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Old 18th November 2004, 11:06 AM   #1
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Unhappy S.E Problem.Urgent help Please!

Hi Everone,
I have recently finished my single ended mono block power amp project. They are fantastic until now. The power tubes are having problems.
The amps run 6x 6550 Valve Art power tubes. Not the best , but hey !
Onced powered up they get a purpley/blue colour to them and the speakers start to buzz and crackle away and it doesnt sound too healthy at all.
I e-mailed the designer and he said that it was the power tubes themselves and that they had gas in them. Another person suggested to messure all the voltages. They all tuned out to be at acceptable levels.
So what do you wreckon is going on... Can one tube affect them all ?
How can a tube develope gas inside and what effect does that have on the circuit? For example: I hooked up 1 tube and it ended up frying a 150v@5watt zener diode.
I also have the output tranny on the secondary side tapped at 16 ohms and it sees a 6 ohm load. Do you think that that has anything to do with it?
All help will be happily recieved.
Thanks heaps
andrewe1
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Old 18th November 2004, 12:27 PM   #2
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If you suspect gas, check that the getters in the valves remain silver or black. If they are white the valve has lost its vacuum completely! Also, see if there is a pinkish glow around valve structures (as opposed to dark purple/blue fluorescence of the glass bulb), this also indicates gas. Measure the voltage across the grid resistor of the output valve. It should be zero. If it isn't, you almost certainly have gas current (I have assumed this is a class A1 design).

On the issue of loading a 16 ohm winding with a 6 ohm speaker, this means that you have decreased the primary load impedance of the OPT, which may result in the valve running out of current on negative peaks, possibly causing distortion and reduced output power. Also, the lowered impedance ratio will increase output impedance. But I don't think this would be the cause of the buzz/crackling you describe.

Hmm... burning out a zener diode? Where?

A schematic would help!
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Old 18th November 2004, 01:14 PM   #3
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Hi , I don't think that it is not something with the tubes, because as I understand you have this purple color on all of them , right? I have seen this before on some old VHF transmitters and it was a matter of wrong bias voltage. I agree that a shematic could help alot.

Chris
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Old 18th November 2004, 01:27 PM   #4
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I should have said this earlier. Just because there is a colour on/in the valves it doesn't mean there's something wrong with them. Some colours are harmless, and some signal bad valves. It is a matter of experience to tell which is which.
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Old 18th November 2004, 09:46 PM   #5
SHiFTY is offline SHiFTY  New Zealand
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I would double and triple check your wiring, solder joints and voltages. Do you have a schematic? If the tubes are cathode biased (auto-biased) measure the voltage across the cathode resistor and work out the plate dissipation. Also check that your tubes are balanced.

Also a deep blue glow right on the glass is 100% normal, especially with new tubes.

If all is well then you might just have a noisy tube, or a loose solder joint...
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Old 18th November 2004, 10:45 PM   #6
EC8010 is offline EC8010  United Kingdom
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If it's a rather pretty soft purple/blue near the glass envelope then it's gas. I don't consider gas to be normal. I've just met my first gassy valve (a 6AS7G).
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Old 19th November 2004, 06:53 AM   #7
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Default Thanks to all

Thank you to all that given there help to aid my amp situation. I will get a schematic and post it as soon as possible.
Thanks again.
andrewe1
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Old 19th November 2004, 07:41 AM   #8
Enzo is offline Enzo  United States
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SOmetimes tubes glow when they are conducting excess current. Have you scoped your output? I would consider the possibility that the amp is unstable and may be cranking out a bunch of RF.

When amps go RF, they often have a noisy output and the audio can sound weak. Scope it and see. or at least hang a small value cap on the tube to squash any high freq stuff and see if that tames it.
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Old 19th November 2004, 03:51 PM   #9
morfeas is offline morfeas  Greece
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Quote:
I also have the output tranny on the secondary side tapped at 16 ohms and it sees a 6 ohm load. Do you think that that has anything to do with it?
So the primary impedance is 2.6 times lower. Not the value the designer had in mind This can stress the tubes heavily but maybe is not the cause of your problem.
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