s isn’t any free lunch. You will have to solder the SMD device on 45 degrees. Which is tricky, see my previous post here.
- If you want to avoid the hassle of SMD soldering and need +30mA current in your stage, then go for either 2N7000 or BS107A. Both TO-92 devices fit on the board and perform really well up to 1MHz.