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Old 20th May 2012, 12:58 AM   #31
NZkeith is offline NZkeith  New Zealand
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Default Really good article and hints....

Hi there to ALL the contributors on this article/thread.... VERY helpful and well explained... Thanks to ALL.... What a great forum and I appreciate all the selfless inputs... Being a bit of a 'newby' at valve/tube equipment, the knowledge of others is always so welcome.... Best wishes to diyAudio and its readers... Cheers.... Keith.... New Zealand
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Old 20th May 2012, 01:11 AM   #32
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Yes. A well-needed bracer!

Thanks to all!
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Old 23rd May 2012, 07:29 PM   #33
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Quote:
Originally Posted by nazaroo View Post
--
(1) A variac on most amps would also drop the heater voltage,
and this directly contributes to tube death.


So much so that RCA did a long comprehensive study
and warned designers not to vary the heater voltage
from recommended values more than 4%:
LOWER VOLTAGES especially KILL TUBES QUICKLY.

Click the image to open in full size.

With 85% of the rated voltage on a tube, it goes from a 5,000-hour+ tube to a 3-hour tube!.
Would this go for all tubes or specifically for power tubes? I remember that undersea tubes were deliberately starved in heater current to increase the life time (no maintenance possible ...).
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Old 25th May 2012, 07:55 PM   #34
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I'm also sceptical about this starved filament stuff. I'm thinking DHTs because that's all I use. It is just not believable that DHTs last 3 hours at 85% voltage. I have them on all day for weeks and months. Thinking 26 in particular here. Plenty of evidence of lower distortion at starved filaments also.

Is RCA talking about indirectly heated tubes here?

Andy
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Old 26th May 2012, 03:45 PM   #35
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Here's data on starved filaments in DHTs

DHT with starved filaments.
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Old 26th May 2012, 04:32 PM   #36
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by triode_al View Post
Would this go for all tubes or specifically for power tubes? I remember that undersea tubes were deliberately starved in heater current to increase the life time (no maintenance possible ...).
- its not "power tubes" vs. "others";

The study included currently manufactured power tubes, signal tubes, sweep tubes, TV tubes etc.
It presumably included all tubes with modern cathodes and filaments, and those with coated cathodes and anodes.

The large directly heated triodes of the previous era were probably not even under consideration. The reason would have been that RCA did not make them. I think for a while that Western, and Taylor continued to make the older tubes, thoriated tungsten, carbon anodes etc., and probably RCA continued to make its 813s and 814s for a while under army contract for transmitters and radar.

But the study refers to common tubes made and promoted in the 1960s and beyond.

Not 'Power vs Signal', but rather 'modern vs. pre-war'.
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Old 26th May 2012, 06:18 PM   #37
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Good thread. Two points though. On DC heating of, say a 12AX7 tube tube where you have a center point in the heater, would feeding +6.3V from either side and exit from the middle not be the best route to go?

Also on cathode stripping. What is the mechanism that strips the coatings? I have seen the migration of elements in metals (used to work in a metallurgical lab in the aerospace industry, also took care of our vac furnaces) but I find it hard to believe a voltage between two points will cause atoms in a vacuum to fly off one surface and presumably deposit themselves on the other. And if the surface coatings have such a tenuous grip on their reality when cold why would they not be more inclined to leave the surface when they are hot? Almost in the realm of thermal spraying done on turbine engine parts, mind you all those require the melting of the source material.
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Old 27th May 2012, 12:27 AM   #38
nazaroo is offline nazaroo  Canada
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Originally Posted by Printer2 View Post

Also on cathode stripping. What is the mechanism that strips the coatings? I have seen the migration of elements in metals (used to work in a metallurgical lab in the aerospace industry, also took care of our vac furnaces) but I find it hard to believe a voltage between two points will cause atoms in a vacuum to fly off one surface and presumably deposit themselves on the other. And if the surface coatings have such a tenuous grip on their reality when cold why would they not be more inclined to leave the surface when they are hot? Almost in the realm of thermal spraying done on turbine engine parts, mind you all those require the melting of the source material.
I was thinking this may have more to do with the primitive state of chemical coatings in the 50s, 60s. Everything must have been experimental, but mostly unguided.

I think RCA did the study in the first place because the army forced them to.
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Old 27th May 2012, 08:44 AM   #39
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Quote:
Originally Posted by Printer2 View Post
On DC heating of, say a 12AX7 tube tube where you have a center point in the heater, would feeding +6.3V from either side and exit from the middle not be the best route to go?
You mean from both sides (pins 4 & 5), I assume. That's the only way of feeding 6,3 Volts a 12,6 volt heater with a center tap (pin 9).

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Old 7th June 2012, 06:52 AM   #40
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Default Filaments run on AC or DC

Hi

While all you are saying is basic and true, you do not know enough about tube amps to really write a tech article covering a lot of time span where various tecniques were used for circuit design.

A. The amp in the picture is a 1935 Seebburg jukebox amp, has a nice sweet tone, very good sonics, good frequency response and dead quiet.

There is a hum balancing pot on the front between the #80 rectifier and one of the #45 tubes. I found this pot had a spot where the hum just vanished, and I mean audiophile quiet!

This amp uses one #45 backwards to provide negative bias for a fixed bias, class-A output, and a very tricky circuit.

This amp is similar to a SE amp, driver transformer with one #45 driver, two #45s out, one #45 bias rectifier

Early amps and Radio transmitters were designed by radio men that knew tubes, and early gear had no hum, a complete myth, as they used good wiring, grounding and tube design to make it work.

This amp, with it's looney wiring, you would think would hum and be unstable, but it is quiet as a mouse, and rock stable.

This amp, is, compared to some of the early amps I have re-stored, actually pretty nice inside, Silvertone small guitar head amps a total mess inside, and others, but all the high-gain guitar amps I have worked on have almost zero hum, AC filaments.

Part of this is the tubes themselves. The mike preamp with the four tubes is one of a group of early style preamps I built, all out in studios, all used every week, and no re repairs, blown tubes or any problems in seven years!

These us a 39/44 very early pentode a #37 buffer, and two #76 triodes in parallel to drive the out transformer so they can run up to +12DB out/600 ohms.

These five-pin tubes were the first group with cathodes, and cathodes were designed to combat the AC hum problems and mostly disconnect the audio or RF signal from the filament.



These preamps use DC for the filaments, and all my gear uses a transformer, bridge, three stage filtering, then a TIP31C transistor as a "choke" filter, for noisless DC to the tubes

I would always use DC, but I am saying that if you had the adventures of being old enough like me to have been inside so many old radios, amps, transmitters, you would know that tubes have a sneaky way of not doing what all your CAD design programs say!
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