EA Tremolo - Why does it get more depth when you remove the ground from 68k?

[patrick398]

Hello,
first post so not 100% sure of the etiquette but this seemed like the right place to come to and i couldn't find anything else relating to my question (even though there's a lot of stuff on the ea trem generally)
It's pretty simple really; i just put an EA Tremolo on breadboard which works fine and sounds great (always a bonus) and i was fiddling around trying to get more depth out of it and found that if i disconnected the ground from the 68k resistor coming off the depth pot it became much choppier, which i like. I used a little switch so i can change between the normal depth and the much choppier tremolo but i just can't work out why disconnecting the ground does this. Like i say it's not really a problem but i'd just like to understand why its happening.
Thanks a lot! Hope you're well!
Patrick

P.S this is the schematic i'm working off:
https://scfxguide.files.wordpress.com/2014/02/ea.png

[merlinb]

Because you've basically disconnected the bottom of the depth pot, so it no longer reduces the size of the LFO signal. The LFO signal is now being injected straight into the JFET at essentially the maximum available level.

[patrick398]

That makes sense thanks very much!

[duck_arse]

patrick - you can shift your pot range by shifting the 120k and 68k resistors. swap them for a start, see how you go.

[PRR]

Like they say: R10 VR2 R11 are a "volume control" but for the wobble instead of the guitar. Open R11, the network does not even try to turn-down. (The Gate of Q2 is essentially infinite impedance, so 0-250K or more doesn't change the drive.)

The published values work well-enough for most parts and most users. They can be changed to suit you and your parts. Turn VR2 full up, trim (change) R10 for the MAXimum wobble you could ever want. Turn VR2 full down, trim R11 for the least wobble you can hear (or would use). Some interaction so repeat a few times.

If you want CHOP, you may not also be able to get a smooth range of medium wobble depth. Your idea of a switch is probably the best way to get WOBbleWOBble without major re-think.

[Transmogrifox]

If you really want it to chop, you might look at this:
http://cackleberrypines.net/transmogrifox/rEAgeneratedTremolo/demo/cleantrem1.mp3
Rev B goes deeper than the demo, so it can get even more choppy than what you hear.
There is a rationale document if you visit the page for it:
http://www.cackleberrypines.net/transmogrifox/rEAgeneratedTremolo/pages

[patrick398]

Great, thanks so much for you're responses. Once i finish up my fuzz i'll get this back on the breadboard and start fiddlin'

[patrick398]

Actually while i'm here, i've just had a look at that schematic (looking forward to it already) but realised i don't actually understand P1. I don't think i've ever seen that before and it's confused me enormously 

[PRR]

> don't actually understand P1

If you mean, on Transmogrifox's diagram....

"Plug" which consolidates some of the off-board connections (input, output, power, ground). Makes it convenient to build the board alone, wire the case without the board in the way, then plug them together. Not always done in small DIY, pretty common in mass production.

[Transmogrifox]

As Paul said, P1 identifies off-board connections. 

On a physical PCB this would look like a header that you would plug a wiring harness into which would be wired the jacks and batteries.  Alternatively it might be left as a row of through-holes into which the respective wires to off-board connections are soldered.

On your breadboard you can disregard P1 because it isn't a meaningful symbol in that context (unless you want to plug a header into your BB and connect to it with a wiring harness).

[patrick398]

Oh i see, that's pretty nifty...might help me tidy things up a bit.
Thanks again!