Lets settle the b+ on cold tubes issue!

[kenpeter]

Play old tape on scope, does it look like Kansas City Standard?
Or alternating clock and data pulses (Trash-80) ??? You might
could still load the sound file into MESS...

http://www.trs-80.com/

[DualTriode]

Hello,
About B+ on a cold tube, Sy thanks for bringing it up. One of my first tube projects in the last 10 years (I took a break) was a knock off of a EAR from MapleTree Audio. The output tubes were NOS RCA 12B4A’s. At cold start one of the tubes would always have lightning in a bottle with large sounds from the $1.00 test head phones for a couple of seconds. I replaced the offending tube with a used Ebay GE tube. There was no more lighting. There was only 330 or so cold volts on the tube. I used the same offending tube where it did not see cold volts at startup and it worked fine. Now that tube is lurking unidentified in the junk bin.
At the time I was plexed between “bad tube” and the stuff I had read about “cold starts” being hard on good tubes. That was the only tube I ever had a cold start problem with.
DT
All Just for fun!

[richwalters]

Originally Posted by SemperFi

[B]For more than 10 years of building amps I'd been petrified of applying high tension to cold tubes.

There is no harm done when applying high voltage to cold tubes!!!

The 5U4 was the standard equipment rectifier in the post war years; worse was to come when evil smelling selenium rectifiers were used. The metal rectifier was the norm in lower voltage TV sets.
Nearly all the tube amps of the late 60's started to use sil rects. Did the instantaneous Bmax destroy tubes ? Nup. No evidence of stripping at anything used in consumer equipment.

Enclosed, anything cheap to save was the norm. One of those nasty rabid cap-input filter circuits. ca 1967. The description mentions KT88's should ahve a life of 10,000 hrs....

richy

[Joshua_G]

Yet, in Morgan Jones' book, on page 441, Fig. 6.30, he puts delay relay for the heaters of the HT rectifiers and also on page 483 Fig. 6.47 and page 484 Fig. 6.48 – so I wonder.

[SY]

I don't have his book with me, but if these cites were related to the Scrapbox Challenge, then one must absolutely let a 6528 or similar tube heat for >30s. Has nothing to do with cathode stripping, it's gettering. Mercury tubes also need a preheat.

But the longevity of tubes in old Audio Research amps, Dyna SCA35s, Berning amps and preamps, and classic Marantz gear (e.g., 9, 8b) are certainly data points in favor of the "it's irrelevant" point of view for normal tubes operated at normal voltages.

[Joshua_G]

Fig. 6.30 refers to the "Scrapbox Challenge", Fig. 6.47 refers to the "Crystal Palace" with 13E1 OP tubes and Fig. 6.48 refers to the "Daughter of Beast" with ECC91, EL84 and 12SN7.

[Poindexter]

The reason I delay B+ voltage in my circuits is in order to avoid overvolting the power supply capacitors. One of my longstanding fave caps only comes up to 400V rating, and two others to 450V, and this is not enough for an open circuit with, say, a 325Vac transformer secondary. Now that I think of it, I'll probably lose the second switch on the Mœbius, since I'm now using a voltage doubler supply at 150Vdc or less final. It has been conclusively shown, as far as I am concerned, that there is no damage to the cathode of the tube by applying B+ before it's warmed to emitting temperature. I'll be reviewing the voltage possibilities of the Machine circuit as well, since why complicate things when it's not necessary?

Aloha,

Poinz
AudioTropic

[Wavebourn]

Originally Posted by Poindexter

The reason I delay B+ voltage in my circuits is in order to avoid overvolting the power supply capacitors.

A bleeding resistor is your friend. You may add a nice LED in series.

Aloha!

[Merlinb]

Having read a LOT of original material about the life of valves, I have only come across two references to standby / warm-up switches. One is in Tomer's book, and rather dubious. The second is from "Valve Reliability in Calculating machines" by L. Knight. Electronic Engineering, Jan 1954.

"Another unconfirmed supposition is that it may be mildly injurious to the cathode to apply any anode or screen potentials before the heater has fully warmed up. Thus it may be beneficial to arrange for a delay of about two minutes on the HT supplies."

Note the use of the words unconfirmed, supposition, may be and mildly. In other words, there is no evidence for it at all!

(And two minutes? Heck, why not make it two days...)

[Joshua_G]

Well, there may be no evidence, however it's unproved possibility.