I am confused as to why you built your BM around some unknown internet schematic. As you know, I had one here where I measured and documented. Why don't you follow that one instead of messing with possibly incorrect stuff.....
You get them in line Scott !!
Now go do 500 lines on the chalkboard.
'I must always follow what the extremely large guy did, never will I dare to disrespect him by changing a single component value as I might like it more"
Just kidding bro...as clearly that wasn't the case here. You are right, always start with something that will get you in the ballpark.....then have fun.
I am confused as to why you built your BM around some unknown internet schematic. As you know, I had one here where I measured and documented. Why don't you follow that one instead of messing with possibly incorrect stuff.....
You get them in line Scott !!
Now go do 500 lines on the chalkboard.
'I must always follow what the extremely large guy did, never will I dare to disrespect him by changing a single component value as I might like it more"
Just kidding bro...as clearly that wasn't the case here. You are right, always start with something that will get you in the ballpark.....then have fun.
Seems that amp was exhibiting some similar stuff. Maybe heisthl will chime in here.
Dave
Thanks Dave,
Been through that thread... don't know if Heisthl did anything about it or left it as is.
I feel like I've got a bit of a perfect storm with this setup... the particular guitar contributes to it and the emminence speakers definitely contribute too with their 'bass hump'... but I can clearly hear the low-tone being 'produced' in the OD2 stage.
I've gone as far as I dare with the cathode bypass cap... down to 0.68uF now (from 4.7uF) and I feel like I've gone full circle, I just did a couple of recordings to a/b against the original problem recording and it seems the progress I thought I was making has evaporated.
I was thinking of playing with the plate/cathode combination again to see if shifting the clipping points & symmetry can help alleviate the problem... I noticed Heisthl was using the 220k & 150k as well.
Just noticed that you were on that thread -oops... I just spoke with Henry on the phone and he said it was all about the presence and where it was set. He was saying he kind of dug the effect at times. He could dial it out when the presence was set at certain levels.
Yeah the HRM that had that "2 Octave Below" MXR BlueBox effect only did it when presence was off (No NFB). The effect was almost as loud as the fundamental. With the presence turned up at all the effect was gone. I was using alternate values in that amp - schematic below:
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heisthl wrote:Yeah the HRM that had that "2 Octave Below" MXR BlueBox effect only did it when presence was off (No NFB). The effect was almost as loud as the fundamental. With the presence turned up at all the effect was gone. I was using alternate values in that amp - schematic below:
Thanks Heisthl,
I started down the path of looking at the presence, but my amp has always had the presence on and varying it doesn't seem to help the problem.
I also tried varying the amount of negative feedback... increasing it until the output of the amp was seriously reduced with no effect on this double-stop tone.
The stethoscope seems to have pin-pointed it to being produced as part of the overdriven output of OD2, and I have nearly exhausted all my ideas about changing the plate/cathode/bypass cap values around that triode trying to minimize the IM product. I've tried 150k/2.2k and 120k/1.8k and 0.68uf, 1uf, 4.7uf. Drastic measures like 0.1uf and 0.22uf do seem to rolloff the bass note, but change the response curve a bit too much I think, but I may end up going there yet!
Attached is a plot of some more drastic examples from simulation:
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what about PI values? I always attributed part of this phenomenon with the fact that I had no trimmer and a good spread between the plate resistors of the PI.
heisthl wrote:Take the preamp out and plug it into poweramp in of another amp - still have the problem?
Thanks Heisthl,
Your suggestion prompted me to try out a number of combinations this morning... I think I have reduced the level of this double-stop low tone sufficiently to be comparable to other amplifiers now.
I tried preamp out into a couple of other amps and was able to duplicate the problem with my worst-case test... but I could also duplicate the problem into a Roland Cube on all the models and even a single-ended ultralinear champ I had built a while back. I have an overdrive pedal that I built after Kevin O'Connor's modified hotbox but I had made a number of non-HRM mods to it... the low-tone is much fainter but still evident in that preamp too.
Bottom-line, this is a natural product of double-stops and overdriven tube distortion so it will be in the mix. Changing the OD2 bypass cap made a substantial difference (I just need to settle on 1uF vs 0.68uF). In playing around I found the 120k/1.8k sounded a bit more 'musical' than the 150/2.2k I had on OD2, but this could just be a result of shifting the clipping point and symmetry a bit.
I want to clean up the board and properly mount the changed components and then I think I'll try some tube substitutions... I think that could really change the character of these IM products too.
