strange noises from ECL86 amp

[woodturner-fran]

Hi All,


I have an old (early 90's) audio innovations series 300 amp that uses 12AU7/12AX7/ECL86 tubes and delivers about 10WPC P/P.

Over the past number of months, there has been an odd noise from the left channel - kind of a rustling, rumbling noise that usually goes away after the amp has been on for 15-20 mins. However, today its been on for an hour and the noise is still there. I love this amp, so want to keep it in good order and original too.

This is meant to be the schematic, but I haven't traced it yet to make sure.

What would the noise be? i take it that there will be capacitors that may need replacing, but which ones should I be looking at first - the 400V/470nF ones?

Any help would be greatly appreciated,

Fran

[kevinkr]

Film capacitors don't typically generate these or any kind of noises, typically failing/old carbon resistors, bad socket contacts, and tubes do.

Sometimes an electrolytic cathode capacitor if drying out may generate some noise as well.

A badly oxidized pot wiper can generate noise, particularly if there is grid current flowing through it. If fitted is the volume pot at all scratchy with or without music playing?

I would suspect tube, socket, and passive parts in that order. (From easiest to hardest)

[Tube_Dude]

...and also cold joints.

[woodturner-fran]

OK,

well I'll open her up later tonight and clean the sockets/cold joints. I have a spare set of tubes that I can try too. I'll look and see what there is in the line of electrolytics, but not much IIRC.


For checking the resistors: If I do a visual exam, would I just look for signs of burning/heating? or is there some other sign I should watch out for?


Thanks for the hep so far guys...

Fran

[EC8010]

Dry joint, dodgy small-signal valve.

[woodturner-fran]

OK,

I took out the valves and sprayed in some contact cleaner into the sockets and reinserted and took out and then reinserted the valves again. Problem seems to have more or less gone away now, so I didn't go any further in taking out the PCB and looking for cold joints. None of the components on the PCB look obviously bad, so maybe the problem doesn't lie there.

The small signal valves (12AX and AU7) are relatively new - EI brand ones. The ECl86s are original TFKs, and I have a set of Mullards as well.
So I can probably try these too.

Anyway I'm going to let the amp cool, do the sockets again, maybe clean the pins and have a look to see if any of the sockets seemto have bigger gaps in the pins.


Thanks so much for the help so far,

Fran

[woodturner-fran]

Amp seems quiet now. I took out the valves again and cleaned the pins with a bit of 1200 grit (they were actually quite dirty although they aren't in that long <1 yr). Sprayed in another bit of contact cleaner and reassembled. So far it seems OK, but really I'll only know over time.

So hopefully I won't need to post in this thread again !!!!!!!!!


thanks again to all who helped, excellent diagnosis Doctors....

Fran

[Shoog]

The fact that the pins were quite dirty might suggest some arking over. This manifests itself exactly as the sound you describe. It may prove necissary to improve the grip on the valve socket.

I have had similar issues after to many adjustments and modification to the components attached to the sockets.

Shoog

[woodturner-fran]

The pins on the ECL86s were just dirty looking, but the ones on the 12AX/AU7s were actually a bit "blue" looking - you know how metal looks when it gets a bit hot. Now a light sandpapering removed it (1200 grit and only really a quick rub) so its not like any serious heat got them.

In any case, I've had it on for most of today and not a single noise out of it, so it looks like that was the issue.

Hey Shoog, whereabouts are you, Irish DIYers seem in short supply, only myself yourself and john kenny! (and I don't know if I even qualify).


Fran

[Shoog]

I'am from England originally though my family is Irish. We're based out in the North of Roscommon near Boyle. I have spoken to John Kenny and he seems to have the same thinking as me. I have a local neighbour who's into radio, but has a side interest in Audio. We're going to do a speaker project together and then he's thinking about a valve amp as well.
Where are you. Distances aren't so huge that it wouldn't be possible to have an Irish meet (of three).

Shoog