Fiber board bites me in the ass for the LAST time

[Randall]

As some may remember, I had my heart broken several years ago on a DRRI tear down and rebuild when I used Mojotone black fiber board, and had to rip it all out because it was conductive from day one. I said I would never use it again, but I had heard the grey board Traynor used wasn't a problem, and maybe it was the carbon black that was the issue. So, I had a piece of the grey on hand when I was building a Bassman combo a couple of years ago, so I used it. I sold it to a guy who really loves the amp, but he lives on the water, and since he is a native Floridian fishing charter captain, doesn't really see the need for air conditioning much of the year. You know where this is going.

The amp came in for a check up, and the freaking board is conductive. I hit it with my heat gun, and that just made it go nuts for a while, voltages everywhere, noisy pots, funny sounds. I let it set for a day and it is better, but I know in my gut it's screwed. This dude plays all over town, and sings my praises to other players. Even if I never build another one, I can't have this amp acting like this, so I have to tear this board out and put in garolite.

It's just a bassman, so the board isn't very complicated, but damn, I am madder than a hive of bees shut off before closing time. I will NEVER use any kind of fiber board EVER again!

F++K!!

[Chuck H]

That's new to me. I live on the water now but I gave up on the fiber board material before I ever got here. I never had a problem with the grey board Traynor stuff. I did have a Traynor with a black board that went hinky though. So my experience is with the black boards and drying them out never helped. This is a solid confirmation for me that moisture can play a roll though. It was never the case in my travels but I'm very aware that it's been reported. This is the first time I've read about a probable moisture issue with a board that isn't black tinted. And being from you, that's good enough for me. Sorry for the trouble you're having. But what you propose is how I've handled it any time it came up. Bad board = new board that won't fail. (<period)

[J M Fahey]

I had it the other way

I started making Guitar amps in 1969, using Pertinax, a brown phenolic paper material which is *excellent*, even by modern standards.

All the time dreaming about using "that black fiber material FENDER uses"

Wow! ... if LEO uses it, then it must be NASA quality or better!!!!

Boy, was I disappointed when I finally got some

Sometimes when I was out of Phenolic, I used a piece of Formica which I could get around the corner.

Very good too, only problem is you have to be somewhat careful on penalty of cracking it, ugly radial cracks emanating from each eyelet if you apply way too much pressure.

I already mentioned that the first Guitar-amp-dedicated tool I ever bought was a bench type eyeletter which keeps working faithfully for the last 52 years now, go figure.

[Mick Bailey]

I gave up with fibre and modern SRBP after repeated problems with new builds. I switched to black FR4 that I take the shine off with a Scotchbrite pad. Never had a problem since. It's interesting that you mention Formica - I had a very early Two-Rock for repair that was built on wood grain Formica

[Chuck H]

Being a "form follows function" guy I bought a 4'x4' sheet of un tinted fr4 years ago. Well, I don't know that it's un tinted actually. It's that dull yellow/green, washed out Gatorade color that's ubiquitous for that material. Not the deeper green. I've only built maybe twenty boards from it so I still have a lot left. If anyone I build for ever asks for a color for the board I suppose I'll need to buy something else. Otherwise "I" think the stuff is lovely because it's never caused me a problem

[Helmholtz]

It doesn't take much board conductivity to upset a tube amp.

Even a board resistance as high as 100 Megohm between a plate connection (at 200V) and a typical 1M grid circuit will raise the grid voltage by 2V.
That in turn may more than double plate current and lower plate voltage by >100V.

[Randall]

On a side note, I'd like to use eyelets on my garolite instead of turrets. Is anyone aware of there being the proper sized eyelets for this application? My garolite is 1/8".

[Chuck H]

On a side note, I'd like to use eyelets on my garolite instead of turrets. Is anyone aware of there being the proper sized eyelets for this application? My garolite is 1/8".

Hoffman sells "tall" eyelets for 1/8" board material.

[Randall]

Thanks Chuck. I just grabbed 100. Clunky website, tho.

[Mick Bailey]

It doesn't take much board conductivity to upset a tube amp.

I was wary of using black FR4 until I saw it being used in the Fender hand-wired reissues and that's when I started using it. The caveat is there's a black ESD material that has a lower resistance - fine for SS circuits at low voltage, but unsuitable for tube amps. The 3mm board I looked at was given as 100M Ohm at 1cm for the ESD version, but the type that was recommended by my supplier was stated as having a resistance of 200G Ohm at 1cm. This doesn't factor and moisture absorption though. All types exhibit some, though how this affects the board resistance I can't say. It's never been an issue for me with either the green 'natural' boards, nor the black. For my own personal builds I use the green because it's more readily available and cheaper. The black gets used in replica builds to give a similar appearance to fibre board. Well, it's black and that's as far as it goes - it doesn't go wavy when bolted up.

[Helmholtz]

I was wary of using black FR4 until I saw it being used in the Fender hand-wired reissues and that's when I started using it. The caveat is there's a black ESD material that has a lower resistance - fine for SS circuits at low voltage, but unsuitable for tube amps. The 3mm board I looked at was given as 100M Ohm at 1cm for the ESD version, but the type that was recommended by my supplier was stated as having a resistance of 200G Ohm at 1cm. This doesn't factor and moisture absorption though. All types exhibit some, though how this affects the board resistance I can't say. It's never been an issue for me with either the green 'natural' boards, nor the black. For my own personal builds I use the green because it's more readily available and cheaper. The black gets used in replica builds to give a similar appearance to fibre board. Well, it's black and that's as far as it goes - it doesn't go wavy when bolted up.

Regular FR-4 (typically green colored) is the top grade PCB material in the electronics industry.
In my professional life I never heard of problems with FR-4 moisture absorption, i.e. except from surface contamination effects.

ESD material needs to have some conductivity to avoid build-up of static charge.

I think the problem with (some) fiberboard materials is the residual content of lignin.
The lignin is contained in the wood fibers the fiberboard is made of.

Lignin is hygroscopic, meaning that it attracts and holds water moleules.

[Mick Bailey]

Moisture absorption of FR4 at prolonged high humidity is fairly well researched, but for most practical purposes it's classed as non or low-absorbent.

[loudthud]

Back when oscilloscopes used tubes exclusively, Tektronix built their own ceramic terminal strips. These worked pretty well until scope bandwidths got up to the 50 MHz region. The input attenuators, which are very tricky to build, were mostly made on multi-wafer ceramic rotary switches. When the transition to solid state circuits was made, it was found that the input attenuators (with PCB mounted switches) could not be made on regular FR-4 because even the slightest amount of moisture affected high frequency response in ways that made calibration impossible. Although pure water is an insulator, it has a very high dielectric constant and is non-linear.

In guitar amps, FR-4 usually isn't a problem unless you need some combination of very high resistance, (>100 MegOhms), low capacitance (<20pF), and/or high frequencies (>20MHz).

[jack briggs]

Lignin is hygroscopic, meaning that it attracts and holds water moleules.

Lignin is more hydrophobic than hygroscopic.


cheers,

[Mick Bailey]

I wasn't aware there was any electrical fibreboard made from wood - I always understood it to be processed from cotton.