807's: How "Blue" can they "Do"?

[WA2ROC]

The modulator tubes in my Viking II are showing a slight blue glow inside the place cavities.  Yes, they are probably as old as the rig itself and we cannot expect them to hold a perfect vacuum forever.

And I believe 807's can be gotten at reasonable prices these days.

However, how much blue can be tolerated? 

And I realize the consequences of having gassy tubes arc over.

[Steve - K4HX]

I've seen a small amount of light blue in perfectly good ones.

[WA1GFZ]

I would think you are fine as long as the mod plate current doesn't take off when the tubes heat up.

[WA2ROC]

So far, the resting and peak plate currents stay within their ranges during hours of operation. 

When they start to exceed these ranges (60ma resting and maybe 150ma peak for both tubes), I'll replace the tubes.

Guess I better get a pair now just in case....

[The Slab Bacon]

I've seen a small amount of light blue in perfectly good ones.

I would think you are fine as long as the mod plate current doesn't take off when the tubes heat up.

Dont forget that in a stock V-2 the modders are running AB1. A little blue glow around the innards or on the glass is not at all uncommon on audio tubes running class A or AB1. Check your biass and screen voltage just to make sure all is well otherwise.
An 807 is basically nothing but a 6L6 with a plate cap.

Guess I better get a pair now just in case....

And.......err.........furthermore........... What good boatanchor enthusiast doesn't have a few spare 807s laying around    

[WA2ROC]

Yes, I must be a good boatanchor enthusiast because I do have another pair of 807's.

But they're slightly Blue Too!

[PA4WM]

Hi,

The 807's that I used, did all have a faint blue glaze....
Especialy when you darken the room, they realy show.
I believe that is what they are famous for.

[Todd, KA1KAQ]

The way it was explained to me eons ago, blue glow on the glass is okay. Blue glow in the structure, not so much. But I'd guess so long as things stay within their parameters as mentioned, they should be fine. Sometimes if the gas is minimal and the tube is run for long enough periods, it's re-gettered. I'd guess a fair amount of heat would be required.

One thing to keep in mind when trusting a metering circuit is the accuracy of the shunts. They're not always spot-on after 50-60 years and might give a false sense of security. Generally not a big deal unless you're dealing with something that could zorch and do damage.

I've got a passel of 807s around here along with 1625s, all surplus. The 807W was used as a driver in the KW-1 and clamp tube in the T-368, along with the Bandmaster and numerous other rigs. They are still quite plentiful and probably will be as long was we need them.

BTW Dick, a new pair of Injuns arrived here this past weekend from Joe, WA2PJP. They look so good, I couldn't resist. Gene/'PGI has my old set on the air regularly. These appear to be late production with spinner knobs, and the Mohawk actually sounds pretty good! You'll probably hear them soon. 

[k4kyv]

If the blue glow appears to be coming from the inner surface of the  glass, or emanating from the mica or other support structure inside the tube, it's OK.  Not gas, it is simply stray electron bombardment striking impurities in the glass, and the substances making up the insulation. The same phenomenon that allows CRT screens to work.

If the blue glow appears to be coming from empty space inside the envelope, like the blue glow of an 866-A does, then it probably is gas, and the tube should be discarded.  I have rarely if ever seen blue glow from gas inside an 807, but I have seen it numerous times in larger transmitting tubes, particularly the tantalum-plate Eimac-like tubes such as 250THs and 304-TLs, which lack a visible getter.

Nearly EVERY 807 will show this blue glow to a greater or lesser extent. 

[WA2ROC]

"Film at 11", or should I say, photos tomorrow morning?

[Steve - K4HX]

Not really. One is a side effect, the other a specific design. CRTs are coated with material, namely phosphor. specifially designed to produce light when struck with electrons.

The same phenomenon that allows CRT screens to work.

[Rob K2CU]

It isnot uncommon to have the glass flouresce (blue) from electron bombardment from electrons escaping the plate structure. The 6360 in my Tecraft 144 does. See if you can deflect the glow with a small magnet. If so, it's on the glass and you can deflect the beam....after all, it is a beam tetrode.

[WA2ROC]

So, Don, if "Nearly EVERY 807 will show this blue glow to a greater or lesser extent", should I:

1- pull these out right now and chuck them?

2- wait until they really turn deep blue and then chuck them?

or

3- keep using them until some new ones arrive?

That's the $64,000 question for the day.

Thanks for all the replies, fellow AM-ers

[Steve - K4HX]

Does the rig modulate 100% with the specified amount of distortion? If yes, keep and enjoy. If not, get new ones.

[WD8BIL]

Yup, if it works don't fix it!

[WA2ROC]

The waveform on my scope looks as good as my other AM rigs look, with my voice as the input.  

I'll put it on the air this evening, watch modulator plate current and see what people say about modulation quality.

I did, however, order a pair of NOS 807's.  Decent price, not those matched sets the audiophools are looking for

[WA2ROC]

Buddly is absolutely correct.  I can't count the times I've screwed stuff up by trying to fix it!

[k4kyv]

Not really. One is a side effect, the other a specific design. CRTs are coated with material, namely phosphor. specifially designed to produce light when struck with electrons.

The same phenomenon that allows CRT screens to work.

True, one is a side effect and the other a specific design.  But still the same fundamental principle. A mineral substance is struck with electrons and produces light. The only difference is that one is a glass impurity, the other a phosphor coating intentionally applied to the glass surface.

[Steve - K4HX]

Yep, kinda like people glowing when they are in the sun too long.

[WA2ROC]

No need for photos, yet.  Powered the Viking II up last evening and allowed the heaters to warm up for about 10 minutes.  Switched the output to a dummy load and tuned it up the same way I always do.  100 watts or so out and same indications on all meter positions.

Then I looked at he 807's during transmit and could not see one tiny piece of evidence of any blue glow at all.

Set the VFO to 3705, hooked up the loop and proceeded to talk to at least one station near Chicago who commented that my modulation sounded fine on my 10 to 15 over S9 signal.

Thanks for all the comments and suggestions, but it looks like the problem went away.

Old saying: " Problems that go away by themselves usually return with a lot of their friends"

[WD5JKO]

Years ago I was maintaining a AM broadcast carrier current transmitter (actually 17 of them) in college dorms. One in particular used an 807 in the final with 6L6 modulators. I took the 807 rig home on a Christmas break, and worked it over. I bridged the HV supply and raised the RF output from ~ 10 watts to about 40 watts. Added NFB to the modulator, and on 560 Khz that thing looked great on the scope and sounded real good on my monitor receiver.

When hooking this thing back to the dorm 3 phase grid I noticed the tuning was real touchy. Oops, I never changed the L-C Pi-Net when reworking the RF power. So as the dorm went from daytime to night time the load on the transmitter would change, and this would move that 807 off resonance. 

So realizing my mistake, but having no time to fix it, I had to visit that dorm boiler room every day to dip the plate current. Problem was that there was no plate meter. So going into the boiler room, without turning on the light, I focused my eyes until I saw that red plate. Then I'd walk over trying not to hit my head on a nearby hot pipe. Once at the rig I'd tune the plate for minimum red, and then fine tune for minimum blue. That 807 lasted running at ICAS ratings for many months. After I graduated and left I wonder how long that 807 lasted. Tough tubes.

Jim
WD5JKO

[w1vtp]

Rob - magnet:  Yeah, on the end of an insulated stick or something.   

Rich, Dick or whatever: consider yourself "degettered."    Congrats!  Nothing like the nice warm fuzzy feeling of a gettering.

Looking forward to getting my Viking 1 on the air.  Maybe this weekend I will get that belly button connector replaced with a 1/4" jack so's I can get with the PTT circuit.

Al