Tube Matching - LoneStar Special

[johnnysshed]

Hey! Question on "matching tubes" .... I've read the manual for my LSS and can't find anywhere whether or not I need "matching" tubes. The amp runs EL84s and 12AX7s .... do I need to have them as matched pairs or quads, or can I replace them with individual, single units. The reason I ask ... my Mesa Boogie supplier sells unmatched singles at "half the price" of matched pairs!! If anyone can advise me, that would be great - Thanks!

 

[RR]

Anyone? Anyone?

This is a good question, the LoneStar Special is Class A, not Class A/B "push/pull"where matched is just about mandatory.

I dunno know :? Just bumping up your post :wink:

 

[TheRazMeister]

It looks like most sites refer to everything "Matched".... Eurotubes has a kit for the Crate V3112 (class A) and he specifies matched quads of El84's.

Anyone? Anyone?

This is a good question, the LoneStar Special is Class A, not Class A/B "push/pull"where matched is just about mandatory.

I dunno know :? Just bumping up your post :wink:

 

[gremmert]

I've wondered about this myself.

It appears to me that MB only sells el84's individually. For example, the pro net price list doesn't offer matched sets of el84's. All other power tubes MB uses in their amps are offered as matched sets. (The Web site does not mention offering matched el84's as a product either)

Given the above, I'm thinking that it's not critical for the LSS.

Just my .02, I don't really know either.

 

[gremmert]

FWIW...

I just google'd on "mesa boogie matched el84"

One of the hits was this thread that confirms that MB does not match their el84's. I'm thinking a matched set isn't a requirement when replacing tubes in the LSS. For sure not when running in 5 watt mode.

http://72.14.203.104/search?q=cache:G3y3GpJba8EJ:www.musicplayer.com/ubb/ultimatebb.php%3F/ubb/get_topic/f/19/t/001711/p/81.html+mesa+boogie+matched+el84&hl=en&client=safari

 

[ylo]

The LSS uses a push-pull, class AB, cathode biased topology in 30 and 15 watt modes. Only the 5 watt mode is truly class A.

For the best performance, you should use reasonably well matched tubes if possible. Mesa power tubes are rated, and probably close in performance, even if not precisely matched. Many people have tried the JJ EL84s and really seem to like them.

 

[johnnysshed]

Thanks very much for your advice and help guys - really appreciated! J

 

[fatboy135]

If your amp works in a push pull mode you must to configurate it with matched tubes. If your amp was only a class A no matched were needed, but like your amp is an hybrid that works in two kind of classes distint you must to acomodate it to the worst specification, the class AB ( push-pull )
Then I have a question did anyone knows how is the circuit that modifies the bias from class AB to class A??? and if the tubes that are used on AB are the same the uses on class A or are different tubes settled on the lonestar chassis????

 

[BottyGuy]

Just a word of correction here; based on Mesa's description the Lonestar Special is all Class A. Class refers to the bias setting of the amp which (along with some other parameters) determine whether the tube amplifies the positive and negative half of the signal (class A), just one half (Class B), or one half and a bit more (class AB).

Class A amplifiers can be operated single ended since it amplifies both halves of the signal, or they can operate in a push-pull arrangment for higher power.

Class AB amplifiers can only be used in a push pull arrangement so that you get both halves of the waveform.

The Lonestar Special is always class A, the tube biasing is set so that all the power tubes amplify both halves of the signal. in 5W mode it runs one tube single ended, in 15W mode it runs two tubes push-pull, in 30W mode it runs two more tubes (4 total) in parallel push pull.

Tube matching is very important in a class AB amplifier because the designer has selected a preferred crossing point where positive and negative halves of the signal ampifier cross over, and bias level is really a current setting that could very significantly from tube to tube. I'm not sure about the importance in Class A, although I assume it make some difference in absolute maximum power output, but I don't know enough here, so I really can't answer the original question.

good links on Tube Amp Class and Arrangement:

I like Aikens explaination, way more technical.
http://www.aikenamps.com/ClassA.htm

Randall Smith has the best pictures:
http://www.mesaboogie.com/US/Smith/ClassA.htm

 

[brianf]

EL 84's do come in Matched quads and pairs if you check out sites like thetubestore.com and Tube depot.

brianf

 

[fatboy135]

does anyone knows if there is any mesa amp that is able to change between class AB and class A????

 

[BottyGuy]

does anyone knows if there is any mesa amp that is able to change between class AB and class A????

Once again, according to Mesa the Lonestar Special does not switch between Class AB and Class A. (see note) It is always Class A. It does switch between a Single Ended and a Push Pull output configuration.

In a quick search could find no other Mesa amps (or guitar amps) that switch between signal ended to push-pull. Although there are some "Audiophile" tube amps like the Manley Neo-Classic that do this. (and at $7200 for the single channel Manley we should consider the $1700 Lonestar Special a good deal).

http://www.manleylabs.com/containerpages/seppneo.html

(note) without measuring or simulating the circuit and seeing if the amp is actually working in Class A when set to push-pull operation we can only take Mesa's word for this. The fact that the amp is only 5W signal ended yet 15W push-pull is suspicous, but they must be using a special output transformer to operate in both modes in any case. Also I don't see the benefit of a Tube rectifier if the amp is really all Class A (there shouldn't be any Sag in Class A since current draw is relatively constant, I'm probably somehow wrong about this but it looks like a costly marketing gimick to me). But, I'll take there word for it. (/note)

 

[ylo]

Every manufacturer uses the term "class A" to describe cathode biased power tubes, and this has become standard marketing hype. Vox AC30, Matchless and other other cathode biased EL84 push-pull guitar amps are NOT class A in reality, but class A/AB at best, that is, operating in class A mode only some of the time, at very low power levels. Check out alt.guitar.amps for technical discussions of this, and consult the tube amp engineering books. The LSS is strictly speaking only in "pure class A" when operating single ended (5 watts, one tube).

Mesa changes classes and power levels in the LSS by disconnecting tubes: 4 tubes (30 W), 2 tubes (15 W), 1 tube (5 W). You get an impedance mismatch to the output transformer when you do this (read the LSS manual), but apparently this does not affect the tone much, and the tubes can tolerate it.

The tubes in the LSS are self biasing (cathode biased), but will have slightly different characteristics at the same bias point unless matched, so you should get better tone with matched tubes. Whether the improvement in tone is noticable or worth the exta cost in this case, I have no idea, not having tried it.

Even though I maintain that "pure class A" is Mesa marketing hype except for the 5 W mode, don't get me wrong, I love the sound and flexibility of the LSS. I'm not knocking it.

Chris Taylor
an old tube amp guy from Boston

 

[fatboy135]

It does switch between a Single Ended and a Push Pull output configuration.

Are you sure???? the push pull configuration is used to split the signal into two phases and then you could set the bias point to the cathode current near 0 allowing you to take advantage over all the range of the amplification curve.
When you are setting a class A you are just setting the bias point just in the middle of the amplification curve, that means that you only have the half of the total power amount to amplificate. To set the bias point in the middle you need a continous current that allows the tube be polarizate even if no signal is present

 

[fatboy135]

ok ok then a single ended configuration doesn´t needs any driver tube. The push-pull configuration needs a tube driver to split the signal into the two phases. If you are right I want to know how mesa is able to do this tech miracles.
I´m agree with some brands says that they amps are class A with a cathode biased, even a cathode biased is not enought to set a class A.
But my little knowledgement saids to me that a class A in a single ended configuration is imposible to be implemented with a push-pull stage because single ended is for a class A and the push pull are for class AB,B and the implementation is totally different. Maybe I´m wrong?. Could anyone explain it to me???

 

[ylo]

Hello Fatboy

The old Fender Champ, THD Univalve and several other amps on the market use a single output tube in true class A mode.

I have not seen the circuit, but Mesa likely uses relays and / or LDRs to disconnect three output tubes and reconfigure the output transformer for single ended operation in the LSS. Mesa has a boatload of patents on clever ways to configure power tubes and output transformers, for example Simulclass. No other manufacturer seems to have as many innovations in this area.

Chris Taylor
Boston

 

[fatboy135]

yes the fender champ uses a true class A single ended configuration. I think that mesa maybe uses a single el84 in a single ended configuration and then with optos you could reconfigurate a second total different circuit in a push pull configuration with more tubes

 

[fatboy135]

my question is how could mesa implement a real class A with a push-pull configuration

 

[ylo]

They don't run true class A in push-pull modes, except possibly while at idle power. If Mesa biased for true class A, the available power would be much lower than 30 or 15 watts. Anyway, this is just a technical / marketing debate. The amp sounds great, right?

 

[fatboy135]

yeah the amp sounds right, but the marketing doesn´t sounds true :wink: