Variations in Filament intensity

[fredcapo]

I recently borrowed a BK 747 tube tester from a friend to run through the pile of used and nos tubes in my collection. I notice that the observed intensity of the filament appears to be different for similar tubes. Is this difference by design, or is it a function of the condition of the tube?

In particular, some mullard 12Ax's glow brightly at the instant of power-up, then instantly dim appropriately. Are these tubes defective? They test OK. Thanks.

[Tom Phillips]

...I notice that the observed intensity of the filament appears to be different for similar tubes. Is this difference by design, or is it a function of the condition of the tube?

It's due to differences in the design (i.e. physical construction of the tube) and also to things such as how much of the heater element is directly exposed to view and the amount of blackening on the inside of the glass.

...In particular, some mullard 12Ax's glow brightly at the instant of power-up, then instantly dim appropriately. Are these tubes defective? They test OK.

This is normal and other brands do that too. The effect is more observable in some tubes due to the reasons discussed in your question #1.

[Bruce / Mission Amps]

It's due to differences in the design (i.e. physical construction of the tube) and also to things such as how much of the heater element is directly exposed to view and the amount of blackening on the inside of the glass.

This is normal and other brands do that too. The effect is more observable in some tubes due to the reasons discussed in your question #1.

Somebody will know, but isn't one of the design elements of the later 12AX7s, and the 7025 version supposed to have slow start up filaments in it?

[Enzo]

SOme tubes have a "controlled warmup characteristic" or something similar, I couldn't tell you which. Usually it meant the tube took 11 seconds to hit operating temperature.

[fredcapo]

I noticed a range in which tubes reach full conductance, however many other factors involved since these tubes ranged from late 30's to present.

But some of the Mullard 12ax's seemed to ignite with a flash on power up then come own to a warm glow in a second.

[Enzo]

When you bias an amp, you should wait until it has been running 20 minutes or so so all the tubes are at 100% warmed up. However, the circuits operate after only a few seconds. They may not yet have settled down, but the cathodes are hot enough to emit and the thing works. Like your car on a winter day. it might take ten minutes for the temp meter to come up to normal, but the car runs fine while the needle is still way low.

[km6xz]

Yes, 11 seconds was the standard controlled warm-up time for receiving tubes that have indirectly heated cathodes. It was required for reliable operation of series connected heaters in transformerless radios, TV and other its in the early 50s and later. Indirectly heated rectifier cathodes were usually used so B+ could come up delayed also. Directly heated cathodes emitted electrons almost immediately which was not good for tube life for a number of reasons. Any miniature or octal tubes available today for amp building will be controlled heater types.
The exceptions are direct heated rectifiers like the 5U4. The indirectly heated cathode 5AR4 and GZ34 have controlled warm up time also. I always liked the GZ34 because it was so efficient, and slow in applying B+ so no standby switch is needed. They will outlast a directly heated rectifier 3 to 1 or more. There is so little forward voltage drop that there is little heat dissipated on the anode compared to the high forward drop 5U4 and 5Y3. Some people like the poor regulation and lower output voltage of these latter rectifiers however, even if they have to replace them more often.
Heaters are the most complex part of a tube, and vary the most between designs. It is an area that QC and material selection result in significant differences in tube performance and life.

[52 Bill]

The term for that early glow is heater flash. Mullards seem to do it the most, and Telefunkens maybe too.

I remember seeing that when I was just a boy working on a Grundig radio in my dad's shop. I thought that it was going to ignite or something. Dad told me that it was common in European tubes. Don't see it that often, but when you do it will draw your attention to it.

[Justin Thomas]

I've got some Amperexes that have done it for years. No problem. I've noticed a trend - if you look at the top of the tube and see two or four raised ridges, I bet it'll do it. I'll check my Mullards later. I've seen a few Sovteks do it too, but jot near as bright as those Amperex.

[km6xz]

Tubes that flash on turn on are not controlled warm up type heaters. They pull a lot of current when first applying power but as the tungsten heats up, resistance increase, positive temperature coefficient, current decreases. The heaters are pretty tough and can handle that, to temperatures well over their 1200C operating temperature. During manufacture, the heater is run at an elevated temperature to about 1800C for a few minutes in a process called "flashing" but that is still well below tungsten's melting point of 3400C to set the thorium which is mixed in tiny quantities with the tungsten. The brightness of the flash is determined by the heater wire construction. Bare tungsten is coated with a layer of Alumina which acts as an partial insulator. Most heaters are a long helix of wire folded back on itself or a double helix interwound. The tendency to flash visibly is due to two point, the alloy positive temperature coefficient and the thickness of the alumina insulating surface coating. A controlled warm up tube is one with a lower positive temperature coefficient which means the operating resistance and cold resistance are close in value. Some heater wire configurations do not require as thick of alumina insulating coating which means the glow is more visible. double helix heaters need more insulation and it the most common configuration for small receiving tubes like the 12AX7. The biggest advantage of the double helix is reduced hum since the interleaved coils of opposite current flow cancels magnetic fields when using AC heater supplies. Tubes with single helix folded back onto itself to get the required total length tend to appear brighter because their insulation layer does not need to be as think, but it also makes them more suited to DC supplies for maximum hum reduction.
So there is quite a range of apparent brightness between heater designs, and warm up current in-rush. If someone is concerned about high inrush current, and for good reason, it lowers component life, you can use in-rush current limiters in series with the power transformer primary. These are thermistors with negative temperature coefficient resistance. They have highest resistance when cold and very low resistance when warm. This has the added benefit of eliminating the need for a standby switch when using solid state rectifiers or directly heated cathode tube rectifiers, such as the 5U4.

[Mick Bailey]

When you lay out a selection of 12AX7s of various brands and age, there's a big difference in plate size, design, surface finish, colour and all other factors. The only thing it needs to do is comply with a specification and how this is achieved varies (like the internal circuitry in a 555 timer). Cathode and heater construction are also variables. Some heaters glow so brightly the first thing I do is check the heater voltage to be sure there's no fault. When you think how brightly the tungsten element glows in (say) a 100w bulb, and for how long, the initial flash in some 12AX7s isn't remarkable.

In some Hi-Fi builds I run DC heaters and use a voltage regulator configured to give an extended soft-start, in an attempt to extend the life of NOS tubes that 'flash' when powered.

[Aloha_Mark]

Besides differences in initial filament intensity, I find it curious that on a tube tester, some tubes can jump close to their peak gm value, after you push the test button, while others have to slowly crawl there. I always give a tube a minimum 10 secs of time for filament heating. Probably the slow reaction of some tubes is due to the momentary application of plate voltage by standard testers. It is switch controlled, right?

[km6xz]

Since small tubes, called receiving tubes, that include small glass power tubes such as 6L6(in the world of power tubes, audio tubes are tiny), have controlled warmup time, standardized at 11 seconds, any reading before that waiting period is going to be low. If a tube tester is showing high readings immediately with a cold tube, it is not testing emissions but maybe shorts or some other test method since the tube can't conduct much until the heater can get the cathode up to temperature for emission. Directly heated heaters, without a cathode sheath do emit almost immediately but those are limited to some rectifiers and large power tubes such as 3-500's or 833 which are used as modulators in small AM transmitters(1-2kw). On the larger amps I've built for RF, it is not uncommon to shut off the heater in standby mode and when power out is needed, just turn the amp on, it will be generating full power in fractions of a second from a cold start. If a heater is dissipating 50-200 watts by itself, system and room cooling requirements decrease. If we used directly heated tubes in guitar amps we could cool them by turning off the whole power section(heaters and B supplies) any time there was a 1 second pause in playing.
But for the tubes commonly used now in amps, wait 11 or more seconds before reading anything or else it will be an false reading.
Tube testers are not needed at all however, the amp itself is the best testing environment because it has the realistic operating conditions present, which are no in a tube tester, except possibly a curve tracer which are pretty rare in repair shops.