Alembic Stratoblaster boost / buffer output level question

[nocentelli]

Hi. I made an Alembic Stratoblaster JFET booster this weekend, and was well impressed: I've never really bothered with boosters in the past since I use mainly solidstate amps, but the boost from the JFET gives my ODs and fuzzes a really nice edge. The biggest revelation was the increase in clarity and brightness of the clean sound with the gain at minimum, presumably from the buffering effect (I do use a long pedal chain). I'm going to build two stratoblaster circuits into one box: one will be a stock stratoblaster [see schem] with the gain control and the other will be a unity-ish level buffer, with the gain contol removed and maybe a trimmer to set the level relative to unity. Here's the strato schem -



My question is this: Unity gain appears to be around the minimum gain setting on the 50k pot. If I remove the gain pot and cap entirely, can I just tinker with the gain level by adjusting the 12k source resistor value? I have no idea about bias issues as pertaining to JFETs - I know that BJTs can easily sound horrible, but I've vaguely gathered this isn't such an issue here. I'm using a 2N5457, btw.

[Mark Hammer]

The gain pot provides gain insomuch as it provides a more efficient path to ground for AC content through the 10uf cap than through the 12k resistor.  So, small wonder that a large portion of the rotation appears to provide little benefit.  So, several suggestions:

1) Stick an 18k-22k fixed resistor in parallel with the pot to reduce its maximum resistance from 50k down to the low-to-mid teens.

2) Use a 10k pot instead.

3) Use a 20k reverse-log pot, that will get you through the highestresistance portion of its range quickly.

Another idea is to keep the 50k pot, tie its wiper to where the 12k resistor meets the JFET.  Stick the 10uf cap to ground on one of the outside lugs and a .22-.47uf cap from the other outside lug to ground.  In mid-rotation (well, more realistically, the 10:00-2:00 range) you'll have unity, and increased gain as you rotate away from there.  Rotate towards the larger cap and you'll have full-range boost.  Rotate towards the smaller cap and you'll have gain for the mids and highs - a treble booster.

[PRR]

There probably should be a 1Meg-5Meg resistor gate to ground to establish gate bias when the source has no DC path.

In some situations, 33K in series with the gate is wise. (Reduces radio reception and protects against accidental high voltage inputs.)

For near unity-gain: remove 50K pot and 10u cap. Replace 22K with about 15K. Or maybe easier: tack a 33K 47K or 100K across the 22K.

[nocentelli]

Another idea is to keep the 50k pot, tie its wiper to where the 12k resistor meets the JFET.  Stick the 10uf cap to ground on one of the outside lugs and a .22-.47uf cap from the other outside lug to ground....  Rotate towards the larger cap and you'll have full-range boost.  Rotate towards the smaller cap and you'll have gain for the mids and highs - a treble booster.

That sounds great, I'm going to breadboard this asap and see if I like it better than the stock booster.

There probably should be a 1Meg-5Meg resistor gate to ground to establish gate bias when the source has no DC path.

Excellent, I've already put a 1M8 from gate to ground, as I'd seen it on another version schematic (maybe JD Sleep's "improved" version).

In some situations, 33K in series with the gate is wise. (Reduces radio reception and protects against accidental high voltage inputs.)

Ah, I see: I've seen 68k on some other schematics - I was wondering what that did. Tried it with + without, couldn't notice any different, but better safe than sorry.

For near unity-gain: remove 50K pot and 10u cap. Replace 22K with about 15K. Or maybe easier: tack a 33K 47K or 100K across the 22K.

Awesome stuff, this is what I was after. Many, many thanks!

[PRR]

> I've seen 68k on some other schematics

The original (tube Fenders) has two jacks and works both 68K in parallel when using one guitar. 33K is good for a single jack.

> Tried it with + without, couldn't notice any different, but better safe than sorry.

Does nothing until the night you do a gig near a radio tower or a truck-stop, near strong radio transmitters.

[Mick Bailey]

If you replace the 12k resistor with a 10k pot and connect the 10uf capacitor from wiper to ground you'll get a very effective variable gain, with a good graduated control that doesn't bunch up at one end of travel. I use this method quite a bit in this type of booster, though it does crackle a little when the pot is rotated. I've experimented quite a bit with different configurations and this is the one I like the most. 10uf can give too much flabby bass, though. In most cases 1uf is plenty - less gives an even tighter bottom-end.

A 2N5457 will bias a little differently from a J201, and FETs are bad for variable production tolerance anyhow. Measure the voltage from drain to ground - should be roughly half your battery voltage. With a 10k source resistance you may find with some FETs that you need to increase the 22k resistor - sometimes to as much as 33k or 39k. This has a beneficial effect on battery life (mine measure 125uA, which is great when they're built into a guitar), but increases noise slightly.

[nocentelli]

Great stuff, guys: I love this place. PRR - Point taken, I'll include it. Mick: I'll try the 10k pot: The potential for crackle doesn't really bother me, might even be a selling point!