Tube Amplification Question

Hello folks,  i found  an old  60s spinet Farfisa Foyer organ that uses tubes for preamp and amplification.  So, the question could go for any  tube  circuit,  old tube radios etc.
I noticed when turned the beast on,  that two tubes (i think the rectifier and preamp ones) were turned on/ light on  for about 2-3 seconds and then were immediatellt tuned off / stopped to light.  Indicator "ON" lamp lights all the time and of course no sound. 
What could this symptom be?  I will fully re-cap it and swap old tubes with new, but i would like to know just before doing so.

PS. I think i ve read the same symptom on tube circuits here or in other forums, but cannot clearly remember (smthng like rectif and pre tubes cannot "keep their power" or smthng like that...)

Any input is welcome

many tubes light up more just at power-on - it's because they draw more current until heated.. no problem in that, I think?

You re right i hadnt left it powered up more than 5 secs cause i was scared, now i re powered it and noticed that they just glow more at power on and then glow normally , but the problem is that i get no sound at all for about 10 seconds, and then suddenly a buzz sound (50-60hz) comes out of  the speakers and gets increased and increased very hard, so i turn immediatelly power off  :-[
Any thoughts?

PS. Its not from "out stage"/speakers etc as i also tested with headpone out and got the same loud buzz noise that increases...

EDIT: Hum does not increase, well it increases but then stays stable.  It's 100 HZ ...

It uses  a EZ81 for rectify, 2 ECC83 for pre, UL84 for amp and one ECC82 (i dont know for what), unfortunatelly no schematic available on net

I found that 100hz noise can be due to dead selenium rectifier bridge. How could i know if thats it?

Do you have a bridge in there???

Are you talking about one for the heaters??

Yes it has a selenium bridge rectifier one of those old that they used in 60s.
Its a Farfisa Foyer (spinet organ). I just open it up, all tubes etc seem fine, but i only get a loud 100hz hum, no sound at all by pressing any key at the keyboard.  100hz noise that does not get affected from turning the colume knob at all, it remains loud and stable. So it is before volume.
I know from my friend that gave it to me that  half a year ago it was fully working and it had never been opened, i was the first that opened it and "see inside" (nice feeling ).

It uses 1xEZ81, 2xECC83,1xECC82,2xUL84

it's not common to tell  how a tube is based on looks. there are exceptions such as broken glass or what looks like white power at the top which indicates a broken vacuum.  The only way to tell if the tubes are good for sure,  is to test them on a tube tester. If you can find a schematic you should be able to track down what the problem and we can even help there.  If you have no output and 100hz hum, I would suspect it needs  new filter caps as 100hz is an octave of 50hz mains frequency which is common over in Europe. you may also have power supply issues which is why there is no sound. Depending on the circuit it may take 30-40 seconds before you have any audio other then hum. your best to leave it on if it looks safe and let it warm up. then see if it produces sound.

thanks pucho812 for the reply. yeap i will fully recap everything just to be "safe" , but i wait for the components to arrive from mouser, so as i m waiting i search for other things that may help. Unfortunatelly the big minus is that i cannot find the schemo anywhere, now i wait for a guy he maybe has it and send it to me, i hope.

you may also have power supply issues which is why there is no sound

Click to expand...

such as? only thing i want and hope to avoid is a dead PT.  I will give it a try and leave it on as you suggested when go home. I have only left it for about 1  min. max but no sound from any key at all.

Ok if the psu is not sending the correct voltages, it will effect tube operations. For example  Too low a heater and the tube does not work....

You run the risk of collateral damage if a part has failed, and you have no test equipment (ie. voltmeter at least) or awareness of the circuitry (ie. schematic).  You can fish around and may come across something, but that is not the same as confirming all the circuitry is functioning properly.

I couldn't find a schematic - posting on organ forum would be a good move (there is a request thread I recall).  And some nice photos at https://organforum.com/forums/forum/electronic-organs-midi/home-organs/50560-farfisa-foyer

Thanks trobbins. I have voltmeter but its mechanical orientation chassis etc is so that you cannot really have tubes mounted, power on and have access to underneath of it (at the components area) so its most likely " i do things first and then put all tubes back, power on and see what happens". Its hard to attach alligators/probes/multi while tubes are mounted and power on to measure volts in there, really.

Yeap, i know the site you posted, we talk each other with the guy long time ago, he has done an excellent job with some decent photos there, you have really  to check his site if you re interested in combo vintage organs
He also believes my issue comes from bad filter caps. And thats the place i read about 100hz buzz-selenium bridge rect problem and was wondering.
My components arrived today so i will start from recapping first and will be back with results

The fact that this 100hz buzz does not get affected by the volume control, does mean that hasn't to do with preamp-power stage?

Good news! I recapped all power electros and boooom...  100hz buzz dissapeared !!   Only a tiny kind of buzz  (maybe 50/60hz, i was so tired to test its freq) and i got sound, all keys/tabs are working perfect/tuned, but the sound i get is very low/weak and volume knob doesn't functions at all.
Just a very low output signal (speakers and headphones too, same) that remains stable even if you turn volume around. Any thoughts on that? It may has to do with volume pot, bad tubes maybe?

Perhaps other bad caps in the signal path?

Bri

Bri other caps seem ok, they are those mustard  that rarely go bad generally . I dont think its that.
http://claviers.mezei.fr/dossiers/FarfisaFoyer.A53-124/FarfisaFoyer.A53-124.html

Other thoughts?

Is the amp chassis what you are focusing on fixing (FarfisaFoyer.A53-124.82.jpg and FarfisaFoyer.A53-124.80.jpg) ?

I strongly suggest you try and prepare a schematic diagram - that may be beyond you - how much have you checked?  The easiest way is to update an existing schematic, but that requires experience.

From a schematic, others can then advise what voltages are expected to be measured around the circuit, and what simple tests can be used to work through what is working and what isn't ok.

That bridge diode is likely CuO, not selenium.

Yes trobbins that's it. I have exactly the same model to fix. I m trying to find the schemo, no results yet .
It has a red old style pot in the chassis bottom for master volume (you can see it in the site's pics) that doesn't work at all. It might be that?  I will check with multi when go home later.

wow, lots of development on this...  Are you sure the tubes are good? are you sure there is no lose connection or faulty solder joint?

pucho812 i have cheched all connections, joints etc all seem gfine. i will recheck later as i m working in 3 other farfisas (no tubes) in parallel with that. 
About the tubes, i have not a  tube tester and the 2 power UL84  seem expensive and a bit hard to find locally (only ebay/internet etc). One thing i noticed is that some tubes doesnt sit well on the sockets, they are a bit loose and you can move them with angle a bit (but i guess that couldnt be a problem as i ve in the past worst situations and worked well though, furthemore i did checked for continuity even with angle and it was OK).
My though is that it has smthng to do with the volume master pot in the bottom, or maybe foot pedal / knee lever issue ...