It's the stock fiberboard that came with the kit. Same with the tweedledee that's fiberboard as well. Not getting voltage on either board.
No voltage, zero volts anywhere on the board? That isn't even possible with vulcanized fiberboard. There is always going to be either a little or a lot depending on the quality of the board material and I have had nothing but bad luck with Mojo fiberboard. What you may try is lift the 68k input resistors off the board and run a wire straight to the tube. If the voltage and static go away then most likely the plate supply voltage is leaking a little onto the 68k resistors.
CW
If that does fix it, then what is the purpose of the grid stoppers? Pardon my ignorance. Are those in the circuit to block that DC? Or is it something else? Seems like it's unavoidable if you use those boards then.
Sorry, i'm still learning and the 5f2a is my first build. I'm just trying to learn.
Grid stoppers help stop radio frequency interference into the signal and oscillations etc. they still pass DC just fine.
charriman wrote: ↑Tue Sep 22, 2020 2:54 pm
So.. short of scrapping the boards and rebuilding them, is the "adding a cap to the input" a decent way to filter out that DC? If so, just a .047 from tip to ground on one input the way to go? Or in line with the wire going from the tip to the grid stoppers?
You would need two caps since you have two input jacks. Not gonna be simple.
The better solution is to remove the 68Ks from the board and put them directly on the input jacks. Then connect a wire from the junction of the two 68Ks directly to the tube. This is how Fender does it on the blackface amps. Very easy and neat. Just follow this pic. You'll likely have to rotate the input jacks to line up as shown in the pic...
Did Fender make that change in the BF amps to allieviate that issue? IOW, was DC leakage on the input a common enough problem with tweeds that they changed that?
charriman wrote: ↑Tue Sep 22, 2020 4:38 pm
Did Fender make that change in the BF amps to allieviate that issue? IOW, was DC leakage on the input a common enough problem with tweeds that they changed that?
No and no. DC leakage was not a problem with the new boards that Fender used. But it became a common problem years later when the boards began collecting moisture. Once a board becomes conductive from moisture all kinds of gremlins would appear. Weird stuff that can be difficult to troubleshoot if you don't know about conductive boards.
The only way to be sure your board is conductive (ie, leaking dc) is to remove all components from the suspect eyelet. Then check to see if there id voltage on the naked eyelet. If so, the board is conductive.
Your quick/easy fix is to follow the pic I posted. And be aware... The dc on your input jacks may be coming from the tube. It's not uncommon for cheap 12AX7s to leak just enough voltage to the grids to cause your symptom. Easy to check though. Measure the voltage on your jack, then pull the tube. If voltage goes away, then it's the tube. If voltage stays, then it's the board.
charriman wrote: ↑Tue Sep 22, 2020 4:38 pm
Did Fender make that change in the BF amps to allieviate that issue? IOW, was DC leakage on the input a common enough problem with tweeds that they changed that?
No and no. DC leakage was not a problem with the new boards that Fender used. But it became a common problem years later when the boards began collecting moisture. Once a board becomes conductive from moisture all kinds of gremlins would appear. Weird stuff that can be difficult to troubleshoot if you don't know about conductive boards.
The only way to be sure your board is conductive (ie, leaking dc) is to remove all components from the suspect eyelet. Then check to see if there id voltage on the naked eyelet. If so, the board is conductive.
Your quick/easy fix is to follow the pic I posted. And be aware... The dc on your input jacks may be coming from the tube. It's not uncommon for cheap 12AX7s to leak just enough voltage to the grids to cause your symptom. Easy to check though. Measure the voltage on your jack, then pull the tube. If voltage goes away, then it's the tube. If voltage stays, then it's the board.
Well pulled V1 and fired it up. I guess it’s not the tube and is the board.
Do I just unsolder the existing grid stoppers and mount them to the jacks as above. Can I use just regular wire or should I get some shielded wire to run from the jacks to pin 2?
charriman wrote: ↑Wed Sep 23, 2020 8:31 pm
Well pulled V1 and fired it up. I guess it’s not the tube and is the board.
Do I just unsolder the existing grid stoppers and mount them to the jacks as above. Can I use just regular wire or should I get some shielded wire to run from the jacks to pin 2?
Unfortunately, I have a feeling from reading this thread that the issue is not going to be rectified by rolling preamp tubes to find one that doesn't have DC on the input grid, but I think you are on the right track with a large DC blocking capacitor like .047uF or .1uF in series after the apex of the 68k grid stoppers. The issue sounds like the plate voltage in that first input tube is too low and is causing DC to leak through to the grid and down the cable to your guitar making the pot scratchy. Take a look at the attached input circuit for this Kay K-506 amplifier (one of the greatest and most underrated amps ever made). It has a different arrangement to the typical Hi/Lo jacks that we are all used to withe the difference being that after the grid stoppers the incoming signal travels through a .05uF cap to block DC from traveling down the cable. I would move the grid stoppers to the jacks as well just to get them off the board and eliminate the conductive eyelet board as a variable. I'm pretty sure you can do a similar arrangement with the 5E3 circuit with a single blocking cap that would block the DC from the plate voltage dropping too low. Shielded wire from the input grid of that first tube to the added capacitor wouldn't be a bad idea just to be on the safe side as the input grid wires should probably be shielded just to keep the noise down any which way you wire this circuit. With a pedal buffer in the signal chain, the output capacitor of the buffer will block leaking DC from reaching the guitar volume pot which is actually the most "non-invasive" fix. However, the buffer must be engaged for that to work which may not be ideal for your application.
This is also the way to fix DC on the guitar volume pot if you run a Marshall (or other amp) on a Variac and the overall B+ is lowered. Blankenship did this with their Vari-plex, which was ostensibly a 50 watt Marshall Plexi run on a supplied Variac, but with an additional filament transformer so the heaters remain stable at the right operating voltage. The scratchy guitar volume pot can also occur with Jose (or other high gain) mods where the plate resistor for the first 12AX7 gain stage is relatively large (330K) and the overall B+ is low like a late 70's JMP. This is what happens in the Kay and what i am assuming is happening with your two tweed Fenders.
Hope that helps!
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"Genius manifests itself. You got a hammer. You either build a Cathedral or you build a shithouse." - Carl Schroeder
Another vote for conductive boards. Since both amps have similar fiberboards I think that's more likely the issue than them both having faulty tubes, but the tubes are easy to rule out. If you probe around on the circuit board with a probe to ground, can you get voltage readings? I mean from the fiberboards themselves.
All my Silverface amps had boards become conductive. Even cleaning and heating them only worked temporarily. Parts of one of the boards I could get ~50VDC readings, inches from the eyelets. It gets very humid where I live, which is why I will use fiberglass boards for everything. I was under the impression the modern 'fiberboards' wouldnt have issues, but fiberglass seems to be more rigid and less likely to become conductive.
It's true i've lost my marbles and i cant remember where i put them
JD0x0 wrote: ↑Mon Oct 19, 2020 11:16 pm
Another vote for conductive boards. Since both amps have similar fiberboards I think that's more likely the issue than them both having faulty tubes, but the tubes are easy to rule out. If you probe around on the circuit board with a probe to ground, can you get voltage readings? I mean from the fiberboards themselves.
All my Silverface amps had boards become conductive. Even cleaning and heating them only worked temporarily. Parts of one of the boards I could get ~50VDC readings, inches from the eyelets. It gets very humid where I live, which is why I will use fiberglass boards for everything. I was under the impression the modern 'fiberboards' wouldnt have issues, but fiberglass seems to be more rigid and less likely to become conductive.
I moved the grid stoppers to the input jacks. That fixed it. I wonder why those mojotone fiber boards are so susceptible to being conductive? It wasn't an issue right after I finished it a couple of months ago. It just stared all of a sudden. I guess they just need the amp to be turned on and used a few times to start the conductivity. Hmm.
