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]

Originally Posted by 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

Thanks Bob,

New hard plastic isolation 'top hat' washers and rubber grommets kindly supplied by Matthew and the hum is now a very faint purr. No gaskets!

Now to test it and find out if will actually play music!

Thanks very much to everyone!

[TimN]

Just an update to say I've had my 6550 in the rack for about a month now running my FB1s and it sounds absolutely fab!

It has the Tung Sol 6550s, Svetlana Flying C 5U4s and Ei ECF80 valves that it came with running at the moment and it sounds great. I see no reason to change them at the moment.

I had a few teething troubles that manifested themselves during testing due to a couple of wiring mistakes, one of which I think melted one of the channels of the volume pot but that has now been replaced.

C9 & C10 and C15,16,17 & 18 had the Blackgates that came with it replaced by some current Audionote specimens as I thought they had probably passed their best. Without knowing what the Blackgates sounded like I am quite pleased with the result nonetheless.

Also, I had not realised just how HOT it would get! It seems the hotter it gets, the better it sounds, though! Also, I will be able to turn the central heating down when the weather gets cooler.

So in summary, thank you all very much for your invaluable help. Without it my first foray into valves would not have had such a happy conclusion.

Next project is to get these KLS3s I got from eBay up to scratch once my back has recovered from transporting them.

Thanks again

Tim

[Greg.]

Good result, Tim. Glad you’re pleased with the outcome. Don’t forget, give me a shout if you need any help with the KLS3’s.

[BUTUZ]

Originally Posted by TimN

Just an update to say I've had my 6550 in the rack for about a month now running my FB1s and it sounds absolutely fab!

It has the Tung Sol 6550s, Svetlana Flying C 5U4s and Ei ECF80 valves that it came with running at the moment and it sounds great. I see no reason to change them at the moment.

I had a few teething troubles that manifested themselves during testing due to a couple of wiring mistakes, one of which I think melted one of the channels of the volume pot but that has now been replaced.

C9 & C10 and C15,16,17 & 18 had the Blackgates that came with it replaced by some current Audionote specimens as I thought they had probably passed their best. Without knowing what the Blackgates sounded like I am quite pleased with the result nonetheless.

Also, I had not realised just how HOT it would get! It seems the hotter it gets, the better it sounds, though! Also, I will be able to turn the central heating down when the weather gets cooler.

So in summary, thank you all very much for your invaluable help. Without it my first foray into valves would not have had such a happy conclusion.

Next project is to get these KLS3s I got from eBay up to scratch once my back has recovered from transporting them.

Thanks again

Tim

I've got physical transformer hum on my KIT88 I need to re-read this thread from top to toe methinks!!

[TimN]

I have noticed a slight hum coming from the speakers connected to my 6550 that I have not noticed before. It is very slight. It gets louder when I turn the volume up. It isn't loud enough to be heard over the music - at least not to my ears.

There is a slight buzz coming from the transformer which has been there since I bought it - but was greatly reduced with the help of this forum - see this thread! I have been under the bonnet and tweaked the transformer mountings again which has reduced that mechanical buzz. I changed the caps C1 - C5 during the renovation work.

I also gave it an inspection at the same time and nothing untoward that I could spot. But still the slight hum through the speakers remains.

I have tidied the cabling and checked all connections - all OK. No induction from mains cables etc. I removed all the interconnects and source components but still the hum remained.

Is this hum normal? Is it to be expected from a valve amp? The music sounds great but I would like to rectify it if possible.

[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.