Conductive phenolic board ?

[GainFreak]

Hi,

To make the story short there's a phenolic eylet preamp board in a tube amp. It has a relay that switches the channels and it pops. Traced the DC potential causing the pop to the relay. Now it gets interesting. With everything disconnected from the relay it has 1.8VDC on the contacts and the coil. When I turn the high voltage off (several nodes from 330 to 440V, 3 to 5 inches away from the relay) the potential disappears which bears the question is this phenolic board conductive?
When I touch with my DMM around the board edges I get anything from 50mV to 15V. The board looks as new, no burns, dirty spots or similar.

[Enzo]

So clean it. Take some Qtips and some isopropyl and wipe it down good. Any change?

[GainFreak]

I did some cleaning around the HV nodes and the relay without any effect. I also noticed that the more the amp works the DC potential goes up a bit but no more than ~2.5V. I'll wire a relay externally to check if it does the same and if not maybe move it off board but if the board is conductive that means all other parts have some DC as well although the amps sounds OK.
I have a fiberglass eylet board by hand for a project that never happened and when connected to the HV all eyelets have zero potential even those in close proximity. As a final solution I'm considering of making a replacement board using fiberglass although I would be glad to avoid that if possible.

[Helmholtz]

AFAIK, phenolic boards are not typically known to become conductive. Could you post a picture of the board/material?

[Mick Bailey]

Some phenolic board can absorb atmospheric moisture; I have a full sheet, bought new from a UK manufacturer and selected on the basis of voltage rating. The first amp build I did with it had scratchy pots and leaked voltage onto my guitar and made that scratchy. I did a low bake on the removed populated board and that worked fine for a while then went back to being conductive. I could get a reading anywhere on the board. There are older posts on MEF where other builders have encountered similar issues and I'd previously posted my own findings and observations. In the end I replaced the board with FR4 with no further problems.

[Chuck H]

Some phenolic board can absorb atmospheric moisture; I have a full sheet, bought new from a UK manufacturer and selected on the basis of voltage rating. The first amp build I did with it had scratchy pots and leaked voltage onto my guitar and made that scratchy. I did a low bake on the removed populated board and that worked fine for a while then went back to being conductive. I could get a reading anywhere on the board. There are older posts on MEF where other builders have encountered similar issues and I'd previously posted my own findings and observations. In the end I replaced the board with FR4 with no further problems.

I just looked up "phenolic board" and it's indicated that it's express purpose is moisture resistance and most is made from phenolic resin and cellulose fibers! Cellulose fibers in a substrate used as moisture resistant? I suppose that's because it's cheap and readily available? FR4 is made with glass fiber so there's nothing to absorb any moisture at all. But yeah, it seems to me that there must be the possibility of conductivity due to moisture if a phenolic board made specifically for electrical insulation isn't chosen. New information to me.

[Enzo]

Don't forget even the least absorbant surface, such as glass, can still have a contaminated surface. Just because the phenolic itself may not be absorbent, doesn't mean the surface can't be. Actually the term would then be adsorbent.

[Mick Bailey]

I used to experiment a lot with high voltage stuff - Tesla coils, Wimshurst machines and Ruhmkorff coils. One of the main issues is leakage across surfaces of otherwise excellent insulators. Dust, even finger prints can cause charge loss on a Wimshurst machine and the class capacitors (Leyden jars) won't hold a charge if contaminated. Much higher voltges than tube amps, but the same issues exist even at a few hundred volts and I always first check for contamination before blaming the substrate.

There are so many brands of phenolic board, but the main distinction I make is between the cotton or linen reinforced boards and those made from paper. I can't recall seeing problems with SRBP (synthetic resin bonded paper), but the fabric boards seem to have too much variation and would always need testing before using any of this material for a tube amp build. Most commercial amps are SRBP.

For better electrical and thermal properties asbestos bonded with phenolic resin was sometimes used for electrical work (and as a filer in Bakelite). Worryingly, it's still available

http://kitprom.com/engcatalogue/962

[GainFreak]

I spoke with an old tech I know and he said back in those days of tube TVs phenolic boards were fire hazard as sometimes they would catch fire. They got rid off them as as soon as fiberglass was available.

[Chuck H]

The material I use is FR4. I bought a 4x4 sheet of the stuff and I still have most of it!!! It's amazing how far it goes when you're not making amps for a living At this rate I may never run out

[Mick Bailey]

FR4 is the stuff I now use and it's superior in every way to phenolic. The remaining phenolic sheet I have gets used up for general engineering, or building low-voltage space-charge regen radios where it gives a nice vintage look.

[GainFreak]

To conclude the topic after cloning the board on 2mm fiberglass all problems are gone. No DC, no pops and/or other weird problems.