Hum problem....

[Richard]

Originally Posted by Richard

I only have experience of working on this amp once and ran out of time trying to stop it bursting into oscillation at 19W output. It sounded fine on music as this level was probably not reached but it showed every time on test despite moving wires around and swapping valves.

To clarify this so no-one is concerned about their own amps unnecessarily. The amp was nice and we'd been listening here when the owner asked if we could just check it through on the bench. The oscillation showed as one valve going blue and one half of the scope trace sine wave breaking up. Just one channel and when the input was turned down it stopped. We suspected a wire lying too close to another wire and moved some around but didn't get around to detaching and re-routing anything. Nor do I know how it was wired from build or if the ground was lifted etc.


There's quite a lot involved and the instructions seem to have changed regarding the ground lift. Perhaps Bob can confirm if the 10R lift resistor (R47?) is in the build instructions he has or if it's simply shown in the parts list? Also, if the later instructions are to use the fb braids to ground the speaker black posts or if they get their own route?

[Chris]

Thanks Richard and Greg for your support and help, once time permits I will proceed with these next steps.

Chris.

[bob orbell]

Richard, yes the build papers say that R47 is connected to earth post and cL9, feedback screen from black speaker post to tag 12 (bR1) at R18, and signal at tag 15 (bR1) at C22. All feedback connections on inner side of tag board. Bob

[Chris]

Have taken out transformers and sprayed with Plasticoat as suggested by Greg, I noticed a detached fb wire from the tag board to binding post, am just hoping this is the cause of the hum and not something that has happened while removing the transformers.

Chris.

[John Caswell]

Hi Chris,
If it was the feedback wire as opposed to its screen, then that would have caused a massive increase in gain which did not appear to be the problem. However if it is the screening wire disconnected then that could have caused the hum. Reconnect that wire and everything else, fire it up, and let us know how you are getting on. Do not earth the feedback cable screen at both ends, that way you will get a hum loop.

John

[Greg.]

Well really hope this is the cause of the problem. We need to think about this logically. An amp that worked well and then suddenly something went wrong. There has been no suggestion of component failure. I asked Chris to think about anything he might have done to cause a change, but there was no response to this. Furthermore it appears Chris still has not carried out the tests suggested by John.

As much as optimising the earthing is worthwhile, nothing to date has suggested any inadequacies here that might have been the cause of the problem, so a worthwhile exercise but not necessarily relevant.

Obviously, reattach the feedback wire and test. I hope it works!

[bob orbell]

I have been watching and replying to this thread and I agree with Greg that Chris has gone about this problem in completely the wrong way, it was working good then a problem occurs, John suggests a test but Chris appears to go his own way, I have given some information from my build instructions but it seams to be questioned. To be honest I have now lost interest in helping . Bob

[Richard]

I think Chris is doing the right things. Surely if he'd known any obvious thing that might have caused the hum he'd have tried that first.

As he didn't build the amp the next thing is to have a look inside for anything obvious.

The original build sheets do not detail the 10R lift resistor so he checked that.

He has now found a detached fb wire. As John says this could have different effects depending on if the signal or braid or both are detached. He should re-attach the wire and check again.

John says "Do not earth the feedback cable screen at both ends, that way you will get a hum loop." which needs clarifying. Going by the original instructions the fb braids are the signal ground for the black speaker posts and need to be connected at the black speaker posts and the tag boards as per the instructions. I raised this in post 39 and suggest he disconnects the fb braids at the black speaker posts and wires the blacks back to star earth on the C2 negative tag (so it depends on if Chris has done this. If he has then don't connect the braids both ends, if he hasn't then connect as original or implement the separate connections).

[Richard]

Originally Posted by bob orbell

I have been watching and replying to this thread and I agree with Greg that Chris has gone about this problem in completely the wrong way, it was working good then a problem occurs, John suggests a test but Chris appears to go his own way, I have given some information from my build instructions but it seams to be questioned. To be honest I have now lost interest in helping . Bob

Bob, if you mean the info about R47 no-one is doubting you. My sheets dont mention it, yours do. Presumably it was added later. Earlier WAD designs didn't lift earth, maybe they found they needed to. Chris has now found a 10R in the Ground to Earth connection which he wouldn't have if he hadn't investigated.

That was partly what brought me to suggest the change of black speaker post grounding. Early WAD kits earthed everything back to one single chassis point which was at Mains Earth - no lift resistor. It worked fine for KEL34 and I think Kit88. It looks like they tried the same thing with Kit6550 but found it didn't work so fitted a lift resistor. This left the black speaker posts grounded back to the input grounds which is not so good.


Normally heavy current grounds go straight to the earthiest point (neg C2 in this case) so I suggested disconnecting one end of the fb braids and running separate ground wires to C2 neg. I checked the rest of the grounding priority on the tag boards and they all seem in accordance with usual bus bar relative impedance practice.

[Chris]

Just to push the point, I really appreciate the help given, having very little time to spend it seems that am dragging my feet or not taking advice seriously..after following John's advice to use shorted phono plugs, I started reading about Earthing, basically you need a degree to fully understand it, having read the importance of safe Grounding to the chassis, I bought a new earth pillar and have bolted tightly the Gound to the chassis using locking nuts and washers etc..So one issue has forced my hand to check the importance of securing the safety aspect of building and maintaining audio amplifiers.
When I make any changes to my amp I do it in stages so if any problems do occur I know where they I most likely to have happened.

Chris.