Tube pre - weird issue

[Daniel75]

Hiya

Im experiencing weird issue with my tube pre amp (superb Deva 500) - it plays as it should but after being idle for couple of hours - right channel is getting very quiet. After switching it off and on again - it goes back to normal. Any idea what could be the reason for this behaviour? Could one of the tubes be broken? Why it happens only when amp is idle?

I did check rest of my system and everything seems to be ok - mac mini server, squeezebox, cables and speakers - all are fine and the only thing which helps is switching the pre off and on again.

EDIT - or maybe it is an issue with mac mini and logitech media server? but then restarting pre wouldnt help?

Thank you

[AlexM]

Daniel,

I don't know if that pre-amp has a muting circuit, but it could possibly be a problem with the muting relays. I presume that they could be cycled when the power is turned off and on again, so that could explain why the intermittent contact is resolved temporarily.

Does it click on power off/on?. If you can locate the relays on the board you could try tapping them with a penci eraser when the channel goes down to see if that has an effect.

Looking at this picture (http://www.hifiwigwam.com/showthread.php?72646-FS-Morgan-Audio-Deva-500-Tube-Preamp), I see two relays over by the pre-amp out - could be them or a dry joint?. Does toggling mute on/off have the same effect as switching on/off?

Good luck,
Alex

[Spectral Morn]

I was thinking dry joint too



Regards Neil

[Daniel75]

Does it click on power off/on?. If you can locate the relays on the board you could try tapping them with a penci eraser when the channel goes down to see if that has an effect.

Looking at this picture (http://www.hifiwigwam.com/showthread.php?72646-FS-Morgan-Audio-Deva-500-Tube-Preamp), I see two relays over by the pre-amp out - could be them or a dry joint?. Does toggling mute on/off have the same effect as switching on/off?

Yes - does "click" on power on/off. I will try "tapping" the relays first. Is there an easy way of identifying dry joints? There is a mute option and funny enough switching it on and off made right channel slightly louder - still way too quiet though.

Also - thats how the right channel and relay leading to pre out looks like - is that the one which possibly is "broken"?

http://img690.imageshack.us/img690/6752/calt.jpg
http://img856.imageshack.us/img856/8432/w45x.jpg
http://img90.imageshack.us/img90/3509/jfir.jpg
Thank you

[AlexM]

I can't see from the picture which relay leads to the channel with the issue.to be honest, I'm surprised that the muting relay isn't a double pole type, I.e. shorting both channels to earth. there seem to be a lot of relays there!. sometimes you can stimulate a dry joint into revealing itself by tapping or wiggling a component.

it could also be a problem with the valves or their sockets so do try to swap the valves between the channels to see if the fault moved to the other channel. try cleaning the valve bases and have a close look at the pins in the socket.

check the wiring to the output sockets and swap the interconnects to confirm that they aren't at fault.

if you can solder then you could reflow the joints but I am leaning towards a relay problem. try cycling the mute button to check which relays are clicking. don't forget to take necessary precautions against shock as there are Ht voltages inside the amp. you could also loosen the output sockets and clean up the oxidation between the socket and the mounting panel in case you have an.intermittent ground. Cycle the muting relays on and off many times to exercise them as it might reveal a difference in the output if the relay is sticking or there is a contact problem. sealed relays are usually pretty reliable, but you never know!
try some basic diagnosis to try to localize the problem and let us know how you get on.

[Daniel75]

pfff I gave it few more days and finally discovered that issue is caused by one of the tubes - golden dragon E82CC to be precise. When I moved it to the left channel socket - left channel died. When in right channel socket - right channel died. So its definitely this wee evil glass thing.

Not sure why I didnt catch it when I was swapping tubes earlier - it seemed to be ok until now when its consistently broken although there is no visible signs of any kind of trouble with it. Is it normal? - tube gradually getting worse and affecting sound level of right/left channels? Im not very experienced with tubes.

Replacement is ordered so hopefully it will be all good now.

[Daniel75]

Help! :)

I wanted to replace Golden Dragon 12AU7A/E82CC. I bought a pair of Electro Harmonix 12AU7A/ECC82 EH

When Im trying to use it - they dont lit at all. Im not getting any sound either and It blew the fuse in my power cable (for the power amp - pre was surprisingly fine)

Are those two tubes not compatible????

Thank you

D

[Daniel75]

Im gonna answer myself here - its probably blown fuse. Pfffff. Off to Maplin tomorrow then :)