Sovtek 6550WA?

[Ty_Bower]

Does anyone here have any significant experience with the Sovtek 6550WA? I've got one that is showing some signs of red plate at the seam where it meets the fin. I've only got 56 mA running through the thing. I haven't checked the exact plate voltage, but I know from previous measurements that it is somewhere in the neighborhood of 420~460 volts. That suggests the Sovtek is dissipating no more than 26 watts. I would have thought it could handle this.

Sitting in the socket right next to it is a Sovtek 6550B. It's running just a little higher at 60 mA, and its plates look perfect.


http://i69.photobucket.com/albums/i4...E/P1130044.jpg

[kevinkr]

In the past I have had similarly puzzling experiences with SV6550B and C.. (St. Petersburg) Probably comes down to quality control issues and/or internal electrode (beam forming plates) misalignment.

I'd ping Jim McShane and see what he has to say about this issue.

If this is a new tube I would try to return it as a defective tube.

[primbech]

sure that it isnt oscillating ?
is it fixed or cathode bias and waht is the value of the grid leak resistors ?

[tubelab.com]

Does anyone here have any significant experience with the Sovtek 6550WA?

Yeah, about 10 years ago. The Sovtek 6550WA was my favorite tube to use in the "cranked to 11" guitar amps that I built back then. Most would take my abuse while the Chinese tubes of the day would often explode!

I seem to remember an occasional Sovtek tube that would glow. I am guessing that the internal alignment is off forcing most of the tube's current to flow in one small area (where it glows). The Chinese tubes from 10 years ago had really bad alignment, obviously bent cathodes and grid rods. If they didn't just touch each other and create sparks, the grids would warp from the heat and sparks were inevitable. I also have some Reflector sourced Russian EL34's with severe misalignment.

I don't remember finding that in the Sovtek 6550WA's, but I didn't take too many apart. They didn't blow up. I still have a few of the old Sovteks (they work), and about 30 of the Chinese firecrackers. Some day when I am bored I may light some off!

internal electrode (beam forming plates) misalignment.

A bent G2 support will do it. It makes all of the plate current flow where the grid is closest to the cathode. Yes, I take apart the tubes that I blow up!

[Ty_Bower]

Originally Posted by primbech

sure that it isn't oscillating?
is it fixed or cathode bias and what is the value of the grid leak resistors ?

I can't say I'm "sure" it is not oscillating, but it sounds fine. I've got a 15MHz scope - I can stick that on the output. I've never had any oscillation issues with this amp (Simple SE) in the past. There is a 100 ohm grid stopper on the finals. There is no global or cathode feedback, at least not intentionally. I'd expect the circuit is fairly stable. I'll swap the power tubes and see if the issue follows the tube, or stays with the socket. I will say the previous set of tubes in this amp didn't show any problems.

It is cathode biased, with 810 ohms cathode resistance. The grid leak is 220K ohms. The power tubes are triode strapped, with 350 ohms connecting the plate to screen.

I can get a pretty good look inside the plate through the nice big round holes. As best I can see, both screens appear dark with no visible hot spots. This tube (and his cousin, the 6550B) are not new and are most likely well used. They came out of my "pile o' tubes", so sending them back to the dealer isn't an option. If it doesn't work out, I won't cry over it. It is a shame, since I like the look of these guys and I have no other partner for the 6550B.

[TheGimp]

If you probe teh screen, or grid, the capacitance of the probe may stop ocillations.

You need a broadband scope (probably 200MHz BW) and a sniffer probe to see the oscillations(a simple one can be a section of coax with the shield pulled back, or inner conductor formed into a loop to the shield) .

[Ty_Bower]

You really think it is an oscillation issue? Wouldn't the tube draw an unreasonable amount of current (which would be noticed at the cathode resistor) if this were the case? If the full oscillation draw isn't significantly different than the idle draw (class A, and all that) then why is the problem manifested as excessive plate dissipation?

I swapped the left and right channel power tubes. The red followed the tube, and did not stay in the socket. I think it's just a crappy tube. Does anybody need a ten watt 6550?

[tubelab.com]

You really think it is an oscillation issue?

No, I have never seen a Simple SE oscillate. Oscillation has been seen in some Tubelab SE's before I raised the grid stopper value on the 5842's. I am betting on a crappy tube. If it was mine, I would just use it until it died, as long as it sounds OK. It will eventually become gassy, causing increased distortion, and ultimately runaway. I have seen tubes glow like that for years, and I have seen them die in a few days. Keep an eye on the glowing spot. If it gets brighter or bigger, discontinue use, unless you like fireworks!