What Amplifier Specifications Do You Feel Is Beneficial?

[agedhorse]

Agedhorse:

Kudos to you for creating this thread! Many TB'ers have already listed some good spec parameters, so I won't repeat those.

If the amp in question has one or more preamp tubes in the gain stage, I am always interested in the voltage that those preamp tubes are receiving. I say that because in my experience, preamp tubes that receive a healthy voltage of around 300v provide much better tone (IMO) than starved plate designs.

And if the amp in question has power tubes, I would like to know the B+ plate voltage being fed to those bottles as well.

Thanks!

There are many applications where high voltage is anything but a guarantee of good tone... in fact one of the most god-awful sounding amps (no, not going to say which) was marketed almost exclusively on "high voltage tube preamp", and one of the very best tube preamps I have heard used a lower voltage on the plates. Starved plate has earned a questionable reputation among hobbiest/DIY types precicely because they don't understand the physics and math behind the various equations governing load lines, emission linearity, operating points and distortion parameters of not just the various tubes but also the circuits that they are used in. In fact, the 12AU7 (and to some extent the 12AT7) was designed specifically as a low plate voltage tube and its operating parameters are quite remarkable in this aspect. One look at the operating curves and it should become immediately obvious WHY this tube class is so interesting.

I have designed both types of circuits (high voltage and low voltage), but I do have an affinity and also an advantage when looking at the "lower voltage" applications (probably not as low as most might consider "starved plate" whatever that means), and you have to also remember that one of the ideal design targets of the 12AU7 tube as spec'd by RCA is with a plate voltage of just 100V per the designer's manual) because my dad, who was also an EE worked extensively in the aerospace industry, and kind of specialized in lower voltage designs for use in military, avionics, and even on navigational instrumentation used on the Mariner Project (first Mars spacecraft) in the 1960's. There are some mind blowing applications that really extended the life of tube circuit designs in applications under these conditions, BUT you have to really understand what's going on at the physics level to know WHY these circuits are so relevant. Many early J-FET circuits were derived directly from this work, and these J-FET circuits found themselves inside popular (and still collectible) bass amps of the late 60's through the 70's. To ignore this stuff , especially with access to my dad's (now declassified) work on the subject, would be to walk away from some of the most authentic vintage high performance circuits in the world. Voltage is just a number, without any reference, it's essentially meaningless as it's ALL about the circuits themselves.

Now the 12AX7 doesn't share the same special properties, and while it can be made to work quite well, the 12AU/AT7 is an entirely different animal which a very different application history.

As far as power tube B+, that too doesn't matter without all the other aspects of the design equations because the voltage alone just doesn't mean anything itself. The voltage interacts with the reflected impedance at the primary of the OT, the screen control characteristics, the grid drive impedance, how the bias is derived and stabilized, the various design targets and margins used, etc. I for one tend to use a slightly lower plate voltage and reflected impedance for plate load to extract better long term life and operating point stability. these circuits also tend to be a little more forgiving of non-linearities on the tube operating curves... something we as designers and you as users have absolutely no control of. Tube parameters are what the manufacturers produce, the distribution is what it is. I'm not talking about the common parameter or two that tube sales guys "screen" for, but all the "between the lines" parameters that real tube designers fret over with regards to long term performance and reliability. There are thousands of tube power amps out there (not Mesa's) with my designs in them running day after day with very few tube problems. Many, even after 10 years or more are still using the same output tubes and the amps still meet their original performance specs. This approach is good for owners of amps where the cost of changing out a set of tubes might be substantial...

I don't know if I'll ever design another big tube power amp again, I just don't know if there's enough demand or need for another. I might enjoy the challenge, but I'm not holding my breath. Is there any interest?

I hope this helps you understand some of the WHYs behind various designs. I don't know what the future holds for any of my tube designs, we will have to see.

 

[agedhorse]

Oh, dear friend agedhorse, your quest evokes two maxims from my years at university, not quite back in the horseless carriage days:
"A camel is a horse designed by a committee." By the time you publish all the info that people want it'll be a camel, indecipherable to all but ... well, it'll defy scrutiny. But they're sure they want it, aren't they?

"Oh little girl, with your nose pressed against the windowpane of life, there is no jelly donut." This came from an economics professor and it's a really interesting [to me] way to say that you can't always get what you want, even when you know what it is.

I suspect that most people will expect and demand that you provide them with information that will support their prejudices. One quote I encountered recently is germane: "More words explain more, less words confuse less." Author forgotten.

Good luck, Don Quixote.

Very interesting perspective, with a fair amount of real world relevance too. I like this a lot

 

[wneff]

Hmmmm....so here is the physicist speaking.....
The problem with most spec is that they are not what matters. I almost think it would be better to have an application matrix, like:

 

[Al Kraft]

Hmmmm....so here is the physicist speaking.....
The problem with most spec is that they are not what matters. I almost think it would be better to have an application matrix, like:
Room size Quiet (Acoustic, Jazz) Medium (Rock, Blues) Loud (Heavy Metal)
<100 100 W amp+SW112 200 W amp + SW115 400 W amp + SW115
100 - 300 200 W amp +SW112 400W amp + SW115+SW112 800 W amp + SW115+SW112
>300 400 W amp +SW115 800 W amp +SW115+SW112 2x 800 W amp + 2xSW115+2xWS112

While there are some obvious variables that cant be fully or accurately accounted for in your example, I like the concept a lot and this is an interesting direction that I've not heard expressed before.

 

[SactoBass]

There are many applications where high voltage is anything but a guarantee of good tone... in fact one of the most god-awful sounding amps (no, not going to say which) was marketed almost exclusively on "high voltage tube preamp", and one of the very best tube preamps I have heard used a lower voltage on the plates. Starved plate has earned a questionable reputation among hobbiest/DIY types precicely because they don't understand the physics and math behind the various equations governing load lines, emission linearity, operating points and distortion parameters of not just the various tubes but also the circuits that they are used in. In fact, the 12AU7 (and to some extent the 12AT7) was designed specifically as a low plate voltage tube and its operating parameters are quite remarkable in this aspect. One look at the operating curves and it should become immediately obvious WHY this tube class is so interesting.

I have designed both types of circuits (high voltage and low voltage), but I do have an affinity and also an advantage when looking at the "lower voltage" applications (probably not as low as most might consider "starved plate" whatever that means), and you have to also remember that one of the ideal design targets of the 12AU7 tube as spec'd by RCA is with a plate voltage of just 100V per the designer's manual) because my dad, who was also an EE worked extensively in the aerospace industry, and kind of specialized in lower voltage designs for use in military, avionics, and even on navigational instrumentation used on the Mariner Project (first Mars spacecraft) in the 1960's. There are some mind blowing applications that really extended the life of tube circuit designs in applications under these conditions, BUT you have to really understand what's going on at the physics level to know WHY these circuits are so relevant. Many early J-FET circuits were derived directly from this work, and these J-FET circuits found themselves inside popular (and still collectible) bass amps of the late 60's through the 70's. To ignore this stuff , especially with access to my dad's (now declassified) work on the subject, would be to walk away from some of the most authentic vintage high performance circuits in the world. Voltage is just a number, without any reference, it's essentially meaningless as it's ALL about the circuits themselves.

Now the 12AX7 doesn't share the same special properties, and while it can be made to work quite well, the 12AU/AT7 is an entirely different animal which a very different application history.

As far as power tube B+, that too doesn't matter without all the other aspects of the design equations because the voltage alone just doesn't mean anything itself. The voltage interacts with the reflected impedance at the primary of the OT, the screen control characteristics, the grid drive impedance, how the bias is derived and stabilized, the various design targets and margins used, etc. I for one tend to use a slightly lower plate voltage and reflected impedance for plate load to extract better long term life and operating point stability. these circuits also tend to be a little more forgiving of non-linearities on the tube operating curves... something we as designers and you as users have absolutely no control of. Tube parameters are what the manufacturers produce, the distribution is what it is. I'm not talking about the common parameter or two that tube sales guys "screen" for, but all the "between the lines" parameters that real tube designers fret over with regards to long term performance and reliability. There are thousands of tube power amps out there (not Mesa's) with my designs in them running day after day with very few tube problems. Many, even after 10 years or more are still using the same output tubes and the amps still meet their original performance specs. This approach is good for owners of amps where the cost of changing out a set of tubes might be substantial...

I don't know if I'll ever design another big tube power amp again, I just don't know if there's enough demand or need for another. I might enjoy the challenge, but I'm not holding my breath. Is there any interest?

I hope this helps you understand some of the WHYs behind various designs. I don't know what the future holds for any of my tube designs, we will have to see.

Agedhorse: thank you for that *VERY* informative reply! I'm a civil engineer (nearing retirement), and I know very little about EE. So I appreciate the info you provided.

You asked if there was any interest in a big tube power amp. Well, I can only speak for myself, but I recently picked up an Arkham Zephyr tube preamp (single space rackmount), and it would be nice to find a rackmount all-tube power amp to go with it. I am aware of the current offerings (Aguilar, Demeter, etc.....I have owned those in the past) but I seem to gravitate toward the Ei transformer type for strong bass tone. I know about the old Ampeg SVT-300 all tube power amp (the power section of the SVT-II non-pro), but I can't seem to find any of those for sale. And I don't really need quite that much power. I think a rackmount 200-watter (200 "clean" watts) made with Ei transformers would interest me.

 

[agedhorse]

OK well then 400 watts RMS with <2% distortion from 30 hz to 5khz continuous +- 3db, something like that is what I look for in modern amps and never see it, I haven't checked Mesa amps though. Maybe the damping spec might be nice too.

Ok, this provides an interesting predicament... one aspect of some of these designs, and I've mentioned it before, is that distortion (as well as distribution of harmonics) changes with both level and frequency which gets back to 3 dimension plots that are both confusing to most people as well as (if they are accurate) containing information that would be helpful if reverse engineering what is really going on with the IP behind the circuits themselves (which is proprietary). They are also unbelievably time consuming to generate. Since I have had my fair share of designs copied (at least on the surface), and so has Mesa, we are understandable reluctant to disclose information at this level as I'm sure you would appreciate.

The damping factor is an interesting spec. to consider... though while people seem to be obsessed with stupid high marketing numbers here, some of the most popular bass amps in the world do not have such "marketing impressive" high numbers. In fact the numbers are downright low and non-linear. This is a good one to ponder some more.

As I've mentioned before, if you want to provide your customers accurate information about what they're buying (or have already bought), you need to include a power vs distortion curve.

I've explained this before, and recapped it in the post above.

As far as your explanation of saying that THD varies with frequency to emulate an overdriven amp, I'm not buying it. I'm sure there are players that want overdrive but I bet that there lots more that want it clean and solid. Are these magic distortions created in the preamp or amp stages?

When you become an amp designer and have to deliver products to the market that players want to play and want to buy, you just may feel different about this. I don't care if you "buy my explanation" or not, it's not your call as they aren't your designs is it?

There are (or can be) all kinds of distortion elements generated throughout the preamp, the power amp and how the two interact. While the early amp designs (of the 1950's- 1970's) may have ended up with these as happy side effects of the specific circuits and devices available back then, there is a lot of careful thought put into this by quite a few designers (certainly not all). In fact, there's an entire line of pedals and an amp that appear to be based almost exclusively around this concept. From what I have seen, they appear to have done quite a nice job achieving their target tone.

I am going to remind you about being respectful in this thread. I am providing folks here with the apparently rare opportunity to participate in something that many have been critical about in the past regarding the depth of specifications that are commonly provided. Some of the participants have provided interesting scenarios or questions that deserve a more in depth discussion of what underlies the spec. or even the basic concept. I don't mind spending the time because it makes me think about things that maybe I haven't thought about before, or in a way that is different, and that might prove beneficial to me.

 

[funkinbottom]

Perhaps voltage gain?

 

[Hoochie Coochie Man]

A complete schematic and bill of materials.

 

[navijaz]

Another vote for information on the most neutral EQ setting on a given amp. It will obviously not be highly accurate but I'd like to at least get a general idea.

 

[agedhorse]

Hmmmm....so here is the physicist speaking.....
The problem with most spec is that they are not what matters. I almost think it would be better to have an application matrix, like:
View attachment 2475268

Interesting, very interesting.

 

[agedhorse]

A complete schematic and bill of materials.

Good for a laugh, but that's exactly what some manufacturers that specialize in copying popular products would REALLY appreciate

 

[agedhorse]

Perhaps voltage gain?

In fact, this was added to the D-800+ spec. sheet and will be included in future products.

 

[basscooker]

Oooooh !
What about a list of recommended common spec parameters for cabs that would compliment the intended design goal of the amp?
This would also help sell cabs within a line.

Conversely, it could also "rule out" cabs they already own, so there's that. Ok maybe not so awesome an idea. But posted anyway. Hey, spitballing....

 

[Joebone]

I ain't no engineer, but here are characteristics I'd love to see measured, if possible...

** Honest power ratings. I guess this means RMS, over some range of frequencies and into 2, 4 and 8 ohms, rather than one cherry-picked frequency.

** I'm not quite sure what "headroom" really is, but note that amps I like seem to have a lot of it...

** Harmonic overtone analysis. Distortion happens, but is it even order or odd order, my understanding being that even order is euphonic and pleasing. This goes to my next point...

** Harmonic integrity. This is critical for me - I can't deal with equipment that does not cleanly portray each given pitch. I have no idea how you measure this, but if it can be measured, then please do so by all means! Pitch integrity is the most impressive feature of the Thunderfunk amp that remains at the top of my heap, even if I don't lug it out for many situations.

** Quickness and cleanliness of attacks and releases. I don't know if this is best represented as a some sort of square wave, through damping factor, or what...but I want an amp that will hit hard, fast and clean on the attack, but that also will manage the release of a note so the woofer is not flopping muddily about.

** Insight as to tone control effects. What does the Q look like? Are the tone controls interactive? How flat is "flat," and how do flat control settings look, on a frequency v. volume graph?

Edit: is it possible to clarify the rate of taper on gain and volume stages?

Interestingly, even if one manufacturer adopts useful measurements, there is no guarantee that others will do so as well, particularly if their stuff does not do well by these measures, so comparison based on specs might still remain difficult.

That said, this thread provokes thought - thank you, Aged Horse.

 

[gustobassman]

Ok this may sound simple but maybe examples of acceptable interconnect cables for varying items that may be used in an Aux input such as the D800. Like connecting a laptop or other device. Or maybe even examples of different possible connections.

Might be useful for non-technical types. Or new players interested in your product.

 

[Daniel Piper]

the spec sheet for my new, updated, light weight, 900 watt walkabout just needs the basics thanks.

Joking.

I am definitely not smart enough to be constructive in this thread!

 

[Spidey2112]

I would love to see a scope reading (if that's the right term) of the amp with all eq controls at noon. If the response is not basically flat at those settings (e.g. if it has a baked-in eq curve like the GK heads), I'd also like to see what knob settings give the closest to flat response on the particular amp in question.

If this is also going to be considered for cabinets, I'd like to see actual response graphs, on and off axis, rather than some mystical "sensitivity" number provided with no information on what frequency it occurs at, or what the response curve as a whole looks like.

Where have you been all my life?

 

[Jack]

I think part of the problem with asking us here is what I call the 'forum effect'. By nature of us being here, TB suffers from a major case of self-selection bias. We're not representative bassists.

As an example, many of us here might be pleased to see a specification that says something like 'High pass filter: -24dB octave starting at 50Hz' but the majority of bassists would look at that and say "I'm not buying a bass amp with no bass, you won't be able to hear the E string".

I can think of similar issues with distortion specs (sometimes more is better, ask an SVT), actual, meaningful power measurements (2000W peak at 1KHz from company A might actually be the same as 500W RMS 20Hz-15KHz from company B but I'm buying the one with MORE thanks) etc etc.

I think that any specs published should come with some prose explaining them. Your problem is neither 'it sounds good so it is good' luddites or self-confessed TB experts, its the dangerous people in between who don't know how little they know. The people that say 10" drivers are punchy or that bass sound waves need a lot of space to develop.

 

[Spidey2112]

I think many of them are virtually meaningless to the overwhelming majority of players.

@agedhorse , unless "It's extremely reliable!" and "Sounds great!" can be added to a spec sheet, I'm pretty sure I fall into the category, stated above... even with a technical background, I never went farther than price, power, and pizza, with regards to buying an amp, cab, or combo...

... now, if I was building something myself, that would be a different story, as I would research it to the point that I may never finish...

 

[lbbc]

For me (not an engineer)....
1. Center dent EQ (optimum sounding)
2. Actual "honest" watts at 8 & 4 ohm
I haven't used an amp in about 2 years, but these would be my 2 deciding factors when purchasing a new rig.