Lightning 10K Bass ctrl wiper to ground resistor

[Rick]

I have had one buyer say he removed this resistor and it improved tone. It would seem that it is leaking signal to ground, but I'm not sure of all the consequences such as effects on tone bandpass and what it really does to the eq since this is the Bass Treble active eq circuit carried over from the AC15? I believe.

Any illumination or comments on what this does or if it should remain or how it could/should be adjusted?

[Badside]

It is leaking signal to ground, that's its use. It's basically your missing mid control, hardwired in a mid-scooping position.

Depending on whether your Bass control bottom lug is grounded or not (technically, it should not, but the classic Top Boost circuit had an "error" there that is now part of its sound), the response will be different.

You could load the Duncan tone stack calculator, go to the Vox model, and enter a really high value for that resistor (like 10M) and see what changes.

I'm guessing there is less mid scooping now.

[Rick]

Thanks Badside, yes, putting in 10M (essentially removing it) puts a huge deep, pointy dip in the mid range. I don't think that is what I'm after. People forget, a guitar is a mid range instrument, chopping out the mid range is like chopping off a runners foot. Thanks for the tip!

[Badside]

Thanks Badside, yes, putting in 10M (essentially removing it) puts a huge deep, pointy dip in the mid range. I don't think that is what I'm after. People forget, a guitar is a mid range instrument, chopping out the mid range is like chopping off a runners foot. Thanks for the tip!

Actually... when putting a 10M for the mid resistor in Duncan TS calculator, I get a huge mid boost. Unless the Bass is at 10 where there is indeed a huge dip at roughly 800Hz, but this is the same with the stock 10k (it is effectively bypass when the Bass knob is at full)

That resistor bleeds mostly mids to ground, so removing it = mid boost

[Rick]

OK, I used a lower case m and that is the milli prefix which caused the deep dip. It needed the upper case M for the mega prefix. thanks for clearing that up! So if I understand it right now, increasing that resistor will BOOST mids. I'm still not sure how I want to change that, I love the harmonics and harmonic feedback tones the original circuit gives and that probably has to do somewhat with the scooping effect that 10K gives.

Then again, maybe adding a 50K or 250K pot in series with a 1K resistor in place of the 10K would give much more flexibility to adjust the mid section pretty much any way the player wants, I may try that but I want to make sure of what I'm doing before I go and drill a hole for a pot.

PS, I went to a 22K 2 W resistor in place of the 10K 2W, HOLY COW! It opened up the BASS drastically. With the bass turned up now you can over whelm the tone with bass but you can still back it out. I gained 3 dB of power it seems with moderate bass. It seems a definite improvement.
I see why the 30 W lightning seemed to have so much less power than my 4-EL84 TMB marshalls based amps, the mids were being bled right out of it losing a lot of gain.
Rick

[Rick]

As a final note in case anyone ever see this, I removed the 220K attenuating resistor to the PI, I removed the phase cancelling MV and replaced it with a central labs 10k pot. I put in an 8.2K 2W resistor in place of the original 10K and ran that in series to the 10K pot so that this mids bleeder resistor can be adjusted from 8.2K to 18.2K. The results are amazing. This gave the 30W lightning much more power, much brighter, chimeyer tone, and gave great control to the previous (original design) mid scoop that the 10K mids bleeding was giving. The smooth breakup and distortion is fantastic. It is not the same amp any more, it is much, much better. The amp is glorious now. Mucho happyo

[Badside]

As a final note in case anyone ever see this, I removed the 220K attenuating resistor to the PI, I removed the phase cancelling MV and replaced it with a central labs 10k pot. I put in an 8.2K 2W resistor in place of the original 10K and ran that in series to the 10K pot so that this mids bleeder resistor can be adjusted from 8.2K to 18.2K. The results are amazing. This gave the 30W lightning much more power, much brighter, chimeyer tone, and gave great control to the previous (original design) mid scoop that the 10K mids bleeding was giving. The smooth breakup and distortion is fantastic. It is not the same amp any more, it is much, much better. The amp is glorious now. Mucho happyo

Hehe, for a second there I thought you meant you had placed that 10K pot in the same place circuit-wise as the MV, then I figured you meant "on the chassis!"

That's a cool idea, I'm guessing it gives a rawer sound

I find the phase-cancelling MV to be rather uninteresting (all PPIMVs are uninteresting to me, they seem to limit the bandwidth), so this is an interesting idea. I'd propose it to the guy I built an AC30 for recently... but he seems to enjoy the PPIMV (hard to get some real gain going without it, even with the two channels jumpered). And there's no more room for an extra control

As for removing the 220k resistor. Since I'm mixing the two channels before the Top-Boost circuit in my version of the AC30, I'm not using that either (added it before the Top-Boost circuit though, with a small bypass cap on the Brilliant channel). Doesn't seem to be creating any instability or motorboating.
Marshalls do fine without an extra resistor there. I tend to avoid series resistance as much as possible. Especially when you're trying to get some chime.

[Rick]

Yeah, that MV was like driving with the parking brake on. The best thing I did was to remove that puppy. I never tried it before in 5 or 6 other lightning style builds. But man, it really cuts out the BS. The amp flies now, the tone is open and present, not fighting to get out.