True bypass ROG thunderchief squeals while bypassed!??? What on earth!?+other

[rockgardenlove]

So I build a ROG Thunderchief...
http://runoffgroove.com/thunderchief.html is the link to the shematic, etc. 

What happens is that when the effect is true bypassed(3pdt), it squeals and warbles as you turn the gain or volume pots.  I'm stumped...anyone have any ideas?  When its not bypassed, its fantastic...just while its off, it sounds like some synth gone mad.  Any ideas?  It happens with any guitars, on any amp...so I know thats not the problem...

Thanks!

[B Tremblay]

The circuit has so much gain that the input sometimes needs to be grounded when bypassed.  GGG has terrific diagrams for this type of wiring, in the Switching & Wiring sub-category of the Tech Pages section.

[rockgardenlove]

http://generalguitargadgets.com/diagrams/switch_lo_dpdt_input_grd.gif
That looks right, correct?

Alright, finished with that...works great now.

I just figured out another problem though, when I use my Gibson 490T pickup in the bridge position of my SG, it squeels like crazy when over ~60% gain...doesn't do this with my Epiphone neck pickup, my Strats Bridge HB, or my Strats two single coils...what gives?

[JimRayden]

Perhaps it's old enough to be unpotted and the windings are shattering loose... Have you tried it with other high gain devices?

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Jimbo

[rockgardenlove]

Its a pretty old pickup...and it does this sometimes...I feel stupid, I didn't think of that.


So, I have more problems.  I biased the JFET drains to 4.5 volts...and now whenever I play something, and then mute it, it makes a big screech when the note dies...needless to say, its really annoying...and it only happens if turn up the volume on the effect.  Its like feedback, but only for a split second after I mute the note...

Thoughts?

[rockgardenlove]

Just remembered, I omitted the 470p cap..might this be causing the problem?  It is C3 on the PCB layout.

[erick4x4]

Try putting any pedal with a buffer (most non true bypass) in front of the pedal. Tube screamer, tuner, compressor, whatever.

I find without a buffer in front I definately get squealing after 60% gain, but with a buffer in front, I can turn the gain all the way up without squeal. I find many pedals we all do around here to respond better with some buffer in front. Everything I use live goes through a NS-2 then tuner, so it always fixes my problem.

[rockgardenlove]

I put it behind my of my wah....but I had to turn it on, because I've true bypassed it.  I messed a bit with the biasing, and its alot better now.  I think my next build will be a little buffer box for my pedalboard.

[RDV]

It would be easy enough to put a little buffer on perf in there with it. I just put up with the quirks in mine cause it sounds so good. It's about the only distortion I've been using lately. It just really sounds good into the low-gain channel of my JCM900 and also into the same channel on the JCM2000 ones too. It rocks.

RDV

[B Tremblay]

It's about the only distortion I've been using lately.

Ricky, it's unbelievably flattering to hear that from a working musician.  Thanks.

[RDV]

I mean it too, it compliments Marshalls really well and sounds better to me than their fizz-o-matic lead channels.

...and you're welcome.

RDV

[rockgardenlove]

I managed to fix it by biasing the third FET a little bit low...3v instead of 4.5.  Didn't sacrifice too much tone(if any) and now I can crank it