AB165 Bassman buzz

[Randall]

Same one I replaced diodes in, I replaced the filter caps because I thought the buzz I was getting thru both channels was probably 50 year old power supply caps. Nope. Here's what it does. Quiet with both volumes down, but raise either one and I get what sounds like an input cord plugged in with no guitar, or an unused input jack not shorting to ground. To make things simpler, I pulled the second preamp tube and am focusing on the first channel for now.

In tracing this noise, I find that grounding the first grid of the first channel makes no difference, but grounding the second grid (or volume pt wiper) silences it. The volume passes signal as it should with guitar, but the noise follows it. Grounding the opposite lug of ground on the volume pot kills signal, but not noise. Lifting the wire from volume wiper to grid at socket makes it go into full noisy loudness. It does it with different preamp tubes.

Must admit I am at a loss here. I have checked grounds around the preamp circuit.

http://www.kbapps.com/audio/schemati...ssmanab165.gif

[The Dude]

If it has one, check all of the ground connections on the brass plate under the pots. Those tend to crack loose once in a while.

[Chuck H]

1) Have you tried replacing the preamp tube?
2) Are you sure you have you checked every ground in the preamp/s?
3) Are you sure the filters/decoupling caps were wired correctly when you started?
4) Do ALL of the tone controls work as they should?

[Randall]

I have checked and desoldered all ground connections in preamp and brass plate. I have tried a different preamp tube. I have confirmed filter caps are wired correctly. Tone controls work in first channel as they should, second channel tube is pulled so I think that is not in play.

Why would the noise be so loud when the grid has no connection, as I describe above? I should think it would be quiet. I'm wondering about filament noise now.

[Enzo]

A floating (disconnected) grid is an antenna picking up any noise in its vicinity.

[Randall]

OK, got it. While thinking about filament noise, I realized this PT was a replacement, and along with the perfectly sloppy wiring (no twisted pairs, just spaghetti with heater and secondary wiring tangled together) it had neither a heater CT, nor virtual tap. So I rewired everything neatly and added two 120 ohm resisters, and viola! I also found and corrected that the power tubes heater wires were opposite of each other.

And while I was at it, I corrected the 3 prong AC cord wiring, which was switching neutral, because that was the length of wire that would reach, and had safety ground connected to transformer bolt via unsoldered wire nut. A real mess this was. I blew a lot of my own time chasing this one, but I learned something, so it's good.

[Chuck H]

OK, got it. While thinking about filament noise, I realized this PT was a replacement, and along with the perfectly sloppy wiring (no twisted pairs, just spaghetti with heater and secondary wiring tangled together) it had neither a heater CT, nor virtual tap. So I rewired everything neatly and added two 120 ohm resisters, and viola! I also found and corrected that the power tubes heater wires were opposite of each other.

And while I was at it, I corrected the 3 prong AC cord wiring, which was switching neutral, because that was the length of wire that would reach, and had safety ground connected to transformer bolt via unsoldered wire nut. A real mess this was. I blew a lot of my own time chasing this one, but I learned something, so it's good.

Great! But...

The fundamental change was to the filament lead dress and circuit. You reported buzz, not hum? So what corrected the buzz? And was hum ever an issue? So what was learned?

[Justin Thomas]

I (still) don't have a scope and didn't have much of a meter at the time, but my Bassman 100 lost the artificial CT for whatever reason (I foget - power tube, I think?) and it was DEFINITELY a buzz, and not a hum. Me playing at such "CRAZY" (thanks, Bob!) volumes, I usually don't notice hum right away. But buzz, oh yeah... Cuz I play strictly single coils in a digital wireless world. I would imagine the "timbre" of the noise may be affected by how good your power is where you live or other such factors, but me losing that CT was a buzz, alright. Like, "pissed-off-giant-hornets-in-a-paper-bag" buzz. I've also heard that in some places, the mains can resemble a sawtooth wave more than a sine wave?

Maybe one day I'll get adventurous and disconnect the resistors to remind myself; maybe my memory is faulty, but I'm pretty sure it was a buzz.

Justin