Son of Ben Bias Trimmer Range

[terribleamps]

Hi all,
Feeling like a bit of a noob with the issues I'm experiencing with a pair of SOB builds on the bench - I've never built with J201's before, and was worried about the "fake JFET" thing.... I've got a multi-component tester and verified that the 10x thru-hole J201's I bought (from pedalpartsaustralia) were indeed legit, and was able to sort two trios that were fairly closely matched.

What I'm finding is that I've had to ratchet the (single-turn Bourns) 50k trimmers hard-to-right to get the voltage at drain - and therefore output level - up... both builds seem to be anaemic on the maximum voltage 'permitted' - on one build, I get Q1: ~4.1v max and Q2/Q3: ~3.2v. The other build is worse - can't get Q1 up past about 2.5v.

One of the two is essentially sounding "allright" - but I'm not really hitting what I think I ought to, and it seems really odd that I've gotta dial the trims all the way up. I've removed the 10uF caps on both boards to no effect. The trimpots are definitely 50k all the way through, and I'm seeing ~9.24v off the diode.

Thoughts? JFET's not quite up to scratch? Different value trimpot worth considering? I soldered the actives straight in, foolishly - have a desoldering pump and can carefully remove/add sockets/try the shebang again with the few other spares I've got and see if I get better luck....

 

[terribleamps]

So the contents of this thread made a significant difference - one of the two boards is now close-enough-for-jazz sonically, but Q2/Q3 still won't spike above about 3.6v with the trimpots maxed out. I can live with that - just strange....

The other board seemed to have a wonky Q3 trimpot, as it'll spasmodically change - it's the only one of the lot that offers a correct-looking range of voltage sweep - but after setting an acceptable 4v (I'm running audio through it via a TC Electronic Wiretap 'looper' - incredibly handy bit of kit for pushing guitar signal through!) - and getting a good sound - it'll suddenly jump up to 8-9v and you can hear it freak right out. I'm going to replace the trimpot - but the fact that both pedals are giving such strange performance with regards to taper/voltage range gives me cause for concern...

 

[Chuck D. Bones]

A few points of clarification:
The JFETs don't need to be matched.
The drain voltage (Vd) is a little low because the JFETs you got were "hot." This is a good thing. If you can get more from that source for a good price, I'd jump on it.
The target Vd are just guidelines. If it sounds good, then the bias is correct.
If the trimmers are full CW and you want Vd to be higher, reduce R5, R9, R12 to 4.7K as req'd.

 

[terribleamps]

Thanks Chuck! I'd hoped that might be the case. Sonically, the first of the two sounded great, and I was happy to abandon the "science says it should be this" in favour of "ears say it's nice-like"... my only real worry was that I'd fundamentally be risking something with long-term performance. I'm a valve/tube amp guy..... poor adherence to voltages rarely ends well in that realm

 

[Chuck D. Bones]

As long as you stay well under the absolute maximum ratings for semiconductors, they will essentially last forever. Look how long the Voyager spacecraft has been operating, non-stop: 43 years and counting. According to the datasheet below, even with an 18V power supply you can set the drain voltage to anything you want and it won't hurt the J201s.