-0.8V DC on amp input with no B+ appied

scottosan

scottosan

Well-known member
I have been troubleshooting why my guitar pot is scratchy for the first few seconds after I take it off standby. What I have found is that I measure -0.8v DC on the V1a input while only the heaters are powered. I could understand if I had the B+ applied, but we are talking AC heaters hear. A few seconds after B+ is applied it goes to zero, but annoying none the less. I know I can resolve this with a cap on the input, but I prefer to diagnose the problem .

I have swapped all tubes. I have removed all tube except V1 and it's still there. I have swapped input jacks and validated my 1M load resistor is fine and I have redone grounding

:confused:
 
RACKSYSTEMS":7icwdols said:
What amp is it? What brand of preamp tube in the V1 slot?
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it 2203 based Metro kit. I inherited I added a few Cathode bypass caps for gain and that's it. It may have been there before I did anything I am just noticing since I am working on it. I am confuse why DC when the heater are AC. The DC goes away once B+ is applied
 
RACKSYSTEMS":1h6sr02u said:
What the preamp tube in the first slot?
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. I have tried JJ's and chinese gen 5 and multiples of each. I actually get -VDC on all of my grids for that matter with only AC heaters applied.

Thanks,
Scott
 
Post a gut shot.
Does it have a heater center tap, or faux center tap to ground? if so lift it and see what happens. Also if its not grounded, ground it, and check.
Does it have elevated heater circuit applied to the heater center tap, or faux center tap?
 
You get DC on the grid because the heater is heating up the cathode, causing it to emit electrons, which must go somewhere. It doesn't matter if the heater is run on AC or DC, it will still heat up the cathode the same.

Normally, when B+ voltage is applied, the plate will attract the electrons. However, with no B+ applied to the plate, the electrons gather in a cloud around the cathode. The grid, which is grounded through a large grid-to-ground resistor, pulls the electrons away, which causes a voltage drop across the resistor. That voltage drop is what you are measuring.

When you apply B+, the high voltage at the plate attracts the majority of the electrons, so the grid voltage returns to normal.
 
Yeah, just tested one of my builds, I get -0.4vdc (actually rises as the the heaters get hot) with heaters only. Totally normal, Brotater Chip!
 
CrazyNutz":nvqnnd6m said:
Yeah, just tested one of my builds, I get -0.4vdc (actually rises as the the heaters get hot) with heaters only. Totally normal, Brotater Chip!
Click to expand...
So, this is what I get for only having this amp and only using my favorite guitar. The crackling noise ended up being my bad volume pot. Replace and i dont get the scratching
 
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