Tube Amp Bias

  • Thread starter Thread starter daluyon
  • Start date Start date
D

daluyon

New member
Does bias reading depends on the tube that the tech used to adjust the amp's bias setting? The reason I'm asking is, I took my amp to a tech to get it retubed and rebiased. However, according to him the amp's problem was not the tube but the bias was set too low which is right coz the amp original sound came back. The tube sets I took with me weren't used at all.

Anyway, since the bias has been corrected, can I replace the tubes myself in the future without going back to the tech or I need to return to him since my new tube is of a different brand? I wish I could adjust it myself but my Marshall 6100 is a bit tricky. Besides, I don't have the inclination to do that right now. I might fry myself in the process.. :eek:
 
Tube bias is an adjustment of current to the power tubes,...... if it is set too low,... the amp will sound crappy due to the tubes running too 'cold',.. if it is set too high, the amp sounds crappy because the tubes run too hot,.... in addition, the tubes will wear out quicker in either senerio.... usually if the bias is set correctly for ..... say EL34 power tubes,.... then changing tubes is no biggy,.... as long as the tubes you change are the same brand, and type as the ones you replace,.... HOWEVER,.......

Some amps are much more likely to have issues than others,.....rule of thumb,... EL84 based amps are less likely than EL34 amps, and 6L6 based amps sort of fall in between,.....

Pre-amp tubes are not an issue in any of those,... and can be swapped any time with no problems....

DO NOT,.. I repeat,..DO NOT UNDER ANY CIRCUMSTANCES OPEN THE AMP AND EXPOSE THE GUTS UNLESS YOU KNOW WHAT YOU ARE DOING,......THERE ARE DEADLY VOLTAGES IN THERE<<<<<<, and it is very dangerous to fiddle around without knowledge of what is what,......

Replacing tubes, or a fuse, or knobs etc is OK for any one to do,.....

hope this helps,....

more if needed,...

Steve
 
Thatupstateguy said:
Tube bias is an adjustment of current to the power tubes,...... if it is set too low,... the amp will sound crappy due to the tubes running too 'cold',.. if it is set too high, the amp sounds crappy because the tubes run too hot,.... in addition, the tubes will wear out quicker in either senerio.... usually if the bias is set correctly for ..... say EL34 power tubes,.... then changing tubes is no biggy,.... as long as the tubes you change are the same brand, and type as the ones you replace,.... HOWEVER,.......

Steve
Click to expand...
The current tube right now is Groove Tubes 6L6 and the extra tubes I recently bought was a balanced quad JJ 6L6. Does it mean that a re-biase is neccessary when I have to replace the tubes in the future?
 
generally .... yes....


if they were the same type and brand,..... no,... but in most cases different brands are built different on the inside,... bigger/smaller cathodes,... bigger/smaller heaters, etc etc,... and as such will react differently in any given amp,....so yes you will have to have it re-biased to sound the best,....

however,..... just by switching the tubes,... as long as they are the same type, IE, 6L6 swapped for a 6L6,..... you will not ruin your amp by doing it without a re-bias,....

you will just end up with your amp not sounding at it's peak if the tubes are not biased correctly,.. it is always a good idea to have it tuned up every time you switch to a different brand,....


the differences are enough to warrent that...


Steve
 
Thatupstateguy said:
generally .... yes....


if they were the same type and brand,..... no,... but in most cases different brands are built different on the inside,... bigger/smaller cathodes,... bigger/smaller heaters, etc etc,... and as such will react differently in any given amp,....so yes you will have to have it re-biased to sound the best,....

however,..... just by switching the tubes,... as long as they are the same type, IE, 6L6 swapped for a 6L6,..... you will not ruin your amp by doing it without a re-bias,....

you will just end up with your amp not sounding at it's peak if the tubes are not biased correctly,.. it is always a good idea to have it tuned up every time you switch to a different brand,....


the differences are enough to warrent that...


Steve
Click to expand...


There are tubes and amp designs that do not require re bias. Components degrade with time for the best perfomance you should always rebias unless the amp is designed other wise it, especially in an amp that has a few years on. Make sure you find somebody that knows what they are doing. Thatupstate guy is quite right.
 
There are also some amps out there that have an adjustable bias via a pot,.... and some players actually LIKE the sound of an amp with the tubes running a bit hotter,.....

altho it does burn em right up,.... and if you are talkin NOS stuff,... That habit can get expensive,......


Steve
 
I donno...when I buy new tubes there's bias number on the tube...get two that aren't matched and stick them in the sound sucks...rebias the amp and all get happy again. Every tube in my experience is a little different, you can get matched pairs but if the bias numbers aren't the same as what was in the amp in the first place, all bets are off.

My personal rule of thumb...new tubes...rebias. I do my own so it doesn't cost a trip and labor. But one thing is for sure...if you're not 100% sure about what you're doing...don't even open the case unless you've already got your Last Will and Testament completed and your life insurance is up to date.
 
I know of no manufacturer, or distributer that writes anything other than tube rating on any tubes,.... there is no such thing as tube bias numbers written on any tubes.....

I am quite cetain of that......


Unless one has started to do that recently,....

If you are refering to the rating numbers that Groove Tubes puts on it's tubes,.... that number refers to a rating system that they have of tube cleanness,... and rates tubes from 1-10,.... the lower the number the cleaner,.or brighter the tube,... the higher the number,... the darker,.. or dirtier the tube,.....

Steve
 
most modern tube amps have a bias pot and a bias test point....biasing a tube amp is something that can be done very easily IMO. You don't need a tech. You should have a bias test point, on the amp (probably have to take the back off), thats easily accessable. You set a multimeter (10 bucks or so at Walmart) to get DC current reading, use an aligator clip to clip the black test prong to the amp chassis, turn the amp on, touch the point of the red test prong to the bias point,and use a rubber handled screwdriver to set the bias with the bias pot...normally turning this pot to the right increases the current and turning it to the left reduces current. Click the multimeter through the different DC settings until you get a reading, and bias according to recommended bias setting (*a little hotter doesn't hurt.....just don't go overboard)


Remember to be careful, because tube amps carry alot of current and can kill you. That being said, you should always use a rubber insulated handled screw driver, rubber soled shoes, and be cautious of not letting any part of your arm or hand come into contact with the amp chassis when doing this


Everytime you change tubes you should re-bias.
 
Thanks Thatupstateguy,

I might not be certain of the meaning of the markings on the tubes but I've got a box of various tubes which were tested/rated and each are marked. I'll have to go back and find out where I got them...some are vintage some are NOS and some are well...just tubes. I've got a pretty decent collection of tubes for experimentation with my Univalve (which doesn't require biasing BTW which is nice), but I also change them out on my B-52 AT-100 and my Mesa Triple.

Example of just a few...most, not all of my 6L6's, 6CA7's, EL34's and EL84's have similar markings on the base but none of the "little tubes" 12Axx have these markings:

Ei EL34 = 27/5.0
Russian EL34 = 26/5.0
Russian EL34 - 27/4.8

It was my understanding that these numbers allowed to compare same type tubes and their performance for matching puposes. I'm no expert in this area but for sure, when I mix-n-match EL-34's that weren't sold as "matched pairs", until I rebias, things sound funky, flat sorta "farty".

If someone could explain these numbers to me, I would be grateful.
 
looks to me as if someone has tested the tubes you have,.... the first number would be the bias setting,.. and the second would be the clean/dirty rating,....

At least thats how I read it,....

I would be curious to find out just how they were tested,... and what kind of tester was used to arrive at those figures,....

Steve
 
All of the power tubes I've ever used had the same markings. The markings are made at the factory when the tubes are tested. The figures are used to sell the tubes in matched pairs and sets. This is useful when re-tubing a non-adjustable bias amp because you can match the tubes to your amp (very simple explanation for those who know what I'm talking about). Very important...DO NOT mess around inside the amp around if you don't know what you are doing. You can be killed...Not fun...
 
I pulled up an old email from a biz I purchased a few of the tubes from (not all of them...just some). I'm told;

"The numbers are the plate current and transconductance readings for the tubes which were tested in the Maxi Matcher tube tester. Tubes with matched readings should bias the same." The email goes on to say that the numbers represent bias range and "Groove Scale". Again stating that tubes which are tested and rated can be more closely matched for use in parallel amplification circuits for maximum efficiency. The email also states that the bias numbers will vary depending upon circuit application and efficiencies and that the amplifier manufacturer specifications and bias procedures should be closely adhered to. More blah blah blah about electrical shock and death if you don't know what you're doing.

I'd like to say that biasing isn't hard to do but different amps are done differently. Safety first...if you don't know what you're doing...don't even think about it! I've you've got both hands on the amp...RIP my friend! I've been doing this a long time and don't recommend it to a newb. Some amps have easy access to the circuits...other don't. The seemingly simple act of removing an amp chassis from the cabinet could get you killed or worse...damage your amp :eek: ( it's a joke ).

N E Ways...just thought I would share. I guess the numbers are helpful and meaningful...in a relative kind of way. I get in jist what the numbers represent but I'm still a little fuzzy. I'm guessing that you'd need to have the same tester used on every tube for the numbers to be absolute for your given amp/application.

Peace :)
 
Anyone have a Marshall 6100 here? That's what I have. I opened it in the past and can't recall seeing a "bias adjuster" as I'd seen in other amps. I'm just curious how the tech adjusted the bias.
 
daluyon said:
The current tube right now is Groove Tubes 6L6 and the extra tubes I recently bought was a balanced quad JJ 6L6. Does it mean that a re-biase is neccessary when I have to replace the tubes in the future?
Click to expand...
Yes, each tube set needs rebiasing.
 
Thatupstateguy said:
however,..... just by switching the tubes,... as long as they are the same type, IE, 6L6 swapped for a 6L6,..... you will not ruin your amp by doing it without a re-bias,....
Click to expand...

Careful with that approach. Some tube manufacturers market tubes as a certain type of tube (6L6, EL34, 6V6, etc.) but the tubes themselves have vastly different performance from what you would expect from that tube type. If you swap in one of these odd-ball tubes thinking that a 6L6 is still a 6L6 or an EL34 is still an EL34, the difference between two brands might be big enough to cause damage to the tubes and/or the amp. For example, there are some 5881 tubes out there that are not 5881 tubes at all... they are repackaged 6L6-GCs. I think this used to be true of some rebottled KT66 types. Another instance: a guitar shop I went to a few months back was selling JJ E34L tubes in a grab-bin as a stock EL34 tube. However, the JJ E34L has a very different current draw than a standard EL34 and may cause damage to your amp if you think it is a standard EL34 and do not rebias.

If you own an adjustable fixed-bias amp, it makes sense to have it rebiased if you install new tubes. In fact, having a tech check the bias in "self-biasing" cathode-biased amps cannot hurt every now and then. If you always run a certain brand of tube in a cathode-biased amp, it might even make sense to have a tech check and adjust the cathode-bias setting of the amp by rewiring to set it up for that particular type of tube. A good tech can check bias in a few minutes of time, which is a lot less than the time it takes to find and replace a new output transformer or repair a damaged amp. :) Good luck and play on!
 
You must log in or register to reply here.
Back
Top