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Author Topic: Proper method to power-up vacuum tubes?  (Read 2575 times)

Posts: 225


« on: November 30, 2007, 09:49:58 AM »

Hello Elmers!

I’ve been under the understanding that the proper way
to power-up a vacuum tube, is to bring the heater up
to temperature prior to applying the B+ voltage.

I notice that in some of the 1960’s ARRL Handbook transmitters
that the application the AC Main power-on routes the power
supply’s 6.3VAC and B+ to the vacuum tubes, simultaneously.

When a solid-state bridge rectifier is employed, the B+ comes
up very quickly as compared to the amount of time required to
fully heat the tube filaments.

So, what is the current consensus on this methodology?

Should one employ a “standby” switch in their designs in
order to allow the filaments to heat to operating temperature
prior to supplying the B+, or is this practice not harmful enough
to be concerned with?

My most expensive tube is a 6146 :-)


--Tom Nickel KC9KEP

Posts: 1

« Reply #1 on: November 30, 2007, 10:17:22 AM »

Tubes that have thoriated tungsten can be have high voltage applied with heater voltage.  Oxide cathode tubes need to have a warm up time before high voltage is applied.


Posts: 2527

« Reply #2 on: November 30, 2007, 10:22:20 AM »

Your in luck!
Tom has information on his web site.

Posts: 4464

« Reply #3 on: November 30, 2007, 10:23:31 AM »

In the 60's tubes were relatively cheap, in the 70's even cheaper as they transitioned toward obsolescence. Adding a delayed start to the B+ line wasn't worth the cost and in the case of consumer electronics, where the same folks selling you a TV also sold replacement tubes, there was a bit of Gillette / Kodak marketing at play. I.E. Sell the razor or camera cheap, ring the register for blades & film for years to come.

Many Tektronix 5xx series 'scopes featured a 45 second delay relay in the B+ line to extend the tube life as there were scads of the little glass devils inside. In the case of mercury vapor rectifier tubes the warmup sequence was 5 minutes or longer... The tube filament had to build enough heat to vaporize the drop of mercury inside before it would conduct full rated current.
Considering the cost of tubes today, a separate switch for B+ is well worth the effort. A delay circuit is even better as it removes the temptation to speed up the process.

The end of the world will occur on April 23, 2018 ( the day after Earth Day. Go Figure ).  If you're reading this on April 24th look for updates coming soon.  If you're reading this after June first, fuhgedaboudit.....

Posts: 4380

« Reply #4 on: November 30, 2007, 10:23:42 AM »

Untold thousands of vacuum tube transmitters and receivers and TV sets have had the filament power and B+ turned on at the same time (with tube rectifiers the B+ comes up slowly as the rectifier heats) and have tubes that have lasted for years.  I have never worried about it.  I have a 75S-1 (about 1956 vintage) with the original 6146's which still put out full power and an TS-830S likewise tubed and still purring away.
About the worst thing that can happen is that the coating on the cathodes may get stripped off if the tubes are abused.  If you homebrew your equipment then you can switch things as you desire, even bring up power slowly with a variac.  Don't worry be happy.

Posts: 6646

« Reply #5 on: November 30, 2007, 10:25:34 AM »

You shouldn't have any problems with 6146s, but "read and heed" if you ever get one of the bigger power amps... they care!


Posts: 3592

« Reply #6 on: November 30, 2007, 11:04:55 AM »

    Be very careful.  Word has it that tubes will soon be skyrocketing in value as production drops almost to nil.  Chinese production may help in filling the "vacume" though!

Posts: 21764

« Reply #7 on: November 30, 2007, 11:24:27 AM »

Most tubes don't require heater power applied before other operating voltages.  I can't recall any receiving tubes that require this, for example: Radios and TV sets used millions of tubes for decades and always applied all power simultaneously.

Most transmitting tubes don't care, either.  Popular PA tubes like 811As, 572Bs, 3-500ZGs, older 4-250, 4-400 and 4-1000 family, 3CX1200A7/D7 and lots of other "power transmitting" tubes normally have all operating voltages applied simultaneously and these are all "instant on" (no warmup time required) tubes.

Some power tubes like 4X150/4CX250/4CX300/4CX350, 8121/8122, 8560A, 8873/8874/8875, 3CX800A7, 3CX1500A7/8877 etc. have warning statements regarding applying heater power two minutes prior to transmitting, but that doesn't necessarily mean the B+ can't be applied along with filament voltage.  Often, B+ is applied along with filament voltage; but the T-R system is locked to prohibit actually driving the tube or trying to create any output from it.

The popular 8877 is one of those "must be warmed up" tubes.  When I had four 8877 amps operating in my shack (for 50-144-222-432 MHz) they all shared a common power supply and when I turned that on, all four tubes had 4000V on them instantly, whether the filaments were on or off.  I ran that way for 20 years and it didn't seem to bother the tubes.  To "use" one deck, I'd turn on its filament and wait two minutes before transmitting.  But the B+ was always on.


Posts: 1697

« Reply #8 on: November 30, 2007, 12:14:39 PM »

Some cathode types will be damaged if current is passed at low temperature. Having HT on when they are cold doesn't matter, it is the (albeit short) time when they are hot enough to support emission, but insufficiently hot to do so without stripping active material.

Indirectly heated cathodes are the most sensitive to this problem, but delaying cathode current until the cathode is sufficiently hot will not hurt any tubes.

Whilst you will find plenty of applications that did not delay application of HT to indirectly heated cathodes (eg just about all consumer electronics), cathode life will be enhanced by delaying HT until such cathodes are sufficiently hot.

An approach to automating delayed operation of tubes in an amplifier (the commonest application of power tubes in modern ham stations) is at .

Interestingly, interest in this article is strong amongst audiophiles playing with tube power stages. Probably a result of the increasing cost of tubes.


Posts: 1041

« Reply #9 on: November 30, 2007, 12:26:51 PM »

Tom, I think you may be reading more into this
than necessary. As many have stated, there are
a few exceptions such as high power ceramic tubes
and the Mercury Vapor rectifiers that require a
warm up period before applying HV voltage. However,
for the most part and as many have stated, it is quite
safe to apply HV to a tube's plate while the filament
warms up. This includes the 6146 as many final
amplifiers using this tube simply do not conduct until
the transmitter is keyed. The final amplifier's
circuit design prevents it from conducting until
it's keyed.

This can be further understood by reviewing the
section of the ARRL handbook covering tube theory.
It is quite simple once you understand what each
element in the tube is and what function each
tube element is there for.


Posts: 9749


« Reply #10 on: December 02, 2007, 06:41:44 PM »

Mike and Alex hit the jackpot.

The ONLY tubes that possibly benefit from delayed HV application are certain special tubes.

Metal Oxide Cathode tubes always run a risk of poisioning the cathode if the protective electron cloud is stripped away during operation. This generally does NOT apply to small receiving tubes or lower power transmitting tubes, but mainly to HV power grid or certain power rectifier tubes.

You can soft start or delay the HV to your 6146 until the cows come home, and any change in life would be lost in the other failures. The tube manufacturer's data will warn you about possible problems.

Most small tubes fail from loss of emission, and that's a function of CATHODE operating temerature. Then there are shorts and air leakage to contend with, as well as accidental over-dissipation of elements by poor tuning if it is an RF power amp.

When you get into power tubes, then you have to watch cathode "stripping" of oxide cathodes. Too much HV or positive grid voltage or too much current can pull the protective electron cloud away from the cathode. This is why you NEVER want to have current demand exceed the available cathode emission or HV exceed rating in a large MOX cathode high power tube.

A 6146 fails through any fault external to an actual internal tube shortfall, it will not be because of warm-up or filament inrush from a normal filament supply.  It will be because somehow somehow exceeded the limits of some potentially fatal operating parameter.

866's and 3CX1500A7's are different than 6146's and 6AQ5's.

73 Tom



Posts: 1697

« Reply #11 on: December 02, 2007, 10:18:24 PM »

Tom wrote: "You can soft start or delay the HV to your 6146 until the cows come home, and any change in life would be lost in the other failures. The tube manufacturer's data will warn you about possible problems."

Noting that the issue is about cathode current rather than application of HV.

Notwithstanding Tom's view, an RCA data sheet for their 6146B specifies a minimum heating time of 60 seconds.

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