It hasn't been my month for tube amps... Yellow_Light_Colorz_PDT_21
Last week I wrote about my stock ST-70. This week it's my other one with the DIYTube board that I wrote about here:
http://www.diytube.com/phpBB2/viewtopic.php?t=2208
Now, every part in this amp is brand new except for the trannies. The amp worked flawlessly and silently for 2 months until this morning. I turned it on and was greeted with a low-level buzz coming from both speakers. Pretty low in level but loud enough that I heard it from my listening seat. So, OK, it's in both channels so I'm thinking power supply. I did quickly try replacing the 12AX7 on the driver board just for the heck of it since it is 'common' to both channels but it's essentially two tubes in one so I didn't really expect anything. Got what I expected..hum still there.
My first guess is the quad cap but it's brand new (I know, not unheard of for it to be bad). It's an 80/40/30/20 from Dynakitparts. I shut off the amp, made sure it was discharged, and did the resistance test. It tested OK. I also tried replacing the rectifier tube just for the heck of it. Hum still present.
I'm still thinking it is the quad cap. I suppose it could also be the power transformer as it is original to the amp.
This drives me nuts as the amp is essentially only 2 months old. Yellow_Light_Colorz_PDT_09
Anyone have any other (less painful) ideas?
Thanks,
Joe
EDIT: Ah yes, almost forgot...I also tried it w/shorting plugs on the inputs so I know the hum is in the amp.