Happy valentine's to everyone. Please remember to care for your missus or girlfriend between builds too...
11.12.2022 - Note that this version of this layout has been proved unverifiable. The schematic was up in Aron Nelson's forum, but we don't have access to this anymore. There may be issues with the Dragonfly's schem/design or with my layout - we just can't say at the moment. If anyone has a copy of the schematic, please link it in the comments. We sure would like to have a decent sounding BJT->JFET overdrive in the library.. Or someone could design a new one that works and sounds good - and i'll draw it?
Right, this is working in a very strange way. Without the JFET it works but it's fairly quiet, with a 2n5457 with the legs twisted it doesn't work but that what the pinout says, I've checked the scheme and your layout definitely uses the correct pinout. Here's where it gets strange, I'd I put the 2n5457 in with the legs as normal, dsg, then it works but sounds like it's poorly biased, if I take the gate leg out then it works great and it's got a decent volume! WTF is going on? I'm guessing that I've just screwed something up but I'm off out to a gig so I've not got time to investigate, any ideas?
ReplyDeleteI think it's just a biasing problem. Can you try breadboarding the 3K resistor going to Q2 drain? I believe a custom, suitable value there should gine you an answer..
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Sorry for the late reply dude, I was feeling a little partied out after last night :D I think you might be right as I used a 3k3 in place of the 3k, though it still doesn't explain why it's not working when I twist the legs of the jfet to match the layout's pinout (which is definitely correct) but when I leave the legs untwisted it works?!! I don't know, I've gone over the layout a couple of times but my hungover eyes may be playing tricks on me. Anyway I've just got an SM57 which is itching to be played with!
DeleteI breadboarded this with a J201 as the JFET, but could not get it to sound good even trying different values for the 3k resistor.
ReplyDeleteIt seems that some signal from Q1 passes through the 1.5k and the 3k resistors, bypassing the second stage. Just as Madferret found, the signal will still pass with decent volume if you pull out the JFET. Also, when everything is in place, the quietest part of the gain pot is somewhere in the middle of the rotation, rather than completely counterclockwise. I think there is some phase cancellation going on there.
Yup. Nine years without no-one getting this working. Can't even find the schematic anymore. I recall this was up on Aron Nelson's forum back in the day. That page is still cached on search engines, but i don't seem to have access to the pages.
DeleteHmm.. At a first glance today, one could try lowering the 100K from gain 1 to grouns. Maybe a 1K? And the 3K could get the voltage straight from the 9V rail instead of the same point that BJT collector gets its voltage. I'd try on a different types of FETs for q2 as well...
Sadly i can't seem to get to the schematic...
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I tried connecting the 3k straight to the voltage rail that solved the weird behavior on the gain knob. The JFET drain voltage was around 6.5 volts for both a J201 and a 2n5457. However, the distortion sounded kind of farty and blown out, even on low-gain settings. I raised the 100R resistor off the JFET to 390R and then 470R, which helped a little. Rolling back my guitar's volume did clean up the sound to get a good light OD (I tried both single coils and humbuckers), so maybe some series resistance or a voltage divider on the input would work?
DeleteSomeone with enough patience might be able to tweak this into something good, but I do not know if we can really verify this. I cannot find a schematic either, or even a demo to compare my results with.
Hmm. I think i should just take this post down as there's little point in keeping it..
DeleteMaybe if i add a note that the schematic is needed? So if anyone has a copy, they could link it to us?
There could be issues with the schem or with my drawing. We simply can't know it..
Idea of a simple bjt->jfet od is quite interesting and could be a nice thing to have in the library. Sadly this one - at least as it is, won't do what is promised..
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DeleteHi Miro,
DeleteHere is a link to the original Valentine schematic by Andy Carrel (Dragonfly).
https://drive.google.com/file/d/1uOzf2501pQAIq3PXDosOgE6Yat4r6f2P/view?usp=drive_link
I quickly compared the schematic with your layout and they appear to be 1:1 and also don't see anything wrong with the circuit topology.
I thought the schematic could clarify the mystery, but I think it still remains unclear...
Dude, Thank you so much for everything and take care.