Tube Matching

[Bry]

I have read lots of opposing opinions on tube matching.

Could someone explain the proper terms when refering to tube matching?

I think what I want is tubes that are matched for current draw?But nothing else matched so I get more harmonic content without the hum.
I was recently told the hum I hear when my amp is turned all the way down(not from speaker)
Is the tubes humming because of mismatched current draw.
And that I should get better matched tubes or use seperate cathode resitors and match the dissipation by adjusting the resistors.
Does this sound correct to anyone?
Couldn't I use a trim pot on one tube to match them together?

[CurtissRobin]

Does this sound correct to anyone?

No.
If the hum goes away when you turn the amp all the way down it may be a bad tube or a mismatched pair. If the hum is still there when volume's at zero then it's most likely, by far, to be the power transformer though lead dress could cause it to be picked up somewhere after the volume control and that wouldn't change with volume. The transformer is a surprisingly complex electromagnetic structure and the AC voltage applied to it causes a usually small vibration that's heard as a hum rather than felt.

Does the hum get louder/softer with the volume control?
Does the hum change if you switch standby off and on with the volume all the way down?

Using a trim pot to match tubes won't work.

KennyO

[Bry]

I am talking about hum emanating from the amp itself.
Like you said from maybe a small vibration.Not what is coming through the speaker.
Of course if I turn the amp up you can't hear the amp humming in to say the noise emanating from the amp.Becuase the amps own slight noise at louder volumes is louder.If that makes since.
The amps sound is fairly quite compared to most amps.
In fact it didn't make any "noise" until I routed my high voltage through
the standby instead of the center tap.Now it has a normal small amount of hum at loud volumes if you leave the volume up and dont mute the strings.
No ground loops or bad lead.
I thought I was hearing the PT hum the way you hear a transformer on a power pole hum.Only on a smaller scale.
Someone else said it was the tubes not matched.
And the dissipation is a little off about a 1/4 watt difference between tubes.

I guess I am asking about how tubes are matched?
What the proper terminology is and what it means?
A link is fine
And if the tubes themselves can give off noise when they aren't-or
if I am just hearing my PT making noise sorta like a line transformer.

[zaphod_phil]

If this isn't hum that's coming through the speaker, then it has *nothing* to do with the power tubes being matched or unmatched. In fact it has nothing to do with any of the tubes or the amplifier circuit.

I'm pretty sure you're hearing the natural vibration of the PT. Some PTs seem to vibrate more than others, and the sound can be amplified by the chassis, so you hear it louder. So you were about right when you said, "I thought I was hearing the PT hum the way you hear a transformer on a power pole hum."

[kedsinger]

Does this sound correct to anyone?

Using a trim pot to match tubes won't work.

KennyO

You can use unmatched tubes and bias individually with independent trim pots and bias network. If thats what you mean.

[Peabody]

Regarding matched vs. unmatched tubes, this topic has cropped up once before just a few months ago. It seems that matched tubes are not an absolute requirement. And a matched pair will "drift" over time, eventually becoming unmatched. From the previous thread, I gathered that most new production tubes are "close enough". And I gave it a try w/two JJ EL84s that I had pulled from a couple of valve junior's I had modded. The result of using an unmatched pair in my 18 watt was awesome!!! It's the best it has ever sounded... can't say that's due to a mismatch, or just the JJ's. But it worked. So I am no longer towing the party line on matched tubes. If it sounds good, it's good...

[CurtissRobin]

For most of us tube matching falls under the same category as using 5% or 2% resistors throughout (i.e. overkill). You need to keep the perspective of Fender, Gibson, & al. using 20% resistors when they designed and built the amps we're copying or tweaking or gold plating. If you're going for absolute max power then matching is de rigueur; when pursuing tone, mismatch is better.

KennyO

[rjgtr]

RE: Matching.

It is not necessary, except you don't want the pair too far from each other (> 15ma?), because one tube will pull more current and wear faster. As long as they are close enough, a mismatched pair will work fine.

My rule of thumb is that if I want the cleanest tone, then a well matched pair is worthwhile. If I'm going for distortion, then somewhat mismatched works well.

I tend to buy matched tubes for fixed biased 2 output tube amps so that I can get a quad and not have to bias when the first pair gets old.

Another way to go in a cathode biased amp (like the 18watt) is separate output tube cathodes resistors/caps for each tube. Then the EL84s can be pretty unmatched but still keep each tube happy.