'97 Classic Graphtech Ghost Conversion- check my plan?

[parkinthepark]

I snapped a ribbon cable in my '97 classic and (thankfully) it still works in mag-only mode, but I'd like to take this opportunity to update & modernize the electronics while I get everything back in order, with only minor changes to the original functionality.

This forum has proven invaluable in making the plan and sourcing the parts, but before I spend the cash and start the process, I'm hoping that some of the experts here can point out any flaws in my plan, or give any pointers on the actual process. I've highlighted specific questions clarifications in bold.

• Piezo Saddles: Graphtech Ghost Parker model
• Piezo preamp: Graphtech Ghost Acousti-phonic
• Magnetic volume: Standard 500K audio taper pot, 3/8" shaft dia. I assume these need to be short-shaft pots, not long?
• Magnetic tone: Standard 250k audio taper pot, 3/8" shaft dia.
• Master Volume: 10k audio taper, dual gang, 3/8" shaft dia. Graphtech recommends to wire the mag side of this between the other mag controls and the magnetic input of the preamp, which makes sense to me. But if I wanted to put a treble bleed on the mag volume, would it make more sense to put it here, or on the original mag volume?
• Piezo Volume: 250k audio taper, 3/8" shaft dia., with push-push switch to alternate between the 2 graphtech preamp voices (sourced here: https://guitarelectronics.com/250k-audi ... sh-switch/ )
• Output Jack: Original It looks like the original jack has tip, ring, sleeve, and switch lugs, which is what the Graph Tech needs- any reason this won't work?
• Mag Pickup switch: 4PDT to match the original switch positions (sourced here: https://www.ebay.com/itm/175369585942)
• Mag/Piezo switch: 4PDT, using patzag's wiring here: viewtopic.php?p=848#p848 (same switch source as above)
• Knobs: TBD- not planning to re-use the originals, they're pretty chewed up at this point, and I'm not going to use the stacked pot for piezo vol/tone.

A general question: Most of the switches and pots in the original cavity are miniaturized to some degree, if I go to full size switches/pots, will I run into any space issues?

Thanks!

#ElectronicsMods
#SwitchReplacement

[mmmguitar]

I assume these need to be short-shaft pots, not long?

Make sure your pot shaft bushing (thread) lengths are at least 5/16" (7.95mm) - Measure the bushings of the pots you're replacing, if you want to be 100% certain (or recycle the old ones where you can, if they're still working). I mention this because the Fly has a carved top of varying thickness (which can be compensated for by carefully thinning out the cavity-side of the control holes with a forstner bit - But you may be more comfortable with tracking down a pot with a bushing length of 10mm or more for the thicker areas of the top). I wish I could be more helpfully specific about the clearances - But my '97 has had its original holes filled and re-drilled. 3/4" (19mm) pot shaft length "long shaft"/Les Paul pots are indeed much too tall to fit in a Fly.

Graphtech recommends to wire the mag side of this between the other mag controls and the magnetic input of the preamp, which makes sense to me. But if I wanted to put a treble bleed on the mag volume, would it make more sense to put it here, or on the original mag volume?

How are you running the piezo side of the master volume pot? Are you running the output from the 250k piezo pot to the input of one gang of the 10k pot? I like that Graph Tech (I assume it was Gray) prescribed a means of retaining the Fly master volume pot functionality with the Acousti-Phonic setup - But I'm skeptical that it's going to taper the same as it did in the Fishman setup (subsequent experimentation with resistor values to tweak the taper might be required; and I don't know that we have firsthand data from anyone else performing this mod).

Concerning the treble bleed and master volume pot: Use the treble bleed with the mag volume pot - The master volume pot value being 10k is going to attenuate minimal high-end, and you might end up deciding to tailor a different high-pass filter for it later on, in the event it doesn't taper the two signals to your liking.

It looks like the original jack has tip, ring, sleeve, and switch lugs, which is what the Graph Tech needs- any reason this won't work?

Make sure you've read the following links concerning the Graph Tech "Stereo Switched" jack:

https://help-center.graphtech.com/en-US ... nic-764474

https://help-center.graphtech.com/en-US ... ack-764603

Basically, Graph Tech *really* wants you to use their switching jack - But it requires you to replace the barrel jack on your Fly with an oval jack plate after drilling out the hole (more surgery). So the compromise is to rewire a switching barrel jack as described in the second link. I don't know offhand if your Fly's current jack is set up with the parasite plug or other modification potentially preventing it from being used in the "switched barrel jack" scheme linked above. In the worst case scenario, you'll notice your Fly's battery draining despite being unplugged, then correct the switching issue.

Most of the switches and pots in the original cavity are miniaturized to some degree, if I go to full size switches/pots, will I run into any space issues?

The pots and switches of your Graph Tech setup will take up far less space than the '90s Fishman ribbon assembly and preamp - But you will have a spaghetti factory of extra wire running between everything included in the Graph Tech kit components unless you take the time to zip-tie and/or shorten the wire runs through clipping and soldering. I doubt the size of the pot housings will be a problem, so long as you're not going with giant CTS solderless PCB pots or anything. The safest thing would be to go with pot housings similar in size to the originals you have on-hand to reference (they don't need to be square or anything).

Though it's been a year or two since I last did any Graph Tech Fly installs, feel free to jog my memory once you're in the thick of it. One word of warning is that there's a chance the Acousti-Phonic preamp will end up creating a ground loop hum in "mix" or "piezo" mode which disappears when you ground the strings by fretting them (or by taking the preamp out of the mag circuit via the bypass switch position) - It's a rare issue I've personally experienced and seen reported here and there over the years by installers and owners of guitars with factory installations of the preamp (e.g., the Gibson Alex Lifeson Axcess Les Paul). I only got as far as narrowing the cause down to the preamp itself - I didn't confirm whether shielding and insulating the preamp in a pocket of copper and electrical tape managed to mitigate it to any extent (it was piggybacked with the Hexpander in a guitar which had already been drilled for the 13 pin setup and additional controls; so I wouldn't have been able to leave it wrapped or isolated if it had worked).

[parkinthepark]

Thank you thank you thank you, just the sort of advice I was looking for!

Make sure your pot shaft bushing (thread) lengths are at least 5/16"/7.95mm - Measure the bushings of the pots you're replacing, if you want to be 100% certain (or recycle the old ones where you can, if they're still working). I mention this because the Fly has a carved top of varying thickness (which can be compensated for by carefully thinning out the cavity-side of the control holes with a forstner bit - But you may be more comfortable with tracking down a pot with a bushing length of 10mm or more for the thicker areas of the top). I wish I could be more helpfully specific about the clearances - But my '97 has had its original holes filled and re-drilled. 3/4"/19mm pot shaft length "long shaft"/Les Paul pots are indeed much too tall to fit in a Fly.

I think the thinning option makes more sense- looking through Mouser the longest bushings I can find for audio taper pots is 9.5mm, and I measured the stock Mag/Piezo switch at 11.25mm bushing and the piezo vol/tone knob at 11.7mm (which seem like they go through the thickest areas of the top). I'm sure with enough digging I could find the right size oddball parts, but counterboring the holes would make the guitar more universally compatible with standard parts (in case I want to try other control configurations in the future), and give me a few more mm of space within the cavity.

What size forstner bit would you recommend? If the original holes are 3/8", should I go for 1/2" for a little extra clearance? I'm thinking I can use a zip-tie or bit of tape around the circumfrance of the bit to mark the maximum depth and prevent overshooting.

I'm guessing I wouldn't need to reapply any of the shielding paint for such small cuts? Or maybe I could just patch it with some shielding tape...

How are you running the piezo side of the master volume pot? Are you running the output from the 250k piezo pot to the input of one gang of the 10k pot? I like that Graph Tech (I assume it was Gray) prescribed a means of retaining the Fly master volume pot functionality with the Acousti-Phonic setup - But I'm skeptical that it's going to taper the same as it did in the Fishman setup (subsequent experimentation with resistor values to tweak the taper might be required; and I don't know that we have firsthand data from anyone else performing this mod).

Here's the diagram Graphtech sent me (it was indeed Gray!):

The taper consideration is interesting, I'll have to see how it works out. For my purposes, getting the taper 100% correct isn't really necessary b/c I tend to use the master knob as an on/off thing, but once I've got everything wired up I'll report back.

Basically, Graph Tech *really* wants you to use their switching jack - But it requires you to replace the barrel jack on your Fly with an oval jack plate after drilling out the hole (more surgery). So the compromise is to rewire a switching barrel jack as described in the second link. I don't know offhand if your Fly's current jack is set up with the parasite plug or other modification potentially preventing it from being used in the "switched barrel jack" scheme linked above. In the worst case scenario, you'll notice your Fly's battery draining despite being unplugged, then correct the switching issue.

I think I'm going to go with the Switchjack- the Parker Technical bulletin seems to confirm the sizing is right, and they're compatible with the Acoustiphonic per your link. That seems like it will be minimally invasive, and if I run into trouble I can always fall back to the traditional jack method.

[mmmguitar]

What size forstner bit would you recommend?

I recommend cutting for the footprint of the pot, rather than increasing the shaft hole diameter - Either 25 or 30mm, depending on the size of the pot housing you're accommodating the footprint of (switch footprints will vary more). You'll be using the existing hole for the pot/switch shaft to center the spur of the bit, and applying minimal pressure as the teeth do their job of counterboring some extra mounting depth to afford thread length while ensuring sufficient surface area for the pot housing to be secured against once the shaft nut is secured to the top (wobble room can cause knob and shorting issues). I've had success fitting pots with very short bushings into Flys in past experiments with this method (due to the only pot I could find for a particular job being intended for PCB-mounting) - In your case, you can just test-fit the pot or switch every few rotations of the bit until you're confident there are enough threads showing for the washer and nut to be able to secure things.

Concerning shielding: Though I've never noticed the bare wood from the counterboring introducing any extra noise, it comes down to whatever conductive material you decide to cover the work with - So long as the multimeter is showing continuity from the strings to the shielding to the ground lug of the jack, you can assume the shielding's golden until you hear something to the contrary.

And thank you very much for sharing Gray's diagram: Not every Fly owner will be willing to scrounge for "vintage"-correct parts; and even fewer do their own tech work - So your retrofitting project having an illustrated diagram is going to set a number of minds at ease (including the odd tech scrambling to find any info they can).

[parkinthepark]

As I did some more reading to help understand the original Fly wiring, I noted that the original master volume control appears to be wired into a part of the Fishman preamp, which I think is what necessitates the 10k resistance for the master volume pot. But that 10K resistance is really hard to find in a pot with standard dimensions (that will also accept a standard knob etc. etc.).

However the Graph Tech diagram puts the mag side of the MV control in line after the mag's regular passive controls, so (I'm assuming) a more traditional value can be used here. It's similar to a Gretsch wiring scheme that has individual volume controls for the bridge & neck pickups, followed by a master control for both. In those schemes, I see 500k pots in both the individual pickup positions and the master position.

So I would think I could follow a similar protocol here and use a more traditional guitar value?

My limited understanding of pot/treble interaction would suggest that the first pot the pickups "see" is the one that's going to be responsible for bleeding treble to ground, and as long as the 2nd pot (in this case the mag side of the master control) is the same value or higher, it won't have an impact on the treble response... right?

[mmmguitar]

@parkinthepark, everything you posted seems to be correct concerning the treble bleed and wondering why Gray included a 10KΩ mag volume pot prior to the preamp input stage (which is why I had the disclaimer about the taper not behaving the same). Because you had stated that you use the master volume pot in an on-off/kill switch capacity, I left it alone - That pot value will still effectively shunt everything to ground once turned to 0. Though I'm not privy to your conversation, I gave Mr. Bramwell the benefit of the doubt that he did so on the assumption that you would either be transplanting the old Fly pots over to the new circuitry, or replacing the pots and switches 1:1, including Ω values.

Though the technician's trick to finding the right pot is to build your own for the particular job (i.e., transplanting a compatible resistor track of the desired value into your pot of choice), you can absolutely go the Gretsch route with the magnetic pickup circuit; and have both gangs of the dual gang pot have the same, more common value (One benefit of the Graph Tech preamps is that it buffers the piezo signal for use with a common guitar tone pot value (250kΩB/linear taper) to be repurposed as its' volume pot (rather than using 5, 10, 25, or 50kΩ pots, as competitor's piezo preamps are designed for). Fly owners modifying the Fishman Powerchip for use as a master volume, for example, have more headaches to deal with; and have reported mixed results trying to cover the needed bases with a dual gang pot of identical Ω values.

In your case, you can try a dual gang 500kΩ pot as your master volume (again, with the treble bleed on the mag volume pot ahead of it in the signal path), and solder an additional resistor to the piezo gang of the pot if you want to tweak its performance.

[parkinthepark]

@parkinthepark, everything you posted seems to be correct concerning the treble bleed and wondering why Gray included a 10KΩ mag volume pot prior to the preamp input stage...Though I'm not privy to your conversation, I gave Mr. Bramwell the benefit of the doubt that he did so on the assumption that you would either be transplanting the old Fly pots over to the new circuitry, or replacing the pots and switches 1:1, including Ω values.

I think that's right- when I initially wrote to Graphtech I assumed the master volume would sit between the preamp and the output, so when I asked about pot values I assumed something like 25k, because that's what I've seen for active pickups... and then when he wrote back to that request he linked me to a pot available on Mouser that has more in common with the original than just value. So he probably assumed I was doing more of a restoration than a conversion. I've ordered dual-log pots in 100k, 250k and 500k and will tinker.

Though the technician's trick to finding the right pot is to build your own for the particular job (i.e., transplanting a compatible resistor track of the desired value into your pot of choice), you can absolutely go the Gretsch route with the magnetic pickup circuit; and have both gangs of the dual gang pot have the same, more common value (One benefit of the Graph Tech preamps is that it buffers the piezo signal for use with a common guitar tone pot value (250kΩB/linear taper) to be repurposed as its' volume pot (rather than using 5, 10, 25, or 50kΩ pots, as competitor's piezo preamps are designed for). Fly owners modifying the Fishman Powerchip for use as a master volume, for example, have more headaches to deal with; and have reported mixed results trying to cover the needed bases with a dual gang pot of identical Ω values.

That's part of the reason I've ordered a bunch of pots- if necessary I can jury rig the MV pot to have 500 for mags and 250 for piezo, but I've also got the resistor trick tagged in case that's sufficient... I've got a similar concept wired up in an HSS strat, using fixed resistors to trick the humbucker into thinking it's wired up to 500K pots vs. 250k.

All of the necessary parts and tools are on their way to me now- I'm hoping I can have everything put together & documented after a weekend or two worth of tinkering. If I can get it right, it could be a useful guide for other pre-refined owners looking to restore functionality or make their guitar a little more repairable.

[mmmguitar]

If I can get it right, it could be a useful guide for other pre-refined owners looking to restore functionality or make their guitar a little more repairable.

I’m excited for your success; and look forward to any part of the project you’re willing to share with us - This kind of work is most enjoyable for me when I’m not the one having to perform it .