ST-70 Rebuilt with VTA Octal Board

Completed the rebuild of an ST-70 recently. A kit build with original transformers, cloth covered wire. Power transformer had a stamp of 11/64. Upgraded with VTA octal board and new everything else. Went with Gold Lion KT-77 output tubes. Very pleased with the sound from the first listen. Have not kept close track of the listening hours, probably around 100.

Read some accounts of KT-77 tubes requiring time to reach full potential. At the time did not put much faith in an actual change in sound after some number of hours. Well, a couple days ago experienced a "what the heck just happened?". Familiar tracks were more alive with dynamics and small details. Vocals more intimate like moving to the front row of a performance. Definitely do not buy into our brain and ears suddenly creating a very different listening experience. Skeptical also about tubes requiring 10s of hours to fully bloom. However, something happened.

texbychoice wrote:Completed the rebuild of an ST-70 recently.  A kit build with original transformers, cloth covered wire.  Power transformer had a stamp of 11/64.  Upgraded with VTA octal board and new everything else.  Went with Gold Lion KT-77 output tubes.  Very pleased with the sound from the first listen.  Have not kept close track of the listening hours, probably around 100.

Read some accounts of KT-77 tubes requiring time to reach full potential.  At the time did not put much faith in an actual change in sound after some number of hours.  Well, a couple days ago experienced a "what the heck just happened?".  Familiar tracks were more alive with dynamics and small details.  Vocals more intimate like moving to the front row of a performance.  Definitely do not buy into our brain and ears suddenly creating a very different listening experience.  Skeptical also about tubes requiring 10s of hours to fully bloom.  However, something happened.      

I have noticed this many many times throughout the years that when you let a new vacuum tube stay hot for a while and allow the tube to brake in it starts to perform a lot better.

It's part of my old spark-plug theory as to why I believe this is happening.... I have thought about this a lot over the years.

Let me explain my theory.

When you first buy a brand new spark-plug all of the metals on the plug are fresh new and clean without debris. Essentially the metals have never passed any electric current or high voltage.

When you first install the spark-plug the spark emanates from everywhere on the plug to where it's not at it’s best efficiency and performance. But after a few miles of driving the spark finds it's best efficiency and performance on the plug for continuity to ground and it stays there instead of bouncing around all over the plug to find ground. Sometimes when you pull a spark-plug out of an engine you can see a small amount of etching overtime that the spark has made on the metals in just one spot to ground .

To me this is sort of what happens with a vacuum tube when it’s new. All of the metal components in the tube are brand new and of course some tubes have a few hours on them from breaking in from the salesman or the company where you bought the tube.

When you first plug in the tube the flow of electrons from the cathode to the anode is somewhat sporadic and all over the place but then (like with the spark-plug) with time, and a little mileage, the electrons on the cathode will find that sweet spot on the metal plate to efficiently transfer its electrons to the anode.

I don't know if this theory is correct but that's the way I think of it and the way that it's working and why tubes need a few hours on them to sound their best.

Have a good day and stay safe everyone.

Tom.

Tom I like your sparkplug theory!
New tube emissions are less stable at first and settle in after several hours.
I think there are several things that happen.
One is the that the cathode bakes and its emission surface stabilizes its emission across the surface like the spark plug analogy.
Also the getter continues to absorb any free oxygen molecules.
And finally the stresses in all the metal parts normalize. When anything is cut, punched, welded, bent, etc. there is stress in the metal at the point it's been worked.
after heat soaking those stresses relax and all the parts will settle into their final working positions.
To use another car analogy think of heat shrinking the metal in a fender after pounding out a dent to get the crease to settle.