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Rostex Turbo Metal - no sound at the output

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Rostex Turbo Metal - no sound at the output

Postby maz » December 22nd, 2019, 8:04 pm

Here's another really good sounding circuit, the (discontinued) Rostex Turbo Metal (1991).

I found a nice vero layout there -> http://tagboardeffects.blogspot.com/201 ... metal.html

It's an agressive distortion using a dual opamp, led clipping, bass/treble tonestack and a fet as output buffer. If like me you love that loud edgy metallica sound, this one's for you !! I'm so surprised how it sounds good, well over several metal pedals I tried before, I would say that it has some Russian color in the tone, which I love ! Thanks to Mr. mirosol at tagboardeffects for this great one ! :great:

I first built the 3 knobs version (I don't like circuits with too much parts), but after having tried the 4 knobs version I should say that it's a must to have treble AND bass control on this pedal, or you will feel like :wdim: She's good to play metal but you will not be limited there, playing with the bass, treble and distortion knobs can lead you into lots of different worlds, from a cool overdrive to a huge massive edgy metal sound.

Parts are available, in mine I used a JRC4558D IC. FET choice is not that important since it acts just as a standard unity output buffer in this case, I used 2N7000 as suggested in the link above because I had a bunch of them.

Now I want to share my built experience with you and also report a part that I think is missing in the layout above, although with the addition of this missing part, the layout is perfect and works as it should.

The problem I had as I first fired up this project was ... no sound .. and after having triple check all the vero board, links, cuts, parts, I was like.. everything seems to be good! Swapped from one FET to another, still no sound at the output. But since I could see the 2 leds switching ON when I was playing on it, I though that the problem could be at the end of the circuit, so I looked on the scope to see where the signal could be interrupted and found that it was okay until the output FET, so I removed it, placed a jumper and paf !! Sound appears like magic but was missing that little oumph from the output FET, for sure.. I was so excited to hear the good sound coming from this pedal that I played for about half an hour without the FET :lol:

Okay so Drain voltage is correct but no signal is passing through the FET. No worries.. I looked at http://www.muzique.com/lab/buffers.htm to see if I can find the problem and realised that the gate wasn't properly biased, so I just added a 1M resistor from Drain to Gate on the FET and Banggg !! Everything works as it should ! :D You can see this buffer on the AMZ web site mentionned above, 4th image from top and it is very well explained.

Also, I used a A500K for the bass pot and B500K for the volume pot, finally I just swapped those two pots, which in my opinion makes much more sens to my ears.

Some pictures of the circuit :

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Since I didn't find any schematic for the Rostex Turbo Metal I drew it by myself based on the layout I found on tagboardeffects blog, so it can be used as reference. I have include the missing 1M resistor from Drain to Gate and the 1N5817 protection diode has been removed.

I'm not sure if it's an error since this resistor seems not to be there, based on the schematic of the Rostex Distortion I found online which is almost 1:1, but in my case I had no sound at the output, so if like me you experience this problem, just add a 1M resistor from Drain to gate, and the layout mentionned above works perfectly.

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So that's it, a great distortion pedal with such a huge sound, I LOVE it !! 8-)
All together, bigger we can be!
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