Hum from my Tube amp (300B) - Please help!

[Saffy]

How to eliminate hum from my 300B amp?
I've tried both a cheater-plug, and disconnected everything except amp and speakers - didn't help
Any suggestions are most welcome
Thanks

 

[raindance]

You probably need to adjust the hum balancing potentiometer or otherwise balance the heater grounding path. Is the hum about the same in both channels?

How much hum is there (from how far away can you hear it?) and what speakers are you using? 300B amps are not typically 100% hum free.

Unless something else is faulty, of course. Bad tubes, bad power supply capacitors, bad rectifier, the list goes on.

 

[kevinh]

You probably need to adjust the hum balancing potentiometer or otherwise balance the heater grounding path. Is the hum about the same in both channels?

How much hum is there (from how far away can you hear it?) and what speakers are you using? 300B amps are not typically 100% hum free.

Unless something else is faulty, of course. Bad tubes, bad power supply capacitors, bad rectifier, the list goes on.

Since the 300B is a DSirect Heated Triode you best bet is to adjust the bias on the tube, like raindance said.

 

[JeffS7444]

Got a schematic? And was it previously hum-free?

 

[raindance]

Since the 300B is a DSirect Heated Triode you best bet is to adjust the bias on the tube, like raindance said.

I wasn't referring to bias. Commonly 300B tubes are used in cathode bias circuits and bias isn't adjustable. But hum balance should be. Does it have adjustments?

Of course, bias could be an issue, depends on the design

 

[Saffy]

You probably need to adjust the hum balancing potentiometer or otherwise balance the heater grounding path. Is the hum about the same in both channels?

How much hum is there (from how far away can you hear it?) and what speakers are you using? 300B amps are not typically 100% hum free.

Unless something else is faulty, of course. Bad tubes, bad power supply capacitors, bad rectifier, the list goes on.

Thanks, no serviceable parts or adjustments available. Hum only heard from 12" away from speaker cone at 6/10 volume. Amp fairly new as are tubes. Rectifier appears fine, though is the weak part of the design. Tried other outlets with no success. Thank you

 

[SIY]

Who made the amp? Not everyone grounds things properly, and layout can also impact things negatively..

Is the hum a pure low tone or does it have some buzziness to it?

 

[raindance]

What amp is it? I've repaired many, but this is like flying blind.

 

[Saffy]

Who made the amp? Not everyone grounds things properly, and layout can also impact things negatively..

Is the hum a pure low tone or does it have some buzziness to it?

Who made the amp? Not everyone grounds things properly, and layout can also impact things negatively..

Is the hum a pure low tone or does it have some buzziness to it?

Audio Nirvana, yes there is definitely some buzziness to the hum.

 

[Saffy]

What amp is it? I've repaired many, but this is like flying blind.

Audio Nirvana 300B thanks

 

[SIY]

Audio Nirvana, yes there is definitely some buzziness to the hum.

OK, that points more to grounding than transformer pickup. Same in both channels?

 

[raindance]

I looked at a photo of the inside and it appears to use DC heaters, so no balancing issue. Maybe it just is the way it is. Does it sound OK?

 

[raindance]

You should also make sure it's designed for the USA mains voltage (120 volts +) and not 110 volts. It may seem trivial, but 10% high on heater voltage is bad. 10% high on HT is very bad and can blow smoothing caps on switch on. It also causes a cathode bias amp to run too "hot" and can cause hum and premature tube failure.

 

[zelig]

There is no cure for an indecisive amp.

 

[raindance]

And hum balance pots are possible with DC heaters, so there's that. There's a discussion at the asylum on this amp.

 

[Saffy]

I looked at a photo of the inside and it appears to use DC heaters, so no balancing issue. Maybe it just is the way it is. Does it sound OK?

Sounds great actually

 

[Saffy]

You should also make sure it's designed for the USA mains voltage (120 volts +) and not 110 volts. It may seem trivial, but 10% high on heater voltage is bad. 10% high on HT is very bad and can blow smoothing caps on switch on. It also causes a cathode bias amp to run too "hot" and can cause hum and premature tube failure.

Its for sure designed for the US

 

[Saffy]

OK, that points more to grounding than transformer pickup. Same in both channels?

Yes same both channels, actually sounds better with cheater plug om PC

 

[Saffy]

And hum balance pots are possible with DC heaters, so there's that. There's a discussion at the asylum on this amp.

The actual transformers hum on start-up, then the hum source goes away after a few minutes. I will try switching out rectifier tomorrow to see if that makes a difference

 

[Saffy]

Hi All, I still have a low frequency hum through my speakers using an Audio Nirvana 300b amp. The hum was not always there. Cheater plug helped, and have gone over my PCs and interconnects and can't find the culprit.. Any suggestions?