A Thor with all J201's

[rock_god_dan]

Anybody tried this? I did a quick search and found nothing to this topic. I'm planning to build a Thor to get that Van Halen Super Lead all at 10 sound, but i can only get my hands on j201's. Would it sound overly compressed and buzzy? I'm thinking this kind of build would be really noisy, so please help? Any help will be greatly appreciated. Thanks!

Dan

[RDV]

Use sockets for the FETs, then if you don't like it you can change it.

IMO you'll like J201s fine.

RDV

[rock_god_dan]

Thanks! I'll try to build one and make a build report after. Hope the gain's still controllable and not buzzy..And hope it can do metal bwahaha (I'm not really expecting this  ) 

[km-r]

IMO, you might wanna change the source resistor of the first 2N5457 if you wanna fit in a J201... its a fetzer stage y'know...

[rock_god_dan]

IMO, you might wanna change the source resistor of the first 2N5457 if you wanna fit in a J201... its a fetzer stage y'know...

Uhhh... Should I bias it colder or warmer? Sorry, super noob here  Tried reading up on ROG's fetzer valve article but couldn't get through it.

[km-r]

the front end of the thor is a cascaded fetzer valve... try copying the source resistor of the J201 stage...

[stratcat]

I have a thor kit on the way from OLcircuits and its my understanding that this pedal is designed to be a faithful recreation of the SL which means its a medium gain pedal - perfect for crunchy rhythm tones. The ONE good clip of this pedal (Martymart) just has that classic Plexi bark NAILED. I think you might be barking up the the wrong tree trying to gain this thing out with 201's. Although its certainly do-able I have read just a ton of info on this site that j201's are ALL OVER THE PLACE in terms of gain and consistency from unit to unit and if you are a total noob it might be a bear trying to dial these suckers in. Plus the older thunderchief circuit that was the ancestor of the thor used 201's and the extra gain doesn't sound nearly as good to me as the lower gain Thor.

If you are looking for the boosted marshall VH tone then do a search on the BSIAB II pedal by Ed Guidry. BSIAB stands for Brown-Sound-In-A-Box. Its also a jfet transistor based pedal like the thor but it is  higher gain and get TONS of props from this board and alot of other guitar forums I frequent like the gear page, Musictoyz and Harmony central. I also have this pedal on order from CMATMODS as they make licensed versions of DIY pedal circuits. I plan on using the thor for AC/DC style rhythm with a boost pedal for leads and The BSIAB for VH gained out rhythm and leads.

Here some clips of the BSIAB. The first 2 are from some scandivian dude running a rat set for clean boost into the BSIAB into a music man amp (SS pre/Tube power amp). These are PHENOMENAL IMO altho it helps that my man can PLAY.

http://web.telia.com/%7Eu27817977/lidberg3.wmv
http://web.telia.com/%7Eu27818002/lick4.wmv

Here's one from the CMATMODS website
http://www.cmatmods.com/SoundClips/Brownie.mp3

Here's a Les Paul thru a  germino club 40 (marshall plexi clone) set mostly clean then the BSIAB kicks in
http://www.germinoamps.com/audio/9club40.mp3

Here's a couple lo fi clips that start with a Judas Priest Riff you may have heard before; followed by a white snake riff that was on the radio when I  had long hair and couldn't purchase alcohol legally; followed by a rendition of one the original brown sound riffs. These clips are more about showing the potential gain of this circuit than how good it might sound.
http://www.mrdwab.com/john/BSIABchords.mp3
http://www.mrdwab.com/john/BSIAB2clip1EQ.mp3
http://www.mrdwab.com/john/BSIAB2clip3EQ.mp3

You should get the idea by now.

[PerroGrande]

For a Fetzer stage, you really need to calculate the Idss and the Vp of your particular J201 and adjust the source resistor accordingly.  I agree with the previous poster -- the existing J201 stage is a decent starting point, though, if you want to wing it.

The calculation and Rs determination is discussed in the RoG article on the Fetzer valve, along with a quick & dirty way to compute Idss and Vp.

[mojo_hand]

As for J201s being "all over the place" in terms of specs, I can confirm that this is true.  JFETs are often that way, and J201s more so than most.  But that doesn't necessarily mean that they can't be used without testing.

In practice, there are probably two things you're measuring, although for reasons that will be clear in a moment, you really only have to test one, unless you're wanting to double-check.

Out of a batch of 107 Fairchild J201s I tested a couple of weeks ago, here is the breakdown.

1: seemed defective.  Tossed.

3 had Idss of around .2 mA, and those all had Vp of .47-.50
13 had Idss of about .3 mA, and those all had Vp of .54-.61
19 had Idss of .4 mA, all those had Vp of .63-.70
16 had Idss of about .5 mA, and all had .70-.75 Vp
21 had Idss of about .6 mA, VP was .77-.85
18 had Idss near .7 mA, all had Vp of .85-.91
14 had Idss around .8 mA, Vp was .92-1.0
2 had Idss of .9, and Vp of 1.0

So, as you can see, the distribution is fairly random, but Vp and Idss follow each other in a neat little continuum.  If you test one, you know about what the other is, with a high degree of certainty.

Even if you don't test them, and are itching to follow Danyuk's theory about keeping FETs in the triode range of operation, you can do reasonably well by assuming that they're all average.  The lowest Idss & Vp examples would need a source resistor of 2.0K (@ 9v) per Danyuk, the highest Idss & Vp would need 920 ohms.  The drain resistor would vary quite a lot more, but if people are setting that with a trim pot, it's impossible to go wrong.  Using those resistor values the gain would range from 3.8 to 9, with 101 out of 106 functional FETs falling between 4.0 and 7.4.  So you can probably use untested J201s with a 1.5K resistor with impunity.  And you can be pretty dang sure that your gain's going to be 5.7, + or - 1.7.

So it's not as dire as it sounds.  Yes, some have 4.5 times the Idss of others, but in a typical circuit, the difference in gain is almost always going to be less than 2:1.   Vp doesn't vary by as much as 2:1, either.

[stratcat]

All I'm saying is that if his goal is VH tone it might be easier to start with a higher gain circuit like the BSIAB than to modify a medium gain circuit like the THOR. Or just build both and see which one he likes better.

I'm getting both because I don't want to choose. And I have every intention of modding both pedals to suit my taste.

[rock_god_dan]

Yup that's the intention - EVH tone. I've already built a BSIAB2, and made a few mods, like removing the last two caps and the last two resistors from the circuit. Still sounds too nasal for my taste, and if I dial down the tone knob the sound loses its bite. Anyway, I just wanna build the Thor so that i'd have something to compare the BSIAB2 against, and I'd use 5457's anytime, but the only problem is that we don't have them here in my country. What if I substituted q1 with a j201 and q3 and q4 with 5458's? I'm not sure if q3 and q4 has anything to do with adding a significant amount of gain, though. 5458's have very low gain, although not as low as mpf102's. You guys think this'll work in creating a gain level closer to the original Thor?

Dan

[PerroGrande]

Mojo -- your results are very similar to my own with regard to testing J201's.  This is kinda the nature of the beast with JFETs.

1.5K with a trimmer to adjust the bias point gives one a statistical likelihood of hitting the triode region, at least for the majority of the operating range of a randomly chosen FET.