12ax7s in power amps but not in heads/combos?

[talltxguy]

Hey ya all,

I understand the Mesa power amps contain 12ax7s in addition to the power tubes.

Do the power sections of the Mesa combos and the Mesa heads contain 12ax7s in their power sections? If they don't, then could you please explain why they don't and why the power amps do!?

Thank you for any insight,
a confused talltxguy

 

[crane]

As long as the power section is a push-pull, a 12xX7 is used as a dephaser (could be some solid-state component used for that, but not in Mesa's).

 

[elvis]

To elaborate a bit, push-pull output stages require a phase splitter, meaning that one set of tubes gets turned on as the other gets turned off. So one tube dives the power tubes on their proper phases. This is done in both combos and power amps. Combos also have 2-4 more tubes for their preamp functions, and volume control is generally built into that. In order to allow for a good input buffer, plus to give additional volume control, most power amps include a single preamp gain stage as well, so they have another preamp stage.

Remembering that each 12AX7 has 2 preamps in it, Mesa's stereo power amps are set up like this (for the most part):

V1a Preamp channel A
V1b Preamp channel B

V2a & V2B Phase splitter/driver channel A

V3a & V3b Phase splitter/driver channel B

So 3 12AX7 for 2 channels.

 

[MesaGod666]

I thought that the 12AX7's were used for the presence controls.

 

[msi]

The presence control works by returning a very small amount of signal from the speaker jacks back into the phase inverter. What happens is they filter this negative feedback so that it affects only the higher frequencies, and because it is wired out of phase the higher frequencies get reduced. It's a passive tone control.

 

[talltxguy]

Thanks for the feedback, guys!

So Elvis, if a power amp isn't push-pull, then it's going to be single ended?

It sounds like push-pull designs would be better for players needing as much distortion as possible?

Is there any advantage to putting 12AX7's into a single ended amp or is that just impossible given the type of output?

 

[ando]

It "sounds" like that might be the case but it's really the opposite. Push-pull designs actually offer much more headroom without clipping or distorting the signal. With a push-pull you can devote one tube for when the current is flowing one way and another tube for when it's flowing the other way. Distortion is produced by pushing tubes beyond their clean range so you can have distortion on any amp design. Audiophiles actually prefer push-pull for high fidelity faithful sound reproduction without distortion.

 

[elvis]

Quick background:

Single power tube amps are biased so that they pull the maximum current all the time, even with no signal. As they turn on a bit more, they pull the output one way, as they turn off a bit, they let the load pull the output the other way. They are typically biased so that they are running in the more linear region of possible operation, and they are the highest fidelity in terms of distortion. They also waste the most power, so they run hot and need the biggest transformers for a given power output level. Therefore they tend to be the most expensive to make. They are called "Class A" amps.

A cheaper, more power efficient design is "Class B". There are pairs of tubes, and the signal is pulled one way by the first tube, and the other way by the second. They pull no current when there is no signal. These have lots of "crossover distortion", as the tubes go in and out of bias as the signal crosses zero. That's bad, as it creates a nonlinear distortion that can be pretty horrible. So the fidelity is the worst.

In between is "Class AB". These amps have pairs of power tubes, but they are biased so they don't turn completely off at the zero cross. This gives a good combination of power efficiency and fidelity, as they can be run at a much lower power level than Class A, while having much less crossover nonlinearities than Class B. This is the most common design.

A given tube type can be designed for similar output and headroom in any of these ways. The differences are efficiency (which drives cost) and fidelity.

A Class A design does not require a phase splitter, the other two do. The use of an additional preamp tube for any of these is a design choice.

 

[elvis]

To avoid confusion, crossover distortion is not the kind you want. If you are looking for "good" distortion, that is the compression you get from the tube when it becomes nonlinear as it reaches the end of its voltage and/or current capability at the peaks of large signals. That characteristic is different for each amp design, based on the region of operation in which the tube is biased, plate voltage, load impedance, etc. It is also affected in any given amp by how the bias is adjusted. It's hard to argue that one amp type is more likely to give compression than another, as they all compress when they get as loud as they are capable of.

If you want lots of power amp compression, get a low-power amp. In general, you can get about the same volume from a 20 Watt amp as a 50 Watt amp, but the 20 Watter will be much more compressed. This is why the 20/20 has been popular.

Personally, I find that an amp like the 20/20 sounds pretty mushy, so I prefer a 50/50, as it preserves the dynamics. I get my saturation from the preamp, and maybe just a bit from the power tubes. It's more "punchy" that way. YMMV.

 

[ando]

Love your work, Elvis. Very thorough.

 

[elvis]

Thanks!

 

[talltxguy]

Elvis, Ando, and Crane...thanks for the insight! This clears up some things for me.

I am a bit concerned about the mushy thing though. I've got a 1X12 Roadster cab that I absolutely love. It's got the Vintage 30 in it. I'm wondering if I will need to get a 25 watt speaker to replace it if I go with the 20/20.

Elvis, when you were getting the mush with the 20/20, do you remember the speaker wattage ratings of your cab? Could a higher wattage speaker with a low watt amp like the 20/20 be responsible or partly responsible for the mush?

 

[ando]

Elvis, Ando, and Crane...thanks for the insight! This clears up some things for me.

I am a bit concerned about the mushy thing though. I've got a 1X12 Roadster cab that I absolutely love. It's got the Vintage 30 in it. I'm wondering if I will need to get a 25 watt speaker to replace it if I go with the 20/20.

Elvis, when you were getting the mush with the 20/20, do you remember the speaker wattage ratings of your cab? Could a higher wattage speaker with a low watt amp like the 20/20 be responsible or partly responsible for the mush?

You have a few ideas that are askew, talltxguy. The wattage of a speaker is the maximum of what it can handle. You don't need to match wattages of amps to wattages of speakers - it's not like matching impedance (measured in ohms). In fact, most people have speakers that are rated well beyond what their amp will push through them. The reason is that if you go over the limit for that speaker, it will fail - either destroy the cone or burn out the coil.

Speakers do play a role in the sound of an amp though and they do change their characteristics depending on how much power is run through them. Some people like to drive their speakers hard, and in doing so, they achieve a certain form of distortion - sometimes known as "break-up". This isn't universally agreed upon as being desirable in the same sense that overdriving a tube is. Personally I don't like the sound of speakers breaking up so I run cabs that have plenty in reserve because I like a crisp clear sound. My cabs are 150-300W. on the other hand, if you never play at high volumes, running a 300W cab is not necessarily the best idea either. High wattage cabs mated with low wattage amps often sound very stiff and cold. It is generally a good idea to be somewhere close to a speaker cab's sweet spot - which might be anywhere from 25-90% of it's rating depending on whether you like break up or not. Some speakers may sound better when driven hard, others may sound unpleasant when they break up. That's up to you to decide. The only certain thing is that you shouldn't drive a speaker beyond what it can handle.

In my opinion, a 20 watt amp should have a cab of at least 50W, possibly even more given that some speakers are don't handle what they ought to be able to handle. I'd run a 75W or so, myself. Why? Because I don't like break up. I think most 75W cabs sound good at around 30-40W. Given a lot of tube amps put out a bit more than what they are rated, that seems about right to me. But if you ask somebody else, they might say go for the a 25W Greenback or something similar. Whatever you do though, don't run a 20W speaker with a 20W Mesa amp - Mesa's are loud!

To answer your question, I don't believe that a high wattage cab would be responsible for mushiness, In fact high wattage speakers often sound clear but sterile. I think speaker type has more of an effect than wattage.

I'm sure Elvis will add some much more elegantly put ideas.

 

[elvis]

Ando, you pretty much nailed it.

I would only add that amp power output is often rated at like 1% distortion. For a solid-state power amp, the saturation is very hard - they hit the power supply and that's all they can push. So power rated at 1%, 10% or any level is pretty much the same. For tubes, the saturation is softer, so an amp rated at 1% distortion can often produce quite a bit more power when pushed harder. That is a likely reason that tube amps are considered to be "loud" as compared to other amps, even other tube amps. I've seen comments to the effect that "tube Watts" are different than solid state Watts. Power is power, but we do hear what we hear.

For speakers, in order to survive the heat and stress of running at high power, they are built heavier. This often makes high power speakers less efficient. So you may notice that high power speakers are not as loud with a low power amp as low power speakers. BUT most people do not run their amps at anywhere near peak power (you would be very surprised how loud 1 Watt continuous power is!) So we ALL pretty much run over-rated speakers for the power levels we actually use.

Wow - that was really long-winded. Just pick a speaker you like. It will probably be fine at whatever volume you use. You may find a practical limit: 1000W speakers for a 5W amp are probably not a good match. But most readily-available equipment is compatible.

 

[talltxguy]

Ando & Elvis,

Excellent, excellent! Thank you for the insight. Can't offer any good advice in return LOL but I will have a studio preamp with a brand new reverb tank up for sale around Christmas and will be putting it on this board first before going to the BAY and Craigslist just because I've received some great insight, not just from my posted questions but from the hours I've spent reading the posts of others.

Thanks!

 

[talltxguy]

Got my Studio Pre back from the Mesa Tech. He not only put in a new tank but basically refurbished her so that she sounds better than when I dropped her off. So there is no way in hell I'm letting go of the Studio Pre. However, I will still buy cold beers for the advice and insight I've gotten. :lol:

What was I thinking :?