Name that hum - vibro champ build

[zbones]

hey board, my first post here - although i've been led here for answers for some time!

i've completed a vibro champ build and i've got some unwanted HUM! I need help identifying it so i can cure it!

Here's the symptoms:

- turns up with volume

- goes away with power tube unplugged

- becomes horrid and loud with the guitar cord unplugged from the amp!


anyone experienced this before or have any prescriptions? thanks in advance.

[Chuck H]

Everyone has experienced build hum. The trouble with this is others experiences aren't likely to help you. Hum has many sources. The first thing we need to know is what the frequency is. (60Hz or 120Hz)

The amp humming with an open input jack means the ground there isn't connected properly. You may have other grounding errors too. Recheck grounds. It doesn't matter if you already have. Do it again. And keep doing it until you find mistakes.

[zbones]

Thanks for the pointer man. Im a noob n appreciate the help! Will recheck all grounds connections this aftnoon.

Since the hum is close to a B in pitch, that would make it 60hz? I was able to lower it by repositioning the pre tube grid leads, but it . Maybe the artificial heater CT is in order as well

[Chuck H]

If you have an artificial CT make sure the resistors are of the correct value. I've seen this mistake a couple of times now where the resistors are rated in kohms rather than ohms. So check that too.

Was this built from a kit with instructions? If so, which one? I ask because an artificial CT wouldn't apply to the original circuit.

[zbones]

I was gonna ask how important were the exact values in the artificial CT - as i have 680 ohm resistors around, but no 100s... will go pick up a few later today probably. not k

the build is happening on a mojotone board and a custom chassis i had available. i omitted the tone stack altogether. it's def a noob build and i've made some learning mistakes, like the filter cap placement - i feel that hanging it down with the tubes wasn't the greatest idea as i'm crossing extra wires at the power supply. but other than the hum issue which i'll get thru it's sounding quite great (even with a cheapo 6v6 and a 10" 8ohm speaker i've salvaged from a peavey practice amp). it took me a bit of time to figure out the power tube bias and I'm not sure if the preamp tube can be biased for more gain (im not getting that much breakup after 5, some but i've heard other builds with a considerable amount more).

I've added a couple photos so you can see what i've got so far. I plan to make face/back plates for the chassis as well once I have the cabinet built.

EDIT: i've been sourcing the parts locally for the most part and reusing some hardware from some older amps I have around

[mort]

those control knobs are sexy. what are they off of?

[zbones]

not sure, i found these at the local electronics store a while back. i remember seeing something similar on an amp on google but i can't recall...

[loudthud]

Radio Shack sold a knob like that back in the 70's.

[zbones]

Wicked. Saw a bag of these on ebay a while back for good price but they were from china... Last time i ordered a batch they didnt fit on any of my pots unfortunately

[loudthud]

I know what you mean. I got some pots and "Marshall style" knobs from Weber. The knobs have a 0.251 hole but the pot shafts are 0.235. The knobs wobble so bad when you put them on the pots, I doubt they will stay on.

[Chuck H]

I know what you mean. I got some pots and "Marshall style" knobs from Weber. The knobs have a 0.251 hole but the pot shafts are 0.235. The knobs wobble so bad when you put them on the pots, I doubt they will stay on.

That's one of the things masking tape is for

[zbones]

Lol, masking tape is keepin half of my house together. My knobs are less than useful the other way, i got a bag of knobs with openings a tad smaller then the size of my pot shafts so they just refuse to fit altogether

[Chuck H]

I've carefully drilled out knobs with a hole too narrow too. If the difference isn't extreme it usually works.

[zbones]

hey Mort, saw ur post on the 'flying' ground bus, nice work! http://music-electronics-forum.com/t32412/

[booj]

Is one of your jacks internally switched to ground with no input? If not, that could cause stray hum to be injected to the first stage.