Switch output tubes by switching filament voltage?

[R.G.]

Never mind. Did some research.

Oxide evaporation happens, but it's primarily an issue of temperature - the hotter the cathode, the faster. Mild under temp on the cathodes leads to longer life from this one effect, and even small reductions make it almost nil. So maybe a reasonable approach is to switch in some resistance to lower the heater (and hence cathode) temps a little, but leave them in the emitting temps range so there's no waiting for the tube to start emitting when switched in.

Interface resistance is murkier. Most of the things I found said it's primarily a high frequency effect except in really advanced cases, and the only references I found that talk about slowing it down seem to indicate that it happens with or without voltage on the plates, or at least full voltage. One paper had tubes made to run with Vp at 60V to be able to develop means to test interface resistance and its growth.

So far all the papers I've found seem to indicate that interface resistance is an issue of aging. One of them talked about raising the cathode temp by running the heaters at 4X rated power for a while to bake the cathode and hence interface layer.

My conclusion so far is that ordinary guitar amp use is worse in terms of life reduction than either effect.

[R.G.]

Good reading. Getting The Most Out Of Vacuum Tubes

[Alan0354]

In my book, you can argue whether it hurts the tube in the long run....yada yada!!!

For me, If I go through the pain of putting two sets of tubes, I want option of using one pair or the other OR combine to get more power. That is able to switch on the fly. I would light up both filaments so I can instantaneously switch. The hell will the life of the tubes.

[Justin Thomas]

What Alan said!

That's fine as long as it's the bitches of guitar amp design... but seriously, I think I pretty much read most of the books on this, including ghe one R.G. posted above, and my opinion is, I'm not going to worry about it. I bet they&re going to die of a screen short from getting dissipated to death, or dropped on the floor, more than any esoteric thing we can cook up. I can see ion bombardment/cathode destruction ifvyour bias runs away and the tubes redplate... Usn that why tubes that have redplated usually don't work anymore? Because the gases released cause all kinds of current problems? I'm summarizing there... most of my tubes die from post-traumatic-guitar-player-induced-stress-disorder! Especially the EH ones I've tried, and any UOS 6L6GCs I run across...

Justin

[Yelly]

In my book, you can argue whether it hurts the tube in the long run....yada yada!!!

For me, If I go through the pain of putting two sets of tubes, I want option of using one pair or the other OR combine to get more power. That is able to switch on the fly. I would light up both filaments so I can instantaneously switch. The hell will the life of the tubes.

Yeah, combine or even have a mix potentiometer is my way too. Would leave the heater alone as well, what´s the point if it takes 10-15 seconds for the tubes to "switch" signal anyway.

[Chuck H]

Funny how esoteric things previously discussed tend to be forgotten and recycled even by long time members. Not that "I" remembered much of the above discussion other than it HAS been covered here before.

Other than that, and to rehash another subject that has been covered here, there's yet another option. Rather than lift the cathodes or switch the filaments one could also increase the cathode resistance to hold the unused tubes in cutoff. Following up even further, one could also just switch the grids to ground. I can't remember how, or if, either of these options affects the observed OT primary impedance. I could guess that any method of stopping a tubes conduction basically removes it from the circuit. Since we're getting finite on the pros/cons of one particular method I thought it would be fitting to open the discussion to alternate possibilities to hash out how things like OT impedance and miller capacitance, things that DO affect clipped amp tone, may figure into any final design considerations.

[R.G.]

I personally like the option of a rotating magazine of output tubes, where the "next" tubes in the rotation are kept hot but unused. When you step on the footswitch, ka-chunk, the wheel rotates in a fresh pair of outputs.

[Merlinb]

Yes, it was originally "duck tape", not "duct tape". It was provided to soldiers in WWII to seal up moisture sensitive things, like ammo cans. They denoted it "duck tape" indicating its water imperviousity. Only later did people find it was useful for sealing ducts.

Hmm... Tale of the tape - The Boston Globe

[eschertron]

Okay, since this thread is already off past Neptune, I'm going to ask what's been on my mind for some time now:

If I use an SSR or some other high-resistance device to let a cathode-biased power tube's cathode 'float', why doesn't the cathode-heater potential become a problem? I'm trying to decide if a 'cathode mute' (as described) or a 'screen mute' (using the SSR to allow the screen to be pulled down to ground) is better. Ease of design, parts count, the cathode version is in the lead. But I am still confused by the cathode-to-heater voltage. The spec sheet I looked at (JJ EL84 for example) lists 100V, about the same for any other tube that I know of. Comments?

[Chuck H]

Well the cathode of an EL84 (for example) doesn't need to float so high as 100V to hold the tube in cutoff. So my vote is the cold cathode method.

[R.G.]

Hmm... Tale of the tape - The Boston Globe

Kewl! I was right!

[R.G.]

If I use an SSR or some other high-resistance device to let a cathode-biased power tube's cathode 'float', why doesn't the cathode-heater potential become a problem?

Well the cathode of an EL84 (for example) doesn't need to float so high as 100V to hold the tube in cutoff. So my vote is the cold cathode method.

Good question, good answer.

I believe that some amps use the method of simply opening the cathodes, so it's at least not immediately disastrous. There is a third alternative - UN-short a resistor of some value in the cathode circuits and let the cathodes float up till the tube is inconsequential in the sound, but not so high that the heater/cathode insulation is compromised. I've used 10K for this purpose, and that works, too.

There will likely be an infinity of opinions about what is audible and whether it's good or not.

Hmmm. One could mix output tubes by varying low-value cathode resistors... Nah, Randall has probably re-re-patented that.

[Chuck H]

Hmmm. One could mix output tubes by varying low-value cathode resistors... Nah, Randall has probably re-re-patented that.

The old saying about "cream rising to the top" isn't nearly as accurate in todays world as "shit floats". I think most people in the know have similar opinions about Mesa. Good business model. First rate customer service. But... Relentless business practices and mediocre product performance, design and construction. Randall's patents demonstrate the latter.

[Merlinb]

If I use an SSR or some other high-resistance device to let a cathode-biased power tube's cathode 'float', why doesn't the cathode-heater potential become a problem?

Because if the cathode rises to a high potential it is only because of puny leakage currents, too small to break down the insulation.

Kewl! I was right!

There's no edivdence that soldiers called it duck tape or that this had anything to do with water repellency!

[J M Fahey]

There's no edivdence that soldiers called it duck tape or that this had anything to do with water repellency!

Why do you say so?

1) there's no evidence that soldiers called it duct tape, to begin with.
Unless you have some link which proves so.

2) the tape issued to soldiers was adhesive and was called duck tape. Period.

Read it straight at Johnson's and Johnson's official History site.
http://www.kilmerhouse.com/2009/08/d...invented-here/

Just a couple excerpts:

It’s strong, it’s sticky, it’s in practically every home, and it’s been used to fix almost everything — from airplane wings to houses to chair legs. ..... It’s duct tape, and it was invented by Johnson & Johnson during World War II as a waterproof sealing tape.

Notice they use the modern name interchangeably, for convenience.

the Company’s long history of making adhesive tapes .... Adhesive tape (or adhesive plaster, as it was called a very long time ago) is one of the Company’s oldest products, dating back to the late 1800s

They recognize the basically same product had different names along History.
That's not enough for anybody to scream: "hey .... unrelated !!!!!! "

Given the Company’s long expertise in making adhesive tapes, the military asked Johnson & Johnson to have one of its operating companies make a waterproof, strong cloth based tape that could keep moisture out of ammunition cases.

These pressure-sensitive tapes, easy to handle and versatile in use, saved valuable time in manufacturing and packaging war materials. A wide variety of tapes to serve a multitude of particular purposes were made for the aviation industry alone. Actually hundreds of thousands of miles of special waterproof tapes were used on tanks, planes, and ammunition destined for overseas.” [Johnson & Johnson 1945 Annual Report]

Notice the adjectives "pressure sensitive" (which means contact adhesive) and "waterproof" are used in the same phrase, referring to the same subject.
How somebody can claim those were 2 different products, specifically that the waterproof tape was not adhesive, is beyond any reasonable mind.

I'm referring to:

There's no edivdence that soldiers called it duck tape or that this had anything to do with water repellency!

As a side note, I would rather believe the official story stated by the original inventors and makers than any hogwash "posted on the Net" by unqualified writers.
Namely, a "newsguy" who had to fill a page with "something" and his purported backup, a certain "Reader Mike Webster " .... who???

Back to Johnson's and Johnson's themselves:

The tape was originally called duck tape, for its water-repelling properties. (Duck…water…get it?)

Incredible as it seems, some people does not "get it"

And, as the story goes, the fabric used to make the tape was called cotton duck. Soldiers soon discovered that the tape was incredibly useful in repairing just about anything that needed repair, from jeeps to planes to tents to boots. As time went on, “duck” morphed into “duct” because of its use in the postwar building industry to help connect…you guessed it…ductwork for heating and air conditioning.

Meaning:
Post: "after"
War: war (duh!)

So postwar means after the war ... what RG said, by the way

Oh well.