DC Leakage Mystery

[nickb]

understood ..well when I pulled the tube the dc voltage wasnt present to begin with. In anycase I switched out the tubes , and the phenomenon has come and gone I unsoldered one end of the the cap to check that way. No leakage. there must be something else going on. I will double check pulling the grid side tube the next time the symptoms come back(probably tomorrow unfortunately). THanks Enzo! I really value your expertise!

Here are some more suggestions.

Is the 100pf cap connected as shown? Any chance it goes to the plate and is leaky? Verify your have things wired up as you have drawn.

Put your meter on DC current and clip from grid to ground. Note the reading.
Now start lift/pulling connected things to find out what reduces the current.
If that fails could there be contamination on the V2 base?

On reflection, I have my doubts that average meter will be sensitive enough for this to work. I guess you can try and let us know. It really sounds like the grid is floating since it takes some time to go wrong.

[vmazz]

the 100pf bypasses the 470k, does not go to plate, replaced already anyways. The one odd thing that might be a clue is that the dc voltage im measuring fluctuates between 2-6 volts at the node between coupling cap and 470k resistor between from 2nd stage plate to 3rd stage grid. When I pulled tubes v2 i got more dc volts up to 10 volts, same when I pulled v1 . Any ideas?

[nickb]

the 100pf bypasses the 470k, does not go to plate, replaced already anyways. The one odd thing that might be a clue is that the dc voltage im measuring fluctuates between 2-6 volts at the node between coupling cap and 470k resistor between from 2nd stage plate to 3rd stage grid. When I pulled tubes v2 i got more dc volts up to 10 volts, same when I pulled v1 . Any ideas?

1) With the V2 removed the voltage should be zero. Since you have replaced the caps we can eliminate them. That only leaves the other things I mentioned. You have to get in there and remove things one at a time- the purpose is to isolate the fault.
2) Something else is bothering me. V2 grid cannot really go higher in voltage than it's cathode due to grid conduction. What voltage do you see on the cathode when this happens? I have a suspicion there is an open in your grounds somewhere.

[vmazz]

When you say "open in your grounds" , do you mean a ground connection that should be grounded but is not?(silly question I guess but want to make sure I understand)

[nickb]

When you say "open in your grounds" , do you mean a ground connection that should be grounded but is not?(silly question I guess but want to make sure I understand)

Yes, exactly that Take a look at the cathode of V2a. That should be 820 ohms to (all) grounds. What is the DC voltage on it?

[vmazz]

1.1 vdc at pin of cathode v2a .3 vdc at pin of grid of v2a , 1 vdc at junction between previous coupling cap and 470k resistor...???THanks very much for your input Nickb.

[nickb]

1.1 vdc at pin of cathode v2a .3 vdc at pin of grid of v2a , 1 vdc at junction between previous coupling cap and 470k resistor...???THanks very much for your input Nickb.

This is getting confusing

In post #1 the grid was reported at 5V and now it's 0.3V? Still seems high and the 1V isn't right either. That suggests the current is coming from that path.

Since the voltage at the 22n coupling cap is higher, disconnect the plate end of it. Measure and report the DCV at

(1) The grid of V2a
(2) The junction of the 220k and the two 470K's
(3) The junction of the 470K and 22nF

[vmazz]

Sorry ..it is confusing. In the first post I was referring to the junction between coupling cap and 470k resistor...and ...the voltage has come and gone , and seems to fluctuate. I will take measurements per your directions and post back . Thanks!

[vmazz]

1).5 vdc
2).3 vdc
3) 1 vdc (although it fluctuated between 2.5 and 1 vdc for a minute, I didnt see what other voltages where at the time)

[nickb]

1).5 vdc
2).3 vdc
3) 1 vdc (although it fluctuated between 2.5 and 1 vdc for a minute, I didnt see what other voltages where at the time)

Just to confirm - you did have the 22nF open at the plate before measuring?
If yes, then something else is leaking/connected to node (3). Very carefully check connections, look for incorrect wiring or contamination.
If no then disconnect and repeat the measurement.

It's unexpected to me to have the grid node (1) higher than node (2). Any chance this is an error?

In an earlier post, you said the DC resistance from grid to ground was 480K. I didn't twig it at the time but that is wrong. It should be 690K (220k+470k). Again, check the wiring to find out why that is. It might help if you were the measure the DC resistance to ground at the three nodes. Can you do that too please?

[vmazz]

Well I did pull the 22nf open at the plate. And measured again....1).001 vdc 2) .0 vdc 3) .001 vdc but the cathode pin was still 1 vdc? Thank you very much for your help with this.

[nickb]

Well I did pull the 22nf open at the plate. And measured again....1).001 vdc 2) .0 vdc 3) .001 vdc but the cathode pin was still 1 vdc? Thank you very much for your help with this.

Those voltages are good. The cathode should be 1V.

I need you to measure the DC resistances to ground at all three nodes, since the earlier measurement wasn't right, just in case.

If those check out OK then looks like you have a batch of bad 22nf capacitors. Can you give us the part number, description and where you got them?

[vmazz]

1)670k 2)460k 3)880k . I dont think its the caps, I have used many different brands and tested on Eico 950B with leakage test. I think its conductive old turret material( I recycled the original amps turret layout, this is built from a Magnatone MP3 carcass!) I will replace first 3 stages boards and post back. Thanks!

[nickb]

1)670k 2)460k 3)880k . I dont think its the caps, I have used many different brands and tested on Eico 950B with leakage test. I think its conductive old turret material( I recycled the original amps turret layout, this is built from a Magnatone MP3 carcass!) I will replace first 3 stages boards and post back. Thanks!

Those numbers makes more sense Is it worth a trying good clean with iso-propyl alcohol before doing the surgery?

[vmazz]

Definitely was dc getting across a conductive turret board surface. Up to 30 vdc on the board itself! Anyways no need to worry about batches of bad caps…whew! Thanks for all the help guys! Another victory for the Music Electronics Forum posse!