Using 3mm MDF board to make Turret Boards for guitar amps.

[DrNomis_44]

Here's something I've been doing some thinking about....


Okay, in some Fender guitar amps, the components are mounted on eyelet boards that use vulcanized fiber board as the insulator, some amps, like early Marshalls, use Turret boards that use an insulator made from Phenolic, and some use Turret boards that use FR4 Fiberglass as the insulator, which got me thinking, is it at all possible to use 3mm MDF board as an alternative insulator material?, my biggest concern is how much voltage 3mm MDF board can withstand before there are voltage-leakages between turrets spaced 3/8 inch, or 10mm apart?

[DrNomis_44]

I just did a quick search on Bunning's website and found some 4mm Brass eyelets, they sell packs of 50 X 4mm Brass Eyelets for Au$1.98, now Brass is not only a soft metal, it also happens to be solder-able, so that got me thinking if it would be possible to make my own Eyelet boards using some pieces of 3mm MDF board as the insulation material, and some 4mm Brass eyelets, Bunnings do also happen to stock sheets of 3mm MDF.

[Simon Barden]

It’s hard to find data on the resistivity of wood in any easily interpreted form, and MDF even more so. I’ve seen figures of 10,000 Meg ohms/cm for pure wood at 12% moisture content, dropping to 10 Meg ohms/cm at 20% moisture content. The glue layers in plywoid are supposed to be less resistive than wood. MDF is held together by Urea Formaldehyde, but again, initial searches give little info. Plus I have no idea how thickness affects the readings because you effectively get a lot of resistances in parallel, which gives a lower overall resistance. Moisture absorption from the amp sitting around unused in a damp or humid environment would be my biggest concern. It doesn’t take more than a few mV DC in the wrong place to start causing noisy pots on the amp, and getting towards a volt can affect preamp tube bias etc.

Even old fibreboard turret boards can be affected by moisture or other conductive fluids that have been spilt on them (beer probably being a culprit here), that may appear to have dried out but leave behind a patch of residue that’s far more conductive and causes operational issues with the amp.

There is also a much greater fire risk with MDF than say glass fibre.

I’d suggest that whilst it may be a cheap material to use if purely prototyping on a test bench, any incorporation into a proper amp is best done using standard materials.

[DrNomis_44]

It’s hard to find data on the resistivity of wood in any easily interpreted form, and MDF even more so. I’ve seen figures of 10,000 Meg ohms/cm for pure wood at 12% moisture content, dropping to 10 Meg ohms/cm at 20% moisture content. The glue layers in plywoid are supposed to be less resistive than wood. MDF is held together by Urea Formaldehyde, but again, initial searches give little info. Plus I have no idea how thickness affects the readings because you effectively get a lot of resistances in parallel, which gives a lower overall resistance. Moisture absorption from the amp sitting around unused in a damp or humid environment would be my biggest concern. It doesn’t take more than a few mV DC in the wrong place to start causing noisy pots on the amp, and getting towards a volt can affect preamp tube bias etc.

Even old fibreboard turret boards can be affected by moisture or other conductive fluids that have been spilt on them (beer probably being a culprit here), that may appear to have dried out but leave behind a patch of residue that’s far more conductive and causes operational issues with the amp.

There is also a much greater fire risk with MDF than say glass fibre.

I’d suggest that whilst it may be a cheap material to use if purely prototyping on a test bench, any incorporation into a proper amp is best done using standard materials.

To combat moisture ingress into the MDF board material, I was thinking of sealing it with some clear-coat spray, as used to clear-coat wood.


10,000 Meg Ohms is equal to 10 Giga Ohms, which to all intents and purposes is an open-circuit, but yeah I understand, I did some quick googling on the electrical properties of 3mm MDF but couldn't find much in the way of data either.


I have seen Formica being suggested as an alternative insulation material in at least a couple of online forum threads, I think Bunnings sells sheets of Formica.

I just read in another online forum that if I'm going to be making my own eyelet boards, 1/8 inch diameter eyelets are the ones to use, 1/8 inch is about 3.1mm in metric, 4mm would be roughly 1mm larger in diameter.

[Simon Barden]

These two videos from my favourite YouTube amp guy describe the issues he has with an area on an eyelet board on a modern reproduction of an old Fender amp, that's fairly conductive.

https://youtu.be/RMtCKjZ4Gwg

https://youtu.be/eJ2_YHyO61k

As he says, just because they used a material on a classic amp, doesn't mean it's still the best material to use.

I'd be wary of a varnish style spray as I have no idea whether it would improve or reduce the board resistance. Plus, around the eyelets, it's going to get hot and burn when you solder them, and the carbonisation certainly will reduce the resistance considerably.

I'd suggest if you want to carry on with the idea, you start with a simple test piece with a few eyelets in a group around 1cm apart, apply high DC across a couple (say 500v DC, so you'll need a suitable transformer and rectifier circuit to hand) and measure the DC voltages on nearby unused eyelets. Then repeat with your clear-coat spray on top and bottom and sides. Solder and desolder the eyelets a few times on each test piece to represent real-life usage and heating of the board and finish (e.g. you may find the eyelets become loose). Really no point progressing beyond any testing until you are certain the MDF is fit for purpose.

[McCreed]

As I have noted numerous on this forum, I know sweet-bugger-all about the technical aspects of electronics. I usually depend on you & Simon to edumacate me on such matters.

So my question is why are you looking to use MDF when you can blank turret boards for what seems like a very reasonable price to me? (Evatco)

There is also a much greater fire risk with MDF than say glass fibre.

This was my first thought even with my low level understanding of amplifim-ma-cation and electricity.

[DrNomis_44]

As I have noted numerous on this forum, I know sweet-bugger-all about the technical aspects of electronics. I usually depend on you & Simon to edumacate me on such matters.

So my question is why are you looking to use MDF when you can blank turret boards for what seems like a very reasonable price to me? (Evatco)



This was my first thought even with my low level understanding of amplifim-ma-cation and electricity.

I was mainly seeing if there were some suitable alternative materials I could buy locally, rather than order them online, of course, I could just order some sheets of FR4 fiberglass from Evatco, but the sizes of FR4 sheets they stock aren't really big enough for my purposes, I was thinking of making some Eyelet boards for my Soldano SLO-50 50W amp head project.

[Simon Barden]

I'd do it properly. You'll invest a lot of money for good components and valves, and you might save a couple of $ using MDF, but end up with a load of issues once you've put it all together. Why risk it?

[DrNomis_44]

These two videos from my favourite YouTube amp guy describe the issues he has with an area on an eyelet board on a modern reproduction of an old Fender amp, that's fairly conductive.

https://youtu.be/RMtCKjZ4Gwg

https://youtu.be/eJ2_YHyO61k

As he says, just because they used a material on a classic amp, doesn't mean it's still the best material to use.

I'd be wary of a varnish style spray as I have no idea whether it would improve or reduce the board resistance. Plus, around the eyelets, it's going to get hot and burn when you solder them, and the carbonisation certainly will reduce the resistance considerably.

I'd suggest if you want to carry on with the idea, you start with a simple test piece with a few eyelets in a group around 1cm apart, apply high DC across a couple (say 500v DC, so you'll need a suitable transformer and rectifier circuit to hand) and measure the DC voltages on nearby unused eyelets. Then repeat with your clear-coat spray on top and bottom and sides. Solder and desolder the eyelets a few times on each test piece to represent real-life usage and heating of the board and finish (e.g. you may find the eyelets become loose). Really no point progressing beyond any testing until you are certain the MDF is fit for purpose.

Leo Fender was a bit of a frugal person, who apparently used to always be looking at ways to cut costs in manufacturing Fender amps and guitars, so maybe he chose to use vulcanized fiber board as the insulator mainly cause it was cheap at the time.

[DrNomis_44]

I'd do it properly. You'll invest a lot of money for good components and valves, and you might save a couple of $ using MDF, but end up with a load of issues once you've put it all together. Why risk it?

Yeah, that's fair enough, and you do have some good points there, I've seen pics of some Turret boards that were made using sheets of white PTFE plastic.