Strange Tone Bender problem - slow start

[ItsGiusto]

Hello everyone! I just built myself a tone bender mkII clone with matched transistors I bought from Small Bear. Upon testing it out, I experienced a really weird issue.

When I switch the pedal from bypass to non-bypassed, the effect takes a good half a second to actually get sound going through. Basically, the pedal is silent for a half-second before my guitar sound can gradually ramp up and come through. Once it gets going, though, it sounds great, although I am getting whine-like oscillations if I run the "attack" knob at full. Having never used a tone bender before, I don't know if the oscillations are normal, or if it's normal to run one with the knobs maxed.

I'm using a standard 3pdt true bypass scheme, with grounded input when in bypass.

Has anyone ever experienced this issue before? Is it possible that the slow start and the oscillations are related? Could it be a transistor issue, a capacitor issue, or something else? For reference, I've attached a picture of the schematic I used. I tried to follow it exactly: no extra pull-down resistors, or anything. Just an added led, and true bypass.

Any advice is greatly appreciated!
Justin

[R.G.]

It is almost certainly one of:

- a bias issue; the transistors leak not enough or too much. This circuit uses the leakage of the germanium devices for biasing. This is actually the least likely possibility.
- the electro caps are inserted incorrectly and leak down when it's bypassed; filling the circuit with signal pumps the circuit back to a more normal bias condition.


Er, you're not switching power off to the circuit when you bypass, are you? That would be bad, and could cause what you describe. I mention this as an afterthought because it's less likely than 1.

[thelonious]

Er, you're not switching power off to the circuit when you bypass, are you?

And that could happen more easily than you might expect---for instance, if you were looking for a convenient location to connect the effect's ground and accidentally connected it to the 3PDT's switched ground (that goes to LED -).

[ItsGiusto]

Wow, might I just say, I'm honored to have the RG Keen reply to my question!!

So I checked these things. The voltage from my negative bus to the ground bus stays at -9v even when bypassed, so I believe that it is not that the power is switching off.

I've attached a picture of my board. The black wires are negative, and the red wires are positive ground.

From what I can tell, the capacitors are in correctly. The negative stripe of the first 4.7u points towards the base of the transistor, and for the others, the negative stripe points away from ground. Does this mean that the transistors are bad? I hate to bother Small Bear about it if that's not the case.

[tonyharker]

Are you plugging your guitar directly into the TB.
Could this be caused by the 4.7uF cap on the input charging up through whatever is on the input every time you operate the switch?
Try putting a 1M resistor between the input cap +ve and ground and see.

tony

[ItsGiusto]

Thanks for the idea, I tried it out. Unfortunately, it seems that when I added the 1M resistor between the input cap's positive lead and ground, I'm now getting no signal through. It was a good idea anyway, though.

I agree that it seems like it might be that the cap takes a little while to charge up. Is it possible that it's merely a bad cap, and I should try another one? On a related note, is there any good and easy way to check capacitors to see that they are good and have the right value? It sure would make my life easier, as I'm someone who likes to triple check everything. Is there something like a digital multimeter for capacitors, or a way to use a DMM to check capacitors?

Also, yes my guitar is going directly in.

[electrosonic]

Unfortunately, it seems that when I added the 1M resistor between the input cap's positive lead and ground, I'm now getting no signal through. It was a good idea anyway, though.

This is a clue. Something is not correctly connected if a 1M resistor kills the signal there.

Andrew.

[ItsGiusto]

Oh, awesome! Do you know what I should do next? I feel like I've traced through the signal path about a billion times, but should I try once again, and make sure that everything is where it needs to be? Is there anything in particular I should be on the lookout for? Stuff that connects to ground or something?

Does this mean that it's likely not the transistor, or capacitor, or anything, but just my own fault somewhere?

Thanks for all your help so far, everyone!

[wildebelor]

what switch are you using?

[ItsGiusto]

The 3pdt? It's just a cheap one that I ordered off of ebay.

But I think I just figured out my problem! I wired the switch to ground the input while in bypass. But I forgot that this was a positive ground circuit. I connected the black, negative wire to the input while in bypass. Therefore when I'm grounding the circuit, I'm actually grounding it incorrectly.  It is causing the capacitor to become fully charged while bypassed, thus blocking the flow of current for the first few seconds when switched, until the capacitor loses some charge.

Unfortunately, this idea popped into my head right as I left and started driving to go on vacation for the three day weekend. Figures. So I can't actually test this hypothesis until I get back. But I'll definitely let you all know how it turns out. Thank you so much for all of your help!
Justin

[PRR]

> I connected the black, negative wire to the input

I can even see myself doing that.

BRAVO for brilliant insight (even if mis-timed).

[duck_arse]

I wouldn't work from the layout in that photo, it seems reversed to me.

[R.G.]

But I think I just figured out my problem! I wired the switch to ground the input while in bypass. But I forgot that this was a positive ground circuit. I connected the black, negative wire to the input while in bypass. Therefore when I'm grounding the circuit, I'm actually grounding it incorrectly.  It is causing the capacitor to become fully charged while bypassed, thus blocking the flow of current for the first few seconds when switched, until the capacitor loses some charge.

Good mental gymnastics! That would sure account for the observed problem.

[ItsGiusto]

I wouldn't work from the layout in that photo, it seems reversed to me.

Yeah, I reversed it so the picture of my board would match better with the schematic for you guys to see.

I finally got around to rewiring the pedal, and it seems that it's working well now, all except that oscillation business I mentioned before. Is that a common problem with tone benders? Is it best to leave it be, or is there some fix that still leaves proper sound of the pedal intact? The squealing noise only happens on about the last quarter turn of the potentiometer.

[Electric Warrior]

Oscillations are a rather common phenomenon with MKII Tone Benders. Sola Sound increased the power filter's cap value from 25µF to 50µF to fix it. You might want to try an even higher value. Not sure what causes your issue. I suspect it's transistor selection. In a stock circuit with 50µF it's not a problem at all.

[Arcane Analog]

As EW mentioned, it is common for the oscilation to occur. Either try the higher cap value, use a limiting resistor or simply dial it out with the Attack rolled back a little.

One thing I would add, and with all due respect to Small Bear, is that they send the trio of transistors out with resistors values to bias the circuit with 4.5V on Q3's collector.

This is not the correct voltage if you want to have a MKII that sounds like a vintage unit.

A properly biased MKII will have ~8V +/- on Q3's collector.