Re: 6-gang FM stereo tuner heads
Yes, it's going to be risky connecting your analyser to valve stuff. As energy stored in the blocking cap is proportional to voltage squared, a lot of energy can be stored in the blocking caps at high HT voltages, and this can produce damaging transients when the energy eventually wants to escape from the cap into a new home. I'd recommend some form of (fast) clamping limiter at the analyser input to minimise this risk.
I managed to find that thesis online, and the technical content isn't that great. Some of it is OK but I'm not impressed by the way the fet probe was developed. The author placed a 5th order LPF at the output and tweaked the values to try and offset some ripple up at UHF. The end result appears to be a network that has smaller cap values than expected. This will make the output network look inductive within the passband of the LPF.
If you place an inductive load at the output of a FET follower, this will effectively lower the input Rp of the FET quite significantly. So, I'm surprised this was done. Perhaps even worse, at higher UHF frequencies I'd expect that network to look capacitive. Putting small capacitive loads at the output of a follower will make the FET input tend towards negative resistance. Up at UHF this may cause the probe to act as a Q multiplier or even become unstable when probing certain loads even with the 47Ω bias resistor present.
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Regards, Jeremy G0HZU
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