Hummmm!

[catch29]

So, finished the refurb' on my Heathkit S-99, all good, but!, there's always a but!. I've replaced the electrolytic and paper caps (yes, I know!). I have left the main 60-75 as it measured quite close, ( passive, not under pressure). It sounds really really nice but for a very low and quiet hum, almost unnoticeable but for my Kef Concerto's. It is on both channels and is not affected by any controls (including the hum controls) or change of input, could it be that big 60/75?.
As a 'side', can anyone sell me a Heathkit MA-12?, don't care about condition as long as it has the transformers. Thank's.

[Radio Fixer]

I would have thought it wasnt the cap. But, if it has silicon diodes for rectifiers, you could safely tack another across the smoother and then power up and see if there is any improvement. If the rectifier is valve you shouldnt do it unless there is choke or resistor between it and the reservoir cap (there must be). Beware increasing the size of reservoir cap which may exceed the valve spec.

Are you listening to anything music, speech and a fair distance away?
With nothing, and the volume turned up, and the head alongside the speaker it may not be realistic. Very easy to spend time tracing minimal hum which for normal listening doesnt matter.

Doesnt sound like an input fault: very easy for external equipment to make hum loops. This often comes down to only having one mains earth to say the amp or turntable and letting the other get its earth via the screening of the audio cable.

Gary

[Lucien Nunes]

It helps if you can characterise the hum, as different kinds of faults produce different kinds of hum.
* Earth loop hums (other than those involving the rectifier and HT secy CT) and those caused by problems with the heater circuit tend to have a fundamental at 50Hz - approxmately the lowest G on a piano - and tend to be 'smooth' sounding with few harmonics.
* HT smoothing issues (with full-wave rectifiers) tend to manifest as predominantly 100Hz because that is the ripple frequency leaving the rectifier. This is one octave higher, so they sound 'high' in pitch relative to classic 'mains hum'.
* Earth loops that are picking up the charging pulses from the rectifier will also be 100Hz and up and might be quite buzzy
* Hum being picked up by a signal pathway is likely to be centred on 50Hz but also include higher harmonics and noise, and sound more 'open' like a guitar amp with its level turned up with nothing being played.

Listen to how it changes as the amp warms up and cools down. Does it fade in first and then out somewhat as the valves warm up and the NFB takes effect? Does it go off suddenly with the mains switch or only fade out as the reservoir runs out of HT?

[joebog1]

Pictures tell a story too!. Any chance of a pic underneath the chassis ?.
Did you change any wiring runs or layout?.

Joe

[catch29]

Thank's for your suggestions, yes could be a number of things. The S-99 uses a GZ34 for rectifier and I must admit I am using a 'cheap' (relative to a Mullard) brand normally for guitar amp use, you could say it is the 'hum' in the background before the band get's going!. I see you can get silicon/diode plug in replacement, I might get one and see what difference it makes. I cannot afford to be a purist!. 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. The hum from this particular S-99 is very low and quiet and admittedly the computer makes more noise!. I am renovating a Heathkit USC-1 pre-amp to go with the MA-12's, And another S-99 to go with another pair of Concerto's. I have, I don't think I will get all this in my 8ft x 10ft room!. I think I will have to bung some of this on Ebay, who knows, in the move the hum might dissapear!. Thank's all.
       

[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.

[Kalee20]

I would have thought it wasnt the cap.  But, if it has silicon diodes for rectifiers, you could safely tack another across the smoother and then power up and see if there is any improvement.  If the rectifier is valve you shouldnt do it unless there is choke or resistor between it and the reservoir cap (there must be).  Beware increasing the size of reservoir cap which may exceed the valve spec.

Actually, it would be OK to do this even with a valve rectifier. If you have a known-good reservoir capacitor of the right (or even a bit greater) value, tack it across the existing one.

It might exceed the valve's rating, especially if the existing one happens to be OK, but for just a few moments use to confirm or otherwise, you're not going to kill the rectifier. Just don't switch off and back on before the valve has cooled down, the greater capacitance value will draw a bigger gulp of charge from the still-hot cathode.