Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Planobilly on October 10, 2016, 06:42:16 pm
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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
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loud low frequency hum
sounds like filament, or 60hz coupling. Any means to measure frequency with a meter or scope?
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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
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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)
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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
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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".
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The ground wire for the filter caps was just hooked on to another wire and not soldered.
Thanks for all the help.
Cheers,
Billy
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Hey man, THANKS for posting the resolution! I appreciate your taking the time to do so.
With respect, Tubenit
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The ground wire for the filter caps was just hooked on to another wire and not soldered.
Billy
Yep, that what do it.
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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