Other Guitars

Martin GT-75 Electric – HELP!

1

It's my first post here, please be gentle...

I must admit I feel a little strange posting a non-Gretsch question here, but my '66 Martin GT-75 is about as close to Gretschness as you can get and still start with letter other than "G" – full hollowbody, dual DeArmond 2000 "DynaSonic" pickup, Bigsby vibrato (and bridge). ANYWAY, here goes:

I'm wanting to clean/renovate the electrics, and the tone controls (one per pickup) are unlike any I've seen: dual "stacked" potentiometer with a large capacitor on one and a small cap on the other. I've heard it referred to as a "parametric EQ", but (being a sound engineer) I have a tough time believing THAT description.

That said, I'd sort of like to know what I'm getting into before I yank everything out of the body. Does anyone have a schematic for a Martin GT-75 or something with a similar type of tone control? Any information would be greatly appreciated.

2

Welcome aboard!

You've piqued my interest. I knew those guitars were Gretschish with the DeArmonds and all, but the tone control setup is a new one on me. They're not too common of instruments. As you're a sound engineer, drawing out the circuit diagram/schematic shouldn't be to hard for you. There'll be someone here that can figure out what's going on with it. You also might try contacting Martin Guitars, too. I doubt the "parametric" circuit as well. It might be a high cut/low cut or tank circuit. I have an old Ampex amp that has a hi cut/low cut tone control. Good luck!

3

Gretsch used that same stacked tone pot circa '56/'57. Which makes me think it was a DeArmond concept rather than a Gretsch or Martin idea. DeArmond supplied complete harnesses to Gretsch, and probably did to Martin too.
Google "Gretsch stacked tone pot" and you'll find more info.

4

Some early 60's Guilds come with stacked tone pots too - even non-DeArmond pickup ones. It's not a great idea, as there's no "neutral" position on pots like that, you're cutting bass or treble whatever you do with them.

Best option, IMO, is to remove the whole original harness and put it in a ziplock baggie should you ever sell the guitar to a nerdy collector type, and put in a new harness with normal treble-rolloff tone controls.

5

I don't even believe the OP owns this guitar. Why? NO PICS!!!

7

I don't even believe the OP owns this guitar. Why? NO PICS!!!

– charlie chitlins

Oh, ye of little faith...

Tried to post pictures of The Red several times to no avail. Why you'd imagine I'd post about a guitar I no longer own is beyond me.

As Rod The Mod once sang: "Look how wrong you can be."

8

from the great dearmond resource-

http://www.musicpickups.com...

cheers

– neatone

Many thx for the excellent pic of the wiring harness - much appreciated. I have one from Reverb that was too lo-res to be of use.

9

Some early 60's Guilds come with stacked tone pots too - even non-DeArmond pickup ones. It's not a great idea, as there's no "neutral" position on pots like that, you're cutting bass or treble whatever you do with them.

Best option, IMO, is to remove the whole original harness and put it in a ziplock baggie should you ever sell the guitar to a nerdy collector type, and put in a new harness with normal treble-rolloff tone controls.

– WB

Excellent advice, since that's what I've almost decided to do. I wanted to give the "stock wiring" a chance to prove itself before ditching it, as someone in a position of authority at some time and place obviously thought it was A Good Thing To Do.

Most likely some Emerson pots and treble bleeds (with tone caps to take off some of the edge) will be The Route I Travel. Does anyone have a good schematic that keeps each pups vol/tone controls "separate" when the pickup toggle is in the middle?

(FWIW - after collecting for decades, I've thinned out the herd to six "keepers", which I'm sure my children will sell once I'm pushing up daisies. Until then, The Six will be hanging on the wall, ready to play...)

10

Welcome aboard!

You've piqued my interest. I knew those guitars were Gretschish with the DeArmonds and all, but the tone control setup is a new one on me. They're not too common of instruments. As you're a sound engineer, drawing out the circuit diagram/schematic shouldn't be to hard for you. There'll be someone here that can figure out what's going on with it. You also might try contacting Martin Guitars, too. I doubt the "parametric" circuit as well. It might be a high cut/low cut or tank circuit. I have an old Ampex amp that has a hi cut/low cut tone control. Good luck!

– wabash slim

Thanks for your suggestions. Correct on schematic sketch, but I'm hoping to get some feedback before yanking stuff out - once I've pulled it (assuming I do) I will certainly share what I find.

I've got a call into Paula Primrose @Martin Service, will let everyone know what she says.

11

from the great dearmond resource-

http://www.musicpickups.com...

cheers

– neatone

BTW - the black guitar shown just above the wiring harness pic on the musicpickups.com page is NOT a GT-75. The single cutaway version shown is a GT-70, while the GT-75 is a dual cutaway.

12

from the great dearmond resource-

http://www.musicpickups.com...

cheers

– neatone

One note: what this pic doesn't show clearly is that each stack has a honking-big reddish-brown cap (I assume) on the left side behind the solder lugs. You can just barely see the edge of it, peeking around the left side.

13

The GT-75 is quite the rare bird. I did some research a year or so ago for a friend who inherited one from her uncle after he passed, and it seems that Martin only produced 1,555 of them, beginning in Jan 1966 with the final example out the door toward the end of 1968.

I have some write-ups and information which I am happy to share if you're interested. PM me and I'll forward them on to you.

Here's a pic of my friends inheritance, actually an F-65, but very similar... somewhere along the way, the original plexi bridge bass was replaced but otherwise, this example is pretty much original. (and NOT for sale at any price)

14

The only thing this could be , in regards to the stacked cap situation "parametric bullpucky"; is a combination treble cut/ bass cut . One pot probably has a .022 cap and the other a .0022 on the opposite lug of the other pot or similar situation. This supplies the illusion that the treble cut pot turns up the bass while the bass cut pot turns up the treble. Very cool approach to tone control. I advise pairing them with resistors as well. G& L have also used this approach before. Google" bass cut treble cut" for specific applications and many more diagrams.

15

Today's update: I finally got a few hours last night to work on the GT. I decided I would attempt to do some pot cleaning and see what happens.

After removing the bridge pup for access, I pulled all four pots out of the body (leaving the toggle switch in place, since I'd replaced it a few months back). All four pots looked relatively good albeit covered in white powder oxidation. Used a small toothbrush and vacuum to clean the exteriors.

Out came the DeOxit D5. Each pot got a good squirt and 120seconds of back n forth. Another squirt and let them sit for about 10 minutes. Another good flush and more BnF. A drop or two of TriFlow, test and re-install (easier to write than to do, since the entire wiring harness is encased in steel springy-stuff conduit).

So far - so good. With the exception of the tone control wiring that allows a single control to affect both pups when the toggle switch is centered, I couldn't be happier. Pots are now very smooth (tones less so but there are two pots to turn) and things for now are sonically as it's Maker intended.

Thanks again for all of the valuable feedback (pun intended) - more as/if it develops!


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