Amp bias How critical?

Lefty · From: Grayson, Ga.

This may be a stupid question for some, but I am no amp tech. I just purchased a new set of Svetlana EL-34 tubes for my Musicaman HD-130.
When I installed these, and powered up after a few minutes the two at the 65 watt side were glowing red hot, and the amp distorted, and lost power. After cooling them down I removed them, and replaced them with the old set, and the problem was gone. I went back to the supplier of the new tubes, and they said without the amp properly biased before hand they cannot guarantee the tubes. Is this legitamate? I know the bias adjustment affects overall performance, but this problem was immediate. Help please.
Lefty
aka Richard Born
Dekley D-10
Sho-Bud LDG
Musicman HD-130

James Morehead · From: Prague, Oklahoma, USA - R.I.P.

It's as important as adjusting the valves in your engine after it is rebuilt. And you don't do it "before" you put your new tubes in, you put your new tubes in and THEN have a tech adjust your bias, BEFORE YOU RUN YOUR AMP. The tubes you put in should also be "matched sets". You will then run "balanced", and get optimum tone and performance out of your amp. Hope this helps! Also, you can (but it's rare) get a new tube that is bad out of the box, but any tech can test them before installation.

[This message was edited by James Morehead on 25 June 2004 at 04:19 AM.]

Rick Johnson · From: Wheelwright, Ky USA

Does the amp have a true bias pot?
Some later Fender amps have bias balance pot
but it is not a true bias pot.
I'm not familar with that design but I
would think it may be a balance pot considering Music Man was the next generation
of Leo Fenders design.
Always buy matched sets of tubes, if you
did there could be one defective.
It would be well worth it have a tech
look at it, he may even let you watch
and ask questions.

------------------
Rick Johnson

John Daugherty · From: Rolla, Missouri, USA

Some tube amps have a screwdriver-adjust pot on the chassis for "hum balance". It is connected across the filament winding of the power transformer. With the gain control set at minimum,adjust for minimum hum.

Bobby Lee · From: Cloverdale, California, USA

Here's an interesting article that Randy Smith wrote on the subject, explaining why Boogie amps don't have a bias adjustment.

http://mesaboogie.com/US/Smith/biasadjust.html

C Dixon · From: Duluth, GA USA

Lefty,

I know nothing of the amp you have. But my guess is the tubes were defective or not matched to your amp. Be wary of any seller quick to blame ANY thing other than the product they sell.

If I had to pick my all time favorite "pet-peave" against sellers, it would have to be this. It is one of the most common "excuses" in business.

Now, lest someone draw the wrong conclusion, I am NOT saying your bias pot (assuming there is one) does not need optimizing. Also, I am not saying that the seller was not right.

What I AM saying is, he is probably more wrong than right and assuming more than analyzing, thus defending wrong as a quick and dirty out. The facts are, you have a set of tubes in there now that sound ok and apparently the tubes are not running red hot.

If you put a new set of tubes in there, they may indeed need a slight adjustment. But I would wager that the tubes were either defective or not matched to your amp, or both. I could of course be wrong. If so, I stand corrected.

But I have simply seen this scenario too many times over my many years. I would have insisted he take them bac; or I would stop payment on the check or credit card, etc. Then I would go somewhere else and order a set of the SAME tube types that are in there, to get a "second opinion".

Good luck and may Jesus bless you in your quest,

carl

Jay Fagerlie · From: Lotus, California, USA

You hit the nail on the head Carl. You can pretty much rule out the amp as having a real "bias problem" because only two of the four tubes glowed. The bias voltage is usually a series connection from the bias supply to all four tubes. That doesn't mean that that specific side of the amp does not have a problem (which it very well could) but you can rule out the bias adjustment as a problem (Although as in all the posts above, bias adjustment is very important)

Jay

Scott Swartz · From: St. Louis, MO

It is fairly common for Svetlana EL34 to require more bias than some other EL34s. As already said, use matched pairs/quads and adjust the bias for each new set.

If the 2 glowing are on the same push pull "side", then the pot is probably a balance pot and the pot is set such that one side was getting a lower bias voltage, enough so to make them glow . Balance pots should generally be set very close to "halfway".

The Fenders with bias balance require changing a resistor to adjust the overall bias voltage (like MESA).

IMO setting the bias voltage to a particular value is not enough, tubes vary and the PLATE CURERENT is what need to be set, the bias voltage is the vehicle to set the plate current, but different tubes will require different bias voltages to achieve a given plate current.

Its not a big deal with a bias probe or some 1 ohm resistors between the cathodes and ground (the MESA article sort of mentions this).

Actually, you don't have to "break the wire to measure plate current as stated in that Mesa article, you can measure it by shunting half of the OPT primary; I will not describe this further since its probably best left to a tech.

The MESA article is part fact and part marketing document, "buy our specially seleected tubes and it won't matter that we made the bias unadjustable". I disagree with their approach, but I work on my own amps. Of course the subject here is not MESA amps anyway.

Set the bias. Its like the idle speed on a car, remember when there was a screw on the carburetor to adjust this?

Jay Fagerlie · From: Lotus, California, USA

I just took a look at the schematic for the 130 and there is a bias trim, there is not a balance control. So either the two tubes you installed were indeed bad, or that side of the output section is suspect.
Personally, I believe it is the tubes, as the old tubes were fine once they were re-installed.

Good Luck!

Jay

P.S. I can email you the schematic if you want it.
J

Jennings Ward · From: Edgewater, Florida, R.I.P.

I did that type work for 40 yrs. Let me tell you I have had some sad experiences with tubes. Especially foreign manufactured. The Check and Sevtlanta seem to be as well as the Chinese, not up to the standards in quality control as some others....Gruve Tubes seam to be the most reliable...None are made in the good old USA.All of the equip went over seas, as it was worn when it got there. Bias. If if is not right it , the tube will not work. And in some cases will self distruct.Same as a transistor or ic...I always checked the Bias with the tube out of the socket before replacind with a new one. Checked screen resistors and plate supply voltage .. then replaced tube and mad proper adjustments. If you dont do it right , be prepaired to pay big$$$$$$$$$$$$$. I have 3 Fenders and an old Show Bud. that were not treated right . Carl, you or any tech knows where of I speak....LOL reguards, jennings

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EMMONS D10 10-10 profex 2 deltafex ne1000 pv1000, pv 31 bd eq, +

Lefty · From: Grayson, Ga.

Thanks for all the information. Upon examination the HD-130 I installed these tubes into was the one with the solid state phase inverter. There is no bias adjustment for this model. After also consulting with a power engineer, and an electronic engineer that I work with it was confirmed that the tubes were faulty, or so far out of tolerance they should not be sold (as many formites told me already). Even if bias adjustment were necessary, it is fairly subtle, and would not cause this immediate effect.
The company, and I won't mention their name, but their initials are SRS out of NY was unresponsive, and insulted my intelligence, and my integrity, by asking a series of insulting questions, and eventually asked for an amp serial number, and saying there was probably another underlying problem. Please, the amp was working fine before. They said I could send the tubes back at my expense (10.50) and they would determine if the tube failure was my amps fault (?), and then ship them back to me at my expense (SWEET DEAL). Please. All for a 37.00 set of tubes.
A learning experience for me. My company and many others (Mesa Boogie included) honor their products without such hassles.
Never again, my brothers.
I feel better after venting this.
Thanks again,
Lefty

[This message was edited by Lefty on 25 June 2004 at 07:30 PM.]

Jody Carver · From: KNIGHT OF FENDER TWEED

I saw this late,,all Music Man amps had NO bias controls. They were designed that way
by LF and Tom Walker.

Jay Fagerlie · From: Lotus, California, USA

Boy, I'm scratching my head here.....
I'm looking at a MM schematic and there it is "bias trim" and there is even a note on the schematic as to how to adjust it........
A solit state phase inverter has nothing to do with an output section, besides supplying an ac signal of proper amplitude to drive the output tubes. The bias section is related to the output tubes only. All tubes must be biased (yes, even preamp tubes) and all tube amps have bias applied, weather fixed bias or cathode bias.
Granted, some companies state that there is no bias adjustment, (Mesa, and now MM I just learned) let there be no doubt, there IS a circuit in those amps that set the bias, and there is a way to adjust it.
I'm not trying to be hard-nosed here, but these are the basics of tube amps that cannot be denied. From the post above, it looks like this is another story of a business that takes advantage of the fact that there are so many 'myths' about biasing a tube amp. (I'm talking about the tube supplier, not MM, although Mesa has a slick answer in regards to biasing)

Flame on!!!

Jay

Jody Carver · From: KNIGHT OF FENDER TWEED

Jay
From the information supplied to myself and all sales personnel, it was not necessary to use what is known as matched tubes and also that it was not necessary to adjust the bias
on any Music Man amp.

These were the words of Leo Fender.

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

One thing, I have to add, I agree that at the time of the amp, biasing may not have been as critical in the Music Man amps,however, the new production tubes out of Russia and China are not as robust as they were years ago.. Sovteks WILL fry in no time in a Music man, Svetlana's are ok, but the amp will need to be opened and biased (yes, there is a true bias pot in these amps), SOME chinese tubes are OK (the Ruby Str-el34-B --note only the B version!) can handle the voltages... thing is, most tubes today are rated to handle no more than 500vdc B+ voltage, and the music man amps ran at a bit over 700v! So if they are biased cold you'll be ok with some tubes...

also if the amp hasnt' been serviced, ti would be a good idea to get the filter caps changed. My advice would be to take the amp to a good tech, drop down about $100 and have them give hte amp an overhaul, and while there, let them suggest and put in tubes for you-- sometimes some small mods to the bias circuit will need to be changed to get these 'modern' tubes to be happy.

Jody Carver · From: KNIGHT OF FENDER TWEED

Gino
Did Leo tell me a fib or what???

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

nah...Leo didn't lie to ya.. leo was a pretty straight shooter, I would say... just that when the amps were designed, current production tubes were pretty consistant, and the amps were designed to run them pretty cold so pretty much any tube was plug and play... Fender still holds to this theory a bit -- if you buy a new Fender, and replace the tubes, if you buy Fender tubes with the same color band (they use red white and blue color bands to determine bias range), then it is still plug and play (I think Groove Tubes also has a cross reference chart for using their product in the new Fenders). -- in other words, Leo probably didnt forsee things as they are today (absolute junk being pushed out of the tube factories etc)

David L. Donald · From: Koh Samui Island, Thailand

If the tubes were good AND coorect replacements for the amp,
then even with a bit of off bias they should have run fine. But woould run better if biased ideally.

Very rarely is an amp so far out of correct bias that it will blow up a tube on start up.
That is likely only with a total rebuild etc. Not a simple change of tubes with the same tube type. It was running before.

That said is is impossible to bias an amp without the tubes installed and turned on.
So the comment about biasing and then running the amp is incorrect.

The symptoms in the 1st post does sound like bad or mismatched tubes.
A tech would be able to tell.

The seller is not correct in refusing to replace them because they were not biased before you turned on the amp...
Because it must be turned on to bias them.

If you played a night and then a tube blew during a show... maybe he has some grounds.

Jay Fagerlie · From: Lotus, California, USA

Hey Guys,
I think I need to clarify my point I was trying to make above.....
All tube amps need to be biased correctly to work.
What I was trying to stress is that even though manufacturers say there is no need to adjust the bias and even if there are no adjustments provided for an easy bias tweek, the adjustment of bias is a tube amp is nonetheless required.

And now back to our regularly scheduled program......

Jay

James Morehead · From: Prague, Oklahoma, USA - R.I.P.

If your refering to me, David, I guess I should have been more specific. Yes, you have to "turn on the amp" to bias it. No you shouldn't "run" the amp until it is biased. In other words, you probably wouldn't want to put the amp to use until it has been biased and checked out. Thus the comment, bias the amp BEFORE you run it.

Steve French · From: Roseville CA

You should also be aware that the name Svetlana was recently purchased by another tube manufacturer. Svetlana had built a reputation as being about the most consistent of the currently manufactured production tubes -- not up to the standard of NOS American tubes, but more consistent and a lower failure rate than its competitors. New Svetlanas may not live up to that reputation. My understanding is that there is a surprisingly high failure rate with all currently manufactured tubes. Perhaps someone else on the forum would hazard a guess as to the typical failure rates of new tubes.

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