Orange Rockerverb Preamp J201 Problems

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KeenVox
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Post by KeenVox »

Hello everyone,

happy to make my first post, sad because it starts with a problem.

I built an Orange Rockerverb Preamp based on the attached schematic from pcb mania. It's a relativly straight forward build using the Rockerverb preamp circuit and replacing the tubes with j201 jfets. I dialed the jfets to 4.5V and everything sounds fine.
However as soon as I play hard on the guitar the sound slowly starts distorting and finally cuts out completely.
I measured the voltage on Q1 and I can see that the voltages slowly rises to 9V while I play. After that the voltage stays consistent and can't be reduced with the pot anymore, till I short the jfet, decharging it.
After that everything works fine till I play, then it starts charging up again, distorting and cutting out.
I switched the pot and even the jfet but the problem remains. I'm using SMD J201.

Any clues what's causing this behavior?
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lcv
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Post by lcv »

q1 gate has no dc path to ground it seems

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Manfred
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Post by Manfred »

lcv wrote: 12 Mar 2021, 21:45 q1 gate has no dc path to ground it seems
I agree that there should be a resistor of 1 megohm between the gate of Q1 and ground.

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KeenVox
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Post by KeenVox »

That would be right after R1 correct?

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The G
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Post by The G »

Can go both before and after, just be sure it provides a DC path to the ground for the gate.. The tubes schematic has it after, but there are reasons for that (http://www.runoffgroove.com/fetzervalve.html).

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KeenVox
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Post by KeenVox »

I added the resistor to ground and now it works line a charm. The voltage sweetspot for good sound is a bit different now but it was easy to dial it in by ear.

Thank you very much for your help!

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sdiazmun
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Post by sdiazmun »

HI KeenVox!

Have you noticed that the capacitor values are different in each section of the PDF? What values did you use?
How similar does it sound to the original amp?

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KeenVox
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Post by KeenVox »

I used a prototype pcb from pcb mania before it was public with an own BOM. If I remember correctly the values were the same like the ones in the schematic. I think the sound comes relativly near to the real preamp. Either way it works really good as a dirty overdrive :). I think this one is a 1 to 1 copy of the original preamp circuit switching only the tubes for jfets.

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wormwood
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Post by wormwood »

What are the "0/1" and "1/1" pads between R5 and C5? I noticed that there is the same junction on the tube Rockerverb 50 mkII schematic that is attached to a SPDT relay. Is this where the clean/dirty channel switch would go?

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