KIT 6550 - hum

[waverley]

The amp was on the other night. Suddenly there was a noise, rather like a crack, followed by a loud mains hum on both channels. The sound was otherwise unaffected. Visual inspection reveals nothing untoward. The amplifier powers up as normal. I suspect the electrolytics, as the amp is over 11 years old.

Can you please offer some guidance on how I should proceed? Is it possible to test the caps, and if so, how?

[bob orbell]

It sounds like one of the rectifier valves may be duff, when did you last change them? if it is a capacitor, it should be visible, are both 5U4G valves glowing? a test you can do to find out if the 5U4G's are both working, is remove and replace them and switch on one at a time it will work with one valve only, but will hum, as you will only have half wave rectification, if you get no sound with one particular valve, then that is your problem. BOB

[waverley]

Thanks Bob. The rectifiers were replaced just over 4 years ago. Heaters are working on both. I have done as you suggested and both valves appear OK, ie amplifier works but with loud hum. I have also tried the old rectifiers,which I kept, in various combinations, and the hum remains.

[Richard]

Hi waverley, yes sounds like a psu problem common to both channels.

C1,2,3,4,5 are all suspect and can be checked with a cap meter but you'd need to un-solder first so at that age I'd swap them for new ones. Even if it's not that it would be a good first move at this age.

Ripple kills caps and the ripple is by far the greatest on C1,2 so just change those if you don't have a meter. A quick check can be made with known good caps crocodile clipped across the suspect ones. If the hum goes you've found the bad cap(s) (careful 500V!).

Otherwise buy a bag of suitable size 100uF 450V and swap all 5.

C6 (small valve DC heaters) is also possible so try removing the ECF80's and see if the hum goes and probably change it with the others if you do them.

Next might be the choke. I had one go shorted turns and it still connects but the inductance is less so the smoothing is lowered and the amp hums. Not easy to check without an inductance meter but a resistance check might show it to be lower than expected (ie 10R instead of 40R should drop 10V).

[waverley]

Thanks, Richard. I shall source and install suitable caps, and then report back. I'm not sure how long this will take, living where I do.

[pre65]

If you get stuck, Philip Ramsey of Bluebell Audio (in Dundee) is a good person to speak to.

He knows valve amps.

[bikerhifinut]

Originally Posted by pre65

If you get stuck, Philip Ramsey of Bluebell Audio (in Dundee) is a good person to speak to.

He knows valve amps.

I'll second that Philip, I had a very good experience buying from Philip at bluebell with a lot of helpful advice when I wanted a pair of chokes for my monobloc amp project.

Andy

[Richard]

Matthew at WD will also probably have suitable caps.

The thing with these (and why I didn't suggest a supplier) is that they are high voltage (limited choice of suppliers) and may need to fit in a particular amount of space (waverley needs to check dimensions).

RS would also be a good source if you know the specs but I'd measure up (length x diam) first as making big ones fit may be difficult

[bob orbell]

RS do do the exact size and make as fitted as standard to the kit, if Waverley cannot get any locally, I have an RS account and will help. BOB

[waverley]

Thanks all for your replies. I have been in touch with Philip Ramsey, who helped me with components several years ago. I have given him the dimensions and await his reply. As you say, space is tight for some of the caps. I'll let you know how I get on.