Quick Q on Max G2/Screen Volts..

[Alastair E]

Ive been messing round with a couple of PL508 / 17KW6 'Frame Sweep' tubes in P-P UL, Cathode-Bias, 5.6K a-a...

The max screen volts is listed as 275 volts, Va at 400.

Its currently running with Cathode-bias, with cathode volts at 30 and Screen at 285 ish...and anode at 310, all measured to '0' VDC,--sounds quite nice but no real proper testing or listening done yet

So, Vk, of 30, from 285 = 255V...Is this correct with regards the Max G2 volts measurement, or should I be measuring to '0' VDC...?

The valves show no signs of any distress or 'overheating' even after 6 hours....

 

[Tubelab_com]

So, Vk, of 30, from 285 = 255V...Is this correct with regards the Max G2 volts measurement

Yes, the tube doesn't know or care what is outside. It knows how much voltage is present between the cathode and G2.

The valves show no signs of any distress or 'overheating' even after 6 hours....

The screen grid voltage rating on some sweep tubes is overly conservative (6AV5, 6LW6) while others must not be violated (6CD6). Turn off the lights and look carefully at the screen grid wires. If any are glowing the tube will die an early death even if the plate is stone cold. Since you are running the tube below the spec, you should be OK.

I have experimented with voltages far above the spec on some sweep tubes. I have noticed a phenomenon with some tubes. As the plate and screen voltages are increased a point will be found where the screen grid wires begin to glow. If the plate voltage alone is increased further the screen current drops and the wires cease glowing. The increased plate voltage attracts some of the electrons that would otherwise be drawn to the screen. This effect does not happen with all tubes. Sometimes tubes of the same type and manufacturer will behave differently.

 

[Alastair E]

Thanks George, I thought that was right.

I always respect your advice and have read with interest your posts on TV type tubes and 'sweep-tubes' in particular.

I have decided to 'torture' these two I have set up, in a similar way to your experiments as I suspect the PL508 is a lot more robust than the spec. sheet indicates. The physical construction and size of the tube suggests more than a 12W Pdiss max, and these tubes were used by the thousands here in the UK and Europe for frame-O/P in valve and hybrid CTV's so are reasonably easy to come by and nice and cheap--I Like Cheap!
A common failure mode of this tube was O/C heaters. I never ever saw a 'low-emission' one or any other issues. Must of changed hundreds in my time.-Just O/C heaters for some reason, and generally they were always Mullard/Philips tubes and were fitted to a particular Decca CTV model, the 'Bradford' (30?) chassis as I recall ...

Currently, I am using some 6.8K resistors to drop the VG2 connected to the UL taps on the trans.

I wonder if a zener string would be better used here in place of the 6.8K...? Any thoughts...?

 

[ilimzn]

Here is something you might want to have a look at:

http://www.tubes.mynetcologne.de/roehren/daten/el508bpt_as_triode.pdf

http://geek.scorpiorising.ca/GeeK_ZonE/index.php?topic=3121.0

PL508 might be a good candidate to use at relatively low G2 voltages.

@Tubelab.com,
George, what are your experiences with G2 ratings when the tube is used as a triode, hence Vg2-p is zero? Some sweep tubes seem to alow a LOT more G2 voltage triode strapped, which does seem logical since pulling down the plate a lot of volts below G2, increases G2 current dramatically - and these tubes were designed to do exactly that. I have the sneaking suspicion that the G2 voltage limit is at least 90% about G2 dissipation, and not about the voltage per se (for instance because of the possibility of internal arcing). With Vg2-p being zero, G2 current should stay a reasonably constant percentage of the plate current, no?

 

[Tubelab_com]

I wonder if a zener string would be better used here in place of the 6.8K...? Any thoughts...?

Much of my "torture testing" of sweep tubes was done during the time that I had a bad case of SET fever. I was looking for a "triode" that falls somewhere in between a 300B and an 845 and doesn't require kilovolt level B+. I experimented with zeners and even 0B2 type gas tubes in series with G2. The idea was to allow triode or UL mode but a higher plate voltage than the screen could handle. This works in small doses. Too high of a zener voltage allows G2 to drop too low as the plate voltage gets pulled down with signal. As G2 gets too close to zero the plate current drops leading to distortion. See the following thread (and the picture in my last post).

http://www.diyaudio.com/forums/showthread.php?s=&threadid=116408&highlight=

George, what are your experiences with G2 ratings when the tube is used as a triode, hence Vg2-p is zero?

Some sweep tubes seem to do well in triode mode. The 6LW6 has a G2 rating of 275 volts. I have cranked up to 500 volts through these tubes in triode mode without issue. I have blatantly violated the plate dissipation ratings too. The versions with the heat radiating fins welded to the plates seem to handle 80+ watts with no visible plate or screen glow. The 6AV5 has a 175 volt screen grid rating. All versions that I have tested (6 differend kinds) can handle 275 volts in triode with some laughing off 400 volts. Increasing the tube current will cause the plate to glow before the screen.

On the other hand I have met some sweep tubes (especially the 6CD6) that will begin to exhibit screen glow at 200 volts. This tube didn't like triode mode at all. Too bad, I have bunches of them.

I have the sneaking suspicion that the G2 voltage limit is at least 90% about G2 dissipation, and not about the voltage per se

I think that you are right since the only sign that things are going wrong is glowing grid wires. I have never had a tube arc from over voltage on G2 unless the grid wires actually melted! Some sweep tubes have G2 extremely close to G1, so an arc may be possible in a TV set, hence the conservative ratings. Remember in normal TV operation G1 gets driven to cutoff by a negative going sawtooth wave that often has a large negative pulse during retrace / blanking. The peak negative pulse rating for G1 in sweep tubes is often -300 to -550 volts. These voltages are not seen in audio amplifier service, so a G1 to G2 arc is unlikely.

I still have about 20 cubic feet of loose sweep tubes to "test". I just haven't had the time recently. I have also collected a big bunch (tens of thousands) of small signal tubes. My next science project is to test them to find a good driver tube.