Dynaco ST 70 with Lo-Gain Board has hum...

[131east23]

My Dynaco ST 70 with Lo-Gain board has a noticeable hum. I have Cornwalls so I'm sure they make it more noticeable, but the hum is related to a vibration in the amp. The left output transformer seems to be vibrating. Is it about to go or could there possibly be something else causing this: bad ground, ground loop, a crappy tube... Any advice would be very helpful.

Thanks

 

[peterh]

Does the hum stop if the inputs are shorted ( with rca shortingplugs ) ? Then it's the preamp OR a groundloop
outside the st-70 that causes this.
If the hum remains with shortingplugs the reason is to be found inside the st-70. You mentioned "lo-gain board" , it's
not original i assume, then the installer of the lo-gain board has missed something.

 

[soundmotor]

but the hum is related to a vibration in the amp. The left output transformer seems to be vibrating.

Thanks

I've had ST70s in the past with loose transformer end-bells. Never noticed mechanical noise from them but tightened them up anyway. You might at least check all the end-bell bolts as well as the mounting bolts to the chassis on all of the transformers just to rule that out as an issue.

 

[bonsai]

Ask in this forum as well.

http://dynacotubeaudio.forumotion.com/f2-basket

 

[dmag]

Sometimes a hum , if its a mechanical vibration can be helped bt loosening the bolts to the chassis a bit rather than tightening. A rubber washer can go between the transformer and chassis as well.