Master Volume help please

[seedlings]

Hello.  Lurker, learner, first post.  I've built 3 amps from scratch with pretty good success, but this time I've tweaked the circuit for a Galaxie 10 http://tinyurl.com/galaxie10 for a friend, and ended up completely rebuilding on turrets.  I really don't have a schematic drawn, just scribbles on the original, but changed values for bypass caps & resistors, coupling caps, replaced tone controls with 330K/330K divider, but it is essentially the same standard amp design, and it sounds very nice... at any setting until the master volume is at 99%.  When the Master Volume gets to 100%, the amp cuts out a little bit and hum/noise shows up.  I replaced the 1M volume pot, but the trouble is the same (made sure to solder with the pot turned down all the way).  If you have an idea, please let me know.  If pictures would help, I can post some.  Thank you.

CHAD

*edit: OT upgraded to a 125DSE

[tonewood]

Do you have R15, the 1.5 K as a grid stopper in yours? If not, you may want to try adding one- could even be larger. 

[seedlings]

Do you have R15, the 1.5 K as a grid stopper in yours? If not, you may want to try adding one- could even be larger. 

Yes the 1K5 is there.  I'm attaching a very crude schem drawing.  I just read 3V leaking past the .01uF cap going to the master volume... how much voltage is normal?  I have less than 1V leaking to the gain pot.

CHAD

[sluckey]

There should be zero volts DC getting to the MV 0r the gain pot.

[seedlings]

There should be zero volts DC getting to the MV 0r the gain pot.

I understand that ideally 0V should be leaking, but do you regularly measure 0V?  I'll be replacing the cap either way.

CHAD

[sluckey]

Yes. Definitely change the cap feeding the MV. 3 volts is not acceptable.

As for the gain pot, you say "less than 1V leaking to the gain pot". What is the exact reading? A few millivolts is acceptable. You would have to have 2 leaky caps feeding the gain pot and that's unlikely. Probably OK with this one.

[seedlings]

After much head banging, come to find out the cap was not passing DC.  New cap, same problem.  There is a few volts from the CHASSIS to the wiper.  NOT from the star ground to the wiper.  I had not grounded the back of the pots, and after doing that, the problem has mostly gone away- probably need a better solder joint though.

Learning continues...
CHAD

[jeff]

After much head banging, come to find out the cap was not passing DC.  

Caps should block DC not sure what you're saying. Try disconnecting the wire to the outer lug of the volume pot and see if you have voltage on it when it's full up.

[seedlings]

After messing around some more last night, I can't quite figure how grounding the back of the pot to the star helped, but if anyone knows why, or has a method/logic for grounding pot housings, I'm all ears.

CHAD

[ThermionicEngine]

After messing around some more last night, I can't quite figure how grounding the back of the pot to the star helped, but if anyone knows why, or has a method/logic for grounding pot housings, I'm all ears.

CHAD

FWIW I run a ground buss wire along the back of the pots and ground it to my central grounding point on the chassis.  Make sure the pot covers will take solder, sometimes they have to be brushed or sanded to make a good solder joint.  I also run grounds from the board to the buss wire so mostly everything ends up at one ground point.  Usually this procedure eliminates ground loops and sometimes mystery oscillations.  Cheers, Larry

[seedlings]

I'm still trying to figure out this volume problem.  To restate, MV works fine up to 9.5, but in the last fraction of a turn the amp gets a buzz like when the cord is plugged in with no guitar.  Here are gut shots.  I realize I have housekeeping to do to secure coupling caps and wires, etc, but I need to get this last annoyance fixed first.  If you have time to invest, please make suggestions, and I'll try them and get back with results.

Left to right in the first shot are:
Power/Standby
Lamp
Low Power switch experiment (two 40K 10W in parallel switched in the HT)
MASTER VOLUME!!!
Boost (grounds preamp bypass caps)
Tone bypass cap selector
Gain
Input




Close-ups of master volume



I tried a different shielded cable from MV to 6L6 screen and that didn't help.  1M MV pot has been replaced.  Re-flowed joints...

CHAD

[seedlings]

With one end of a test lead clipped to the cable shield, the trouble clears.  The other end of the test lead is not terminated.  I'll just have to bury that in the chassis somewhere...?

CHAD