EA Trem debug?

[Cardboard Tube Samurai]

Hi guys, I've built an EA Tremolo from the GGG layout but it's being a kill-joy. The preamp side of things is working great and sounds pretty nice, but there's just no trem. I'm getting no voltage at Q2 - supposed to be 0.0v on source and drain, with variable voltage at gate. I've done the mod for the flashing LED. Any suggestions?

[George Giblet]

Is the LED flashing?

If not flashing check out the oscillator section.

If it is flashing check out the depth pot and the JFET pinouts.

[Cardboard Tube Samurai]

No flashing, no readings at all on depth pot and the JFET pinouts have been checked a couple of times. I've even replaced the JFET, just in case.

Excuse my ignorance here, but which part/section of the circuit is the oscillator section? I have narrowed the fault down to a part of the circuit that is most likely what you're talking about but I'm not sure what to do there because everything seems to be in order

[George Giblet]

> which part/section of the circuit is the oscillator section?

That's the one transistor circuit that connects to the depth pot and the LED.   You problem is around there.  Don't worry about the JFET or the other part of the circuit until you get that LED flashing.

First step would be to disconnect the clockwise tab of the depth pot (pin 3).

Takes some voltage measurements on the base and collector of the oscillator transistor, and check you have the pinouts right.

[Cardboard Tube Samurai]

Pinouts are right. With Lug 3 disconnected from Depth pot, B = 0.53v and C = 6.34v.

I'm using a 5mm Pink LED, vF typ = 3.5v, If = 30mA. I don't know if anything (ie: resistor) should be changed as there is no information as to what voltage is going into the LED. Could this be causing issues?

[George Giblet]

> Could this be causing issues?

It's quite possible that would stop you seeing the LED flash even if the oscillator is working.  I thought you might see *something*.  The easiest thing to do is just stick a red one in there until you have got it working.  As soon as you see a flashing LED move onto other parts of the circuit.

> B = 0.53v and C = 6.34v.

The base voltage looks a tad low to me.  The collector voltage looks a tad high but not too bad (a low base voltage would a low collector voltage).

With the pink LED....You could try shorting the junction of C5, C7, R11 (GGG schematic) to ground then wait about 30 second and see if the voltages change.  If the unit is not oscillating then the voltages won't change - in which case check the speed pot wiring.  If it is oscillating the voltages will change a bit.

[Cardboard Tube Samurai]

Changed LED, still nothing. Shorted junction, voltages stayed the same. Speed pot wiring seems to be fine.

As for voltages being slightly different... I should also have mentioned that I am using a BC549 instead of the suggested 2n5089. From memory, they are pretty much the same but the BC549 might be slightly higher gain

[George Giblet]

It seems like the oscillator isn't working then.

The BC549 should work fine.  Are you sure you have got the pinout correct?  IIRC, the  C and E are reversed on the BC549 compared to the 2N5089.   That would explain it.  It would also explain why your base voltage is low.

[Cardboard Tube Samurai]

Well gosh-darn! Evidently the pin-out I looked at was actually a top-view, not a bottom-view. You were right, the pins were back to front. I checked it a few times, but the datasheet was not clear as to which perspective it was being looked at from.

Thanks for all your help George Giblet, you are worth your weight in gold!

Now I just have to look into making it a more enthusiastic tremolo

[Cardboard Tube Samurai]

So I've been looking around for ways to make this thing a little more "enthusiastic" but to no avail. It is far too subtle an effect for my likings and I want something with more balls to it. Is there any way anyone can think of to boost the effect? The preamp side of things is great, but the effect is... I don't know... too shallow?

[gez]

There's a resistance chain from the LFO to the JFET that controls the modulation.  By messing with this you can squeeze some more depth out of the circuit.  What needs tweaking where?  If you can find the original EA trem article (it's online somewhere) it'll tell you.  Alternatively, link to the schematic you're using and someone here will advise you.

By the way, everything that could ever go wrong with this circuit has been dealt with extensively, it's all in the archives.  You don't even need to read through pages and pages as there's only ever about 3 things that people screw up on.  I'm mainly saying this for the benefit of others who might be contemplating building this in the near future.  If it doesn't work first time, it's all there folks! (including mods/tweaks)

[squidsquad]

JFETs can be *all over the map* in terms of gain & efficiency.  And so...the depth can vary a lot.  You can try a few other JFETs...or try piggybacking 2 of them on top of each other...connecting D to D, G to G, S to S.  I got great depth doing that.

[Cardboard Tube Samurai]

JFETs can be *all over the map* in terms of gain & efficiency.  And so...the depth can vary a lot.  You can try a few other JFETs...or try piggybacking 2 of them on top of each other...connecting D to D, G to G, S to S.  I got great depth doing that.

Ooh, I might just try that. I don't have a vast range of JFETs available to me, so this could be a good option. Cheers, I'll let you know how I go with it

[George Giblet]

Based on the GGG schematic, reducing R7 can compensate for JFET variations to some degree.  (Increasing C4 to 1u might help a small amount on low speeds.)

[Cardboard Tube Samurai]

I made it sound awesome! I was using MPF102 JFETs. I tried piggy-backing them... no improvement. I lowered R7 to 1k5... only very mild improvement. I realised that I actually had some 2n5484s lying around so I threw them in to replace the MPF102s... AWESOMENESS!!! This is a really nice sounding tremolo.

Moral of the story = MPF102s don't do the job.