The main issue with thresholding here is that everything has to be real stable, or the sensitivity varies. You might not care for short term tuning uses, which is where these things shine anyway.
You'll always have some post pulse overshoot as the coupling cap charge changes a little whilst driving the current into the preamp input, and it's gotta go someplace after that. It's just the usual AC coupling artifact. It's one reason there's a range of values that work right for that. The other is current limiting the tube during the pulse. It's expected and OK to have some overshoot time there, as the tube takes longer to recover than the circuit anyway.
For audio (other than PC input) even short pulses are hear-able. And you get additional info from the width/height as far as the ear is concerned - it gets more "bassy". I just use the signal straight for that, no processing at all. On my big-fast 3He tube (1" by 30") I can hear when it gets a double hit from a burst of neutrons, nice.
The gain is set by the negative feedback from the output, through the load R, to the emitter of the input. The circuit is an opamp with open look gain >>50k, so that ratio sets the actual gain. The bias network has no effect if the emitter of the NPN is bypassed as shown -- there is no AC signal on it at that point (the .1 uf being huge in the context of the speed of a pulse). There is, however, some DC feedback there, to help it stay biased in range as the supply voltage changes over 50% or so with a battery. This keeps the bias resistors from having any effect on the AC gain as a nice by product.
(I'm going to go find the schematic so I can see what I put in for the pnp emitter resistor, be back soon) OK, it's 10k/2k, which in the standard non-inverting opamp formula gives gain of 51 (with a hell of a lot of loop gain to make it frequency-flat). You can check that with an audio input and just scope both input and output. Should be dead on at 51 and not vary with supply voltage at all. I used the rather large 10k value as the only bad effect of the high impedance is to slow down the pulse a little bit when driving capacitive loads like coax, and then only on the transistor turn-off side of the pulse. During the onset of the pulse, the transistor drives as hard as need be (that negative feedback loop) so this impedance is different on the rising and falling edges -- this doesn't matter in this case, as the onset of the pulse from the tube is fast, and so is the preamp. During the back side, the pulse decays slowly, and so does the preamp. If you want to use tweaky words, it's an ideal "matched filter" for the actual signal. In this case, it lets the design draw lots less power, which is what I was after.
http://en.wikipedia.org/wiki/Weiner_filterTheres no need (and no desire) to match the Vbe's of the transistors, just bias the pair up right it all that's needed. Here, I specced a fast, low noise, low current "audio" transistor for the front end, and a regular old switching NPN for the output, because noise doesn't matter there, but output drive peak current does -- again, even a tiny cap, like a couple feet of coax, draws infinite current with 1/infinite risetime. I did it pnp input, to match the signal characteristics. The pulse turns that one on (well, both of them), which is quicker in transistors -- on time is faster than off time, which also matches the signal here. While turning on, full current from the tube is hitting that base, during the off time, only the bias R is turning it off, so the speed is asymmetric, again, just like the signal. Gives a little extra noise rejection without actually slowing anything down. The fact that the pnp collector is driving an AC short circuit makes it faster, as it doesn't have to charge any capacitor (internal to the xistor, or external) during the pulse. The input to the second stage looks like a short circuit to ground for AC.
As you can see, there can be a lot more to even a simple circuit (or would that be
especially a simple circuit) than meets the eye at first. This was all pretty well thought out.
I've been using variations on this circuit since about 1972, when I stole it from a Marantz tone control buffer... It's pretty good for many things in the mid low-level domain, as changing the various values can set it up for fast or slow, low noise for a given impedance and so on. If this guy doesn't do it, you can go to a more "heroic effort" but usually you don't need that.
Edit: Added a copy of the preamp schiz so you don't have to flip around to see what we are talking about here:
- edit: modified diagram with battery symbol reversed
- NewDPre.jpg (61.32 KiB) Viewed 10827 times
The protection diodes should be 1n4148 FWIW, unless you have low leakage versions of the germaniums shown.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.