Mains hum

[steveandthecats]

Hi, I am new to the forum, but not new to the world of valve amps.

I have a mechanically noisy mains transformer on my WAD amp
I built it some years ago and it has 4 KT88's and 2 5U4G's and 2 preamp valves.
it has served me well and I want to keep it, I have still not heard anything that I would wish to replace it with.

Recently it has developed a severe mechanical hum from what sounds like either the choke Design no 7500 6550/34 or
as would see more likely the mains transformer Design no 7546 6550 mains.
Two of the the top hat washers seem to have disintegrated but as they only isolate the transformer form the chassis this is not the reason for the now quite loud hum.

Is there anything I can do to quiet the transformer. I have tried different ways of mounting it, tightened and loosened the bolts, tried different ways of isolating the whole amp but the sound still is quite loud.

I'm quite happy to buy a new transformer but it would appear that I would have to have one made. I suspect this would not be cheap.

Can anyone make any suggestions as I am listening with my old integrated, IC amp.... and I want my valves back.....


thanks

Steve

[davebms]

my amp, did that exacly the same thing , i changed the 5U4G's and its been fine

dave

[steveandthecats]

Thanks

I'll send for a couple and give it a go......

Steve

[Richard]

Hi Steve,
I think it will be the choke making the noise due to ripple on the HT. That may be caused by the rect valves (always good to have spares) or failing first smoothing caps C1/2 before the choke.
If the smoothing caps dry up and and lose capacity (as they do with age) they will not be as effective at smoothing the ripple on the HT coming from the rects. The choke then has to cope with rough DC and you hear the result.

[steveandthecats]

Thanks for that... I have a few spare caps... would it bother the amp if I just soldered them in parallel

Thanks
Steve

[Richard]

Valve rects have a limit for the first cap value which might be exceeded by paralleling more caps. I'd remove the originals first and try new ones with the specified values or you may end up creating more problems

[pre65]

Do the valve rectifiers have (or even need) the specified resistance (from the data sheet) in the Anode ?

5U4G = Effective Plate Supply Impedance Each Plate ... 75 Ω

http://www.nj7p.org/Tube4.php?tube=5U4G

That 75 ohm would also include the resistance of half the secondary winding.

[Richard]

Originally Posted by pre65

Do the valve rectifiers have (or even need) the specified resistance (from the data sheet) in the Anode ?

5U4G = Effective Plate Supply Impedance Each Plate ... 75 Ω

http://www.nj7p.org/Tube4.php?tube=5U4G

That 75 ohm would also include the resistance of half the secondary winding.

Good point Philip. I gather this is Kit6550 with parallel rects so Rt should be ok and presumably it was working noise free at one stage. Running the rects though without balancing resistors may give them a hard time especially as they age and start to drift apart in emmission. Whilst it may not cure the noise now it would certainly be a good move to install balancing Rs imho,

[steveandthecats]

I replaced the caps C1..C4 and this made no difference at all.
The new rec valves arrived yesterday (blue dragon) and they have stopped the hum.... well done the forum.

Should I add the resistors as suggested above to prevent a re occurrence.

There is still a very small mechanical hum but not noticeable as it was.
As a general rule I thought that as the transformers were dipped in something originally , would this not 'dry out' eventually and cause it to rattle... in other words do transformers have a lifespan?

I built the amp in 2006 and when I am home it goes on in the morning and stays on for most of the day. I am on my forth set of KT88,s. I still love it.

thanks again

steve

[Ali Tait]

For the mechanical hum, check all the transformer's fixing bolts haven't worked a little loose.