# what is considered matched tubes?



## ezcomes (Jul 28, 2008)

as the title implies...what is considered matched tubes in regards to bias...

i had a pair of 6L6's that were 1mV off from each other...my new EL34's are 4mV off from each other...

is there a line drawn somewhere that says what is too much? If an amp has individual bias points it wouldn't be a problem, but my amp has one bias adjustment...so with the required setting, i stuck it in the middle of the two tubes...one above the setting, one below...

when an amp asks for matched tubes...is it even required??


----------



## GuitarsCanada (Dec 30, 2005)

According to the TubeStore a matched set is measured to a tolerance of 5% of the measured plate current. They say that the plate current is the most accurate measurement for testing tubes.

They give the following advise for biasing.

Amps with fixed bias: Tubes should be in a specific range current draw. 
Amps with cathode bias: No bias adjustement should be required
Amps with adjustable bias: If the amp has a visible meter, set when you put in new tubes. (suggest a tech looks at this)
Amps with auto bias: No bias adjustment required


----------



## ezcomes (Jul 28, 2008)

so...in my case...Traynor tells me the plate voltage would be just shy of 500...so that means??


----------



## CSBen (Mar 1, 2011)

ezcomes said:


> so...in my case...Traynor tells me the plate voltage would be just shy of 500...so that means??


It simply means that your amp was designed with power tubes of that specific rating in mind. What type of Bias is your amp? I'm thinking it's a fixed bias amp, but preferably it'd be an adjustable bias amp. Cathode based as posted above is the no-brainer of them all; that goes without saying, but seeing the manufacturer is suggesting a specific voltage range. I know nothing about Traynor amps : \

Now the point of putting the same spec tubes is to ensure that the amp performs to what it was designed for. If you put tubes in that have more and less of the suggested rating, you'll be running it "cold" or "hot", as it's referred too. It will completely change the characteristics of the amp.

In Canada, the best place IMHO to get tubes are from TheTubeStore. Shoot Jon an email, with your the tube specs that you need and he'll take care of you.

Cheers
Ben


----------



## keeperofthegood (Apr 30, 2008)

Another thing to keep in mind with some tubes like the 6L6 series is that they are not all defacto created equal:



> The 6L6 family...
> The 6L6, 6L6G, 6L6GA, and 6L6GB are all identical in ratings (*19W* plate dissipation), except for their bulbs, as follows:
> 6L6 - MT-10 metal bulb
> 6L6G - ST-16 glass bulb
> ...


The higher wattage is because those makes of the 6L6 can take higher max plate voltages. While they are upwardly compatible they are not downwardly. If the amp takes a 6L6GC (500v plate) then a 6L6G (360v plate) wont do but will be fine the other way around.


----------



## CSBen (Mar 1, 2011)

Keeperofthegood is absolutely bang on.

Ideally you find yourself a solid amp tech who owns a well calibrated tube tester such as say, a Hickok 600A, you could get all your tubes tested for plate voltage, transconductance, plate dissipation. The ultimate would be to have it curve traced so that you really do have exact matching tubes, pre and power, but hey, those guys are pretty rare!


----------



## jb welder (Sep 14, 2010)

ezcomes said:


> so...in my case...Traynor tells me the plate voltage would be just shy of 500...so that means??


Plate current doesn't tell us much unless we know the plate voltage. Plate current times plate voltage tells us the plate dissipation in Watts (all measurements when idling). This is a useful number. When we know what our idle plate dissipation is with respect to the max. plate dissipation of the tube type, we can see what % of max. we are idling at. 
Typical numbers often recommended are 60 to 70% for class AB (push pull) or 90% for class A.
So for example, lets say you measured your plate current to be 36mA. At 500VDC plate voltage you would be dissipating 18 Watts (.036X500). 6L6GC's are rated for 30Watts max. so you would be idling at 60% (18/30). However, if your plate voltage was only 350VDC, you would be biased fairly cold at 12.6Watts or 42% (.036X350=12.6W; 12.6/30W =42%).
I have assumed 6L6GC in class AB for your amp. Other tube types would use different numbers for max. plate dissipation, class A amps generally run close to max. (or sometimes more!).


----------



## WCGill (Mar 27, 2009)

I like to have them about 5ma apart for a "good" match. They usually drift with time, possibly closer but don't count on it. Closely matched tubes are an asset if you play at higher volumes as they are much less of a stress on the output transformer. Tonally, a little mismatch may sound richer in harmonics. Closely matching anything in a guitar amp may not improve your sound, it's more of a hi fi thing. As far as a specific plate voltage, this will rise or fall depending on dissipation.


----------



## nonreverb (Sep 19, 2006)

There are other factors at play as well when considering bias. As stated, the 5% rule applies and is the convention. However, after biasing many amps over the years, I've noticed discrepancies in the output transformer windings which can impact the biasing of matched tubes. You can have a perfectly matched set of tubes, throw them in your favourite (insert conventional non ultralinear amp here) and surprise!! they don't bias exactly the same...Problem is, most tubes amps don't have perfectly balanced primaries on the output transfrmer. In some cases the imbalance can be close to 20%! The outcome is one tube working harder than another. Rechecking the bias after a period of time will result in the tubes bias points being off. Not much you can do about it and unfortunately makes matched tubes argument questionable. Still, the easiest route is a matched set as opposed to trying to specifically match a pair of 6L6's (let alone a quartet) dynamically which is a more difficult prospect...besides it's only a guitar amp 
Some amps like Koch have bias adjusts for both overall bias voltage and left/right adjustment. That way you can compensate for small variation in tube characteristic and amp characteristic as well.


----------



## WCGill (Mar 27, 2009)

I once had an output transformer that was so unmatched there was a 10 volt difference on the plates between primaries. I didn't get so far as to check the bias. The transformer was removed, which required tearing a newly-built amp apart and was returned to the vendor.


----------



## nonreverb (Sep 19, 2006)

Wow! Not surprising in this day and age though. Some of the cheap stuff is pretty bad...I've certainly seem my share of unbalanced output transformers and not all were new ones either. It's not always a bad thing as it can have a positive effect on the tone of an amp.



WCGill said:


> I once had an output transformer that was so unmatched there was a 10 volt difference on the plates between primaries. I didn't get so far as to check the bias. The transformer was removed, which required tearing a newly-built amp apart and was returned to the vendor.


----------

