Another insight gained from experts

Mowgli

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This AM a video from a very sharp electronics expert, Mr. Carlson ("Mr. Carlson's Lab" fame), popped up on my YouTube feed. It was entitled something like, "1959 Electronics; Let's see if it works." He was discussing an old Tektronix square wave generator.

During his discussion of the two rectifier tubes that were employed to supply a lot of current to the downstream circuit he mentioned something that may have addressed a question I wondered about decades ago but forgot!

That question was: "How come low wattage amps don't have standby switches while higher wattage amps do?"

Mr. Carlson may have provided a reason, if not THE reason, for this.

There is a vulnerable junction in vacuum tubes where the individual elements (i.e. cathode, grids, plates) are mated with their outside pins. Think of this junction as a "weak link." Mr. Carlson explained that if "too much current" is quickly drawn across the junction of the cathode-pin in rectifier tubes (Diode tubes) it will blow the junction (burn up) and an open circuit will occur as a result of this excess current burning up the connection.

In "low" wattage amps the amount of current drawn from the low wattage power tubes does not pose a threat to the "weak link" junctions of the rectifier or power tubes.

But in "high" wattage amps, the higher current demands could pose such a threat to these "weak link" junctions if a cold tube was suddenly exposed to a high current.

[NOTE: Incandescent light bulbs, when they fail, almost always fail when you "flip on" the switch. Cold light filaments pose very little resistance. So when we turn on the switch, a rush of high current flows through the cold filament. As the filament becomes increasingly hot, its resistance increases (old oscillator circuits employed light bulbs as current limiters; they could "automatically" limit current gains as a function of the current flow!). But cold filaments having low initial resistance, if they are weak or thinned from use, can blow and the bulb fail! This is why one should strategically use dimmer circuits on expensive specialty and rare incandescent bulbs; start out at a low setting, wait about 20 seconds or so for the filament to heat up at the lower voltage, then slowly increase the brightness; less chance of sudden bulb failure!]

So to avoid this risk of damaging the tube(s) and tube failure, the standby switch and observing a "warm up" period were employed.

Not only does a standby switch and observing the recommended "warm up periods" allow for the filaments to warm up and help extend the life of the cathode coating, but it also allows the "weak link" junctions to warm up through heat conduction, thus increasing its intrinsic resistance and decreasing its vulnerability to sudden high current flows.

I thought this was interesting and worth sharing. I think it addresses the question of why standby switches are not used in low wattage amps but are employed in high wattage amps. It appears to be an issue of sudden high current draws during start up.

Additional comments/insights encouraged and welcomed.
 

King Fan

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Not only does a standby switch and observing the recommended "warm up periods" allow for the filaments to warm up and help extend the life of the cathode coating, but it also allows the "weak link" junctions to warm up through heat conduction, thus increasing its intrinsic resistance and decreasing its vulnerability to sudden high current flows.

The delicate innards-to-pin connection is a new game token -- at least on my board. I'll be interested to see what folks say, tho I worry wide misunderstanding combined with dogmatic belief can make these discussions less than satisfying.

Your mention of 'extending the life of the cathode coating' makes me cite Merlin, who addresses *most* of the folklore about standby, especially that part.

http://www.valvewizard.co.uk/standby.html
 

D'tar

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What is High Watt amp vs Low Watt. In My travels.... the High Watt concernes come in Kilos?
 

boxocrap

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This AM a video from a very sharp electronics expert, Mr. Carlson ("Mr. Carlson's Lab" fame), popped up on my YouTube feed. It was entitled something like, "1959 Electronics; Let's see if it works." He was discussing an old Tektronix square wave generator.

During his discussion of the two rectifier tubes that were employed to supply a lot of current to the downstream circuit he mentioned something that may have addressed a question I wondered about decades ago but forgot!

That question was: "How come low wattage amps don't have standby switches while higher wattage amps do?"

Mr. Carlson may have provided a reason, if not THE reason, for this.

There is a vulnerable junction in vacuum tubes where the individual elements (i.e. cathode, grids, plates) are mated with their outside pins. Think of this junction as a "weak link." Mr. Carlson explained that if "too much current" is quickly drawn across the junction of the cathode-pin in rectifier tubes (Diode tubes) it will blow the junction (burn up) and an open circuit will occur as a result of this excess current burning up the connection.

In "low" wattage amps the amount of current drawn from the low wattage power tubes does not pose a threat to the "weak link" junctions of the rectifier or power tubes.

But in "high" wattage amps, the higher current demands could pose such a threat to these "weak link" junctions if a cold tube was suddenly exposed to a high current.

[NOTE: Incandescent light bulbs, when they fail, almost always fail when you "flip on" the switch. Cold light filaments pose very little resistance. So when we turn on the switch, a rush of high current flows through the cold filament. As the filament becomes increasingly hot, its resistance increases (old oscillator circuits employed light bulbs as current limiters; they could "automatically" limit current gains as a function of the current flow!). But cold filaments having low initial resistance, if they are weak or thinned from use, can blow and the bulb fail! This is why one should strategically use dimmer circuits on expensive specialty and rare incandescent bulbs; start out at a low setting, wait about 20 seconds or so for the filament to heat up at the lower voltage, then slowly increase the brightness; less chance of sudden bulb failure!]

So to avoid this risk of damaging the tube(s) and tube failure, the standby switch and observing a "warm up" period were employed.

Not only does a standby switch and observing the recommended "warm up periods" allow for the filaments to warm up and help extend the life of the cathode coating, but it also allows the "weak link" junctions to warm up through heat conduction, thus increasing its intrinsic resistance and decreasing its vulnerability to sudden high current flows.

I thought this was interesting and worth sharing. I think it addresses the question of why standby switches are not used in low wattage amps but are employed in high wattage amps. It appears to be an issue of sudden high current draws during start up.

Additional comments/insights encouraged and welcomed.
Another insight gained from experts

x= the unkown quantity...spurt= a drip under pressure
 

Mowgli

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High watt v. Low watt? Hmmm. A semantics discussion I didn't anticipate.

Not sure how many kilovolt/kilowatt high production guitar amps are out there but I understand the vagueness of the term "high voltage." The "high energy voltages" used by the Manhattan Project-era physicists is considered very low voltage today. Your point is a good one.

I'm not sure I can provide an accurate number to establish a clear demarcation but I can cite a few examples to illustrate and frame the discussion.

Fender DR, PR and Champs don't have standby switches but the "higher watt" Fender amps do.

Rivera-era Fender amps, the same.

The solid state rectified Music Man-RD-50 (50 watt) doesn't have a standby switch but the HD-65 and up amps (65 watts and higher up to 150 watts) do.

It certainly appears that the decision to employ standby switches was predicated on the power of the amps! Consistently, those higher wattage got standby switches.

So the question remains: "Why is this?"

Mr. Carlson's discussion seems to provide at least one answer that is plausible. No?

Abrupt high current flow through worn incandescent lamp cold filaments don't make them more susceptible to burn out? Of course it does.

Production engineers tend to want to keep costs down to maximize profit. If there is no need for standby switches then is there a widespread institutional flaw in its thinking? Bad "group think" on a mass scale? Are all of these standby switches in the higher watt amps just a costly legacy based on a "myth"??? It is possible but monetary and other considerations argue against this.

Or is there something the vintage industry knew and that knowledge was not passed down clearly?

After all, the older guys are the ones who found most of the flaws in both tube prototypes and mass production tubes. We don't wonder why "tetrode" tubes didn't succeed do we? The old tube guys figured that out fairly quickly. Maybe they knew some things that weren't written down? An inquiring mind wants to know.
 

King Fan

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The small Fender v. big Fender thing is not about EE; see Merlin's section on Standby Switch Origins. Merlin points out that in the history of amplification, only guitar amps ever used standby, and yeah, Fender started it. He details possible reasons Fender started putting standby switches in their bigger amps. He also then notes other guitar-amp companies starting using it only because they were copying Fender, and even nowadays, many amp companies still propagate all the 'folklore' reasons in their literature.

The higher-watt argument *from science* is covered in the Merlin piece, and as @D'tar says, it's about "really high." Merlin says:

"...cathode stripping occurs when particles of the oxide coating are physically torn from the surface of the cathode when it is exposed to a powerful electrostatic field from the anode. This would happen if the valve is operated at saturation, without a usual space-charge of electrons to protect it. Fortunately, this effect does not exist in receiving valves, even when operated at saturation, because it requires an electric field strength of at least 4MV/m (yes, 4 million volts per metre!). No guitar amp ever comes close to this.
Another type of cathode stripping occurs when stray gas molecules in the valve become ionised by the electron stream. The positive ions will then be accelerated towards the more negative grid and cathode. If these manage to miss the grid then they may crash into the cathode, physically damaging its surface. The proper name for this process is cathode sputtering. Sputtering is a known problem in gas tubes and transmitting valves operating at kilovolt levels, near saturation. It doesn't occur to any significant degree in ordinary audio circuits. Note that even the RCA Transmitting Tubes Technical Manual No. 4, p65, states: “Voltage should not be applied to the plates or anodes of vacuum, mercury-vapor, or inert-gas rectifier tubes (except receiving types) until the filaments or cathodes have reached normal operating temperature.... [Receiving valves are the small kind used in radio receivers, i.e audio valves like those in guitar amps, in case you were wondering]. ”
 

Mowgli

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King Fan. I understand all of the cathode stripping info and appreciate you mentioning it here. From what I heard from an EE a few years ago this issue isn't really settled but it does appear that Merlin's argument is favored as the reasoning appears very compelling. The main subject here doesn't concern cathode stripping.

None of this cathode stripping discussion, despite it being worthy of discussion, really addresses the "weak link" junction issue which is really a metallurgy issue at the end of the day.

We're talking about the melting point of the junction/joint where two dissimilar metals mate, how that melting point changes with both heat and the accompanying resistance increase in the conductive joint as the heater filaments heat the cathode-joint-pin assembly while on standby and it's susceptibility to damage from a sudden burst of high current.

Thus, an entirely different discussion regarding the need for a standby switch.

In light of more than ample empirical evidence from simple incandescent light bulb failures, the discussion offered by Mr. Carlson makes great sense to me.

So does or doesn't Mr. Carlson's argument make sense to others and, if not, what is the principaled metallurgical argument against this reasoning?

Carlson cites a diminished voltage potential between the cathode and adjacent filter cap for the added purpose of decreasing high current surges through the joint thus protecting the joint from a high current surge. Again, extending this reasoning to guitar amp tubes and known increased resistance in heated wires makes sense to me and I haven't seen a convincing counterargument yet.
 

teleman1

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THe best reason for a standby switch on a small tube amp is; forced warm up. AS I have been told, it is not a good practice to turn your amp on and immediately start struming waiting for it to warm up to operating temp. Thoughts?
 

El Marin

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No, not again a Standby thread

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