Vulcan build report- I need some help

[audioguy]

So far I realy like the Vulcan- it sounds amazing with the gain full open- BUT I have this wicked scream like a 60 cycle hum only higher pitched... its pitch os ringing somewhere around E (6th string open).
The scream begins when I get the gain to about 90%- No... its not my wife telling me to turn it down....
Any ideas? Oh yeah... has anyone compiled a list of cool mods for the Vulcan?

Thanks Joe for a kick ass toy!

Audioguy

[RDV]

Boxed or breadboarded?

RDV

[spongebob]

Do you have it boxed already? Joe warns on his site that the diodes can pick up a lot of interference in this configuration, I had the same problem...

Diagnosing oscillations is easy, fixing them is the hard part, you could try adding a power supply bypass cap, or using shielded wire for off-board connections.

For a mod you could try a notch filter as described in this thread, sounds great:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=22649

[audioguy]

Not boxed yet- I thought some of the complaints solved by boxing are that of the radio statiions... I'll box it and see what happens.

thanks guys.

[petemoore]

That's a very friendly notch filter...
 a few caps, a couple resistors...gets a very nice notched mids sound from Fuzzes etc. Tunable switchable etc. very good..I like it.

[audioguy]

OK I boxed it up and still get the oscilating hum. the pitch increases as I turn the gain up past 90%. Boxing it did GREATLY improve the over all noise level, but we knew that it would... any other suggestions?

A couple pieces of info- Im not using a switch, no need for it since Im still just in the testing phase- second thing- I accidentally bought a 1M Audio for the gain that has a switch in it. Is there any way to wire it up to be the main on off? Probably not- how would it by pass.. oh well Im just thinking out loud.

[WGTP]

Mine has started doing that squeal too and there is no wife here to complain.  I'm wondering if it has to do with the battery "aging" but haven't checked it out further.  Reducing the gain seems to help and there is plenty of it to cause problems.

The mods I have done reduced the gain and noise, but when I power it back up by adding the by-pass cap back to the 3rd stage, then it starts the squeal.

Might try reducing or removing the .002uf cap between stage 1 and 2 just leaving the 220k resistor.  

I'm still modding, but a nice version is as above without the .002uf cap removed.  The by-pass caps are 2.2uf for stage 1(that's where the treble boost is), 47uf for stage 2, and nothing or 22uf for stage 3.

The current notch filter gets changed every 5 minutes when I sit down to play, but uses 10k and 22k resistors and .005/,01 caps in the signal path and .01/.02/.047 caps to ground depending on how happy/sad/tired/fresh/mad/pleased/hot and bothered/ etc. I am.  

[RDV]

Long wires can cause oscillations in high gain circuits, just make them as long as they need to be.

RDV

[Joe Davisson]

Try:

-100uF or greater power supply filter cap (even with a battery)
-changing the 220k's to 470k's, which lowers the gain somewhat

If those don't help, let me know. Also post the collector voltages if you can. Thanks for the "feedback"

-Joe

[audioguy]

Try:

-100uF or greater power supply filter cap (even with a battery)
-changing the 220k's to 470k's, which lowers the gain somewhat

If those don't help, let me know. Also post the collector voltages if you can. Thanks for the "feedback"

-Joe

Joe- Thanks for the help, but Im new so you have to speak slowly to me... How/where do I put the 100uF cap? Im only using a battery would I put that along the path of the battery before it gets to any of the other components? If so how should I allign it?

I REALLY dont want to lower the gain...

Audioguy

[Michael Allen]

The 100uf cap goes from the + voltage rail to ground

[RDV]

Put the + end of the cap to the 9v rail & the - end to ground.

RDV

[audioguy]

Put the + end of the cap to the 9v rail & the - end to ground.

RDV

Thats what I thought... I just wanted to be sure...

thanks!

[WGTP]

I started with a 100uf cap, so I'm not to ...  I'm right next door to a little Christian radio station and I never hear from them ...  I'm thinking it is a matter of MGM - Massive Gain Management.

Mine is breadboarded, but I don't think it did this at first, so I'm trying a new battery.  Next I would reduce the .002uf cap and see how that worked.  Depending on how strong your guitar pickup is, you may not need on of the by-pass caps.  Let us know what works.  

[Joe Davisson]

If it oscillates than there's some positive feedback somewhere. On the Vulcan this could be in several places:

-collector of Q3 too close to base of Q2
-emitter of Q3 too close to base of Q1
-capacitor self-inductance radiating signal to the wrong place.

First thought is to change the .002 cap to a smaller ceramic value, say 470pf. Another is to add .001-.002 caps from one or more collectors to the +9v rail (or ground), to shunt some of the high frequencies to AC ground.

Layout matters, I usually wire my circuits straight in a line to prevent interactions, but sometimes it's hard to avoid. My pickups are too weak to really push the pedal, does it still oscillate with single coils? Do people even use single coils anymore?

Let me know if that helps/dosen't help. Thanks.

-Joe

[audioguy]

I'm going to try to implement some of these suggestions later this afternoon/evening. I'll need to swing by the store and pick up some caps.
I have yet to play it with single coils- I'll try.

Thanks again Joe and gang!

[WGTP]

Hey, I'm driving mine with a Les Paul Melody Maker with the notorious P-90 pickup.  As long as I'm perpendicular to the amp I'm cool.

Let me suggest a experiment with the Vulcan.  Get a 2.2uf a 10uf and a 47uf electro cap and try them in the 3 different stages as the by-pass caps.  Or leave one or 2 out.  Different "textures" to be had.

This VERY roughly emulates the tube emulations of ROG.  Remember I said VERY.    :twisted:

[audioguy]

OK I just dropped the 100uf cap along thepower supply and the noise is gone!!!! BUT.. maybe its me- but it doesnt feel like it has the same amount of gain. Maybe its me not opening up the amp- the kids are sleeping... but I'll rock out with my c*ck out tomorrow!

Joe- if I reduced the 220k's would I get MORE gain? Im looking for a metal beast... along the same lines of the Death Metal pedal from DOD.
Either way- I love this one. I think I'll take a stab at some of your others as well.. Ive been meening to for some time now!

Audioguy

[Joe Davisson]

It probably did have more gain, oscillation is caused by positive-feedback which increases the gain. Lowering the resistors increases the gain, but also the fizziness. The pedal is not ultra-sensitive, so a buffer before the pedal should help. Glad it works now.

-Joe