What is an acceptable amount of mismatch?

[Mars-Hall]

I've been told by an internet tube seller that up to 10 ma is an acceptable mismatch.  This info is taken with a grain of salt, considering the source.  I've also read, by another internet tube seller, that there is a bit of magic with a mismatch.  Quoting that "3rd and 4rth order harmonics are audible".  Open discussion welcome...

[SILVERGUN]

Here's a great article pertaining to the science behind the "magic"
It doesn't directly answer your question, but acceptable is subjective,,,and opinions vary greatly on this topic
Scroll down to the last 2 paragraphs to "cut to the chase"

http://www.aikenamps.com/Biasing.html

[Mars-Hall]

Here's a great article pertaining to the science behind the "magic"
It doesn't directly answer your question, but acceptable is subjective,,,and opinions vary greatly on this topic
Scroll down to the last 2 paragraphs to "cut to the chase"

http://www.aikenamps.com/Biasing.html

Thanks for the link, it was an interesting article.  The reason I'm asking is, on a Super Reverb amp I recently installed power tubes in, I used some oddballs of the same type and found 2 tubes that were within 7-8ma of each other at idle.  The amp sounds good to my ears, so I wanted to get opinions on the extreme cases.  All the other amps I own, have tubes matched within 1-2ma of each other at idle.  What's the biggest mismatches that sounded good to your ears? 

[stratele52]

I've been told by an internet tube seller that up to 10 ma is an acceptable mismatch.  This info is taken with a grain of salt, considering the source.  I've also read, by another internet tube seller, that there is a bit of magic with a mismatch.  Quoting that "3rd and 4rth order harmonics are audible".  Open discussion welcome...

On Weber's books , Gerald Weber suggest close to 3 to 8 if I remember . But if like the sound wit 10 ma , leave it like that . The target its the sound / tone.

[Ed_Chambley]

10ma is not matched.  Different amps will react differently.  I will return tubes if they do not match because I purchased matched tubes.

  Tube sellers have a lot of tubes they cannot match within 2%, so if they increase their "advertised" matching to 4% they have many more out of that lot they can sell as matched.  Then 2, they may not be matching anywhere near the voltage/current your amp uses.  Before he moved to North Carolina, there was a guy (Steve) that used a maxi-matcher in town here.  I would go there to get tubes and he showed how they matched tubes.  There was a tremendous amount of waste on tubes which weren't good.  At that time, he was selling the majority of Chinese tubes as singles because he could not match them.  All the tubes he would not sell to amp shops and music stores, he sold in bulk to someone who resold them probably on ebay.

"Just stick them in there and if they sound good and do not redplate, your are good to go".  This is what I have heard from some, but that does not address the issue that you are BUYING matched tubes.  I can reach in a untested box of =C= and bet you I can grab 2 which are better than 10 ma apart.  On 2 separate instances Tube Depot has send me badly matched tubes and these were going into an amp which does not tolerate a large mismatch.  I normally get tubes from Upscale Audio, but in this instance I was already ordering so I thought I would save a little.  Well I learned a lot.  I learned that after shipping back the cost exceeded if I would have ordered the and paid shipping.  Tube Depot could not see their way into reimbursing my return shipping.  Before I would not have mentioned the company name, but since this I will.

Looking at a box of GEC KT66 and printed on it is a guarantee of a 1% match, or they consider them defective.  those days are gone.  I have heard this mismatch theory about harmonics and it is complete crap.  I have 2 Super reverbs with the same circuit, speakers.  They sound identical for the most part.  Put a matched set one and a harmonically rich mismatch and a/b them.  The mismatch amp has less punch and bass notes get greasy, but it is a Super Reverb and I don't own them to sound greasy.

I have a strong opinion on this topic simply because I know you can match tubes because I get them all the time.  If you are speaking of the tubes in your amp being 10ma apart and the tube vendor said that 10 ma is acceptable and it is acceptable to you, you got a good deal.  I would have come unglued it they told me that.  I did have the fine folks at Tube Depot tell me that there is not much profit in tubes these days which is another way of saying they have lowered their standards.

Give this a try, as I have done it.  Buy 2 old working tube pulls off ebay  at 2 different times.  If the tubes are working and are old, I'll bet you bet a better match than 10ma.  That is ridiculous.  If one reads 35ma and the other reads 25ma that is almost a 30% mismatch.  Bullhockey.

There are loads of opinions on this, but any opinion that differs is willing to accept substandard quality and also willing to accept not getting what you paid for.

[stratele52]

Since many years I never pay for matched set and those I receive are always matched in spec for me.

[Mars-Hall]

Interesting viewpoint Ed, thank you for your post.  I might add that I have never paid for a set of tubes that did not match.  However, I did return a set that were 5ma apart.  This is when the vender mentioned this "acceptable mismatch" jargon. 

The question came up while retubing my Super Reverb and trying to save some money, by scavanging oddball tubes of the same make and type.  One of these was a brand new set that one tube fell out of the box and broke on my concrete basement floor.  The other was pulled from my 61 Concert amp where the 500v + plate voltages caused the matched pair to drift very far apart.  So I thought I'd give this "mismatch magic" theory a whirl, since it isn't costing me anything. 

I'll see how it goes this weekend at the gig.  I never get a good indication of my tone until I play with the band.  I'll report back early next week.  Please, keep up the feedback.

[Willabe]

Shouldn't we be matching at points similar to the voltages posted by fender.  1kc knobs all max?

That's where matching for transconductance along with current draw comes into play. Not all match both.


                    Brad