quick question: EA trem JFET substitues

[smoguzbenjamin]

I just got parts for the EA tremolo, but the part shoppe didn't have either a J201 or a 2N5088. I opted for two 2n2222s because they were only 20 cents each, but I got home thinking "shit! biasing!" :lol: So here's a question for the EA trem people: Can I substitue Q2 and Q3 with 2n2222's? Or does Q2 have to be a JFET and Q3 have to be a 2n5088?  :? I haven't breadboarded this out yet, going to do so in a few minutes...

[petemoore]

You need a gainy transistor for the LFO [I understand] this from the guy with two non EA tremolo builds...fohh...
 You could maybe try 2n2222 'darlingtransistorizing'  that slot...can get yer gains up that way I think...makin yer own darlington...Runoff Grooves get expl. and schematic how to...in the Bazz Fuss article...

[ErikMiller]

The JFET does have to be a JFET, but I got the oscillator working just fine with a 2N2222.

It's not so much the part number, as I understand, but the gain. If your 2N2222 doesn't work, a 2N3904 should be more likely.

I believe I subbed a Radio Shack MPF102 for the JFET and it still worked as a trem. I got it all working with the correct parts and then started swapping in the other stuff to see what would happen.

[smoguzbenjamin]

Hmmm. I just breadboarded it, no trem and a reduce in volume. Playing with the trimpot doesn't make any difference.

I think I will buy perf tomorrow and perf this one, witch sockets for the trannies. My breadboarding skills are mediocre to say the least. Hey hang on! I still have a 2n2222 left! lemme make a darlington and see if that works.  

[R.G.]

Any transistor with a current gain of over 200 should be fine for the LFO transistor. Darlington-ing works, and is actually a good thing, because one could then raise the impedance of the phase shift networks by a factor of ten to get smaller caps in the phase shift network. 2N4401 is also a good choice, as it's optimized as a bit more amplifier and less general purpose than the 2N3904. Once again, though, 2N5088 is my choice.

The JFET must be a JFET. It is possible to do the tremolo portion of this with an NPN transistor (as is used in the Pulsar tremolo and the Diaz Tremadillo) but that makes the distortion worse and changes other things in the design.

Long channel JFETs make the best variable resistors. These are usually higher Vgsoff devices. 2N5457 and MPF 102 ought to work well. J201 is a switching JFET, optimized for small Vgsoff. This will work with much smaller Vgs changes but is less predictable as a variable resistor.

[Gringo]

Long channel JFETs make the best variable resistors. These are usually higher Vgsoff devices. 2N5457 and MPF 102 ought to work well. J201 is a switching JFET, optimized for small Vgsoff. This will work with much smaller Vgs changes but is less predictable as a variable resistor.

Now, that's really useful for us people having to sub almost every semiconductor in a circuit. It's gonna help me a lot!!

I have some bf245 i measured and they have even higher Vgsoff than the mpf102s, would they work too? How would they perform in a phase 45 circuit?

Thanks RG!

[smoguzbenjamin]

The long channel thing also explains why the 2N2222 isn't doing the job. Nah, time for some substitutes!

When I FINALLY get my site up & running I'm going to make a big table for substitute parts :mrgreen:

[smoguzbenjamin]

Riiight. I found out why my EA trem isn't working.

The LFO isn't oscillating. I attach the battery, and the LED blinks once,but fades while trying to blink. My guess is that the 2n2222 isn't gainy enough. So I looked in my parts bin, and found a bc547c.
The 2n2222's Hfe measured at 194
the bc547c's Hfe measured at 595

But putting the bc547c in the LFO didn't make a difference. What am I doing wrong this time?

[gez]

Take some voltage measurements for the LFO and post them.  Double check all your connections

[smoguzbenjamin]

It's on a breadboard, I allready triple-checked all the connections. Lemme take some measurements:

C - 6.98v
B - 0.05v
E - 0.00v

Someting interesting happened though: The collector is connected to the outside shell (it's a metal head tranny) and when I short-circuited the Emitter and Collector, the LED lit up, obviously. My guess is it's misbiased... Could I maybe reduce the 2M2 resistor over the Collector and Base to make this work?

[gez]

It's on a breadboard, I allready triple-checked all the connections. Lemme take some measurements:

C - 6.98v
B - 0.05v
E - 0.00v

Someting interesting happened though: The collector is connected to the outside shell (it's a metal head tranny) and when I short-circuited the Emitter and Collector, the LED lit up, obviously. My guess is it's misbiased... Could I maybe reduce the 2M2 resistor over the Collector and Base to make this work?

The base should be about a diode drop above the emmiter (0.6-0.7V).  The fact that you only have .05V there suggests that something is wrong!  

Measure for DC voltage accross the 15k resistor.  It could be that the 1u cap between it and the 2m2 is leaky, resulting in the 15k forming a divider with the 2m2 and pulling the base down towards earth.  If this is the case then the reading of .05V on the base would make sense.

[gez]

Also check that you've got that part of the circuit wired up correctly!  If you're using polarised caps check they're in the right way round.

[smoguzbenjamin]

Looks like I %^&*ed up anyway, the 15k resistor was in the wrong place. Now everything's fine, except for that the LED doesn't pulse... Well at least it does pulse in the beginning when I first connect the battery, after that it stabilises and doesn't pulse at all. As if it oscillates until the caps charge up.

The readings now the oscillator is wired up correctly:
C - 3.62v
B - 0.59v
E - 0.00v

I tried substituting the 2n2222 with a bc547c which has an Hfe that's 4 times higher, but no banana, and no oscillation... :roll:

[gez]

Well, at least the voltage readings look ok now!  Other than double checking the phase shift network, you're going to have to sub in another trannie.  People have had this problem before, but when they get the right one it kicks into action (be patient).

[smoguzbenjamin]

I have two weeks to kill (X-mas hols) so patience isn't a problem, I can always sit down and play guitar through the boxes I allready have Just the problem of Madame Girlfriend needing attention now and again and also the fact that shops close at christmas. I'll go to the shop on moday and ask em for the highest-gain transistor they have... In the meantime I'll be substituting every transistor I can find :twisted:

edit:

PS cheers for your help, gez!

[gez]

Smoguzbenjamin, how long have you been speaking English?  I'm really impressed!  Although I speak a bit of German (and I've been learning for years!), it's nowhere as good as your command of English!

[smoguzbenjamin]

My dad's english, so I've been speaking english all my life That's almost 16 years now. My mum's dutch, and my dad used to work for ESA, he's retired now due to heart problems. He's also a radio-amateur but as to how a fuzzbox works he's stumped. Also because of a tense situation at home I've been living at my girlfriend's place with her parents for almost a year now. :roll:  Alas, bad things do happen.

Wow that was the shortest summary of my life ever :mrgreen:

edit: call me Ben

[gez]

My dad's english, so I've been speaking english all my life That's almost 16 years now.

Well that explains a lot Ben, I wondered where you got all the Anglicisms (if that's a proper word!) from.  

I'm half German but still can't speak it very well (don't know what went wrong there).  I'm told I speak it without an accent, it's just a pity it's all gibberish!  Most Germans look at me as though I'm a retard when I talk to them, till I explain that I'm English...then they really look at me as though I'm a retard!!!

[smoguzbenjamin]

Don't get me started on Krauts :mrgreen:

[gez]

Don't get me started on Krauts :mrgreen:

Don't get ME started on them!!!