300b

[ledzep]

Hi,ive just got a apir of 300b pp monoblocks,but there is a constant hum in the background,ive adjusted the humbuck to make it quiet as poss but its there,any ideas? cheers

[John Caswell]

Well, it could just be the design. What make of amplifier are they?
Is it 50Hz or 100Hz hum? Does it vary with volume.

John

[Richard]

Hello and welcome I see from your other post these are WAD monoblocs. I had a pair and they were absolutely quiet. There were some issues when they were first launched about 10 years ago. They were a modification of the 300BPSE monoblocs using a reworked PS and different OPTXs. Tell us as much as you can about these inc build history if you know who built them and when and if they have always suffered hum. Do they hum similar amount on both amps, have you swapped valves between amps and does hum change? What source, preamp and speakers? etc. Oh, and how happy are you working on them, are you familiar with such gear?

[ledzep]

All i know is about them they where factory built,the hum is the same out of both speakers,the chap had the serviced and the hum/buzz is a lot quiter now,i think it sounds like transformer hum,i have adjusted the humbuck on the back to get them as quiet as poss but its till there,these are the 300bpp, rega cdp,croft 25 pre and impulse lali speakers

[John Caswell]

Hi ledzep,
Transformer hum does not manifest itself through the L/S it is a physical property (see magnetostriction) the simplest way to decide whether it is transformer hum or not is simply to disconnect the L/S with the inputs shorted out (a bit of tinned copper wire soldered into two spare phono plugs will suffice), then you can decide where the hum is coming from.
If hum is negligible plug the L/S back in (with the inputs still shorted) and listen for hum in the LS whilst adjusting the humbuckers for minimum. Normally it is reckoned that if you cannot hear hum 6"-12" away from the L/S that is satisfactory.
If the hum is much louder than this then suspect unbalanced output valves, poor decoupling etc.
As you have two amplifiers you can select one and swap valves, especially the 300Bs, to maybe find a pair that give minimum hum, adjusting the humbuckers as always after changing valves. If changing the 300Bs cures/helps the problem then new valves are usually the answer.
Decoupling capacitors can really only be proved by substitution I am afraid.
If you are not sure what you are looking for/at refer the units to someone more experienced or familiar with dealing with valve amps.

John

[ledzep]

lCheers John, L/S? that sounds complicated think isend them to someone to do,do you think they are worth getting done?

[Greg.]

From your reply to John, you don't appear to feel confident to fault find yourself. If that is the case, Richard's post will be even more challenging. No Problem. If you want one of the best professionals to look at your amps, John comes (we call him Dr John) with the highest recommendations. Furthermore, you say you are in Kent. I don't know what part but you may well be in reasonably easy striking distance of John himself. I suggest you PM John Caswell with a view to arranging for him to give your amps his excellent attention. Seriously, you couldn't do better. happy Christmas!

[ledzep]

Cheers Greg

[John Caswell]

Hi Ledzep ,
Received a pm from you but no message in it, how can I help.

John