Tube Preamp Hiss

[taha_never]

Hey all.
got myself a hand built tube preamp (2xRCA12ax7s).
not much high quality components though.
at the beginning everything were working fine and it was quiet till suddenly it decided to hiss! there's a very obvious white noise which increases with the volume control including crackles when turning the knob.
unfortunately the builder has migrated to another country and i can't get in touch with him.
do you have any idea what is the common cause of such problem? dirty board or pot? broken caps or tubes? what can i do to fix it myself? i need your advice and i will appreciate your help.

[JTUK]

I'd swap out the valves to see if they had 'gone'...

[taha_never]

none of the preamp knobs affect this hiss. only the volume knob. is it still a suspicious tube?

[taha_never]

with all the valves removed it's still there. unchanged. so the tubes are ok? so what else should i try? please shed a light

[ahpook]

Is that the output at the top-left of the picture ?

A close-up of the main board and the one at top-left would help I think.

Edited September 27, 2015 by ahpook

[taha_never]

[color=#282828][font=helvetica, arial, sans-serif]hello "ahpook"[/font][/color]
[color=#282828][font=helvetica, arial, sans-serif]yep thats the output. what a messy interior i have here! here are some details [/font][/color][attachment=201665:IMG_6432.JPG][attachment=201666:IMG_6434.JPG][attachment=201667:IMG_6436.JPG][attachment=201668:IMG_6438.JPG]

[ahpook]

I'd guess that if you took out the valves then you should get no sound at all.....I'd start by having a good look at output board....

It's difficult to say without knowing what circuit is being used - it all depends how confident you are at trying to fix it, given the high voltages used in a valve amp.

Edited September 27, 2015 by ahpook

[Passinwind]

You could try replacing the socketed IC; looks like an NE5534 or 5532 opamp (can't quite read the number), which are common and pretty inexpensive. If that fails then further self-repair is not such a great idea IMHO. A good tech could most likely trace that circuit by sight in 5-10 minutes tops.

Edited September 27, 2015 by Passinwind

[taha_never]

thank you JTUK you helped me figure out that my valves are ok. thank you ahpook for your time, yep i don't dare, but as you and Passinwind suggest i will chase the output part. thank you Passinwind for your advice i'll take my unit straight to a techie. have a good time friends

[Passinwind]

[quote name='Yawn_Blah' timestamp='1443375497' post='2874222']
thank you JTUK you helped me figure out that my valves are ok. thank you ahpook for your time, yep i don't dare, but as you and Passinwind suggest i will chase the output part. thank you Passinwind for your advice i'll take my unit straight to a techie. have a good time friends
[/quote]

Cool, and good luck. With the right tools and a little knowledge that should be pretty easy to fix, most likely.

[ahpook]

[quote name='Passinwind' timestamp='1443375911' post='2874230']
Cool, and good luck. With the right tools and a little knowledge that should be pretty easy to fix, most likely.
[/quote]

Very much so - do let us know how it works out.

[taha_never]

[quote name='Passinwind' timestamp='1443375911' post='2874230']
Cool, and good luck. With the right tools and a little knowledge that should be pretty easy to fix, most likely.
[/quote]
yep. in fact i'm pretty strange with circuitries. i swapped some pickups and pots the other day, but when it comes to ICs and such i'm a noob. but yes i'll get a replacement for that IC and give it a try. Thanks a lot again.

[Passinwind]

[quote name='Yawn_Blah' timestamp='1443376449' post='2874239']
yep. in fact i'm pretty strange with circuitries. i swapped some pickups and pots the other day, but when it comes to ICs and such i'm a noob. but yes i'll get a replacement for that IC and give it a try. Thanks a lot again.
[/quote]

Make sure you get the right part number, if it is in fact one of the two I mentioned they are not interchangeable.

[taha_never]

[quote name='ahpook' timestamp='1443376314' post='2874235']
Very much so - do let us know how it works out.
[/quote]
sure. thanks mate.

[taha_never]

[quote name='Passinwind' timestamp='1443376824' post='2874245']
Make sure you get the right part number, if it is in fact one of the two I mentioned they are not interchangeable.
[/quote]
yep i'll read and write it down from the part. cheers.

[BanditSid]

NE5534 OP Amps are designed for low impedance circuits where they are fairly low noise, in a high impedance circuit however they don't perform nearly as well from a noise point of view. A good low noise replacement is TL071/T072, it's not the best but is cheap and reliable and gives good results - if the existing chip is an NE5534 use a TL071 (single op amps) if it's an NE5532 use a TL072 (dual op amps).

Edited September 28, 2015 by BanditSid

[3below]

Tantalising image, as Passinwind notes, it is a 553?..... Looking at the stripboard back, I suggest taking a close look (magnifying glass) between pins 7 and 8 (pin 7 has the brown wire, pin 8 above on LHS of image). Is there a clear gap between the pins (no solder bridge / whiskers)? It is not clear in the image but the solder from pin 8 looks 'blobby' towards pin 7, however this could just be a camera angle artifact