Traynor YBA-1 and Fender Hot Rod Deluxe, Tone Stack Comparison

In the last blog, I discussed the use of a Duncan Amp’s Tone Stack Calculator as a means to motivate the discussion of what makes a Marshall amplifier growl. Today, dear readers, I will back up that theoretical discussion by demonstrating measurements from both amplifiers, because really, what use is a tone stack calculator if it doesn’t *really* tell you about how an amp might sound?

For these experiments, the two amplifiers used are the Traynor Bassmaster YBA-1 and Fender Hot Rod Deluxe. If you really want me to measure a Marshall JTM45, then please, contact me, and send me one ;) These measurements were acquired using XX via the line out feature of a Hot Plate Attenuator. Signal generation and signal acquisition were performed by separate USB sound cards to hopefully eliminate the presence of ground loops or feedback affecting measurements. The amplifiers were driven in their linear regime before the onset of distortion. The nature of distortion will be discussed later and was excluded from this experiment for the sake of measuring only the tone circuit.

Fender Hot Rod Deluxe Frequency Response
Below, you will see plots of families of curves for the Fender Hot Rod Deluxe with one of the tone stack knobs (Bass, Mid, Treble) at levels of 0 (green curve), 50% (black curve), 100% (blue curve), while the others are kept at the mid level. The black curve has all knobs at level 5 and is repeated in all plots.

(a)

(b)

(c)

(d)

Figure 1. Frequency response of Fender Hot Rod compared with theoretical values. Tone control knobs are kept at mid-point if not being varied. (a) Varied bass response, (b) varied mid response, (c) varied treble response, and (d) the theoretical curve.

In making comparisons, one may at first be struck by the difference at the very low and very high end of the spectrum. Rest assured, this is due to the measurement system, which is a sound card running at 44.1 kHz. I have indicated this limitation in the sampling system with red lines in Figure 1d. The sound cards used cannot drive the low frequency signal very high, and sampling at 44.1 kHz only allows samples of signals up to about 22 kHz. This is known as the Nyquist theorem to the engineer but could just as easily be known as the rule of “twice as much is needed to make a useful measurement.” In other words, you need at least twice the sampling rate to capture a signal at half the sampling rate. I will discuss this in more detail in another blog, because some people might be wondering: why are sound cards using 96 kHz sold if humans only hear up to 20 kHz (which requires the 40 kHz sampling rate). I can assure you, the answer is not so we can record dog whistles, but this will come in due time.

Traynor Bassmaster YBA-1
Below, you will see plots of families of curves for the YBA-1 with one of the tone stack knobs (Bass, Mid, Treble) at levels of 0 (green curve), 5 (black curve), 10 (blue curve). The black curve has all knobs at level 5 and is repeated in all plots.

(a)

(b)

(c)

(d)

Figure 2. Frequency response of Traynor YBA-1 compared with theoretical values. Tone control knobs are kept at mid-point if not varied. (a) Varied bass response, (b) varied mid response, (c) varied treble response, and (d) the theoretical curve.

Okay. What the hell is going on with the bass response on the Traynor measurements? The bass frequencies in the theoretical model are almost equal in amplitude to the high frequencies. This is clearly not the case for “guitar” channel on the Traynor. Since the YBA-1 was made for bass, my suspicion is that the Traynor engineers placed a low-cut filter at the "guitar" input of the amplifier in order to make the channel more ‘bright’ and ‘guitar-like’. Next time, readers, this will all be clarified with schematics and measurements. Possibly this is the only place where the Traynor is lacking in the ‘growl’ department—crappy bass. I mean, the HIGHs are greatly outweighing the LOWs in the Traynor as I am currently playing it through the guitar input as measured above.

Fender, Marshall, and Traynor Tone Stacks

I’ve recently begun recording a second disc with the band I’m in, Shayna and the Bulldog. On our first disc, we were a little unhappy with the guitar tones we got, so over the past year we’ve expanded our amp collection and knowledge. The first disc contained primarily a Fender Blues Junior and Hot Rod Deville (4”x10”) for the rhythm guitars, while the lead was done entirely on a Fender Deluxe Reverb Reissue. With a couple more amps and more knowledge, we are now stuck with the dilemma of options. Recently after band practice, the following conversation took place.

Charles: “We could use the Traynor YBA-1 through a 4x10 cabinet for our rhythm guitar tracks. It has basically the same circuit as a Marshall JTM45 and gives that fat, chunky rhythm guitar sound.”

Joel: “Yeah, but … the Traynor it just ..”

Charles: “It just .. what??”

Joel: “It’s missing that growl .. that feeling. Can you do anything to it?”

And so the search began. The question of the hour, dear readers, is what gives an amp that growl? To approach this vague question, I had to consult the small part of my soul that is not an engineer. We opened at an outdoor gig a couple years ago, where I had the opportunity to play a fully cranked Marshall half-stack, which most certainly had “that growl.” This was before I took the model and serial number of every amp I played or even saw, so I have no idea what the model was unfortunately. Anyhow, since that time, I have associated Marshall amps with that growling, mean, distortion that breaks up at just the right part of the attack and makes bar chords sound like Satan himself is beating a drum in your chest. From this abstract idea, I began just to wonder. What *is* it that makes that sound, opposed to the relatively cleaner, chunkier sound of an overdriven Fender? From there, I wandered into thinking about diametrically opposed ideas about the cultures surrounding the amplifiers, such as monarchy vs. democracy, English vs. metric, the Beatles vs. Zeppelin, Churchill vs. Roosevelt, etc… Quickly, I realized how unprepared I was to answer any of these questions, so I returned to guitar amp terms: is it the tubes, the tone stack, the transformers, the cabinets, etc… There are a lot of differences, and I was at least a little bit prepared to answer these questions.

Returning to the debate about the Hot Rod vs. the Traynor and realizing that a true Marshall really “growls,” I guess the primary question is, what is the Traynor lacking, as far as emulating a Marshall tone? I should preface this discussion by noting that I think the Traynor has an entirely unique sound all its own, which should not be discounted in quality, simply because it is not Marshall-like. However, I think my goal is to give the Traynor a more Marshall-like growl. Sure, I could look at the schematics for the JTM45 and make the mods to create a JTM45 duplicate, but I want to answer a couple questions about this ‘growl’ along the way.

In coming weeks, this investigation will quantitatively run through some of the many facets that make up a final guitar tone, such as tone stack, tube types, closed vs. open-backed speakers, and anything else I can think of that can be measured and might be affecting the tone. I will begin today, with a theory-based discussion using Duncan Amps excellent tone stack calculator.

The Tone Stack

For those unacquainted with the tone stack of a guitar amplifier, this is generally your bass, mid, and treble knob. Some people might argue that the presence knob is part of the tone stack, but those people are wrong and this is not up for discussion. Anyhow, a typical tone circuit consists of a very basic RC filter with 3 potentiometers (knobs). However, it is well known, there is more than one way to skin a cat, and Fender and Marshall amplifiers most certainly go about the tone circuit in separate ways. Classical Fender and Marshall tone circuits are shown below:

Figure 1. (a) Shows a typical Marshall style tone circuit, compared to (b) which shows a typical Fender style tone stack.

Okay, great. But what does this mean to the amplifier tinkering individual? The electronically inclined could solve these circuits and come up with nice equations to show the frequency cut-off points, etc... However, I don’t like math very much, and Duncan’s Amp page has created a wonderful tool for analyzing tone circuits, so I will use that for the sake of efficiency. Below, we have the Fender and Marshall tone stack frequency response. Again, the untrained eye can see the difference between the two. If you are not familiar with frequency response curves, the x-axis is a given frequency, while the y-axis tells how loud that frequency will be reproduced.

Figure 2. (a) Typical Fender tone stack frequency response. (b) Typical Marshall-style frequency response.

To relate these plots to the sound of a guitar, one should note that a typical electric guitar can produce fundamental frequencies between 82 and approximately 1000 Hz. Now, if you look at the plots above, you can see that the large dip in the fender plot over the decade between 100 Hz and 10 kHz certainly must produce a different tone than a Marshall, which has a much less dramatic dip. When one considers higher harmonics and subharmonics of a guitar pluck, you can get the idea about just how much this affects the tone. It’s worth mentioning that no matter how you turn the knobs on a Fender amp or Marshall, the two frequency response plots will basically NEVER match one another. They both provide a range of control over the low, middle, and high frequencies, but as was mentioned earlier, there is more than one way to skin a cat. Here, we have identified one possible source of a lack of “growling-ness” from an amp.

How does the Traynor, the bastard cousin of the Marshall fit into this argument? The results of plugging in the stock values for the Traynor tone circuit into the Tonestack Calculator are shown below.

Figure 3. Traynor YBA-1 Frequency response, using stock capacitor values.

Here, you can see the Traynor circuit has a decidedly Marshall-like response (note the small dip between 100 and 1000 Hz) ; however, the very low frequencies are quite a bit different. At first glance you may think the YBA-1 would give a more ‘bass-y’ response than the Marshall. This is not the case, however, since frequencies between 10-100 Hz are generally inaudible. I hypothesize that this low-end “growl” is probably happening somewhere between 100-400 Hz. The Marshall Frequency response is actually greater in this range.

That is all for the theoretical analysis of tone stacks. Next week, we will compare the tone stacks to some experimental data from the Hot Rod Deville and the Traynor YBA-1. Until then, grab the free Duncan Amps tone stack calculator and see what kind of tone circuit your amp has!

Tube Amplification Links

I have waded through a ton of sites and books about tube amplification, searching for accessible information. Some are really good, and others were a waste of time. Below are my favorite sites for learning about tube amplifiers:

Boozehound Labs - this guy has created a lot of really cool stuff ranging from tube mic preamps, guitar amplifiers, hi-fi amps, etc.. He also has a very good tutorial on the basics of tube amplification. Strangely enough, he is also a bike fanatic.

Weber Speakers - Ted Weber and the folks on the forum are incredibly helpful. I have ordered some stuff from them and can highly recommend them.

Duncan's Amp Pages - I found this to be the absolute best resource for simulating tube amplifiers using free SPICE software, and their tube data sheet locator is amazing.

BillM's Blues Junior Page - Although this site is primarily geared towards Blues Junior owners, a lot of the information here is pretty general. Rather than just spewing a bunch of unfounded opinions on what is "best" for an amp, he provides a lot of really legit analysis on the how and why certain decisions might be made for modifying tube amplifiers.

The Guitar Amp Handbook - This is a link to a book about tube amps that is very readable. It covers a lot of the basic ideas about what can be done to get guitar tones you're looking for!


Okay, those are all that come to mind at the moment.

Welcome to my blog about beer and music

And not bikes?? Well, I do love bikes as well, and I will post pictures of fun bike rides I do around California. However, I am not such a gearhead when it comes to bikes, so I just like to ride them and have fun. My intent with this blog is to aggregate information I have learned about audio electronics and home brewing. Primarily because a lot of the things I've learned about these topics has been from other blogs and forums on the net, and I want to leave some 'bread crumbs' for other experimenters in the fine arts of music audio electronics and homebrewing as a thanks to all the other people out there who have taken time to write up very helpful documents on these topic. As I am not an expert in either of these topics, I also hope I can get some input from more experienced individuals.