quote: Disconnect the grid resistor and temporaraly ground the grid - what hum do you get?
Already done that, it's why I test initially with shorting plugs in the input sockets to eliminate external hum sources. Result.......... silence. So I reckon the amp itself is working as planned.
I increased grid leak back up to 1meg but that made no difference, nor has reducing the grid stopper to 1k.
Reduced open loop gain with a more suitable anode resistor, I should have seen that one coming as The original circuit used different anode resistor values depending on how much gain was needed into the phase splitter section. Leak altered their designs over time and I have a comprehensive history here documenting it all.
The argument about designing with a triode is a bit of a red herring, it's already designed around a triode first stage, albeit an EF86 triode wired but it's a pretty good audio triode at that with a gain somewhere between the ECC81 and ECC82 as far as I can ascertain from the data sheet.
The point is this, it's dead quiet until a source is hooked up. The amps predecessors didnt do this, the issue there was mechanical noise from crap mains transformers.
I suspected a hum loop but from where? my main sources are not earthed to the mains, the Rega Cursa is double insulated as is the Cd player so no mains earth pin on the mains sockets. Right now theres a 10 ohm earth lift backed up with reverse parallel rectifiers and i was tempted to wire the signal earth to chassis/mains earth, if nowt else its an added safety factor.
I did have an issue with valve amps with long interconnects to the power amps, so I reverted to the amp on the rack with long (very long) speaker leads and a 1m lead from pre to power. But this made not a jot of difference.
Its 50Hz mains 'um but where is it getting picked up? The toroid mains transformer on the power amp doesnt radiate like the old one did nor like the EI transformer on my KT88 amp does where it needs a good couple of feet vertical separation from any sensitive sources.
I may just be a bit fussy but it strikes me that a hum loop if that is what it is should be resolvable.
Oh for fully balanced connections!
One thing i havent done is connect the screen pin on pin 7 as according to the Mullard valve data its internally connected to pin 2 also. This has me thinking a bit. Its about all that I have left. I also did remember to earth the centre spigot of the base as per Mullards data.
If it goes boobs up then for sure its a dead easy fudge to rebuild as an all ECC83 amp and it will be a Stereo50/60 clone. It'd be no quieter though.
Anyway I need to do some proper measurements now.
