Conductive circuit card

[Colossal]

I had the same thought re roughing up the surface of black G-10 to give it the look of vintage fiberboard. Beware though, as there have been reports of that material being conductive too, perhaps due to the added black pigment. I think buying from a reputable manufacturer and source would minimize the possibility of a disappointment.

Yikes Martin, surely for G10/FR4 to be conductive would mean some kind of counterfeit specification (I'm looking at you China)??

[pdf64]

Thanks for the reply Phil, Hoffman only sells G10 boards, I suspect Marsh amps sells the fibre board, but I haven't gotten a reply from them.

I get the impression Marsh isn't a customer focussed vendor https://www.thegearpage.net/board/index ... t-26378250

[dorrisant]

I have some of that CED gray paper boards and they are terrible to drill through, punching would work.

Mark

CNC cuts them clean as a whistle.

[Mark]

I did some noodling tonight lifting the caps out of the board and putting wire on them to move them to the tone controls.

Without a doubt the board is conductive, with all the tone controls maxed I get 10mV on the top of the volume pot.

I listened for scratchiness and any other ill effects but the amp works quite well despite the conductive board.

I'm thinking it isn't worth the hassle at the moment, thanks for all your assistance and the links. If the condition continues to deteriorate I know my options.

Thank you for all your help.

[M Fowler]

I have some of that CED gray paper boards and they are terrible to drill through, punching would work.

Mark

CNC cuts them clean as a whistle.

You rascal

[pdf64]

Agree, 10mV isn't worth worrying about.
Bear in mind that Vdc can appear in a tube's grid circuit due to its grid current, with some tubes it can be high enough to cause a problem. So the tubes need to be pulled to eliminate that being a source.

[lovetone]

I have had a similar problem with a Bassman 6G6B.

The problem I had centred around the components for the bass channel.

The main cause of the problem was the supply to the two 100K anode resistors, I lifted the connection above the board so that it was floating in the air supported by the resistor leads. This reduced the problem to next to nothing. It was only very slightly noisy when first turned on and was completely gone after about 5 minutes.

I also installed a screen lead from the input jacks to the grid, the problem with this was that the wire ran under the board and also picked up low level voltage.

I spent time removing the dc connections from the board one by one until I found the point where the volts where leaking into the board, it might be worth a go. I dought that its the whole board that is conducting.

[dorrisant]

Only questions I have: What caused the conductivity? And will it continue, maybe in other areas? This is the reason I have stopped trying to repair and had no returns after replacement.

Seriously, if you get to the point that your like to try a replacement board, let me know. I can hook you up.

[lovetone]

I think the conductivity is caused by moisture ingress over the years, this is probably more likely if the amplifier is not used for long periods.

I have seen this only a few times and has always been the boards that have not been wax dipped from the factory.

[Mark]

I thought that too regarding moisture, but I've had the board in an oven to dry out. The other thought is the black colour is made from carbon which is conductive. I've seen phonetic boards that were conductive too.

[lovetone]

I have had a Marshall 1959 a late 60's amp that had the same problem around the phase splitter, for that one I had to remove the 100K - 82K junction and leave it suspended above the board.

There was no visible reason for the leakage into the board.

[Mark]

Lovetone, you are lucky as Chris Merren sells replacement circuit cards. I would be inclined to get one while they are available.

http://www.merrenaudio.com/perf_boards_and_end_blocks

[Colossal]

Lovetone, you are lucky as Chris Merren sells replacement circuit cards. I would be inclined to get one while they are available.

http://www.merrenaudio.com/perf_boards_and_end_blocks

Mark,

I am not sure Chris has any perf board left, or at least that he is selling. I contacted him years ago about it and it was no, sorry. That page has not been updated.

[pdf64]

I’ve come across SFs with waxed boards that leak dc. It’s just a pathetically inappropriate material to use as a tube amp circuit board substrate.

[Mark]

Champ as the high voltage source.

IMG_1219.JPG

Champ as the high voltage source.

IMG_1218.JPG

Here is a shot of me testing the board using the rail of my

IMG_1217.JPG