Hummmm!

[Lucien Nunes]

a GZ34 for rectifier and I must admit I am using a 'cheap' (relative to a Mullard) brand normally for guitar amp use

I wouldn't worry too much. A rectifier is a simple thing that only really has to meet a few basic criteria. If it can withstand the reverse voltage without flashing over and the forward current without overheating, and the voltage drop is reasonable, then it's probably fine. Only in the extreme and unusual case that the two halves of a fullwave rec were vastly different, or one side is entirely dead, is it likely to be responsible for hum. People make much out of the audible difference between rectifiers in guitar amps, without realising that any difference in performance stems from the sag in the HT influencing the operation of the amplifier as the current demand of its class AB output stage varies. One is not hearing the rectifier as such; any rectifier that causes the same sag, be it new or old, cheap imported or premium NOS British, will sound the same.

The hum from this particular S-99 is very low and quiet and admittedly the computer makes more noise!

It might be normal then. But what kind of hum is it?

It could well be a earth loop as I am using the S-99 in question driving a pair of Kef Concerto's, another S-99 with some Kef 104's, a pair of MA-12's with another pair of 104's and an old Sony receiver with Wharfedale Diamond 7's. I run these all at the same time from music off my computer, so the wiring does get a bit complicated.

Mmmm, spaghetti alert! Run wiring back to a common location in star form, power and signal alike, to where all the power converges into one strip plugged into the wall in only one place. You can often resolve loops without danger (e.g. breaking mains earths) and without breaking signal grounds, but sometimes interconnect fudgery is unavoidable.

I think I will have to bung some of this on Ebay

I wouldn't mind buying an S99 but I want one that hasn't been recapped / overhauled, to keep as an original museum piece.