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Looking for that Magic Dumble overdrive

1/6/2005 10:08 PM
Bob Ingram
Looking for that Magic Dumble overdrive
I'm using the schematic found at schematic heaven called "Humble Overdraft Specimen".  
I've got a beautiful clean channel but the overdrive lacks that "bloom". After changing the cathode bypass on V2B from 4.7uF to 2.2uF the overall tone is more balanced. If you listen you'll see that it's pretty muddy and one dimentional.  
I recorded this will all knobs except the master at 5, master lower to prevent ears bleeding, using a PRS 22 Fret w/ Dragon PU's, a great "reference" guitar.  
I'm thinking I need to get more mids through the coupling caps after V1A. By driving the mids into the next stages I think it'll bloom.  
Any suggestions?
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1/7/2005 3:27 PM
Rick Erickson

Before you change the circuit, try a new set of preamp tubes. The Chinese 12AX7's seem to work well in this circuit. The New Sensor 12AX7EH would be my next choice as far as current production tubes go. ymmv  
1/7/2005 4:36 PM
Bob Ingram

Thx Rick,  
I've played quite a bit with the tubes, I'll continue to do that and I have some Chinese 12AX7's.  
I really think there are errors in this schematic, or maybe better put, guesses. I'm hoping for some input to help me nail this smooth cushy tone.
1/7/2005 9:47 PM
Bob, to me it sounds like something is getting too overdriven. To the point that it is just clipping, sounds similar to a fuzz. I don't have any experience with this exact circuit but when I've had that tone with a high-gain preamp, it always got better by doing one or all of the following:  
1) adding some interstage attentuation, but in the right spot.  
2) lowering preamp voltages  
3) removing bypass caps (could be either a cathode bypass cap or a treble-peaking-type)  
But especially #1. At least one of those preamp tube grids is getting slammed too much. Just my stab in the dark!  
PS - I've noticed in a couple of cases that as soon as that clipping is cured (by REDUCING gain), you'll actually get MORE good OD/crunch and sustain. Hard to explain.
1/7/2005 11:18 PM
Bob Ingram

Thx for the message, as you can see I found the problem but you were thinging along the same lines I was, until I started looking at the NFB loop.  
I agree, lots of times folks use too much gain. I like this circuit because of the trim on the overdrive ckt. I've found that my best tone comes from having this trim set low, under 50%.  
Here's another clip, still some tweeking to be done but I've got the silky smooth mids I was looking for.
1/7/2005 11:08 PM
Bob Ingram

I found the problem. The negative feedback was actually positive feedback. I had the plate leads reversed. She's sweet and smooth now.
1/8/2005 7:39 AM
That was my anon post earlier, new computer. That sounds better. I'm actually a little surprised that all of the cathodes have bypass caps. In my limited experience, having them on all the cathodes increases gain and distortion, but not always the sustain (to a point). I think of the OS as a liquidy, sustainy tone without as much distortion. Glad to hear you're headed in the right direction. Every time I've had the leads swapped I had that runaway howl, which is a nice wake up call.:)

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