Guitar Amp Hum Question

[Trout]

Hi Guys,
Ok I am back at it on another project, But I have a couple questions about what to expect.
Primarily, I am building a Deluxe 5E3

I have everything punched mounted and partially wired up. Heres my question. I have the entire Output & Power sections completely done. up to the volume Control. I plan on using a slightly different Pre-amp section so I connected Pin 2 of the 12AX7 to the wiper(center) on what will be the master volume pot.

1 (left)outer pin of pot to ground, Other (right)outer Pin of pot will be my input.
I Decided to power up the unit at this point to confirm functionality of the work to this point.

So as it sits, I have a 12AX7 Driving a pair of 6V6's into a 8 ohm cab.
Should I expect hum without a sources connected?
The Amp does respond as expected when the input pin of the volume pot is touched.

BUT, I thought If the pot was rotated it should kill the open input hum?
Gene

[dhaen]

You're right, with the grid grounded, there should be no hum. Hum could be caused by supply ripple, a bad grounding scheme, or even induced directly into the output transformer.

Assuming no wiring mistakes:
Quick check list:
Is there hum without the 12AX7?
If yes, check for ripple, and check transformer orientation.
If no, does the hum change with the pot setting?
If worse at low volume setting, check grounding.

Are the heaters floating? They should reference to ground some way.

[Trout]

Whew, Got it.

Da olde guy had a brain glitch, I had a sheilded lead from pin 2 (grid) to the pot, The outer shield to outside connection on pot, Inner to wiper. BUT, forgot the ground!!! and therefore the shield only contributed to the problem. Attached ground, Problem solved!

Sometimes I wonder why I just didnt stick to RC Aerobatics, At least I was a national champion at that as well as a top designer. But NOOoo I got bored and switched to this stuff

Eh, either way, I am having loads of fun learning. Your response got me looking at the grounds and bingo!
gene

BTW Grid shorted = Dead silent

[tubelab.com]

RC Aerobatics, bet thats expensive when things don't work right. My buddy flew the RC pylon races. Grown men cry when things go wrong there. He gave that up too, lots of broken parts.

[Trout]

RC Aerobatics, bet thats expensive when things don't work right. My buddy flew the RC pylon races. Grown men cry when things go wrong there. He gave that up too, lots of broken parts

Honestly, In my particular position it wasn't bad. I had a sponsor on the radio equipment during my last 5 years. I flew my own design 5 of those years, And since I owned the kit manufacturing co, It was fairly easy to keep actually costs at a minimum. And of course all trips to competitions were advertising expenses.
In 11 years of competition, I lost 2 Models, Both valued just over $2500.00 each if you had to buy them. But actual cost was about 1/10th.

Really besides regional contests lacking any serious competition, The primary reason I got out was basically age (old eyes). Though I could literally close my eyes and know the orientation of the model it had become an issue due to long term exposure to the sun starring at the sky 2-6 hrs a day " Nearly EVERY Day". Even Now, 5 years later, there is no such thing as a bright enough work area. Had I not used the best protection of the times, No doubt I would be legally blind. I just had my first restriction put on my DL this past May, At age 51??

gene

[tubelab.com]

After 53 years of life in the Florida sun, I have similar problems especially at night. Not to mention skin cancer.