WDKEL84 Transformer Hum

[Ianm2]

1/ no its not likely dodgy.

2/ remove the choke and replace with an equivalent resistor.

why 1? they are cheap, that's the bottom line, they are designed/wound on a budget and are close to saturation, that's the buzz, the core is chattering as it can't cope with the current/volts.

2 will help as it sings along with it.

pay peanuts, get monkeys.

good circuit compromised by parts.

thorsten says " I try to get as many iron cored components in...

I say....I try to get as few in as poss.

[Richard]

Originally Posted by Ianm2

1/ no its not likely dodgy.

2/ remove the choke and replace with an equivalent resistor.

why 1? they are cheap, that's the bottom line, they are designed/wound on a budget and are close to saturation, that's the buzz, the core is chattering as it can't cope with the current/volts.

2 will help as it sings along with it.

pay peanuts, get monkeys.

good circuit compromised by parts.

thorsten says " I try to get as many iron cored components in...

I say....I try to get as few in as poss.

That's not fair Ian, there are plenty of quiet KEL84s around.

Changing the choke for an equiv value DCR resistor (approx 60R) will decrease HT smoothing by a factor of 10. The p-p output stage might cope but the same supply feeds the pentode first stage so it's not really on without more reworking - nor is it necessary.

The noise starts when the led changes colour as that's the point that the HT begins to be conducted by the valves and loads the choke and mains TX.

Ascertain whether it's the choke or mains TX that's making the noise and be sure it's nothing else on the chassis such as the baseplate loose. Then, if you can't quieten it down with the mounting, ask Peter for an exchange. It's probably only the varnish that hasn't penetrated fully.

The TX and choke are both to spec. The choke's specced at 250mA but passes just 170mA. If there are some turns or laminations that haven't been penetrated by the varnish they may rattle or buzz. That's all it is, older designs used to pot them in pitch for similar reasons, now they're mostly vacuum impregnated.

Rich

[Chivvyp]

Originally Posted by Richard

Ascertain whether it's the choke or mains TX that's making the noise and be sure it's nothing else on the chassis such as the baseplate loose.
Rich

Hi Richard,

It's definately not the baseplate, I've not attached it yet.

Is there a way of checking whether it's the choke or tx without putting my head inside the case? I'm pretty stupid most of the time but I doubt having 300v though my earlobe is going to improve matters!

Regards

Pete

[Richard]

Hi Pete,

Try a length of wooden dowel with its end pressed against the TX/choke and the other end against your ear as a stethoscope.

Also try undoing each mounting in turn til they are just hanging loosely to see if the sound changes to identify the noisy one.

Something else that might be worth trying if a bit abstract; connect the choke wires the other way around. It's only an idea, I've never tried it, but will do no harm. It may be that the choke and mains TX hum fields (the choke is carrying DC but has a ripple current) are in phase and if they are quite close to each other reversing the choke connections may cancel some of this. It would be interesting to know if you can detect any difference.

Rich

[John T]

Originally Posted by Richard

Also try undoing each mounting in turn til they are just hanging loosely to see if the sound changes to identify the noisy one.

This is a very good, and in my experience, a most effective tweak. By adjusting the screw/s you can literally "tune" the vibration!

Another very effective expedient is to replace the rubber feet on the amp with wood cones. This effectively "grounds" the amp (as one does a turntable) and stops the amp wobbling about!

[JerryT]

Originally Posted by John T

This is a very good, and in my experience, a most effective tweak. By adjusting the screw/s you can literally "tune" the vibration!

Another very effective expedient is to replace the rubber feet on the amp with wood cones. This effectively "grounds" the amp (as one does a turntable) and stops the amp wobbling about!

Both good ideas but I've found black 3/4 inch hole in the middle tap washers very effective under Mains TX and choke.

Just another thought.

Jerry

[Chivvyp]

Hi All,

Thanks for all the suggestions.

Also fulsome apologies for wasting your time.

I don't know what I was looking at earlier in the week but at the weekend I completely removed the tx and I noticed that the top hat inserts were inserted from the bottom of the chassis.

I haven't been able to spend much time listening yet so I'm not 100% sure everything is now ok but from my initial (very short) test the hum seems to be a lot less.

I'll have an extended session next weekend.

Regards

Pete