KiT 6550 Hum

[TimN]

Hi Richard, attached is a photo of the only identification I can see on the Tx. The laminations are 65mm high.

I have attached some photos of the 6550 cathode resistors. Only one of them is a ceramic box type. The others are axial - I presume wire wound. I don't know what value they are as I am unfortunately colour blind. I noticed the mix the other day and have ordered 4 new 2K2 3W axial wire wounds. I thought the ceramic box one was the intruder!

More photos to follow.

[TimN]

Transformer photos.

[TimN]

Originally Posted by TimN

Hi Richard, attached is a photo of the only identification I can see on the Tx. The laminations are 65mm high.

I have attached some photos of the 6550 cathode resistors. Only one of them is a ceramic box type. The others are axial - I presume wire wound. I don't know what value they are as I am unfortunately colour blind. I noticed the mix the other day and have ordered 4 new 2K2 3W axial wire wounds. I thought the ceramic box one was the intruder!

More photos to follow.

I've just measured the colour coded resistors and they are all 2.2k

[Richard]

That's all fine then Tim.

Your mains tx is 7546 on the label and the 65mm stack confirms it is the later larger one. If it is buzzing lams it may not be possible to completely silence them. They were usually varnish impregnated which you might be able to get re-done if there is a winder near you. Otherwise try various mountings as others suggest.

The cathode resistors are the green 560R ones in your pics which likewise are the later spec and should be fine so find and meter all 4 to check. (Earlier circuit was 500R which would have loaded valves and the tx more.)

The white rectangular and pink tubular resistors 2K2 are the g2 resistors and are all metal oxide despite the white one looking different to the pink ones. The colour bands should be RRR and may have been running hot. If they meter fine leave them or change them if your new ones arrive.

One of your pics also shows green wirewounds on pins 4 and 6 of a rectifier. They would be the balance resistor update in the FAQ so check the values with the meter and for all 4 being present

[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.

[bob orbell]

Hi Tim, without reading through the complete thread, do you have the balancing resistors fitted to the 5U4 rectifier valves, as in the FAQ? how old are the 5U4 valves ? one may be working harder than the other, I doubt you will totally remove all hum due to the heater voltage being AC but the hum should be very low level indeed, I found that changing C5 to 82uF on mine almost totally removed hum. Bob