Tubes tubes tubes

[jackthehack]

If you're replacing tubes with the same model and brand, there's generally never any need to re-bias, but if I had a head that expensive I'd probably have a qualified service tech give it a check up every time I went through a set of tubes.

 

[Music Man 91]

that's what i thought. i'd probably have someone take a good look at it. i got it gently used for $1250-with matching cab for $800. i wouldn't want to have to spend $1800 or so on a new  head if the new tubes f*** it up. thanks for the advice guys!

 

[Alfang]

CB thanks, you confirmed some thoughts and expanded others thankyou.

Have you heard of Dumble? (dumb question) You should take your knowledge, and use it to create an amp of your own.

Call it the dumbass, no offense and just kidding reallly

Reallly, thats a joke, but it;s crazy enough, it just might work

 

[dbw]

I'd buy your amp, CB!  :icon_thumright:

Actually there was one question I had that your essays haven't answered directly: If I don't wait for the tubes to warm up at all, will anything bad happen?  I'm pretty sure the answer is "no" from your first post, but I'd like to be sure.

Oh, and on a related note... when I'm done playing, if I flip off the power and THEN the standby, will anything bad happen?  Again, I'm guessing no, but I'd like to hear what CB has to say

 

[Music Man 91]

i always wondered that too.... nothing bad has happened to me yet... so far...

 

[-CB-]

I've been told I build a good amp.  Let a customer borrow one once (day job customer).  I never got it back.  He said name your price.  I said, new Les Paul Standard.  It was here in three days.

Jack,  even with the same brand and type.... aak on that bias.

Here's the thing.  There's only a few tube makers worldwide.  There are LOTS of tube "badge'ers" though.  Everybody puts their name on things.  There is no Sovtek factory.  No Electro-Harmonix factory.  No Tung-Sol factory.  No Telefunken, no GE, no RCA, no Philips, no Magnavox, no Sylvania, no Fender, no Groove-Tube, no Mesa, no Valve-Art, no Ruby, no Svetlana, etc etc.

There is Reflektor, EI, Tesla, perhaps another in Russia, and at least two factories in China.  Thats all I know of.  EH/Sovtek/Tung-Sol is all Mike Matthews stuff, and he has ties to the Russian factories and may be at least part owner in one.  I've not spoken with him since... oh in eight years or so.  Ruby bought controlling interest in a Chinese factory.  From there... all gets murky.  Svetlana had some legal name troubles.  EI was out then in then out then in business again.  All the rest just put their name on things.

So, from EH/Sovtek/Tung-Sol, Svetlana, etc  When you buy the exact same tube, you can get one that is markedly different, because they do match, but generally dont "grade" tubes.

Companies like Ruby at one time graded tubes.  Groove-Tube does.

Mesa, Fender, Peavey, etc etc etc... all select tubes that are intentionally uniform, and uniformly such that they will require a rather hot bias setup to optimally perform.  The reasoning there is... they'll work, not go bad, not make things bad, even if they dont sound great.  They cover their butts by selecting and selling tubes that have a safety margin built in (not by design, just by how they test out).

Output tubes should be changed in matched sets.  They should be matched for bias (idle) and transconductance.  Matching for idle only is a crap shoot.  In short terms, transconductance is the relationship of grid voltage to plate current.  The normal test is to idle the tube, noting its bias voltage.  Then deflect the bias voltage upward (towards zero volts) and note the change in current passing through the tube.  Some people use a 1vdc deflection, some use 2.5vdc etc.  What you have then, is a picture of how the tube performs with a signal going thru it.  In effect, you can judge the linearity of the tube, or put another way, how it responds to varying signal input.  Think of it as saying, ok the engine idles at 800rpm, now press the gas pedal by X amount.  How fast is the engine going now?  You want the engines to respond similarly, same with tubes.  Having a set of tubes that responds similarly is always a good thing.  One needn't be anal about it though.....

I always liked to see tubes that were within 2ma difference, when idled at 32.5ma, and showed within about 5ma difference when the bias voltage was deflected 5vdc upward.  Thats about as good as one can get for guitar amps.  After that its big buck audiophile stuff (anal regions....).  But, even at twice the 5ma... 10ma, you really have no great issues on tone or operation.

 

[-CB-]

if I flip off the power and THEN the standby, will anything bad happen?

The reason for the standby switch, in Leo's world, was to enable the musician to warm up the amp, then instantly have it available to play with.  The "other reason" was to prevent a thing called cathode stripping.  Cathodes are covered with what is termed an "emissive" layer of rare earth compounds.  When they are heated to a certain temperature, they emit electrons.  Normally the cathode is the "minus" side of the tube, and we think of things as flowing from plate (+) to cathode, where in fact... its the opposite.  What happens is... if the high voltage relationship is present between cathode and plate, and the cathode is not yet up to temperature, the electrons will jump from the cathode to the plate anyway, and in so doing, tear little chunks of the emissive layer along with them.  This is cathode stripping. 

So, when we flip on the power, the heaters light up, getting the cathodes up to the proper operating temperature.  THEN the standby switch can be switch to "play" (or out of stand-by) and things are fine, no stripping.  This is especially helpful on amps like the Twin Reverb, Dual Showman, Bandmaster, etc that use solid state rectifiers.  Amps that use 5AR4 rectifiers are less prone to cathode stripping, as the rectifier takes time to get up to temperature... negating the need for the standby switch (not completely though).  Tubes like the 5U4GB and the 5Y3 are quicker to heat up.....oops need that standby switch again.  Better to just use it all the time.

What happens when you turn off the amp is a matter of the amps design.  Some amps have whats called a pull down resistor in the power supply that rather quickly takes the high voltage to zero when you flip off the power.  Other amps do not.  The rectification matters.  The filtration matters...    The issue is, how long will the high voltage be present when the switch is flipped off, and what will the heater temperature be?  Worst case, a lot of reserve in the power supply, and you get a slow decay on the amp... and... you're stripping the cathodes on turn off time too.  Best case, the pull down resistor dumps to ground and no issues are seen.  Its better to go standby first, then off, as a matter of choice, because it presents no harm, and possibly some good.

Another thing... is the issue of prolonged reliance on the standby switch during breaks.  Every once in a while, someone gets the idea that its really bad, and "proves" it with the case of cathode poisoning.  In short, when the cathode is there and not conducting, and getting hot from the heater, the heater can contaminate the cathode, as there are no electrons flying off to clean it.  The "fix" is to use the tube which gets rid of the contamination.  Cathode poisoning is a bit of a non-issue, as it only occurs after a really long time period of "heaters on" but no use (ie, on standby).  Its of no concern whatsoever to musicians, unless they arrive early for a gig.  Like... arrive and set up, and go "on standby" in March 2008 for a gig in Dec 2010.  You get the idea.

 

[guitlouie]

i remember someone mentioned--can't find the quote--that if you replace mesa tubes with mesa tubes, you will not have to re-bias.

That would be me, and I read and re-read the Mesa literature to be sure that I understood before I re-tubed my Mesa Dual Rectifier.  Their website has all the product manuals in PDF format, so even if you got your head used and did not get a book with it, it is  all right there.  They claim that the bias on all their amps is set at the factory, and all the tubes that they sell under their own name are tested to fall within very strict tolerances, and hence if you use Mesa brand tubes they will swap out in any of their heads without the need for biasing.  I'll let everyone else debate wether this is logical or not, but what I know is I changed the output tubes and both rectifier tubes just like I was changing a lightbulb, old one out new ones in, and a year later and the thing has never sounded better.  I'm thinking that there would be some liability on the company for adverse effects of users changing their own tubes when a claim like that is made in writing in the user manual, so I guess there is something to that claim.  When my Recto needs new tubes again, you can be sure that I will again by Mesa tubes and do it myself.

 

[Music Man 91]

sounds good, i gues i'll keep buying mesa tubes then

 

[-CB-]

What Mesa has done, is specified tubes that test out in a certain way, then done the adjustment (easy btw) and thats that.  There is a range... of what sounds good and what works.  Just know, to make it in that range - the liability thing - they play it safe.  Nothing wrong there.  If it does ok with tone, great!~  A lot of amps will like to be set a certain way, and thats fine too.

Just dont go getting another set of Sovtek tubes and think they'll work like your last Sovtek tubes.  It'll be a Hail Mary shot if they do.

 

[Patrick from Davis]

Hey CB, I was curious, with a high gain preamp how long do the pre amp tubes last?  I have read several times that they are indestructible (maybe I am exaggerating a bit)  But I can't imagine that is true.  Here is a pict of my little Class A high gain amp (5 to 8 watts depending on the output tube (it can use a el84 or el34, I put a kt77 for more definition and less warmth))  But, I was curious about the preamp tubes, and I thought it might be fun to show a class A amp with one output tube. 

Patrick from Davis

 

[-CB-]

all preamp tubes are run in class A operation.  this is what permits them to be cathode biased (they are).  class a/b amplifiers cannot be effectively cathode biased.  thats a fact

so, class A operation give us a constant current through the tube, allowing cathode bias, you can see that there is no greater wear or usage with a high gain circuit - the current through the tube is fixed, and not variable.

more gain STAGES make it higher gain

with exception for some gain changes due to the choice of plate load resistor and co-ordinating bias set (Cathode) resistor