DC voltage on input jacks on 5f2a and Tweedle dee

[charriman]

Hi, I have these to amps. When using either, the controls on any guitar are scratchy. I checked the input jacks and noticed that there is a little DC voltage present on the input jacks. If there is a buffer in the chain, the scratchiness goes away.
As both of these are recent builds (I built the 5f2a bought the deluxe), would this be indicative of something with the grid stoppers? The 5f2a is from a Mojotone kit so that is the layout I used with no mods.

[pompeiisneaks]

Grid stoppers don't stop DC. The only thing that can stop DC is a coupling capacitor. Its job is allowing AC signals to pass at certain frequencies but block DC. It is odd you're getting DC on the input, as both go directly to the grid of the tube, i.e. no DC voltage present that can get to the input?

~Phil

[Stevem]

Get a .47 200 volt cap and place it in series off of the hot of the input jack, issue over and if the amp does not have a grounded power cable you will be safe from getting shocks due to that also as a side benefit.

[charriman]

Get a .47 200 volt cap and place it in series off of the hot of the input jack, issue over and if the amp does not have a grounded power cable you will be safe from getting shocks due to that also as a side benefit.

the cap from tip to ground?

[Charlie Wilson]

Hi, I have these to amps. When using either, the controls on any guitar are scratchy. I checked the input jacks and noticed that there is a little DC voltage present on the input jacks. If there is a buffer in the chain, the scratchiness goes away.
As both of these are recent builds (I built the 5f2a bought the deluxe), would this be indicative of something with the grid stoppers? The 5f2a is from a Mojotone kit so that is the layout I used with no mods.

What material did you use for the circuit board?
CW

[xtian]

It is odd you're getting DC on the input, as both go directly to the grid of the tube, i.e. no DC voltage present that can get to the input?

It is known that some 12A?7 tubes can leak DC to the grid. You can try swapping V1 tubes to see if it makes any difference. This has happened to me.

[pompeiisneaks]

It is odd you're getting DC on the input, as both go directly to the grid of the tube, i.e. no DC voltage present that can get to the input?

It is known that some 12A?7 tubes can leak DC to the grid. You can try swapping V1 tubes to see if it makes any difference. This has happened to me.

Oh interesting, I've never encountered that before... is it considered normal operation or a defect in the tube?

~Phil

[charriman]

Hi, I have these to amps. When using either, the controls on any guitar are scratchy. I checked the input jacks and noticed that there is a little DC voltage present on the input jacks. If there is a buffer in the chain, the scratchiness goes away.
As both of these are recent builds (I built the 5f2a bought the deluxe), would this be indicative of something with the grid stoppers? The 5f2a is from a Mojotone kit so that is the layout I used with no mods.

What material did you use for the circuit board?
CW

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.

[charriman]

It is odd you're getting DC on the input, as both go directly to the grid of the tube, i.e. no DC voltage present that can get to the input?

It is known that some 12A?7 tubes can leak DC to the grid. You can try swapping V1 tubes to see if it makes any difference. This has happened to me.

swapped out multiple 12ax7s to no avail.

[xtian]

is it considered normal operation or a defect in the tube?

It's a defect.

[pompeiisneaks]

Gotcha, but if you put it in a later stage, the leakage won't be a problem due to the coupling caps etc... cool.

~Phil

[Charlie Wilson]

Hi, I have these to amps. When using either, the controls on any guitar are scratchy. I checked the input jacks and noticed that there is a little DC voltage present on the input jacks. If there is a buffer in the chain, the scratchiness goes away.
As both of these are recent builds (I built the 5f2a bought the deluxe), would this be indicative of something with the grid stoppers? The 5f2a is from a Mojotone kit so that is the layout I used with no mods.

What material did you use for the circuit board?
CW

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

[charriman]

What material did you use for the circuit board?
CW

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.

[charriman]

More info...
I misspoke when I said there was no voltage on the boards.

5f2a:
tip to sleeve input 1: 59.8mV
Board to ground: 16mV

TweedleDee:
tip to sleeve input 1 Bright channel: 179mV
Board to ground: 41mV

So yeah, there is voltage on the boards.
I did not build the TweedleDee so I don't know who the board was sourced from. It obviously is not a prefab 5e3 board from Mojo as the eyelets don't line up, but the board stock may have come from there. This amp was built by VVT.
The 5f2a is a MojoTone kit built by myself.

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?

[sluckey]

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...