OK all you Shaka Tube builders, need troubleshooting help

[Paul Marossy]

I mentioned in another thread that I just rebuilt my Shaka Tube. At low volume levels it sounds fine, but at high volume levels, a little bit of a strange problem becomes apparent.

I have no problems at all with the circuit when it is active, but when it is bypassed, things get a little strange. If I am not playing anything, there is sort of a faint, high pitched noise around maybe 16-18kHz, but it's not the "normal" oscillation kind of squeal. It's definitely not a 60 cycle hum.

Anyhow, if I leave the volume pedal wide open and sit the guitar on the stand for a minute, it slowly starts to feedback increasing in strength until it goes full bore. When it's in bypass mode, the gain control will affect the frequency of the noise. But with the gain at 60% or so, the sounds goes away. Weird, huh? Whatever is causing this is also causing my Metal Zone to feedback way too easily when the Shaka Tube is bypassed.  :cry:

So, does anyone have any ideas what may be causing this? Should I shield the input and output wires from the jacks or something?

Other than that, it sounds great!

[Paul Marossy]

OK, here's an update. I used shielded wire on the input and output jack wires, and that seems to have helped with the problem. There is still a little bit of a very high pitched noise around 18-20kHz, but it's in a much narrower range on the gain control, and I don't think it's as prominent as it was before. The real test will be at practice on Thursday...

I'm still a little preplexed as to what is causing this in the first place.

[J.Nadge]

I believe this oscillation can happen when you have no signal feeding a high gain/input impedance device.  Most likely you would not hear this oscillation if you grounded the input of your shaka tube when it is bypassed.  You can do this with a 3pdt switch and LED.   Maybe some one out there has a schem?

Any way, an easy test is to ground the input of your shaka tube when it is bypassed and see if the problem persists.

Good luck!

-J

[Paul Marossy]

I did use a 3PDT switch. I think grounding the effect input when bypassed is the way to go. Why didn't I think of that?!?  

Thanks.  

I've never had a problem like this before in the 20+ projects I have built, most of which are high gain distortions. There's a first time for everything, isn't there?

I've been pondering this for a while, and I still don't understand how this oscillation occurs in the first place. The effect input is phsyically disconnected from the jack when the switch is in bypass position, so how does is pick up an oscillation? Is it because this is a tube circuit? I've noticed that tube guitar amp schematics usually show grounded instrument input schemes, I wonder if this is the reason why...

Assuming that is the case, why doesn't it try to oscillate like mad when the effect is on? It's very quiet when the effect is on. I don't get it...

[J.Nadge]

Well, I don't fully understand either.  However, my hunch is that the issue is having a high impedance input input floating in a circuit that producies a lot of gain.  You don't hear the oscillation when the effect is in precisely because the input is no longer floating.  It is being fed a signal.  If anyone can elaborate or has another point of view I would be interested as well.

I think the problem can be compounded by layout and wiring, which speaks to your point about hearing this oscillation even when the effect is bypassed.  The oscillation must be picked up in the outgoing leads.

Mr. Boofhead, RG, Peter, or the other big guys around here would have to explain things further.

If I can figure out how, I'll upload a pick of my tube box too.  Instead of drilling holes, I filed out some parallel slits-- not that easy either, it turned out.  

-J

[smoguzbenjamin]

If I'm correct it's because when ypu pull the input to ground it's stable. When you leave it alone it's unstable and oscillates.

[Paul Marossy]

OK, if what y'all say is true, then I have two problems. One is the tube section is oscillating with no input, and the other problem is that somehow the bypass section is picking it up. So, it would seem that a grounded input bypass would fix it. I guess I'll try that when I get home from work today.

This is very much like the problems you hear about at guitar amp forums! And, the Shaka tube is essentially a mini tube amplifier.

[smoguzbenjamin]

Isn't it basically a tube preamp? :?

[Paul Marossy]

Yeah, it's a pre-amplifier. It is apparently subject to the same kinds of problems as a tube amp with a pre-amplifier and output section.

In a way it's like a tube amp, the IC chip is like the preamp and the tube is the output section.

[smoguzbenjamin]

Aha!

[aron]

THe grounded input should fix it.

The circuit is probably oscillating when bypassed. Let us know.

Aron

[Paul Marossy]

"The grounded input should fix it. The circuit is probably oscillating when bypassed."

Changed to a grounded input bypass scheme and no more problem. Now I am a very happy camper.  

Thanks for all the help guys! You learn something new every day...

[aron]

Great Paul!!!!!

Make a sound clip when you have a chance!

[Paul Marossy]

I will as soon as I round up a program that will allow me to record guitar directly to an MP3 file...

[george]

THe grounded input should fix it.

It sounds like you guys are using a 3PDT switch to do this.  

Would a simpler (and cheaper) way of doing this be to hook up a pulldown resistor of say 1M to the input as RG does on all his layouts?

That way you could still use the spare poles of the switch for your status LED :-)

George

[Paul Marossy]

Actually, you can do it with a DPDT switch.

Check this page out: http://www.generalguitargadgets.com/v2/index.php?option=html&file=instructions/switching_all.htm&Itemid=51&op=page&SubMenu=

[george]

Paul you da man!

I might actually try the pulldown resistor idea first then this one.

Viva Las Vegas!

[Paul Marossy]

Let us know how the pulldown resistor works.

[george]

hmm it did reduce the oscillation but not entirely, also it changed the tone a bit (as I've tweaked mine quite a bit to the point that I'm *finally* happy with it) probably not in a good way  :evil:

looks like the DPDT switch-input-to-ground-in-bypass thang is the way to go for me too ...

[Paul Marossy]

I thought that would be the outcome. Thanks for letting us know.