Tube Amp Myths

For fun, or in all seriousness, let's see what's out there.

Here's mine: You don't need to wear a hazmat suit to pull tubes. Nor any kind of gloves to protect the tubes from 'skin-oil'. Washing hands is enough, and is even likely unnecessary, unless you just finished a 'meat-lover's slice' or were working o a greasy engine.
 
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cybermgk

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But EL34s, 6L6GCs, 6550s, KT88s, 6336s, 6AS7s, 6528As, 7591s, 6SN7s and many others do have bases.

Scott Novak
Excactly, and the myth about finger 'grease' is that as the tube heats and gets really hot, said residue will cause cracking of tubes.. It dealt with O tubes (the myth). Granted EL 84s are O tubes.

And one should still pull tubes WITH a base, BY the base.
 

grumphh

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If the O tubes are mismatched, you'll never have the amp biased properly for all tubes.
:D :D Another myth from the OCD world repeated in this very thread :D

"Proper bias" for a tube is found within a fairly large range of bias adjustment, where the changes in sound for the most part of that range are negligible.
And even extremes of the bias range are not very far removed from each other sonically.



...as this vid shows:

Just listen to the ultra cold 18 ma vs the hottt 41ma next to each other - think you are going to hear a difference if one of your tubes runs at 27 and the other at 35? :D :D :D
 
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cybermgk

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:D :D Another myth from the OCD world repeated in this very thread :D

"Proper bias" for a tube is found within a fairly large range of bias adjustment, where the changes in sound for the most part of that range are negligible.
And even extremes of the bias range are not very far removed from each other sonically.



...as this vid shows:

Just listen to the ultra cold 18 ma vs the hottt 41ma next to each other - think you are going to hear a difference if one of your tubes runs at 27 and the other at 35? :D :D :D
Actually, I heard differences on each of those settings.

But, it isn't all about tone. Hotter biased tubes have a different operational time than colder biased tubes..

And if you would read/quoute my entire post, I said Need, no, but you may want.
 

grumphh

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Need? no. BUt most amps have a single bias pot. If the O tubes are mismatched, you'll never have the amp biased properly for all tubes.
Darling, i didn't even mention the word "need"... :cheers:
The bolded part of the quote is simply wrong. You can easily have mismatched output tubes and still have a correctly biased amp, as the tubes (in the Marshall case) can be anywhere between 18 and 41 ma and function just fine within their operating parameters.
Only OCD requires that both tubes operate at the same bias voltage. (Or as close as one can get).

As for the sound of the amp - sure, those with golden ears will be able to "hear" differences between tubes biased at 29 ma vs. tubes that are biased at 32 ma - the differences are clear for all to hear in the video - the mundane rest of us wouldn't even know whether they are at 20 or 39 ma :D
 

cybermgk

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Darling, i didn't even mention the word "need"... :cheers:
The bolded part of the quote is simply wrong. You can easily have mismatched output tubes and still have a correctly biased amp, as the tubes (in the Marshall case) can be anywhere between 18 and 41 ma and function just fine within their operating parameters.
Only OCD requires that both tubes operate at the same bias voltage. (Or as close as one can get).

As for the sound of the amp - sure, those with golden ears will be able to "hear" differences between tubes biased at 29 ma vs. tubes that are biased at 32 ma - the differences are clear for all to hear in the video - the mundane rest of us wouldn't even know whether they are at 20 or 39 ma :D
No but the original post on the myth did say need. You might try reading that, as I had the whole post quoted in my reply, ergo why I said Need? No.

And, um, your 'opinion' does not = fact, as much as you may want it to be trhe case. Although a particular tube may operate at a range of bias, sorry that doesn't mean they are running optimal, for that amp's output circuit, for optimal longevity of the tube, and yes, for overall tone and sound. Thus the term I used, 'properly'. What is 'proper'? It varies by amp and user. Hiow do they want that amp to sound, how long do they want their o tubes to last, what is the design of the amp?, etc. You can ride the brakes on your car, all the time, and everythinf wqill 'operate'. That doesn''t mean it isn't burning those breaks out.

But hey, keep on blowing through tubes, it aint my money. After all, you know it all.

As to hearing the difference? That depends on how badly one has damaged their hearing. Because tthe difference IS there, it's a scientific fact on how tubes work. That video, compressed and as crappy a medium as it was, I, easily heard a difference. But, then I also haven't spent years in front of amps, cranked WITHOUT ear protection.
 

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I had a tube go bad in my OR50 so I threw in another EL34 I had laying around and it doesn't sound different even though they're mismatched.
 

HogmanA

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Bias can make a huge difference sonically.
I had a single ended 6l6 amp that sounded truly glorious when redplating. Obviously, valves didn't last long.
After a tech sorted it to run at correct bias it turned into a dull, lifeless, 'cardboard' type sound.
It's like the difference between the sound made by brand new top of the range hairdressing scissors and children's safety craft scissors. Anyone can hear the different e between them, surely! Well the redplating difference was larger than that.
But it's hard getting techs to do what you want if it's dangerous.
 

grumphh

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I had a tube go bad in my OR50 so I threw in another EL34 I had laying around and it doesn't sound different even though they're mismatched.
The "biasing makes a huge difference" myth is a resilient one. Factual information will not make its perpetrators stop their misinformation :D
 

Caoimhin

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The "biasing makes a huge difference" myth is a resilient one. Factual information will not make its perpetrators stop their misinformation :D
Sometimes I go above or below recommend specs anyways.
 

HogmanA

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Just listen to this cold biased amp. Even though it is greatly exaggerated, as well as the distortion, you can clearly hear the lack of articulation, and that is what a cold bias does, as well as a lack of compression (in fact the opposite, as though an expander is used).
 

grumphh

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Sometimes I go above or below recommend specs anyways.
If you have money to blow, sure bias above spec :D

As well as it is a fact that mismatched tubes will do just fine in amps*, it is also a fact that to hot biased tubes have a shorter lifespan, and in worst case scenarios can do damage to the amp.


*Obviously provided that the tubes are not way out of spec, to the point that they can be considered defective...
 

grumphh

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Just listen to this cold biased amp. Even though it is greatly exaggerated, as well as the distortion, you can clearly hear the lack of articulation, and that is what a cold bias does, as well as a lack of compression (in fact the opposite, as though an expander is used).
Yeah, now that you mention it - i could totally hear that sound when Segeborn played the cold biased Marshall...
 

Caoimhin

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If you have money to blow, sure bias above spec :D

As well as it is a fact that mismatched tubes will do just fine in amps*, it is also a fact that to hot biased tubes have a shorter lifespan, and in worst case scenarios can do damage to the amp.


*Obviously provided that the tubes are not way out of spec, to the point that they can be considered defective...
I've been running well above the recommended 70% in JCM 800 for almost 3 years now. Obviously I'm not running it to the point of red plating.
 

HogmanA

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Yeah, now that you mention it - i could totally hear that sound when Segeborn played the cold biased Marshall...
But you did hear it in the clip I posted. For whatever reason, it makes more difference in some amps, less in others like most other aspects of guitar.
So I call bogus on this particular myth! (not a myth)
 

grumphh

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But you did hear it in the clip I posted. For whatever reason, it makes more difference in some amps, less in others like most other aspects of guitar.
So I call bogus on this particular myth! (not a myth)
Sure, your vid is a perfect example of how a cold biased amp sounds, you have totally busted the myth that biasing is completely irrelevant :D :D :D
 

HogmanA

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Sure, your vid is a perfect example of how a cold biased amp sounds, you have totally busted the myth that biasing is completely irrelevant :D :D :D
Ha! You're getting lost in your own sarcasm. The myth is supposedly that bias makes a difference. Keep up man! :naughty:
 

CB91710

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Sometimes I go above or below recommend specs anyways.
Exactly.
And considering that:
1 - "Proper" bias is generally considered 70% of rated plate dissipation
2 - The tubes will run safely anywhere near that level
3 - Testing a box of tubes to destruction would yield very few that failed exactly at 101%

I don't think you're going to hear a difference, or see a difference in tube life, if one is at 68% and one is at 72%.
Fender biases the Blues Jr pretty hot from the factory.
 

grumphh

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1 - "Proper" bias is generally considered 70% of rated plate dissipation
That is another internet myth :D

Some techs would tell you that "proper bias is done with a sine wave generator and an oscilloscope" - in order to avoid crossover distortion yet keep the tubes running cool.

The point is still that the operating range of tubes (in general) is so wide that a moderate mismatch is of absolutely no consequence - unless you have OCD, in which case the amp becomes unplayable, of course :D
 

Scott A Novak

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Push Pull Output Tube Biasing.

Most tube amps over 5 watts output use a pair or pairs of output tubes in a push pull class AB configuration.

Pure class A operation means that each output tube will amplify both the positive and negative portion of the signal. Class A operation has the least amount of harmonic distortion, but it causes the tubes to run hotter, so you must use a lower plate supply voltage for reliability which also results in lower output power.

Pure class B operation means that one tube is used to amplify only the positive portion of the signal and the other tube is used to amplify only the negative portion of the signal and the output transformer recombines the signals from both tubes to power the speakers. Class B is the most efficient with more output power and results in a lower operating temperature. However, it also results in severe crossover notch distortion which most people dislike.

Class AB operation is a compromise between the sonic purity of class A operation and the efficiency of class B operation.

Class AB operation means that one tube is used to amplify the positive portion of the signal and a small portion of the negative signal and the other tube is used to amplify the negative portion of the signal and a small portion of the positive signal and the output transformer recombines the signals from both tubes to power the speakers.

For reasonable efficiency and still have reasonably good sound, you adjust the bias current (Idle no signal current) of the output tubes just high enough to eliminate the crossover notch distortion.

But you can also use more bias current to bring the amplifier closer to class A operation. This will reduce the harmonic distortion. However, the penalty that you pay is that the tubes run hotter, so for reliability you will need to reduce the plate voltage and/or redesign the output transformer to insure reliability, which also reduces output power.

For a specific amplifier WITHOUT modifications to the circuit, such as lowering the plate supply voltage and/or redesign of the output transformer, there will be a maximum idle current for reliable operation. If you try to increase the idle current any further you will reduce the lifespan of the tubes, and/or possibly damage the output transformer.

The minimum idle current is mainly determined by the amount of crossover notch distortion that you are willing to tolerate.

There isn't any one "correct" bias per se. There is only sufficient bias current to reduce crossover notch distortion to a reasonable level and excessive bias current that causes the tubes and/or amplifier to fail.

Scott Novak
 
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