Lovepedal - Dover Drive  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
ozzy666
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Post by ozzy666 »

di-anno wrote:i built the pedal and i had same oscillations issue with gain maxed and/or guitar volume closed. To avoid this i put a 100 Ohm resistor in series at pin 3 of gain pot. So it never goes to zero. i also used hfeQ1<hfeQ2. With this mod I no longer have any noise. However,now, with guitar volume closed randomly the circuit picks up radio frequencies..
i can mod the gain pot that way it never reaches the "oscillation point", but there must be some other way...

ozzy666
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Post by ozzy666 »

i just want to confirm that di-anno's mod worked for me too, i.e. 100 ohm resistor at pin 3, and hfeQ1<hfeQ2. cheers!

ivan H
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Post by ivan H »

ozzy666 wrote:
ivan H wrote:Thank you again. I'm off to get a trimpot now, I'm sure I'll have a resistor of suitable value once dialled in. It does sound good with the gain down low, I'm sure it'll be great when working properly. I used a Russian D9B detector diode for the germanium, measured a heap of all types I had (no 1N34A's) & used the one with the lowest measured drop. Again, you help is greatly appreciated, I'll let you know the outcome. Cheers
I am experiencing the same issue. Did you get it solved yet? Thanks so much!
Hi, sorry I haven't been here in a while, I've had a lot on. To answer the question, no, the trimpot did not solve the problem. I was able to get it a little better but definitely isn't a fix. Haven't had the time to look at since but the idea of putting a 1k trimpot in series with the pot to find a suitable value resistance to stick there was my thoughts on getting it to perform reasonably. I'll get back to when I have time to sort it. Cheers

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Post by ivan H »

Ok, so I have a little time to try dialling this in. Removed the two 2N3906's & put sockets in. Also put a 1k trimpot (that tops out at 886r) in series with the 1k gain pot. Measured a few 2N3906's & stuck with 260 hfe in 1st position (lowest of the few I checked) & 271 hfe in position 2. Made sure the 10k trimpot was set at 2k66. Set the trimpot in series with the gain pot around 1/2 rotation. Success. Could max the gain control without oscillation or any weirdness. Had it powered by a battery that started to get low after a while, making it spitty & lose volume, but it had (or seemed to have) better tone than before. I'll have to power it with my boards supply to properly dial in. I checked a few more 2N3906's & found one at 209 hfe & two at 229 hfe so I'll try them in position 1 to see which performs best, then find the lowest setting of the trimpot in series with the gain pot that will allow it to operate without problems, then remove, measure & replace with a resistor of same value. Cheers

ozzy666
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Post by ozzy666 »

ivan H wrote:Ok, so I have a little time to try dialling this in. Removed the two 2N3906's & put sockets in. Also put a 1k trimpot (that tops out at 886r) in series with the 1k gain pot. Measured a few 2N3906's & stuck with 260 hfe in 1st position (lowest of the few I checked) & 271 hfe in position 2. Made sure the 10k trimpot was set at 2k66. Set the trimpot in series with the gain pot around 1/2 rotation. Success. Could max the gain control without oscillation or any weirdness. Had it powered by a battery that started to get low after a while, making it spitty & lose volume, but it had (or seemed to have) better tone than before. I'll have to power it with my boards supply to properly dial in. I checked a few more 2N3906's & found one at 209 hfe & two at 229 hfe so I'll try them in position 1 to see which performs best, then find the lowest setting of the trimpot in series with the gain pot that will allow it to operate without problems, then remove, measure & replace with a resistor of same value. Cheers
Thanks for your feedback! This confirms that in 1st position should be used the lowest hfe. Plus the resistor with the gain pot helps also.

ivan H
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Post by ivan H »

Thanks for your feedback! This confirms that in 1st position should be used the lowest hfe. Plus the resistor with the gain pot helps also.[/quote]

Yes. I have one transistor measured at 209 hfe so I'll try that in position 1 later today. Let you know how it goes. Cheers

Manfred
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Post by Manfred »

It did not leave me in peace, because It is not acceptable that a stable cicuitry is so greatly dependent on the current gain value.
Therefore, I ran a simualtion of the transistor circuit with shorted input.
I found out that the value of collector resistor must be 3.3 Kiloohms not 33 Kiloohms.
Have a look at the collector DC-voltages there are about 7 Volts at 33 Kiloohms and about 5 Volts at 3.3 Kiloohms.
In the first case the collector circuit clips already at a low input level.
What are the the conditions that a system oscillate?
Enough gain, feedback, signal limitation (clipping).
Feedback is due to the 100 Kiloohms resistor.
Signal limitation is due to the early asymmetrical clipping of the input buffer circuit.
That's why the circuit oscillates, without input signal.
Dover0.jpg
Collector transistor = 33 Kiloohms.
Dover1.jpg
Collector transistor = 3.3 Kiloohms.
Dover2.jpg

artguitar
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Post by artguitar »

Probably error in the original schematic

Manfred
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Post by Manfred »

The sentence contains a typing error, buffer is wrong, it should read:
Signal limitation is due to the early asymmetrical clipping of the input circuit.
artguitar wrote:Probably error in the original schematic
I don't know if the circuit works properly when the 3.3 Kiloohms resistor is applied,
because the original circuit is similar the fuzz face one.

I will carry out further circuit simulation with variation in terms of components.

Manfred
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Post by Manfred »

The simulated circuit drives me grazy.
Oszillation with signal source too.
I think is better to make tests with the real circuit.

PlutoniumBob
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Post by PlutoniumBob »

Manfred wrote:The simulated circuit drives me grazy.
Oszillation with signal source too.
I think is better to make tests with the real circuit.
your original schematic shows a 100uf and a 100n cap decoupling the +ve supply after the 330r resistor try putting these in to see if it stops the self oscillation.

ivan H
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Post by ivan H »

If you look on the layout, directly below Q1 you will see a 100uf electrolytic & a 100n film cap side by side. A wire link goes to the emitter of Q1. That would be them. Cheers

PlutoniumBob
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Post by PlutoniumBob »

ivan H wrote:If you look on the layout, directly below Q1 you will see a 100uf electrolytic & a 100n film cap side by side. A wire link goes to the emitter of Q1. That would be them. Cheers
Hi ivan, I was referring to the simulation that manfred was working on 8)
Regards
Bill

ivan H
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Post by ivan H »

Oh, apologize, I miss understood. So anyway, I tried the hfe 209 transistor as Q1 & while it worked ok the tone suffered (different "brand" transistor). It was more bassy & not as much clarity, sortta thing so I stuck the other back in. That is one thing I'd maybe like to tweak on the circuit, get a little more possible treble from the tone control. On a proper power supply I didn't have any problems, though I didn't have time to look to adjusting either trimpot (on 'board or in series with gain control). Also wondered how it'll go on an 18 volt supply. Might have a look see after dialling in on 9 volt. Cheers

Manfred
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Post by Manfred »

PlutoniumBob wrote:
Manfred wrote:The simulated circuit drives me grazy.
Oszillation with signal source too.
I think is better to make tests with the real circuit.
your original schematic shows a 100uf and a 100n cap decoupling the +ve supply after the 330r resistor try putting these in to see if it stops the self oscillation.
I add the capacitors.
You are right, it stops the oszillation on the simulation, but not with shorted input to ground and low signal levels.
I will run a simulation with the fuzz face circuit for comparison.
Again as I stated above, the real circuit may behave differently.

Manfred
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Post by Manfred »

I should like to apologise for the confusion about the circuit.
My fault, I omitted the power supply components, I insert them , now it works fine.
The oscillations starts for values of the gain potentiometer below about 30 Ohms, depends on the bias setting.
On the simulation the HFE value is ~181.
Change the value to 300, the start point of the oscillation does not change significantly.
Perhaps it may be necessary to have a 47 Ohm resistor in seriell to the gain potentiometer.
Dover3.jpg
Dover4.jpg
Dover5.jpg

di-anno
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Post by di-anno »

i did some research so..the first stage of the pedal is exactly a "pnp negative ground fuzz face..Orman style" with minimal changes to gain pot to make it more accurate. As a fuzz face it must have hfeQ1<hfeQ2. As a fuzz face it has a low input impedance and for this reason there is an interaction between guitar volume pot and gain stage. Common problems related to this are oscillations and radio frequencies capture. I found various tips to prevent each of these problems (R in series to pin 3 of gain pot to limit oscillations, a small 1n cap between B and E of Q1 to limit radio frequencies, reduce feedback 22uF cap to roll off a bit of bass, reduce 330 Ohm power source resistor to give a little more volume and headroom.. ) but I think my next step will be put finally a jfet input buffer. In theory it should decouple guitar from pedal (this might not please someone...) and give more clarity. Cheers

ivan H
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Post by ivan H »

Thanks for the tips. I'll have a bit more of a look into mine over the weekend, if time allows. Reducing the 22uf might serve me well, seeing I was thinking of getting more possible treble. Cheers

di-anno
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Post by di-anno »

i tried quickly a simple input buffer and...wow..now this is a great sounding pedal! the buffer has a huge effect on this circuit for my ears, as a treble booster in front of it, great high gain distortion with great clarity and sustain. There is however a feedback whistle with all pedal pot at max...
dov_buffer.jpg

Manfred
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Post by Manfred »

Thanks, sounds good!
I further step could be to hook in the blend circuity of the Lovepedal Silicon Fuzzmaster.
http://tagboardeffects.blogspot.de/2012 ... aster.html

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