Mysterious hum in a 2203 build (SOLVED...)

[whoismarykelly]

Solved - I'm an idiot. Missing ground on the speaker jacks.

I just finished wiring up a 2203 build and am in the midst of testing the amp and playing it for the first time.

I followed the test procedure in the Metro 2203 instructions for simplicity, although my build is not exactly the same as the one described there. A few different values here and there. Prior to installing the output tubes, all the voltages were correct. When I installed the power tubes and set the bias, the amp has a very loud hum when connected to a speaker.

• Whether or not there is a cable plugged in does not affect the hum
• Neither the preamp volume nor the master volume effect the hum
• If I pull the phase inverter tube the hum goes away. But pull V1 or V2 does not make the hum go away.
• Lifting one end of the NFB resistor on the board also made the hum go away and the amp sounded great.

I initially thought the hum might be related to the output transformer primaries being wired backwards when I was able to make it go away by lifting the NFB resistor. So I flipped the OT primaries hoping that would fix it but the hum was still there. Oddly enough there was not additional hum as well which is confusing. If the OT primaries were correct before, and the hum was from elsewhere, wouldn't reversing the primaries make it even worse? I have not returned the primaries to their previous orientation.

I reflowed all the joints in the phase inverter and double checked the wiring but nothing has managed to fix the hum. So Im somewhat at a loss for what to do. I have swapped a number of tubes around in the PI and preamp slots but nothing changes the hum.

I have also moved wires around in the amp to see if lead dress was an issue but nothing changes the hum.

Any help would be appreciated. Im a little bit at a loss because I feel like the problem must be something obvious yet Im not seeing it.

 

[WaltC]

it would help tremendously to know for sure what frequency the hum is. You can use a phone app or maybe a guitar tuner (If you know how to tell the difference of 60 cycle and 120 cycle hum on it) to tell the difference. It is important because 60 cycle is usually filament hum and 120 cycle is a symptom of B+ (DC) filtering problems. they are treated significantly differently.

the first thing I'd check is the filter cap on the node the PI is on to make sure it's wired correctly, soldered well and functioning as it should. But knowing the source of the hum (heaters/filtering) will save tons of work and frustration.

CAUTION: lethally high voltages are present and you need to be knowledgeable and extremely careful anytime you mess around with such.

 

[whoismarykelly]

it would help tremendously to know for sure what frequency the hum is. You can use a phone app or maybe a guitar tuner (If you know how to tell the difference of 60 cycle and 120 cycle hum on it) to tell the difference. It is important because 60 cycle is usually filament hum and 120 cycle is a symptom of B+ (DC) filtering problems. they are treated significantly differently.

the first thing I'd check is the filter cap on the node the PI is on to make sure it's wired correctly, soldered well and functioning as it should. But knowing the source of the hum (heaters/filtering) will save tons of work and frustration.

CAUTION: lethally high voltages are present and you need to be knowledgeable and extremely careful anytime you mess around with such.

Frequency counter on my DMM says the measurement across the speaker jacks is 119hz so it must be a DC problem then.

Is it possible that this is a ground loop issue in the preamp? I have a buss bar tacked to all the pots in addition to the preamp filter being grounded directly to the chassis.

 

[WaltC]

Congrats, that was pretty quick...

 

[whoismarykelly]

Congrats, that was pretty quick...

Considering it was one wire in an obvious place it took way too long

Here are some pics of the finished build:

 

[Tone_Terrific]

Looks nice.
The school of hard knocks is always open and when you have yourself as teacher the lessons can frustrating.

 

[donnyjaguar]

If only production amplifiers were assembled so neatly.... <sigh>