Fetzer Valve DB (Dumb question)

[jimpajohn]

I'm a newbie - I just built a fetzer valve boost from run off groove. I really like it, but I want more output (db) from it. Can I coax more volume out of the JFET by increasing/decreasing component values? I know I could build another one in series, but I would like to avoid that at the moment. Any help would be appreciated.

[StereoKills]

Try playing around with the value of the source resistor (1k5). Lowering the value should increase the dB output.

[jimpajohn]

Thanks for the reply StereoKills! I'll try that.

[kurtlives]

Put the 1M resistor before the 68K resistor.

[zambo]

change the output volume pot to a 250k or a 500k. I think thats the easiest.

[mrmoo1337]

I'm a newbie - I just built a fetzer valve boost from run off groove. I really like it, but I want more output (db) from it. Can I coax more volume out

I'm assuming you're after 'clean headroom' here?  As in more volume without increasing the distortion? 

keep in mind there's a limit to how large of a signal you can feed into your amp before the amp starts clip the signal and adds distortion.  Perhaps you can just turn up your amp?

or alternatively, are you already driving the crap out of your amps first stage and giving you some nice crunch, and you want to bring this up from crunch to the wickedest distortion level imaginable?  In that case you'd be better off with a pedal that clips your signal along with boosting it to give you the MOAR DISTORTION goods

[sault]

Okay, so let's look at the numbers:

9v supply, Jfet has a rating of -1.5v Vgs. So, you'll want the voltage drop to be no more than, say, 7 volts. With 1 mA, that's what, 7k? With more realistic ratings (-0.8 Vgs and 0.6 mA Idss) the Rd is more like 11k, with a 6-ish volt drop. Let's say you can get a 7 volt drop by selecting your components by hand - you've gained about a decibel. Now, let's say you double your power supply, so you're running 18v into your Fetzer Valve. You still need about 2 volts in reserve (call it 3 to hedge your bets), and you're looking at as much as 15 volts p-p. That's a little over twice as much, so you're looking at 6-7 dB increase.


So :

The answer is that if you want a significantly higher output, you need to raise your power supply. Carefully choosing/measuring your components may give you another decibel or so over the Fetzer, but it's not a large difference, and you still need to be mindful of component tolerances. A +/- 20% resistor, in this situation, could drop your jfet out of saturation mode...

As far as the other suggestions :

Putting a 1M resistor in front of the 68k turns the input into a 1M | 1M voltage divider, meaning that the signal is only cut by half. This will put a hotter signal into the Fetzer, but if you're already pushing a hot signal, it will just add distortion, not increase volume output.

Changing the value of the source resistor can help you dial in that maximum voltage drop, but it will also change your bias point. This can mean more gain, but it will be asymmetrical gain, ie, distortion.

Raising the value of the output pot will help present a stiffer load to the Jfet, which will help keep it from losing voltage on the output. The difference between the two is minimal. You do raise the output impedance, which isn't always desirable - it will interact with cable capacitance.