Fender Bassman clone start up issue..SOLVED

[Planobilly]

Hi,

I have a new Bassman clone someone built that has a loud low frequency hum. The circuit is pretty close to a 6G6 schematic. The voltages are all within reason. The hum is present with all the preamp tubes removed. OP transformer is wired correct. I assume this is some wiring issue but not sure just where to look first. The guy who built the amp did a good job and I don't see any obvious problems.

Before I spend the time to go through the whole amp, any ideas of where to look first?

Thanks,

Billy

[shooter]

loud low frequency hum

sounds like filament, or 60hz coupling.  Any means to measure frequency with a meter or scope?

[Planobilly]

Hi Shooter,

Just by ear it sounds like 60 hz. And yes I have a good scope.

I guess I will bite the bullet and go dig into the issue.


Thanks,

Billy

[eleventeen]

99% of all tube amps need to have a grounded heater CT or an artificial CT (the 2 qty 100 ohm resistors) or they will hum like a piece of construction equipment. It's an easy thing to leave out.....the first time. Then it's pretty tough to forget (on a build, which this is)


By the way....did this amp EVER work (without the gross hum) or did it develop the hum? Just asking you to prod your thinking.


I once worked in a factory that made pretty simple record players and CRT displays and I was the tech who fixed stuff that came fresh off the assembly line that simply failed the very first go/no-go test. It was important to realize that this gear NEVER worked right. It didn't "break". It NEVER worked properly. Many times I could fix 50 things in day if the gal who stuffed D22 in got every single one of them backwards that day. (The union guys hated me for that) 

[Planobilly]

Hi Guys,

The hum is 120.1 hz. The power transformer is not marked...I don't know who the mfg is. It has a center tap on the heater winding and I tested the grounding which is good.

Upon further inspection this looks like a 5F6A design and even perhaps one of Hoffman's. The turret board is blue and there are two turrets that would have  100 ohm to ground resistors for a PT with no center tap on the heaters. The control grid resistors are 1K and not 470 ohm like Doug shows on his drawing. I assume there may be several people who make these turret boards but I am not sure.

I don't currently know if the amp ever worked correctly. The cathodes are connected directly to chassis ground...no 1 ohm resistors and there is are turrets on the board to hook it up that way. Other than that, so far, it looks just like a Hoffman design.

It is getting late...i'll get back into this tomorrow.

Thanks,

Billy

[eleventeen]

If you have actually measured the hum freq and got 120 Hz it can't be anything except the power supply. First or second e-cap. End of story. Especially * 10 if there is still hum with the preamp tubes gone!


It *IS* possible for e-caps to be brand new or near-new and bad. It *IS* possible for them to be marked (and thus installed) backwards. Both really rare, but not impossible and not "never". 

[Planobilly]

The ground wire for the filter caps was just hooked on to another wire and not soldered.

Thanks for all the help.

Cheers,

Billy

[tubenit]

Hey man,  THANKS for posting the resolution!   I appreciate your taking the time to do so.

With respect, Tubenit

[bnwitt]

The ground wire for the filter caps was just hooked on to another wire and not soldered.
Billy

Yep, that what do it.

[Planobilly]

Hi Tubenit,

Your welcome!

Sometimes the simplest issues can be difficult  to resolve. This is especially true for me when I don't know things like, who built this amp? Who provided the components? What schematic was used? Has it ever worked? Was there a gipsy woman involved?...lol

Well...lol...that's why we get paid the big bucks...lol

Cheers,
Billy