buzz problem

[adason]

hi diy friends!
i need your help
i am restoring old magnavox se tube amp from an old console
the output tubes were replaced with new one, but the preamp tube and rectifier are the same, i do not have the way to test the old tubes, i just dont have new ones to replace them
the power supply capacitors have been replaced as well

now the problem is that i can hear the buzz in the speakers when volume pot is around the middle, there is no buzz when volume is on minimum or maximum, it comes slowly with increasing volume and slowly goes away with increasing volume
i could have the volume on maximum and use signal source with volume control, but thats not how i want to use the amp
its going to be part of biamp system so i need volume control to match with solid state amp on woofers

any idea what the reson is for the buzz and how to cure it?
thanks
ed

 

[anatech]

Hi adason,
Sounds like the leads were moved around the volume control. If shielded wire was used, the ground may be broken. They should be grounded on one end only. Watch for heater wires in the wrong location.

-Chris

 

[adason]

hi anatech,
thanks for the advice, I will redo the wireing and i will be very careful with ground

i did not touch the heater wires, but i noticed these are not twisted or shielded, maybe I should do that

plus it would not hurt to make it dc for preamp tube heater, i guess

the other thing is that feedback, those two leads from secondary of otput trafo are not shielded as well, should i use shielded wire? i guess i should, that is very important signal

will report the results, I can't wait to work on it, otherwise its nice sounding amp
ed

 

[anatech]

Hi Ed,
Leave the heaters AC. DC heaters are a pain. If you have a hum problem after the wires are dressed properly, isolate the heaters from ground and bias them up to about 30 VDC. That should pretty much end that problem. If you go DC for the heaters you will end up with extra heat and a 120Hz buzz. Not worth it.

Leave everything original until you fix this problem.

-Chris

 

[adason]

hi anatech,

good news
i found the cause of the buzz
still do not know why the buzz was slowly incresing with volume pot turnig up and started to decrease till full volume, as a matter of fact, it was quitest with the volume all the way up though

anyway, it was not my wireing
i rewired it a couple of times as best as i could with one side of shieled wire grounded as it should be and it did not cured it
i was just about to take a picture and post it how it looks like and desperately seek other solution, but than....

i realized that the magnavox console amp as I obtained it was most likely not complete, it was missing some parts
i noticed that when i put my hand near the imput tube, the buzz was more audible, but when i touched the steel chasis with other hand, the buzz decreased, so there is obviously something missing, maybe the shield or something, so i quickly attached gounded aluminum plate over the imput tube and all the buzz is gone!

now i realize a while back i encountered similar thing with Lafayette integrated amp, there was some sort of steel L shaped plate behind the front, if you removed that, the amp hummed

anyway, I am happy, for the sound was already spectacular and now i can finish my biamp open baffle project!
thanks for help anatech,
ed

 

[anatech]

Hi Ed,
Great! That's using your common sense and your noodle.

I assumed the shields were in place if equipted. There is something to be said for having the unit in front of you. The reason the hum got louder, then quieter is the circuit impedance is the highest where the hum is the highest. So stray pickup is maximized at this point.

-Chris