wobbletron build/any mods?

[donald stringer]

I have this on the breadboard and was wondering if anybody besides myself has had trouble with the mpsa18 on the input? Now I had a good clear signal upon startup but no wobble. I had to bypass that trans. and go to the fet to get  any vibrato. For what it is it sounds pretty promising.

[Processaurus]

I made this on a breadboard a while back, I think I had to put in another .1uF de-coupling cap between the emitter of the MPSA18 and the drain of the FET to get it to work right, if memory serves.  When it did work it was subtle, until you turn the speed up.  More complex phasers seem to do the phase vibrato thing better.

I was trying it as an add on to the EA trem, because they use the same LFO.  I didn't actually get around to trying both together though, who knows, it might have been good...

A version of this circuit that works like the old fender tremolo would be neat, they did a thing where the LFO just turns down the highs, rather than effecting the overall volume.  You could do a weird opposite version of that with this circuit, where the LFO turns the bass down, if you took your signal that goes to the drain on the FET from the collector of the MPSA18 rather than the emitter, along with the other signal that goes to C1.  Make C1 tiny and the cap between the MPSA18 and FET big.  Maybe put a small resistor (equivalent to the drain-source resistance of the FET when its fully on) in series with C1.  That would avoid a constant treble boost that may happen otherwise.

Or figure out how to pan between Hi passed and Lo passed versions of the input signal.  I tried that with a stereo trem and a mixer w/EQ and it sounded cool.

[slacker]

I built it a while ago with a BC108 instead of the MPSA18 and it worked fine. If you add a switch to disconnect C1 you get a nice tremolo, basically an EA trem. I also tried 2 of the audio stages in series with different sized caps for C1 to get a low budget univibe type effect. It worked but it had too much LFO thump to be useful.

[Processaurus]

I built it a while ago with a BC108 instead of the MPSA18 and it worked fine. If you add a switch to disconnect C1 you get a nice tremolo, basically an EA trem. I also tried 2 of the audio stages in series with different sized caps for C1 to get a low budget univibe type effect. It worked but it had too much LFO thump to be useful.

The throb is because the design as posted pans between an ac coupled signal (through C1) with a signal with a DC level (through the FET).  Hence the need for decoupling before the FET gets mixed with C1, or the LFO will show up on the output.

[burnt fingers]

I just laid this one out on a perf board.  I havn't soldered anything up yet.  I will probably get to it this weekend.  I'll post my results.  As with everything I build, I socket most of the caps and the trannys so I'll probably be tweaking in the coming weeks. 

Scott

[burnt fingers]

I just finished this today and I seem to have the same problem.  The signal goes through, but no wobble.  Then I touched the back of the board between the input cap and the first tranny and I got a great woblle.  I spent another 20 min trying to debug but everything loosk right.  Donald Stringer, did you ever get your's up and running. 

Also, the value of the input cap is not on the schematic, does anyone know what the proper value is.  I tried a bunch of different ones but Don't know if that is contributing to my problem. 

Scott

[donald stringer]

One thing I did was the recomendation of processaurus [decoupling cap] please read his post. The next thing was my own idea. I put a 100k trim on the collector of the mpsa18 and turned it until I got sound.I tried a 10k trim but that didnt work.After everything was said and done the sound was allright but I never built it. If I had my drothers I would rather make transmo"s modified ea trem.One other thing the wobbletron is not an vibrato. Let me know if you get it up. I might have some more notes on it laying around.

[burnt fingers]

I'll let you know what happens.  I'll try some more debugging tonight.

Scott

[burnt fingers]

Yahhhooooo success.

I finally got the wobbletron working.  I replaced the j201 with an nte457 and the first mpsa18 with a mps2369.  It works and I get a decent wobbely tremolo somewhat like an old magnatone amp but not as extrem.  It's about what I expected and Im gonna play with the caps to see what kind of variations in the sound I can get.  I'll put together a schem with the changes that I made.

Scott

[donald stringer]

I will be looking for the schem, maybe I "ll try it one more time.