Fibre board just say NO!!!

[Mark]

Okay, I got your attention. This concerns tweed disease. My 6G7a Bandmaster has it as does many old Fenders. In the case of my amp, I can live with it.

In fairness to the person who made this board for me (eBay seller) I did question him about how he made and tested the board. He didn’t test the board with an insulation tester so I knew there was a chance the boards would be conductive. I also was advised by a kind member here that the new boards would be worse that the old boards. Okay so it doesn’t look good for me after this statement, but I thought surely they take time to become conductive?

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The multimeter says the board is fine. However, this isn’t an insulation tester. I don’t own an insulation tester, so I use the amp to provide the high voltage, in my case the DC voltage is 460VDC.

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I suppose this begs the question how much leakage is acceptable?

[Mark]

Next I look at the power supply board to see what readings I get from that board. I assume that the material will be consistent and I will get results in the same magnitude as the diode and bias board.

Again 460VDC is used on the further most eyelet and I will attach the earth connection to the middle eyelet.

I should point out that I considered the material underneath the boards to be conductive. I held the board in free air and got the same measurements.

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Next to use voltages on the main circuit card to see what the leakage looks like.

[Mark]

The main circuit card is looked at here.

Unfortunately this circuit card didn’t fare any better than any of the other cards did. I used the voltage on the voltage that was on the circuit card which was 233VDC.

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The points measured are important as DC voltage on these points results in noisy pots.

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However, seeing this level of conductivity on a board isn’t a good sign.

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I think we can still safely say Garolite G10 is still the preferred circuit card material. Unfortunately, these type of boards were a bad move for Fender, easier to assemble no doubt, but bad for the longevity of the amp.

Thanks for your time and replies.

[sluckey]

I wonder if that white foam is a factor?

[Mark]

I wonder if that white foam is a factor?

Thanks for your reply.
I thought of that and measured them in free air and it made no difference.

[bepone]

@Mark
load your insturment probe with 1Mohm to the gnd and touch all again, this will be better test

[pdf64]

It really is comically ridiculous stuff to use for a highish voltage circuit board substrate.

[Mark]

It really is comically ridiculous stuff to use for a highish voltage circuit board substrate.

Well, insulation testers exist for precisely the purpose of finding conductive paths. I think you need to explain the logic behind your statement. We know the Tweed disease is a real thing, so I don’t see your logic at the moment.

[Mark]

@Mark
load your insturment probe with 1Mohm to the gnd and touch all again, this will be better test

Thanks for your advice. The Fluke 77 multimeter has a high input impedance as it is. Surely, putting a 1 Meg resistor in series with the meter will increase the impedance thus reducing loading of the signal and the voltage present on the pins will be higher perhaps?

[xtian]

Thanks for your advice. The Fluke 77 multimeter has a high input impedance as it is. Surely, putting a 1 Meg resistor in series with the meter will increase the impedance thus reducing loading of the signal and the voltage present on the pins will be higher perhaps?

Forget the meter, just touch it with your tongue!

[bepone]

@Mark
load your insturment probe with 1Mohm to the gnd and touch all again, this will be better test

Thanks for your advice. The Fluke 77 multimeter has a high input impedance as it is. Surely, putting a 1 Meg resistor in series with the meter will increase the impedance thus reducing loading of the signal and the voltage present on the pins will be higher perhaps?

in parralel with the ground i was thinking, load every point with 1M to the gnd, which will discharge some residual or parasit capacitances from fibre board..

also you can measure capacitance between the eyeletts, because this is what is existing on the board, and is charging now with voltages. i think capacitances are very small and can be taken out of account, and this 1M can prove that.

otherwise we will see 

[Mark]

Thanks for your advice. The Fluke 77 multimeter has a high input impedance as it is. Surely, putting a 1 Meg resistor in series with the meter will increase the impedance thus reducing loading of the signal and the voltage present on the pins will be higher perhaps?

Forget the meter, just touch it with your tongue!

I’ll send you my address you can come over and demonstrate the technique.

[Mark]

One thing that may have gone unnoticed is the last post with pictures shows the new unloaded board with the same earth and voltage as is present on my amp. The voltages present on the 250pF treble cap eyelet is 804mV, while putting a 1M resistor from that point to earth may drain away the DC voltage. The eyelet with 804mVDC is attached to the treble pot. This has to result in scratchy noises in the pots, and the voltage may make its way to the next triode.

I have tried this sort of testing on Garolite boards are they perform beautifully, no stray voltage.

[WhopperPlate]

You know this comes up right as I was considering this factor with Bakelite components .

Bakelite resistance can increase with humidity and heat. One study I read showed a decrease from 1000m to 60m at one environmental extreme.

This crossed my mind that this isn’t ideal for components that are often typically mounted on Bakelite terminal strips , like high voltage power supplies .

I am building an SVT style bass amplifier right now with the 700vdc power supply , and I have opted to build g10 turret strips where ever possible for close mounting components at the socket pins . With so many environmental variables , why bother building something that will operate even more inconsistently? It gets HOT and it SHAKES. Inconsistency becomes apparent when you are torturing your gear with regular usage getting thrown around from gig to gig. Might not matter as much for guitar Joe playing crunchy blues rock out of his 50 watt combo, but to get a clean powerful bass response these issues will undoubtedly be more noticeable to a discerning ear .

To me it seems willfully negligent to utilize sub par board material, when we have stuff like g10 . I just use the old boards for making drilling templates for the G10 lol

[Mark]

Funny you should say that as during my first attempt at building my Trainwreck Rocket amp the circuit card was conductive. I got it from TAG RJ he replaced the card with Garolite and problem solved.