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Orange/Red Spot on The Anode of Sovtek 6L6GC

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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?

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 :devilr:
The bass is tighter then previous unknown 6CA7.
Joined 2004
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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.

. . . . . 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.
Thanks for all your replies:)

Some history:

Actually, several years back, I had tried these tubes & made them glow on this amp, exactly the same situation as I have now. They worked like this for a short while, then I put them aside for spares.

Later, a friend got a pair of Leak TL10. The KT61s on them were too old to be reliable (also the other parts). I restored the amp for him & converted the output stage to 6L6GC with different cathode resistor.

The original KT61 ran on 330V B+, 100R of common cathode resistor, -7.5V bias. After converted with 6L6, B+ slightly dropped to 328V, with -32V bias on 330R common cathode resistor.

At that time, THESE 6L6GC tubes were used on his Leak without any problem. The dissipation was 296V*48mA=14W. (Considerably less than mine though.... )

Back to my own amps, I've used the following tubes: unknown fat bulb 6CA7 (with 5 little stars printed on the bulb), JJ E34L, Philips 7581A, and these problematic Sovtek 6L6GC.

With 6CA7 & E34L, the current is slightly less then 6L6 series. The dissipation is about 18W by calculation. ( 400V B+, 29.5V self bias, 98mA for 2 tubes )

By comparison, the 6CA7/EL34 has 25W max dissipation, and they're running OK at 18W on this amp. While the 30W 6L6GC can't handle 20W dissipation? Odd!

The printings on bulbs are mostly gone, barely visible. I misted them & tried hard to find out, only see ...L6G... :xeye:
(big & clear 6L6GC is printed on the box:rolleyes: )

Talking about oscillation, actually I don't have any experience:eek: Now the output stage of this amp has 100k bleeder & 100R grid stopper, self bias with common cathode resistor. Tubes are connected as pentode (as they are).

If it's caused by oscillation, which is good enough to glow the plate, I guess it can also heat up the OPT, right? Ultra sonic frequency can not pass the iron, so the energy should be kept in the primary windings....

I feel the temp. of OPT is very stable & normal.

I do have a oscilloscope, but, it's now my 2-year-old son's toy :cannotbe: (how he LOVES those swithes...) And, since the oscillation can not pass the OPT, so I have to see the anodes or cathodes to find out. But ( yet another but ), the mono blocks have 2 "external" power chokes each. Gee, so hard to deal with in my present living environment.

BTW, the glows are stable, do not follow input signals. By the operation point, I think they are working class A in 99% of the time in my usage.

These "glow" tubes actually sound good. The amp is now serving the bass department (below 160Hz). The bass from these tubes is tight & punchy, way better then JJ E34L's softness, much better then 6CA7, almost as good as Philips 7581A if not as rich & deep.

I placed an order for 6L6EH, should get them in a few days. We'll see how it goes then.
Yes I do:D

These tubes are STRONG:smash:

Their bass is not so 'snapy' as the old "glow tubes". It's richer, deeper, louder & more "full-body" yet still very well controlled, very similar to Philips 7581 to my ears. (I'm only using them in the bass region)

I think maybe the "wrong tubes" were not up to the job, so it dried out at the bottom half & relatively emphasized the upper bass, thus snapy.

And EH did a great job in matching. The OPTs are so cool, cooler than ever:D Or I doubt my previous purchases of the "matched pair" :rolleyes:
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