Hi,
Tonight I went back to my severly buzzing 18watter - my second 18w build.
I undid a few grounding things I'd done with the buss - moving a few wires so they were closer to the ground for that part of the circuit ( i.e cathode resistor ground for that stage etc; I grounded the 1st cap, PT secondary centre tap, filament secondary centre tap and output stage cathode resistor all together at one point, and then moved preamp star earth point to another part of chassis with 1 wire coming from the ground bus at the cathode resistor of the PI stage...............Guess what....................
STILL BUZZING....................
So, I took a deep breath and started looking for something I may have rushed through from the first step of the build.
So, I took a look at the filament supply wound pair - one of the first things I wired up in the chassis - and wondered.......had I matched the pins properly when I wired the EL84 sockets....??
ERRRRR....NO ! So, I switched filament supply wires on pins 4&5 of the EL84s, powered her up and I am now the proud owner of a non-squealing, non-buzzing , bl**dy fantastic 18watt amp ! I just played for an hour non-stop until I remebered I have neighbours..and kids...and that it was nearly 11pm in the UK.
Its taken me 4 days to find the cause, and its a nice reminder to take it steady in the early stages when you just cant wait to get the amp built and hear for real what your dreaming about in your head.
Hope this might help someone else..
Now where did I put that pick....
Rgds
Pedro