Attention has turned back to this project as little 6EW7 sounds so good I'm loathe to turn it off! Also JTS gets the 'ump if we don't have any music on when she's 'broidering
the detour with loading the sec of the IT with a low value resistor has been dumped, in retrospect all I was doing was causing the 2A3 to conduct harder as I was pulling down the grid bias supply with the resistor and causing the 2A3 to conduct harder. Live n learn. So the amp is back to normal.
I'm getting too used to having an amp that doesn't hum at all, 6EW7 on the other thread has none, I like that, a lot.
So I'm hum-chasing on this amp 6C45->IT->2A3.
The 2A3's are AC heated natch but are hum-dingered so I can null pretty well. The 6C45's are also AC heated (for now, haven't tried DC heating yet, might.) but having read the Merlin Blencowe PSU design book he mentions that some hum and rectifier action can be coupled to all other windings if the same TX supplies other supplies. He shows an oscillogram of switching spikes coulpling hash into the grid of a valve.
Here's my 6.3V AC heater supply to the 6C45's...
See the little spikes on both edges of the sinewave? If I power up the amp from cold whilst observing the 6.3V sine on the scope I can see those spikes appear as the GZ34 starts to conduct and supply HT, so those spikes are deffo rectifier induced IMO. It was mentioned in passing a while back in this thread IIRC.
The 6.3V sinewave is a poor shape I know I don't know why this is, I live in the middle of a large town on mahoosive estate with lord knows what being pumped into the mains supply, but the rectifier spikes, there must be summat I can do about them? I've got another stand alone 5V heater supply TX I could try, i also have a pair of Hammond 2.5V heater TX's.
Is it worth trying 3 separate heater supply's ( 1 for each 2A3 and 1 for the GZ34) or is there a way of snubbing the spikes and hope this reduces the slight background hum I'm getting, or am I being a straw-clutcher having tasted regulated smooth DC for 6EW7 amp??
DTB