• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube turned black and rainbow. Cause?

Greeting all
What would cause a 6550 turn black and rainbow at the top? It was supposed to be new, I put it into this Defy 7 (yeah, I know, pls don’t ask) that I’m working on, and it played fine for an hour or so, but then I saw it change color and shut it down. All surrounding circuitry tests fine. Socket leaking?
 

Attachments

  • 1F64DC6A-063D-4B1A-A3A5-D059035EA8E3.jpeg
    1F64DC6A-063D-4B1A-A3A5-D059035EA8E3.jpeg
    60.8 KB · Views: 638
  • C918AD29-D149-4D30-8CC2-B7355A08E09F.jpg
    C918AD29-D149-4D30-8CC2-B7355A08E09F.jpg
    996.8 KB · Views: 647
6550

Looks like only the top left one? If so a knackered tube, bad luck. Get another and soldier on. If its general then there's something rather wrong with the circuit. I imagine these are el34 Siemens/rft tubes?

Hi. Thx. These are “standard” Svetlana 6550C. I can’t find anything wrong with the circuit. Unless the socket itself is cracked? Funny but this tube actually tests nearly new on my BK747... Mystery.
 
Thx

yes, the micas between plate and g2 is the shortest path in between them..

Thank you
You are correct on all accounts.
I think a crack would explain this. When I got the amp, there was a chinese kt88 sitting in this #1 tube position, and it was fine, and still is fine. Of course, because its bias is so much lower when its stuck into 6550-biased line-up. Need to find a good black socket...
 
The colors you see are produced the same way one sees colors on an oil slick, or thin film. Its an interference pattern cause by the varying thickness of the film cancelling one reflected wavelength. What we have is a rainbow minus a wavelength with a different wavelength missing as the thickness varies; its physics!
 
Nice!

The colors you see are produced the same way one sees colors on an oil slick, or thin film. Its an interference pattern cause by the varying thickness of the film cancelling one reflected wavelength. What we have is a rainbow minus a wavelength with a different wavelength missing as the thickness varies; its physics!

Sounds great! But is the tube still good or it’s it caput?
Thx
 
Sounds great! But is the tube still good or it’s it caput?

Thx
It's getter is half knackered. At best it will have a significantly reduced lifespan, at worst it will gas out, lose its bias and run away with expensive consequences. IMHO its not worth the risk in an amp like that, best thrown away or, in extremis, left in the parts bin for eg. Prototyping or similar well monitored applications.
 
Sounds great! But is the tube still good or it’s it caput?
Thx
IF it had been like that for a long time and still working, *maybe* it got an extra thin getter layer, or glass there has uneven thickness, which might have the proper thickness to produce interference "rainbow" patterns ... BUT it happened in a short time, literally before your eyes, that means that the suspected leak must be quite real and significative.

You said:
Tube turned black and rainbow. Cause?

What would cause a 6550 turn black and rainbow at the top? It was supposed to be new, I put it into this Defy 7 (yeah, I know, pls don’t ask) that I’m working on, and it played fine for an hour or so, but then I saw it change color and shut it down. All surrounding circuitry tests fine. Socket leaking?

Your description is worrying to say the least, I would not use that tube again.
 
Thank you

It's getter is half knackered. At best it will have a significantly reduced lifespan, at worst it will gas out, lose its bias and run away with expensive consequences. IMHO its not worth the risk in an amp like that, best thrown away or, in extremis, left in the parts bin for eg. Prototyping or similar well monitored applications.

Thank you
There is obviously a problem with this amp. Position #1 is killing tubes.
 
Amp

we do not know how far of the datasheet operating points this amp was run on....and new production tubes can live a shorter lifespan...


I know a few owners of these amps that have had original Sovteks and Svetlanas in there for nearly 20 years without a single problem. I’m lucky. Or rather the seller screwed ma royally. But that’s a different story.

I have the schematics. Plate voltage is specified at 465v @ 110vac mains. At our normally high 122vac line I measured 485. Bias is specked at 4vdc between ground and cathode resistor (fussed), 180 ohms, which gives us 22mA, not too bad. Voltage on grid resistors (474k) specked at 62v, measures exact. So the only problem I see now is around the socket in tube # 1. If small caps around it were shot, I wouldn’t get correct readings, would I? Arcing in socket seems more and more plausible. Should I use a ceramic socket?
 
Got it

IF it had been like that for a long time and still working, *maybe* it got an extra thin getter layer, or glass there has uneven thickness, which might have the proper thickness to produce interference "rainbow" patterns ... BUT it happened in a short time, literally before your eyes, that means that the suspected leak must be quite real and significative.

You said:


Your description is worrying to say the least, I would not use that tube again.

Got it. Thx. Have to dig deep into the amp...