HUM STARTS 1 OR 2 HOUR AFTER THE AMP IS ON

[JORGETronic]

Dear members, I have finished a RIAA preamp that was working well for a month or so, then last week when I turn it on no problem or hum is present 1 hour or maybe 2 later hum starts in both channels, but if I take one channel out the hum desapiar in the other channel, not mater what channel (took the tube out) I eliminate the other channel has no hum.
Heaterds are DC, LM7812 with diode 1N4007 in the central pin.
RIIA circuit is pasive bethween both amps of 12AX7 (one per channel)
Then 12AU7 one half for each channel DC coupled, the output is taken from the Cathode.
Basically the hum starts with time( temperature), faulty cap that work well and the start to fail? any Ideas.
The HT is regulated ( 250V) comom LM317 protected via zener and TBJ.
The circuit is the Turneraudio: Three triode phono amp, Pasive Riia

[JORGETronic]

passive RIAA.gif

[Rodeodave]

Hum? As in 50\100Hz, or at a different frequency?
Do you have a spare set of tubes, and iff so, is the hum also showing up with these tubes installed?

I have seen 7812 fail before...I don't always trust them, especially if they're without heatsinking.
What happens if you cool the circuit, like with a fan...Does the hum go away? Do you heave some funky resistors whose endcaps could come loose with increasing temp? A bad solder joint maybe? Is something expanding and touching the chassis? So many possible causes...

[speakerfritz]

you didn't post the schematic for the power supply. my vote is a bad diode in the rectifier section. the hottest one to touch. spray it with CO2, and if the hum stops while it's frosted over....you found your gremlin.

[DigitalJunkie]

Perhaps the 7805 is overheating after a while?
Personally,I prefer to use 1 780x regulator per tube,and heat sink them well.This helps avoid any thermal/protection circuit issues with the 780x regulator chip.

[Green77]

Do you have the psu schematic too?

[DF96]

317 overheating?

[Original Burnedfingers]

I'm for the 317 overheating also. Is it heatsinked with a large enough sink?

Have seen this plenty of times in commercial equipment.

[JORGETronic]

well If tell you how I solve the issue you beleive me, today 4am in mirming I was crazzy so as I are using 2 22o-12v tranformers bach to back I sheld them manaually, the fisrt one that get the 220 from the outlet get 10 times hotter that the other ones, doing the math they should work ( 1 Amp each), the number one ( 220 fromthe outlet) have to provide 450ma forn the heaters and and 6 ma for the secondary (HT) of the following transformer wich will demand someting like 130 ma to the primary tranformer.
So the total for the first transformer is: 450 + 130= 580 ma. now add 15% of looses and we are done to much for my poor hand shielded ! am primary transformer...
So with the vice I tight/ press the tranformer and the humm desapiar.
Once getting hot the diferent metal layers of the transformer start oto vibrate and geberate magentic noice as hum.......well this last part is my conclution, the real stuff is that you poress all the metal sheets together and the humm disapiar...
any other diagnistic from beter skilled tude builders?

[JORGETronic]

whatsoever 12AX7 I took out the humm desapiar, in both channels so less current demanded from the power cure the problem, that point me direct to the power supply but in this case heaters and HT are supported directly or indirectlky by the first transformer .......I am open for nay other thinh I culd solve while I can tell...
Thanks a lot fro you collaboration, by the way the sound of this RIIA is very nice and easy to build.....please give it a try and let me know