Orange/Red Spot on The Anode of Sovtek 6L6GC - diyAudio
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Old 12th August 2005, 03:20 AM   #1
CLS is offline CLS  Taiwan
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Default Orange/Red Spot on The Anode of Sovtek 6L6GC

I run them in about 400V HT on the anode, with -33V/55mA self-bias. So the anode dissipation should be around 20W.

The orange/red spots show on 3 out of 4 tubes, on the folded area where the supporting rod is welded (pressed) on. They are about 2cm high x 0.5cm wide.

I checked the datasheet, 6L6GC has a max dissipation of 30W, so it's strange to have red spots on the plates.

When Philips 7581A is used in this amp, no red spot is shown.

How long can they last in such condition?
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Old 12th August 2005, 03:48 AM   #2
SY is offline SY  United States
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Not long. If they're new, I'd talk to the dealer about an exchange.
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Old 12th August 2005, 04:44 AM   #3
CLS is offline CLS  Taiwan
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Thanks

They are not new I bought them many years ago, just for spare & little real use.

When power down, there'd be several crisp crushing sounds coming from those tubes. I've heard such sound from other tubes but never as crisp & loud.

What will happen when they die? Melted plates? Cracked tubes?

If the death won't kill the OPT & loudspeaker, I think I'll keep using them to the end of their lives. Nowhere else to use them anyway.

Despite the spots, they sound good to me though
The bass is tighter then previous unknown 6CA7.
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Old 12th August 2005, 09:26 PM   #4
anatech is offline anatech  Canada
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Have you measured the cathode and grid voltages? Also, try a 'scope to see if you have ultrasonic oscillation.

These tubes are generally okay, the 6L6EH are way better. The 7581A should have a PD of 35W. The red spots indicate that you are beyond the max plate dissipation. Period. Could be defective tubes as well.

-Chris
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Old 13th August 2005, 01:32 AM   #5
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. . . . . or it can be the result of a temporary high current as a result of oscillation. Good advice preceeded this post; do try to put an oscilloscope on. Does the are of the red spot vary with applied signal?

But now; I hope I do not get fried for the following! Be careful of some 6L6s designated 6L6GC. They are not. Let me just say that certain brands can be quite free with designations. I have seen the GC marking on what is actually a 6L6GB - only 23W Pa, though still within your operating conditions. (Jeepers, I have seen a certain brand market a KT66 with the innards of exactly a 6L6 - but costing double the price locally!) If I may point out the difference: If you see inside the tube, that the element construction proper is mounted on a glass tongue as in older tubes, it is a GB. The GC mounting is on (thick) metal rods directly mounted in the base pins - this gives some extra heat conduction to the outside world. Also, the cooling fins mounted on top of the control grids are about 10 x 7 mm for a proper 6L6GC; these are smaller in "lesser" 6L6s, 5881s, etc.

Then, to be complete, you presumably checked the voltage over the cathode bias resistor. I am alluding to the possibility that the G1 voltages may not be zero, due to a tube defect such as internal grid-cathode leakage. You can check this with a DVM of about 10 megohm input resistance, on the mV scale. (Presumably the grid resistors are 470K or lower.) Fundamental advice, but just for completeness sake.
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Old 13th August 2005, 01:57 AM   #6
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If you have oscillation, it will show on the 'scope without connecting the probe to anything. Just bring it near the glass of the output on the 10V/DIV range and watch the screen.
-Chris
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Old 13th August 2005, 03:32 AM   #7
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Quote:
When Philips 7581A is used in this amp, no red spot is shown.
Anyone else think this might be a clue?
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Old 13th August 2005, 12:58 PM   #8
anatech is offline anatech  Canada
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I'm thinking straight plate dissipation. Mislabeled tubes?? I have not had this problem with Sovtek in the past but, you never know.
I'd like to confirm the conditions are as CLS intends.

What are you thinking SY?

-Chris
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Old 13th August 2005, 01:03 PM   #9
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I'm thinking that oscillation is unlikely; 7581 is exactly the same thiing as 6L6GC, yet there's no glow. My first suspicion, then, is that the tubes are not up to snuff. Whether it's from a manufacturing defect or a mislabelling, who knows.
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Old 13th August 2005, 01:08 PM   #10
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Agreed, but since CLS is unsure I want to cover all the bases. Definetly sounds a little over the limit on plate PD.

-Chris
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