DC Leakage Mystery

[vmazz]

I posted this question onanother thread about leaky caps, but I thought this might be a more appropriate area for my search for help solving a mystery....I dont have a lot of experience with leaky cap testing . But I have recently had some trouble with symptoms in an amp which sound like dc leakage on coupling caps. When I test with my multimeter I get around 5 volts of DC on the grid side which should be 0 I think. I have replaced 4 different caps in this same spot and they all end up(after a day or 2) to show dc voltage. I assume this is leakage from the plate? But very wierd that the same spot would all result in leaky caps. SOme are brand new mallory others nos polystyrene. When tested with Eico 950 they dont show much leakage up to 400v (rated at 400v caps). Is there another way that dc voltage would be getting to this node. ITs the v1b to v2a coupling cap in a Marshall 2203 style circuit. Any suggestions appreciated. Thanks.

[nickb]

Is the grid 'leak' resistor open or wiring/solder joints to it OK? In other words is there a path for the grid leakage current to get the ground?

Edit: I'm assuming the is a coupling capacitor so I'm talking about the resistor on the coupled to stage. Do you have a schemo of that section?

[Enzo]

So much better if we had a schematic with part numbers like C1 and R2. But... Is the grid side tube a separate tube from the plate side? If so, pull the grid side tube and see if the voltage still arises. That would mean it was the circuit, and nick's idea of an open grid return is a good one. if however the voltage no longer appears with the grid tube missing, then we suspect the tube is leaky.

[vmazz]

Marshall_Rockstah_Mod_5.pdf This is the basic platform of the circuit. the coupling cap where it says v1a ( I know there reversed? but 2nd gain stage basically)from the plate 22n cap at the junction of the 22n and 470k resistor. is where I am testing and no I am not lifting one end of cap Iam testing as is in the circuit. I am using a FLuke 16 Multi, The wierd thing is the symptoms (lower vol, blocking distortion sound, gated sound etc) keeps coming back with new caps and then it test and get the dc voltage at that point. All other coupling caps test appropriate with dc voltages on the plate side.

[nickb]

With the power off measure the resistance from the V2a grid to ground. Is it ~470K?

[vmazz]

I will check Nick.
Enzo the plate and grid are separate tubes, I will pull the grid tube

[vmazz]

Tube pulled tested good for leakage. I replaced anyway. Grid to ground 480k. Also so far today I havent had any symptoms of leakage since I changed the cap this morning.So no DC Leakage at that point right now.

[Tom Phillips]

You should still do the test that nickb described in post #5. Measure from the tube socket grid pin itself to chassis ground. Not just across the resistor.

It would also be good to pick one of the two threads that you have running in parallel on this subject and post a link in the other thread to close it out. Then everyone can follow in one place AND you will get the best overall help that way.
Cheers,
Tom

[vmazz]

I did the test per nick b suggestion, although its not exactly from the grid because I have an additional 220k resistor on the grid so I went from that junction to chassis ground and get 480k. ....aaannnnddd. by now I am starting to hear blocking/gated sounds shen testing the amp this afternoon. ..almost like clockwork..its starts happening after running the amp a few times and powering down and sitting then come back and have the issue later. I will pull it and run some more test. Any more suggestions greatly appreciated. THanks.

[Tom Phillips]

I did the test per nick b suggestion, although its not exactly from the grid because I have an additional 220k resistor on the grid so I went from that junction to chassis ground and get 480k...

That's confusing. We need to verify the resistance exactly from the grid to ground. Can you post a schematic of your exact circuit?

Edit: Clarification - By "exactly" I mean the circuit path from the grid to ground. Not the exact resistance. The exact value of resistance is not critical but it is necessary that their be a proper grid leak path to ground.

[vmazz]

here is the basic circuit with some of my adaptations.

[Pdavis68]

Is that a switch between ground and the V1a cathode resistor & bypass cap? You can't let the cathode float like that. The resistor should go to ground. I assume you want a switch on the bypass cap. On your gain pot, you might try reflowing the solder on the pins. If the ground wire gets loose, that's going to cause it to float.

[vmazz]

Some of the notes on this schematic are not as it is switches on the cathode yes but always with resistor and other cap in parallel. Mainly posted because Tom Phillips was confused about the grid stopper and voltage divider layout after the coupling cap in the 2nd-3rd stage area (v1a-v2a in the schematic). What else besides leaky cap or grid leaky tube could cause dc voltage to appear at the junction past 2nd coupling cap and 470k resistor? Thanks for your help.

[Enzo]

When I suggested pulling the following tube, it wasn't to have the tube tested, it was to see if the circuit put that voltage in the grid pin while the tube was absent.

[vmazz]

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!