Under rating of Vertical Sweep Tubes

[mashaffer]

From time to time I have heard rumblings about vertical sweep tubes being under rated on their data sheets WRT audio amplifier use. Your take on the degree to which this is true? Would it be in regard to plate voltage, dissipation or both? By how much?

Thinking of 6GF7, 6EM7, 6LR8 etc.

[aardvarkash10]

George is the oracle on over-rating performance, but in general terms the voltage is negligibly open to over rate operation (excluding the screen voltage), but cathode current and hence power is open to liberal interpretation...

[piano3]

I would be interested as to whether the maximum heater to cathode ratings are conservative as they would appear to be candidates for CF output stage.

[smoking-amp]

6CM6: Audio: 12 Watts, Vertical: 8 Watts
http://scottbecker.net/tube/sheets/137/6/6CM6.pdf

6973: Audio 12 Watts
6CZ5: Audio: 12 Watts, Vertical 10 Watts (same tube as 6973 apparently)

http://scottbecker.net/tube/sheets/049/6/6973.pdf
http://scottbecker.net/tube/photos/049/6/6973.jpg

http://scottbecker.net/tube/sheets/137/6/6CZ5.pdf
http://scottbecker.net/tube/photos/137/6/6CZ5.jpg

Some other issues to consider. Even within the same tube type, you will find different size plates, presence or absence of grid cooler fins, different plate construction/material/surface finish. The glass bulb temperature has a rating usually (different glass compositions were used), and spacing/ventilation between the tubes. TV sets usually had the tubes all crammed together tightly. Some ventilation (chassis fan, holes around sockets) should allow some uprating all by itself. Some amateur radio operators immersed the output tubes in mineral oil for cooling even.

[mashaffer]

Thanks. Looks like maybe 20% is in the ball park.

[Ty_Bower]

Originally Posted by mashaffer

From time to time I have heard rumblings about vertical sweep tubes being under rated on their data sheets... Would it be in regard to plate voltage, dissipation or both?

If I were messing around with sweep tubes, I'd keep the screen voltage low and beat the living snot out of the plate dissipation.

[aardvarkash10]

also depends on what your amp bias set up is... if you are going pp run deep into AB, then you can afford to go WILDLY over ratings, since half the time each tube is conducting stuff-all. If you are a Class A purist, you don't have that option...

[smoking-amp]

"Looks like maybe 20% is in the ball park."

Been waiting for George to chime in. I think he has gotten well over +50% on some exceptional tubes. But then you have to wonder how long they will last when they start glowing!

Another trick is to trade off screen dissipation for more plate dissipation. (This would require some limit on the grid drive to the tube, so the plate cannot be pulled down into saturation.) Look on page 4 of the 6JS6C datasheet below. They allow 4 more plate watts for every 1 watt less screen dissipation.

http://scottbecker.net/tube/sheets/123/6/6JS6C.pdf

[smoking-amp]

" if you are going pp run deep into AB, then you can afford to go WILDLY over ratings, since half the time each tube is conducting stuff-all. If you are a Class A purist, you don't have that option... "

That's for sure. No rating upgrades possible if the amp's running in Class A. That would be the same mode as in the TV set. (could still up-rate with forced ventilation though) Only audio power in Class AB can take advantage of the crest factor (ratio of average power to peak power) since the tubes run cooler at low power out. The screen grid is likely the shortest fuse to overheating quickly on extended peaks, so it would pay well to avoid overdriving the tube into heavy screen current (some kind of grid drive limiter).

[mashaffer]

Just to be sure I understand. In the class AB case are you saying that the dissipation at idle can be well over spec? I was thinking that in class A max dissipation is at idle where in class AB the dissipation goes up with output.

I am planning several projects with verticals. The 6LU8 SE for my mains is already decided but I plan on at least a 6LU8 PP (got a bunch of these on hand) for woofers (not sub) and a 6GF7 SE for computer or bedroom system. On the PP it would be nice to manage 30WPC or so from single pair of bottles per channel. I can't remember the voltage available with the tranny I have on hand but I think it was 400V or better.