KiT 6550 Hum

[TimN]

I thought I would test the valve amplifier waters with a second-hand purchase from ebay with a KiT 6550. Having assembled it by pushing the valves into the holes and switching it on for the first time there is a definite hum/buzz emanating from it. At first switch on it sounds a bit like an electric loco powering up. If I put my hand on the surface in front of the transformer there is also a vibration that can be felt.

As I said, I have never owned a valve amp before but I do not think this is as it should be. I can hear the hum/buzz from across the room.

I have played some music through it and it sounds pretty good from both channels. If I turn it up loud enough I can't hear the hum/buzz either! It hummed/buzzed without a source plugged into it also.

Can anybody suggest me a course of action to track down the source of the hum/buzz and eliminate it? I am quite a novice at tinkering with hi fi, but can probably work my way through it with some help. Reading through other posts, I believe that it may not necessarily be the transformer at fault.

Can anybody please give me some help and guidance?

[bob orbell]

Hi Tim, some of these amplifiers did have noisy transformers but, it is worth looking at the transformer mounting, it was originally fixed with plastic isolation washers, rubber has been used by some with good results. Another problem / poor design is the fixing of the chassis bottom cover, it does not have enough fixings and can vibrate by means of the magnetic force of the transformer, an Aluminium or Stainless-Steel bottom cover works good as does extra fixings, but one needs a tool to attach some hank bush nuts. I would also remove the bottom cover to see if this makes any change, also have a look inside and try and post some pictures, there may be some parts that need replacing IE electrolytic capacitors. PLEASE be very careful if you venture inside the amp, there is lethal voltages inside, make sure the power lead is removed. Bob

[John Caswell]

Hi Tim,
Before you start ripping transformers about, is the hum audible in the loudspeakers with the volume turned down, if so is it on both channels.

John

[TimN]

Hi John,

Thanks for your reply.

The hum is not audible through the speakers. It is still there with the volume turned right down. It is coming from the amp itself. It seems to be emanating from around the primary transformer. If I put my fingers on the top surface in front of it, I can feel a vibrating. It remains at a constant volume regardless. It sounds like the noise a transformer would make before Frankenstein uses it to zap his monster with it.

[TimN]

Hi Bob,

Thanks for your reply.

Here are some photos I took when I first acquired it.

I will go back in and have another look at the transformer mountings as you say and try it without the bottom cover, but I don't think the humming/buzzing was coming from underneath. I think the transformer mountings are a more likely source. I was quite surprised at the apparent power it has! The caps look OK to me, but that doesn't mean much. If any more photos of anything else would help, please let me know.

[Mark]

Tim,

I had a similar problem with my KEL84 when I first built it, but not as bad as you appear to have it. IIRC I found that experimenting with the transformer mounting bolt tightness helped. Too tight or too loose caused a problem. Is the transformer tight or loose to the touch ? With the power lead removed (in the unlikely event that something shorts) can you move it ? It may be too tight or too loose. You might find that moving the transformer only a very little significantly reduces the vibration. It might have shifted slightly in transit. Try loosening the nuts slightly (with the mains lead out) and moving the transformer with the power on to see if there is a no, or reduced vibration sweet spot.

Also as Bob has already suggested you might want to experiment with the mountings. Again IIRC, my KEL84 was supplied with rubber grommets and white plastic sleeves that fitted in the grommets. I will check this evening. I think I found it was better with just the rubber grommets.

Finally I would like to underline what Bob has already said : there are high DC voltages inside which are lethal. Be careful !

Mark

[Richard]

Hi Tim,

It could also be the choke which has been a fairly common culprit over the years. Listen and check its mountings in a similar way to the mains TX.

Thin pieces of rubber strip (old bicycle inner tube would do) can be used to stop metal to metal contact.

If the main smoothing caps C1,2,3,4 look old/dusty I'd change them as a matter of course as they could be 20 years old and giving the choke a hard time with ripple.

[Mark]

Tim,

The gap between the Tx and the top plate of my KEL84 was too small to see what the mounting was so I took the bottom off. As I suspected it looks like a rubber grommet only mounting - see photo. Also note the nut is not central on the grommet indicating some room to wiggle the tx into a sweet spot.

It also occurred to me that the situation may be analogous to our office water cooler (!). When it stops dispensing water the solenoid valve slams shut and starts the thin metal case vibrating. This can be stopped with a gentle rap on the side with one knuckle. FWIW you might therefore try knocking your Tx on the front, back or sides gently with a wooden/rubber mallet before taking things to pieces.

If it's the choke as Richard suggests it could be, it might respond to the same treatment, but my KEL84 choke is rigidly bolted to the top plate, so yours may be too.

Mark

[TimN]

Hi Richard and Mark,

Thanks for your reply.

It does seem to be coming from the choke. If I put my fingers on the surface above it, I can feel a vibration. Also, if I press lightly above it, I can change the tone of the hum/buzz.

Looking at the Tx and the choke they both currently seem to be just bolted directly to the top cover without even any steel washers.

I think I'll try some rubber washers and/or gasket as damping/suspension. As you say, I may as well change C1,2,3 & 4 while I am there. Can you recommend a brand/type? Are Mundorf any good?

Thanks

Tim

[bob orbell]

Any capacitor with 105c deg. rating, all these fancy capacitors are snake oil, try by all means, but you will hear no difference. Bob