A Question concerning vacuum tubes

[bsco]

HI all. I'd like to take this opportunity to thank enverybody here who has helped me out so far with troubleshooting issues..I have a question. I have noticed that some tubes glow much brighter than others and thus give off more heat than others. I would assume that this might be due to the design and the manufacture process involved and maybe even the type of material used for the various elements. THe EL 84 tubes I have noticed appear to run very hot...and the different brands of these tubes such as JJ's, Ruby, Mesa-Boogie Brand, and GT brands all run very hot but I have noticed that the GT series glow very bright when compared to other brands. Is this sort of thing normal?? If so,should I have a set of tubes that have the same amount of glow in the same amp for example? Or is it ok to mix the various brands...On another note I had two matched sets of Mesa EL 84's coded Yellow. I installed both of these sets in an amp that required a matched set of 4. I did not touch the bias controls at all.I just installed these and monitored the bias for each tubes. Two tubes from one set drew about 11ma and the other two tubes drew around 20ma. The specs for the amp states that the bias is corect when all 4 tubes drew betweem 16 and 21 ma.In order to bring the two lower tubes up to this range I would have to adjust the bias. Then the other tubes would be much higher than 20ma. There is only one bias pot for the amp. So I had to get a bunch of tubes and keep swapping them until I was able to get 4 tubes that sat within that range without adjusting the bias control. SO I went one step further..I got tubes that were as colse together as possible. Now they range from 16ma to 18ma...Was this the proper thing to do? I still have the amp here but want to return it to the customer soon. It is a Hughes and Kettner Quad EL84....combo. The amp sounds great but some of the tubes glow brighter than others and of course like I mentioned above they seem very hot. So I was wondering if anybody can shed some light on this issue for me...and what does these codes mean...yellow, green, red, etc..then there are numbers on other tubes such as 6550"s which range from 1 to 10. And I am sure that other tubes will have different coding schemes as well...I thought that if tubes had a certain code then they would pretty much have the same characteristics and draw the same amount of current at idle. I take it now that this is not the case. And I appolgize for this long post but this really has be baffled...Cn anybody out there attempt to clear this up for me...somewhat....THanks for your time...
Cheers,
Bernie

[soundguruman]

You are fixing amps for customers?
And you don't know the answer to these questions?
Time to take this amp to somebody who does...

[Jazz P Bass]

Basically, this is why Mesa Boogie recommends that you purchase there own tubes.
Tubes, as manufactured nowadays, are all over the place, spec wise.
I will agree that EL84's run hot.

[km6xz]

The rating system a rebrander uses is not usually public information but should represent a range of cathode current read with a set anode voltage and a fixed mid point bias. That anode voltage will almost always be a lot lower than what an amp would normally run. Within a single brand's products, their rating numbers should be reasonably consistent, by 20% or so....which really is not that close. The numbers mean nothing that can be compared between brands. Some use 2 or 3 digits, some use a color code, others a 1-10 rating which they suggest represents gain.
The most common reasons for 1 tube to glow brighter include slight differences in the length or position of the filament within the cathode sheath so a little bit more of the filament is visible would represent a brighter looking tube. 95% or so of the filament is invisible to you due to the cathode tube covering it so a slight difference in the length sticking out the top of the cathode tube will have a big impact on apparent brightness. The glass envelope of EL84's do seem hot, tubes cool their plates by infrared radiation and the proximity of the anode structure to the glass envelope affects the relative area of glass to heat generating surface. A tube with larger diameter envelope has more surface area to absorb the radiated heat so the temperature per square cm is lower for the same total amount of heat compared to a smaller tube dissipating the same heat. Another cause of tubes to glow brighter is others glowing dimmer due to loose tube sockets or oxidized contacts. The heat and exposure of sockets make them prime candidates for oxidation so any time you replace tubes or suspect differences in gain between tubes you know to be close, de-oxidize and reform the socket contacts. A slight difference in resistance of the filament contacts makes a big difference in filament heat, which makes a big difference in tube gain. When seeing a big difference in cathode current of tubes that should be close, check for oxidized sockets on the filament pins. EL-84 pins have less surface contact area so any oxidation will impact them more than a octal base tube.
Good luck
You will impact the sound by having tubes with dramatically different gain on either side of a push pull AB power output stage, so for a consistent sound between tube changes, selecting tubes that have close conduction characteristics will be needed. Do not mix tubes from different brands in the same power section because there are big differences in basic structure and materials. None of the brands you mentioned make tubes, there are only a few companies actually making tube, the many dozens of brands only buy tubes in bulk and put their own logo on them. From my experience, the brand that seems to test most consistently is Ruby and the worst is Groove Tube but they are ALL a matter of chance. For general repair work, it is a benefit to stick with one brand in a gain setting you decide on so any stock you have has a better chance of matching. For those who think they want a particular brand, usually due to marketing hype, stock those for direct sales but don't use them for general service if you want to have a high reliability rate with consistent tone between repairs for a customer. If you order the same range from the brand and you have 50 EL84's in stock, the chances of having 12 sets of 4 that are very close is pretty good. If you stock a few from a dozen brands, the chances of assembling sets of 4 that are close is remote.
Paying extra for "matching" often is a waste of money since they were matched only for range, not actual gain, and at no where near the voltages or bias level a working amp uses. With a bit of experience you will discover ways of getting good matches, for example setting up your own test jig or a shop amp that you can test tubes in when they are new to come up with your own matching, doing so will mean more consistent sound between repairs for a customer than if you rely on the fairly wide range of values represented by a brand's rating number.
Good luck

[olddawg]

You are fixing amps for customers?
And you don't know the answer to these questions?
Time to take this amp to somebody who does...

Dude...the best tech on the planet started somewhere. All he wants is information. The insults accomplish nothing. When I managed a shop I expected a née technician to mis- diagnose or even damage the occassional piece of equipment for weeks or even months. It's a learning process. We cleaned up the mess. Fixed the units and moved on. Most of these new techs became very competent eventually. At least he is asking and these are logical, reasonable questions. Especially since the old pros that were trained in this technology are few nowadays. Pesonsonally I enjoyed reading the explanations and I have been repairing equipment for 30 years. Chill out.

[popoahi]

while we're discussing tubes I have a newbie question*

why do the tubes on the right side of my panel have these aluminum covers on them???
seems to me this would keep the heat in??? shouldn't we be trying to keep the tubes cool???
BTW my amp is a TW-BF

[Enzo]

They only had so many covers, so they put them on the tubes to the right, and the others were...left...



The small tubes are mainly preamp tubes, and they have all the gain, they take a pretty small signal from your guitar and make it into a fairly large signal for the rest of the circuit. But they are also right next to the power transformer and other sources of noise. So the aluminum covers are shields to prevent the tubes picking up the noise.

heat is not much of a problem with preamp tubes.

[bsco]

You are fixing amps for customers?
And you don't know the answer to these questions?
Time to take this amp to somebody who does...

Hi there SGM...I never noticed this reply.......I must say, you really are one ignorant, s.o.a.bitch...I come here to learn something and you have this stinking ATTITUDE to talk down to people...Let me remind you that at one point in time you didn'r know very much either...It is people like you that actually don't know how to talk to people...I wouldn't want to be one of your customers with an attitude like yours...that's for sure...Why don't you do me a favor(and everybody else).......from now on, don't reply to any more posts....I really don't need to read your bullshit....like in the AMPEG SVT thread I had put up...You have a good day......

[big_teee]

You are fixing amps for customers?
And you don't know the answer to these questions?
Time to take this amp to somebody who does...

Don't mind SGM, He never does Get it!
He thinks to impress guys, He has to belittle them.
If we all ignore him, maybe he will get it, or go away!
T

[bsco]

I can see now why everybody holds the same high opinion about this guy....I think he enjoys throwing his weight around.....so to speak...but at least now I know what he is really like.....he probably treats his customers with the same respect as he treats the people here...I'll know now not to pay any attention to any of his comments....I feel that if a person can't conduct himself any better than that they shouldn't be taking part in round table disucssions.....thank god there are lots of very sensible, people on the forum.....

[big_teee]

Hang in there!
It's only a Movie!
T

[bsco]

Thanks for the moral support......
Cheers,
Bernie

[g1]

I thought maybe he was gone and it was safe? Seems so much more "fool-free" around here lately, let's not poke the bee-hive with a stick.
Oh by the way, "who wanna get paid?"

[Jazz P Bass]

Yeah, what a friggin riot act.

I saw a post that SGM made on another forum.
The OP wanted to know about a Behringer powered speaker tweeter replacement.

SGM told him to go down to Radio Shack & purchase a piezo.

Uhmm, the replacement diaphragm is the same as what is used on a Mackie & widely available.
And it is not a piezo.

[bsco]

hahahahahahahahaaaa.....what a dick head....then he says I don't know anything......