how much difference does a matched set make?

[bob p]

As an aside, I don't think any Mesas are "self-biasing"...they are non-adjustable fixe-bias.

i could have sworn that Mesa made some cathode biased amps. maybe my memory is playing tricks on me.

[Jammin'John]

Blue Angel

I think the Blue Angel is cathode bias.
Kevin Eubanks on the Tonite Show plays one.

JJ

[kg]

i played for over a year with 1 of the 6 kt90s in one channel of the BAGA pulled. its screen resistor had opened up and i hadn't fixed it.

when i DID end up fixing it, and using the full sextet again in that channel, it sounded just like it did before.

i have not heard a hell of a lot of difference in noise/hum when twiddling with bias balance in a pp output stage. i think the unbalance has to be ridiculous before things get really noticeable.

[stokes]

A slight mis-match is okay,and in fact will give a better sounding output,but if one tube is drawing 20ma's and the other is drawing 35ma's the hum will drive you nuts,in a two tube output.Of course if you are talking about a 12 tube output section,one tube isnt going to make much of a difference,since you are talking about the average draw of six tubes on one side vs. the average of six on the other.

[J Martin]

There are good reasons to use matched tubes. One reason I did not see mentioned is that the extra handling the tubes get during the matching process reduces the drop out rate to near zero. I buy a lot of tubes and while the vendors make good on bad tubes, the whole return process is a real pain and takes time away from work. Also I have found matched tubes make the repairs more consistant and biasing easier. Tubes will drift anyway and its nice to know they are drifting from the same value and none will end up too far off.

[kg]

A slight mis-match is okay,and in fact will give a better sounding output,but if one tube is drawing 20ma's and the other is drawing 35ma's the hum will drive you nuts,in a two tube output.Of course if you are talking about a 12 tube output section,one tube isnt going to make much of a difference,since you are talking about the average draw of six tubes on one side vs. the average of six on the other.

the amount of mismatch that will produce a hum voltage at the speaker terminals also depends greatly on your power supply. in fact with a "perfect" b+ rail you don't need the PSRR that differential operation gives you at all.

[Steve Conner]

Mismatched tubes will draw unequal idle currents through the two halves of the OT primary, which will cause a net DC magnetization of the core, making it saturate early on one polarity. I don't know how ordinary O.T.s like this, but I know toroidal ones cope very poorly with it.

[telemoxy]

In marketing-speak, that's what self-bias means these days.

Customers have been brow beat with the "bias must be adjusted at all costs" mantra for so long, they expect it. So saying an amp has non-adjustable bias is a turn off, but self-bias sounds like a good feature.

When I hear "self-bias," I get a mental image of an amp that somehow senses current and then a servo circuit adjusts voltage to compensate.

Just out of curiousity, oes anyone know how the self biasing circuit in a Traynor YCV50 "Blue" works? The idea behind it is that you don't need to use "matched tubes".

[Enzo]

Chances are it is nothing cosmic. SInce unmatched tubes will work fine in an amp anyway. For example the PV arrangement where tubes are simply biased real cool, so prtty much whatver you stick in there will not be underbiased.

Got a schematic?

[telemoxy]

Chances are it is nothing cosmic. SInce unmatched tubes will work fine in an amp anyway. For example the PV arrangement where tubes are simply biased real cool, so prtty much whatver you stick in there will not be underbiased.

Got a schematic?

Yes.

http://www.traynoramps.com/default.a...7&p2=0&p_id=30

See the All Tube Guitar Combos under YCV50Blue and click on the "download" link in the service manual column and a pdf is downloaded. The schematic is included on page 6 to 9. It looks a bit cosmic to me.

[Steve Conner]

It looks like a totally ordinary setup with a pot to adjust the bias. They even printed "Bias 350mV across R91 and R92" on the schematic.

There are lots of transistors, op-amps, MOSFETs and other junk in there, but they don't seem to have anything to do with the bias.

[Enzo]

Looks utterly conventional to me, I don't know why this would be any more self biasing then an old Fender Twin Reverb or a MArshall.

[telemoxy]

It looks like a totally ordinary setup with a pot to adjust the bias. They even printed "Bias 350mV across R91 and R92" on the schematic.

There are lots of transistors, op-amps, MOSFETs and other junk in there, but they don't seem to have anything to do with the bias.

Quite right. In comparing the schematic at this link to the one I have it appears they have made changes. They have removed the auto-biasing. Pity! It allowed quick swapping of output tubes. If you or Enzo are interested I could email you the schematic with auto-biasing.

[Robert M. Martinelli]

Matched/Unmatched controversy

I HATE marketing ( or at least the current concept of marketing, which is something jrfrond explained perfectly : invent all sort of b*@°#!!t to sell whatever kind of crap at all costs! ) - The truth is, as some of you have already pointed out, that when it comes to tube ( or should I say valve, being on the other side of the Atlantic Ocean ) amps, most of the times unmatched pairs actually sound better to the musicians' ears, we all know most of the amps out there in the REAL world are class AB1 amps, so a perfectly matched set would eliminate completely all even-order harmonics; still some tube-badgers claim their selected pairs are best then others, while some amp manufacturers recommend to use only selected pairs in their amps....that' s crap. I own a 1964 AC30, and when I laid my hands on it some twenty years ago it was beautifully sounding - when I opened it to check the innards I found a lot of components to be out of spec, the biases were uneven, the voltages out of spec, but the sound was fantastic nonetheless.... what I would like to highlight is the absolute nonsense of all this matched/unmatched controversy. Ears and taste should be the judge, if someone wants less or little harmonic distortion then he can put in selected/matched tubes, but he should be aware that his tubes will stay matched only for a few minutes or hours, as the surrounding components' tolerances ( which in the old amps was 10% for resistors and 20% for caps ) will make 'em wear at different rates....
It must be also considered that musicians have different needs and expectations from the ones an audiophile might have.

In other words : Replace that tube without worrying too much about matching...you could even be in for a big surprise, it could even sound better to your ears!

Hope this helps

Regards

Bob

[martin manning]

You guys knew this, right?

http://mixonline.com/news/headline/f...ovetubes_0206/

I find that engineers very seldom see eye to eye with "marketeers."

MPM