Glory Hole OD Mods?

Ben Love

New member
Has anyone attempted to mod this one? I'm mainly curious if there's a way to make it high gain? Another thought I had was, what would happen if you put all of the 1N4148 diodes in backwards? Just looking to get some different sounds.
 

Mcknib

Well-known member
There's a coupla posts Ben this one probably the most comprehensive

 
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BurntFingers

Well-known member
Increase r7 but you'll change the tone unless you adjust c10 (which appears to be on a switch).

Alternatively you can reduce r3 From 4.7k to 1k (for example), but would then need to increase c4 to 47n for the same frequency response... I think.
 

HamishR

Well-known member
As to the diode question: If you reverse D100 you will most likely get silence. And as long as you reverse all of D1-4 you probably won't notice any difference. Reverse just one or two and it won't sound very good - It will probably sound broken but not in a good way. Don't ask me how I know... You could try replacing the 1N4148s with a pair of red LEDs for a slightly different kind of breakup - that's an easy and popular mod. Or you could try 1N4001s - I often use a pair of 1N4001s with a pair of 1N4841s like Rockett use in some of their pedals. It's extremely subtly different from 4 1N4148s but I like it better.

Making C4 bigger will make it sound bassier.
 

Chuck D. Bones

Well-known member
I think upping R7 is the simplest and most effective way to increase the gain without affecting the tone. As BurntFingers said, you might want to adjust C10 downwards if you do that. Depends on how much you increase R7 and which switch position you like.

You can also increase the gain of the last stage by decreasing R17. That will drive the FET into saturation which will definitely change the tone. If you want it adjustable, replace R17 with a 10K trimpot.

Changing diodes as HamishR suggests will change the character of the distortion. You can replace one of the diodes with a jumper or LED if you want asymmetrical clipping.

BTW, R14 doesn't do anything.
 

Ben Love

New member
Thanks everyone for your replies! I'm thinking of bumping R7 to 1.2M and dropping R17 to 10K. I like the HI-CUT switch in the cut position in the one I've already built, strangely with the tone knob turned up higher (is the tone knob a frequency sweep or a treble cut?). So how far down would i need to adjust C10? 400p? 390p?
 

Chuck D. Bones

Well-known member
I recommend changing one thing at a time, otherwise it's too easy to get lost.
Raising R7 from 220K to 1.2M will increase the gain at all DRIVE settings by 6x (15dB). Pretty substantial increase. However much you increase R7, reduce C10 to maintain the same cutoff freq. That being said, with that much overdrive, you probably want more treble cut than you're getting now. Start by reducing C10 to 75pF or 100pF and see if the gain range is good. Once you're satisfied with R7, then adjust C10. You may have to go back and forth a couple times tweaking R7 & C10.
Dropping R17 from 12K to 10K will be barely noticeable. Try reducing R17 in 2x steps (6.8K, 3.3K, 1.5K, etc.).

The TONE knob is a "shelving" control. The cutoff freq is constant, but the amount of cut you get is variable. It's gonna be kinda touchy at the very bottom end of rotation.
 

Ben Love

New member
I recommend changing one thing at a time, otherwise it's too easy to get lost.
Raising R7 from 220K to 1.2M will increase the gain at all DRIVE settings by 6x (15dB). Pretty substantial increase. However much you increase R7, reduce C10 to maintain the same cutoff freq. That being said, with that much overdrive, you probably want more treble cut than you're getting now. Start by reducing C10 to 75pF or 100pF and see if the gain range is good. Once you're satisfied with R7, then adjust C10. You may have to go back and forth a couple times tweaking R7 & C10.
Dropping R17 from 12K to 10K will be barely noticeable. Try reducing R17 in 2x steps (6.8K, 3.3K, 1.5K, etc.).

The TONE knob is a "shelving" control. The cutoff freq is constant, but the amount of cut you get is variable. It's gonna be kinda touchy at the very bottom end of rotation.
True.
Oh ok, wow. That's a crazy bump in gain. probably don't want that much right off the bat.
So move up in doubling increments.
Moving down on C10 would that be in half increments, or just move down one level at a time?

Thanks again for the help!
 

HamishR

Well-known member
Actually that's what I've worked out too - try changing parts at 2x intervals. Just going one increment at a time can sometimes be hard to hear. Going up or down 2 increments will at least tell you if you're going in the right direction.
 

Ben Love

New member
Ok, so I changed R7 to 470K and really like the sound of that, but now it's a little quieter. I've looked at the schematic and honestly don't fully understand it, I do know, just by reading the letters and numbers like R and C for resistors and capacitors and the IC has the name, and D1, 2, 3, 4 for the diodes. But, just by following the lines along the schematic I'm gonna go out on a limb and guess lowering R10 would increase volume? its the last resistor before the volume knob is why I guessed that.

I haven't yet tried the lowering of C10 or R17 just for reference to where I'm at so far.
 

Ben Love

New member
I recommend changing one thing at a time, otherwise it's too easy to get lost.
Raising R7 from 220K to 1.2M will increase the gain at all DRIVE settings by 6x (15dB). Pretty substantial increase. However much you increase R7, reduce C10 to maintain the same cutoff freq. That being said, with that much overdrive, you probably want more treble cut than you're getting now. Start by reducing C10 to 75pF or 100pF and see if the gain range is good. Once you're satisfied with R7, then adjust C10. You may have to go back and forth a couple times tweaking R7 & C10.
Dropping R17 from 12K to 10K will be barely noticeable. Try reducing R17 in 2x steps (6.8K, 3.3K, 1.5K, etc.).
I lowered C10 to 100p, and it sounds pretty good. But I'm still having the volume issue. By looking at the schematic I'm guessing that by lowering R10 would increase volume? It's the last resistor before the volume knob is my reasoning for that.
 

Chuck D. Bones

Well-known member
Changing R7 and C10 affects the gain, but not the volume. The diodes limit the maximum signal and everything after that can affect the volume. R10 has such a small affect on volume that you could take R10 all the way down to zero and not hear the volume increase.

As it is now, you should be getting around 5Vp-p output when TONE and VOLUME are dimed. It's hard to believe that's not enough. Reducing R17 will increase the volume, but Q1 does not have much spare headroom. Where is the unity VOLUME setting?
 

Ben Love

New member
Changing R7 and C10 affects the gain, but not the volume. The diodes limit the maximum signal and everything after that can affect the volume. R10 has such a small affect on volume that you could take R10 all the way down to zero and not hear the volume increase.

As it is now, you should be getting around 5Vp-p output when TONE and VOLUME are dimed. It's hard to believe that's not enough. Reducing R17 will increase the volume, but Q1 does not have much spare headroom. Where is the unity VOLUME setting?
Unity Volume is at about 2:30-3 with the gain knob at 0, and about 1:30-2 when gain at 12:00. I've built this pedal before, just the Glory Hole straight up Morning Glory clone, and it has 12:00 unity volume.
 

Chuck D. Bones

Well-known member
With a digital voltmeter. Do you have one?

I'm surprised that unity volume is so far up the dial. Knowing how Q1 is biased might shed some light.
 
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