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Old 25th January 2011, 02:32 PM   #1
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Default Strange Problem In Tube Amp Build

...or maybe it is more common and I don't know how to search for it properly...

so...long time lurker, blah, blah...

Anyway, I just finished building an 18w amp with my own design for the 2 preamps. Channel 1 is simply a single gain stage 12ax7 and Channel 2 is a 3 gain stage trainwreck style with the eq right after stage 1.

The amp sounds amazing but the issue i am having is when the gain for either channel is turned all the way down, the guitar signal still comes through the amp fairly loud. It is really dark and has very little distortion, and when I turn up the gain for either channel it is like the signal engages and acts/sounds normal. I have literally torn the thing apart to find the problem and have traced it to the second or third gain stage on the trainwreck style pre (does not do it when only the first gainstage is running), but no matter what I do I cannot get rid of the blasted signal when the gain is down.

I mean, it is kind of cool in that it can be used as an effect, and I can kill all of the signal by rolling off the guitar volume pot, but this is freaking annoying.

Any ideas?

Last edited by nogoodnamesleft; 25th January 2011 at 03:00 PM.
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Old 25th January 2011, 03:06 PM   #2
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Hi, Most likely you have the volume / gain pot wired incorrectly or the pot is faulty in that it does not go to "0"; there is some residual resistance left when turned all the way down.
Looking at the pot with the shaft pointed towards you, left most lug is ground, center lug goes to the next stage, and right most lug is the "hot" signal.
If you could post a schematic, that would help.
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Old 25th January 2011, 03:13 PM   #3
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It could be crosstalk.

When you say "Turn the Gain down", do you mean turn the input volume potentiometer to the GND end or literally turn down the gain. If you mean the latter then the amplifier will still have some gain even in the minimum gain position.
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Old 25th January 2011, 03:27 PM   #4
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the pots are all wired correctly...and when I simply use the first gainstage in the trainwreck prethe issue goes away. The gain pot is right after the first gain stage and there are no pots after the first gain stage.

i don't have a schematic on my computer (it is handwritten) but maybe it will help to at least show you a schematic it is similar to.
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Old 25th January 2011, 03:30 PM   #5
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Andy...I have the pot wired to I am shorting the whole signal. Signal still comes through.
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Old 25th January 2011, 03:39 PM   #6
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Yea - sounds like either a faulty pot or there is another path for the signal other than through the pot. Get access to the pot with no connections at all to it. Measure from both of the outside terminals to the center connection while adjusting the pot to its extreams. You might find a "bump or dead spot".
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Old 25th January 2011, 03:47 PM   #7
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i tested the pot through only the first gain stage and it worked perfectly...i don't think the pot is the problem
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Old 25th January 2011, 03:54 PM   #8
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It's also likely that with that much gain, the signal could be capacitively coupling between various circuits in the wiring, so that some of the signal is literally bypassing the volume control. Try removing (shorting out) the 820 ohm grid resistor and see if that doesn't help the matter. That will make sure that grid is truly grounded when the control is turned to minimum.

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Old 25th January 2011, 04:29 PM   #9
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In the actual preamp I do not have an 820R there, it is straight through to the second grid. I do however have a 680k resistor right before the grid of the third stage along with a 220k resistor to ground right before the 680k resistor...if that helps.

I went ahead and bypassed the 680k resistor with no change, and I then shorted right before the 680k to ground and that cut the volume in half. In fact there is a lowering of the volume when I short out the signal right before gain stages 2 and 3. When I do it right before 1 or after the coupling cap of 3 then it kills the signal completely. So I'm guessing somewhere between stage 2 and 3 I have a leak?
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Old 25th January 2011, 05:01 PM   #10
taj is offline taj  Canada
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Hi nogood,

The exact schematic is probably the best way to solve this conclusively. Questions, wild stabs and try-this, try-that could go on for ages without it. Just sayin.

Scans of napkin drawings (or whatever you can do) are perfectly acceptable.

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