ROG/OLC "Eighteen" pedal problem

[grolschie]

Hi guys,

I have built a couple of these. My own unit **was** sounding great when I first built it. But currently not so. When I hit the pedal with a strong signal (i.e. from my humbuckers) the pedal seems to create a swell-like effect. As the note decays the pedal seems to function as normal, but the initial attack of the note farts out to a much lower volume than the rest of the note. This effect is more obvious on the bass notes and obviously when played hard. So currently, the pedal is no longer usable to me. :-(

Pedal placement affected this alot. Previously it would do this when my Guyatone ST2 compressor was immediately before the 18 (even when the Guyatone was by-passed), but now it does it even by itself (when it's the only pedal between my guitar and amp). My pedal didn't always sound like this. Is there anything obvious that I should be checking?

cheers,
grol

[tcobretti]

Have you rechecked the voltages on the jfets?

[spudulike]

Battery voltage ?

[wampcat1]

yep, that sounds like a bias problem...tweak the trimpots on your jfets.
bw

[mars_bringer_of_war]

To the issue of tweaking the jfets, I'd say use an external power supply. Tweak for that supply, and use only that supply. I've found that if using a battery, I have to re-set the fets not only for a new battery, but sometimes when the battery voltages is partially depleted. When using a power supply, I haven't needed to re-tweak at all.

[grolschie]

Thanks all. I have been using the same Boss PSA-240 (NZ version) to power all my pedals. I never use battery. Maybe I need to adjust the bias when all pedals are in my chain?  I know how to bias it, but have no idea what the other voltages mean on the other pins.

Does the bias on JFET drift over time? Do they wear out?

[mars_bringer_of_war]

There seems to be some evidence that the bias will drift. As far as wearing out, you probably wouldn't need to worry about that for decades.

[grolschie]

Thanks for that. I will post back with voltages when I get a free moment. Pity about bias drift - that makes it hard to sell a production pedal, right? Not that I sell pedals.

[mars_bringer_of_war]

Thanks for that. I will post back with voltages when I get a free moment. Pity about bias drift - that makes it hard to sell a production pedal, right? Not that I sell pedals.

I think that bias drift is very slight over time, but ask the big guys around here, they know far better than I.
Are there many production pedals that use trimmers for jfet bias? If not, we know why! 

[grolschie]

The RunOffGroove page states:
J201  Q1       Q2      Q3
D      4.61      6.86    4.63
S      118mV  2.19    29mV
G      0          1.63    0

Mine shows:
J201  Q1       Q2      Q3
D      4.51      4.57    4.52
S      0.18      2.18    0.3
G      0           0.59    0

I guess my 0.18 and 0.3 are 180mV and 30mV respectively? The pedal used to sound ok with all FETs biased to ~4.5v.

Anything else that could be wrong?

[grolschie]

Now it sounds really crappy!

MP3 (230k) of the Eighteen spluttering (playing softly -> harder -> softly -> decay):
http://s22.quicksharing.com/v/744759/eighteen.mp3.html

Jpeg picture (27k) of waveform (arrow marks start and end of playing)
http://s19.quicksharing.com/v/6506326/eighteen_wave.jpg.html

When playing really gently it's ok. But playing harder it craps out. Fades back in as a play softer. Bizarre!

[bajaman]

pop it over next week - I'll have a look at it for you
cheers
Steve

[grolschie]

pop it over next week - I'll have a look at it for you
cheers
Steve

Hahaha.  I forgot that you read this forum.  Ok Steve, will do. BTW, looks like there are more people wanting your True MV mod done. 

[grolschie]

Update (after quite a long time):
As recently mentioned in another thread, I have increased the value of the 8.2K resistor before Q3 and the sound has improved heaps! It's starting to sound good to my ears.

The current value I have tried in there is 144.2k (8.2k+68k+68k in series). It starts to sound a little bad again as I crank the level/volume (not the gain), so I wonder if increasing this resistor even more could help? There is still plenty of volume on the level knob remaining. I am using J201s throughout, all biased to 1/2 supply voltage. What kind of ballpark values could I try?  Thanks in advance. 

[kurtlives]

Change the recto tube? Sounds like some sag.

[grolschie]

Well, my Eighteen pedal still sounds like it's sagging when my compressor is in front of it, even when compressor is bypassed. Eighteen by itself seems ok.

[grolschie]

Is there a simple way reduce gain a little at each stage without swapping fets?

[Uma Floresta]

You could try removing the 47uF bypass caps.

[grolschie]

Thanks for that. These are after stages 1 and 3 right? What is the purpose of these caps please? Taking a quick search through the Fetzer article, I see that they increase gain. Is this the only purpose?

[grolschie]

It's been a while. I ended up lowering the gain at each stage by doing the following:

• Increase the 820ohm resistors on Q1 and Q2 to 1K5.
• Increase the 120ohm resistor on Q3 to 360ohm

I also increased increased the 8k2 resistor before Q3 to 10k. I forget why 10k is in there, and I am not sure whether that value had much affect. I guess it can be increased more - but I am not sure if this resistor changes the tone though while lowering the gain.

Anyways, I am really liking this pedal now. I think the original sagging problem I had was because I had my compressor pedal (with a buffer in it) before the Eighteen. It definitely doesn't like my compressor being in the chain before it, even when compressor is turned off.