Very noisy volume pot on Princeton Reverb

[Rascalrick]

Hi there.  I recently completed a Princeton Reverb, which I added a mid control and bright switch to.  Everything works as expected, but the volume control crackles badly.  I replaced it with another brand new 1mA pot and still just the same.  I can measure upwards of 200mvDC when the volume is turned to full.  Pretty much 0vDC at zero volume.  I doesn’t matter which position I put the bright switch in.  The bright simply places a 150pf cap across the upper end and wiper of the volume pot.  Does anyone have any other suggestions of what might be going on?  It’s making me crazy!  :)

[sluckey]

Replace V1 first. If no joy, pull V1 and check for leakage voltage on the tone pot side of the three tone caps. Replace caps as needed.

The original Princeton Reverb does not have a bright switch. Do you have other mods that could be a factor?

Check for possible wiring errors. Some of the thin flypaper boards like Fender used are conductive and will allow voltages to show up where they should not be. Show us some hi-rez pics of your build.

[Rascalrick]

I’m having trouble posting my pics, as they are too large to post.  The only mods I made were the addition of the bright switch and mid control.  I used a Hoffman turret board, and have checked the wiring multiple times.  I always use the schematic and hightlight every connection as they are made, so I am pretty sure there are no missing connections.

[sluckey]

Did you try any of the suggestions?

[92Volts]

pull V1 and check for leakage voltage on the tone pot side of the three tone caps. Replace caps as needed.

Based on 200mV mentioned in the original post, there is leaking voltage. I'd guess a leaky cap-- 200mV is less than I'd expect from a missing cap or other serious error.

I'd second sluckey's advice to check voltage without the tube installed which would determine if the tube itself is the issue. I have seen a tube leak voltage from its grid on one occasion but I think a leaky cap is more likely in an old amp.

Where are you measuring that 200mV? If it's the wiper ("output") of the pot further testing is needed. If you see those voltages at the positive end of the pot, that suggests the voltage is coming from the tube (which needs replacement in that case). It wouldn't change like that with pot position if the voltage were coming from the tonestack.

[Rascalrick]

Hi.  Sorry ti took awhile, but I was at band practice today and am finally home.  I tried swapping V1 and the level of the scratching was reduced but still there.  I took measurements with V1 out, and there is no leakage on any of the tone stack caps.  With V1 back in I took more careful measurements.  At the wiper of the volume pot, 0mV at 0, 220mV at 7, and 107mV at 10.  I then swapped V4 with V1 and the issue remained.  I am using JJ ECC83MG tubes, and the one I tried in place at V1 is an EH 12AX7EH ( the one the had reduced noise ).
For the bright switch, I wired the 120pF cap using coax cable, as I wasn’t sure if the wire length would induce noise.  For the Mid control, I replaced the grounded resistor below the Bass pot with a 10kA pot, and replaced the .047 cap with a .022 cap.

[Rascalrick]

BTW, I took all my pics of the amps construction with my iPad, and apparently the images are too large to upload here.  Is there a way I can get around this?

[sluckey]

BTW, I took all my pics of the amps construction with my iPad, and apparently the images are too large to upload here.  Is there a way I can get around this?

Yes. Set your ipad to take lower resolution pics. Or reduce the size/resolution with software on your end. Or upload the pics to the cloud and just post a link to the pics.

[Rascalrick]

Ok, here are a couple of links to pics of the chassis...

https://www.dropbox.com/s/14apuqowyggemtc/IMG_0314.jpg?dl=0
https://www.dropbox.com/s/4krjhn6vccxdyx3/IMG_0315.JPG?dl=0

Hopefully this works!  I haven’t been able to upload .jpg files with any success.

[Rascalrick]

I just took more measurements with the EH tube in place of the JJ tube.  The readings now are 0mV at 0, 26.4mV at 7, and 4.6mV at 10.  The reason I am show the voltage at 7, is that is the highest reading between 0 and 10.  I am still stumped.  Obviously for some reason there is DC leakage with V1 in place, and it varies quite a bit between the tube brands.  Is there anything I am missing here?  Three different tubes all have DC leakage, so what in the surrounding circuitry could do that?

[sluckey]

The readings now are 0mV at 0, 26.4mV at 7, and 4.6mV at 10.

I'd call that zero. Is the volume pot still very scratchy?

This volume pot wiper should connect to the grid of the tube, probably pin 7. Measure resistance between pin 7 and chassis ground (meter connected directly to the tube pin) with the vol pot set to zero. Do the same with the pot set to max. What are the numbers?

[sluckey]

Nice pics. Found an error that will probably fix the dc voltage on the pot. See attached pic...

[Rascalrick]

Yowza!  Looks like a 1M resistor!  I had them all sorted into values, and I guess that one slipped into the wrong batch!  I’ll get that corrected.  Thanks for catching that...man I can’t believe I missed that when I was checking things.  I certainly know better!

[Rascalrick]

Good eye!  I got the 100K in there, and what do you know? ...it works just like it’s supposed to!  :). Thanks so much.  I’m going to have to be more careful sorting my components on the next build.  Geez...

[pdf64]

Grid current is often associated with low plate voltage (guess that would have been the case with a 1M plate resistor for that stage)

[sluckey]

I just noticed there is no bias filter cap on the board. What's up???

[Rascalrick]

Sorry, the pics I posted were taken a little while ago for the person I am building the amp for.  I had to order another bias filter cap, because the place I ordered parts from substituted a 400v cap which was WAY to big for the board.  Rest assured that cap is there now!