Altering layouts that use 2N2646 Uni-junction transistors, for 2N6027.

[Beaker]

Hi guys,

Given the eye watering price of 2n2646 uni-junction transistors, coupled with their known flakiness, I'm just wondering how feasible it would be for someone to post up modified layouts for any pedals that use that use them (Vox Repeat Percussion, Earthquaker Devices Hummingbird)?

Replacing the 2n2646 with a 2N6027 programmable transistor apparently only needs one resistor value to be changed, and one resistor added to the layout for it to work, as long as you accept some leg bending on the transistor, as the pin-out is different. Of course the layout would need more re-jigging to remove the need for leg bending.

Many clone builders are already using these 2N6027 in preference to the 2N2646, possibly as a result of price and availability - they are MUCH cheaper, but the word is that they are much more reliable, and with less tendency  to cause ticking.

Unfortunately I'm stuck for any thing more detailed than this at the moment.

[IvIark]

I bought some 2N6027 specifically for this and have a schematic for a pedal which is based around the VRP with a 2N6027 so I will be able to see how it has been implemented.  Can't promised when it will happen because I am mad busy with work and other home life things, but I'll certainly do it at some point in the near future.

[Beaker]

Thanks a lot IvIark. I really appreciate all yor efforts.

Maybe someone else knows how to do this and might want to help you out?

I always feel bad that my knowledge of electronics is so poor that I have to rely on other people to do the hard work for me.

[Heath]

This is something I've been wanting to figure out myself...

I found this:  http://moosapotamus.net/images/Skippy-v1-scheme.jpg

I'm thinking that since the Skippy pedal is based off of the VRP pedal, I might be able to stumble onto something workable.

So far it appears that the only real changes are increasing R9 from 1k to 2k, and adding R12 (3.9k) between the K and G legs of the 2n6027 transistor... that and making sure the legs of the 2n6027 are placed correctly...

I'm thinking of testing it out by putting this all on a little piece of vero with three leads mimicking the 2n2646 legs and dropping it into a known good working VPR or Hummingbird.  Problem is, I've only got one and it works like a dream and I'm terrified of screwing it up with my ham-fisted experiments.

Of course I'm typing this at work with nothing in front of my to reference, except that link, so I may be talking out of my ass, but I'll be damned if I'm going to pay the vampires on Ebay what they're asking for 2n2646s.

Anyway, I'll try and remember to post any interesting results here.

[Heath]

I also found this:

"MU4892's also work without modifying the schematic, but they're harder to find, so I assume they're expensive."

I couldn't find this anywhere.  



The following info sounds simple enough.  I can't wait to get home and try it out:

(From R.G. on DIYstomboxes regarding 2n6027s)
"You can sub them into the existing circuit with the following mods:
1. cathode goes where B1 was on the UJT
2. anode goes where the B2 was on the UJT
3. 33R resistor may need to be shorted (that is, made 0 ohms)
4. 1K to B2 changed to 2.7K
5. add new resistor 4.7K from B2 to ground
6. gate goes where E was on the UJT

I've tried several 2N6027s and they all worked. "

[Heath]

Well... I made up a little adapter thingy



and made a couple of changes to the circuit...

Shit don't work.  I think I screwed the circuit up because the one and only 2n2646 transistor I have left doesn't work in it now either.

TobyCat couldn't get it to work either



It's 2am (work at 6am) and I'm frustrated.  I'll try this crap again later with a properly working circuit.

[Heath]

So far I've been unsuccessful.  I've got one more idea to try out before I call it quits.

What I did so far is to build a fresh Vox Repeat Percussion pedal (Modified II)

I socketed the 1k resistor and 33r resistor mentioned above.

I added a row between the 68n cap and the 2n2646 transistor and created sockets for a 4.7 k resistor from B2 to ground, but did not populate it.

At this point the layout is exactly like the published layout with the exception of the unused extra row I added for later.

It works perfectly with the 2n2646.

I removed the 2n2646
replaced the 33r resistor with 0r link
replaced the 1k resistor with a 2.7k resistor
added the 4.7k resistor from B2 to ground

I then installed the 2n6027 transistor with the A leg where the B2 leg was, the G leg where the E leg was, and the K leg where the B1 leg was.

I got nothing but a non-tremolo signal.

I set it aside thinking "hell with this shit" but as I've been sitting at work thinking obsessively about it, I think I might have placed the 4.7k resistor on the wrong side of the path to/from the A leg of the transistor...

I'll test that in a bit before just turning it back into a spare 2n2646 VPR

[johnk]

don't know if this helps but you can test a UJT with a multimeter:

http://www.circuitstoday.com/testing-a-ujt

http://www.sentex.ca/~mec1995/gadgets/ujttest.htm


on a hummingbird, or a VRP you can replace the the 2N2646 UJT with a 2N6027 PUT. G goes to where B2 was, A goes to where E was, and K goes to where B1 was. then replace the 470R resistor (on a hummingbird) that connects to B2 with a 2K resistor and connect it to G, then add a 3K9 resistor between legs G and K. if you use IvIark's layout you just have to twist the pins on the 2N6027 so legs A & G are reversed and insert K into the bottom pin of the socket. i.e the flat side of the the PUT faces in the same direction as the 2N5457 below it.

[Heath]

Yep, Mr. RG must have had a brain fart when he was typing the pin-out relations between the two transistors.

The upside is that I now have 3 working VRPs and can make as many more as I want with the 2n6027 transistors.  I bought 25 so watch out world!

[Beaker]

Thanks Mark, John and Heath for all this, however I an now a bit confused. Can you post up definitive instructions on how how to do this?

[johnk]

Beaker wrote

Thanks Mark, John and Heath for all this, however I an now a bit confused. Can you post up definitive instructions on how how to do this?

I thought that my instructions above were pretty clear, but if you mean that you want to see a vero layout using a PUT instead of of UJT here's how to mod IvIark's layout to do it:



a few other mods:

I changed the 1u (Mode Sw3 cap) in mine to a 2u2 to slow it down so the mode speeds would overlap since there's a pretty large speed gap in the stock unit.

I changed the 22n cap on Q2 to a 68n to drastically reduce the ticking without effecting the overall tone much at all.

and finally, I removed the ground link on the vero and use star grounding (ground all of the grounds to a single point) to also help reduce any ticking issues, which means that you could build it with one less column if you wanted to.

[Beaker]

Thanks John, that is amazing. Yes your instructions were clear - the layout confirms it. My confusion came about over the leg twisting issue.

Many thanks also for the suggested mods. I have been asked to make another of these pedals for a friend, so I will try them out.

[IvIark]

Nice one John.   I've got a schematic for another trem which uses the 6027 so I'll do that (after I've finished your Kraken)

[johnk]

IvIark wrote

Nice one John.   I've got a schematic for another trem which uses the 6027 so I'll do that (after I've finished your Kraken)

what's a Kraken?
i'm waiting for the Mutron V.

[IvIark]

Yes but you said "or, better yet, maybe madbean's Kraken" so I started that one

[johnk]

cool!

[johnk]

okay, so my 2N6027's got here today and I tried it in IvIark's hummingbird modded for a PUT like I posted above. it works the same, but the funny thing is, it actually ticks a little bit more with the 2N6027 PUT than it does with a the 2N2646 UJT so I went back to the 2N2646.

[reggy1]

Necro post, I know, but just a note for any future builders.
Thanks for your vero, johnk.
I noticed a small discrepancy between your verbal description of how to adapt the UJT layout to a PUT one.
You said "..... then replace the 470R resistor (on a hummingbird) that connects to B2 with a 2K resistor and connect it to G.....". The layout shows the 2k resistor connecting to A of the PUT.
When building mine I connected it to G and everything works well with no audible ticking.
Future builders might like to take this into account.
Thanks again.

[reggy1]

Necro-post #2 for future builders.

47k resistor shown going from ground to E of Q2, should go to B of Q2. Seems to make Depth control much more functional.