The tweed covered Fender Bassman is considered by many to be the epitome of tone, and every current tube amplifier myth revolves around this amplifier. Admittedly, they sound great for guitar, but let's back up a little and see as Fender makes a bass amplifier! (Keep in mind throughout this lesson that Fender is in business to make money, and the bottom line is a cheap, mass producible product.)

 

If you were designing a bass guitar amplifier, or if you are a bass player, how would you like an amplifier that featured;

• an open back cabinet design?
• 10" speakers that had a low end cut off of about 90Hz?
• a power supply with a tube rectifier?
• a power supply with 20uF filters?
• a 5/16" baffle board with four speakers screwed to it?

Well, Fender did just that. A company that had been making amplifiers for over a decade (starting with K&F) 'invents' the bass guitar, but has no decent amplifier to go along with this 'new' instrument! It simply cannot be that Fender knew nothing about ported bass reflex enclosures. They had been around for a while from people like Carlson, and many articles in magazines such as Audio Anthology, Radio Electronics, and Popular Electronics featured speaker enclosure projects with all sorts of theory about how to 'tune' the port. And didn't someone ask about bigger filters? How about diode rectifiers? Even the words 'power supply' and 'sag' should have entered everyone's mind, but apparently did not (keep in mind, however, that few played their amplifiers at full volume, which is where 'sag' usually becomes an issue). For guitar it sounds marvelous, but again, we are designing a bass guitar amplifier. The highly favored '59 model was put out largely in response to bass players complaining about their 1X15" Bassman 'flapping out' on the low 'E' string. Another headache was that the single speaker used was barely adequate in handling the power put out by the amplifier! Blown 15" speakers were rampant, and solutions were needed. The simple solution offered was four speakers that could handle the power, and had a resonance one full octave above a bass 'E' string. No more blown speakers! No more flapping! Very sneaky. Had Fender just used a sealed cabinet, with a tighter power supply, they would have solved this 'mystery'. (The 4X10" combo version of the Bassman was quietly replaced by the 'piggyback' version a few years later. The Bassman 'head' had many variations, and to see a brief overview, with some modification suggestions, CLICK HERE.) Who knows how history would have been written had they paid attention to all the advertising for tuned bass enclosures.....

Companies such as Karlson were certainly on the right track when they rationalized that the speaker cabinet designs of the day were not conducive to efficient, quality bass frequency reproduction (an open-back design is the worst option). Magazines such as Audio Anthology were publishing articles post WWII about tuning ports on homemade speaker boxes. A few of the promised 'benefits' of a Karlson Cabinet... '2 more octave bass!... Flatter response-Less distortion!' ...would have been welcomed by more than a few Bassman owners. The Karlson cabinet is still popular today, to the point that there is a website devoted to this brainchild of John E. Karlson; complete with a 'Forum'! Visit it by CLICKING HERE. Back to the discussion at hand; to be fair, Fender was simply trying to mass-produce an inexpensive product. Yet another avenue of thought involves the Bassman and the 10" speakers, with a rolloff of 90Hz. How about.....

Seen right at the same time as the Fender Bassman was trying to sort out its open 'E' string dilemma, Electro-Voice unleashed this 30" speaker. At a claimed low-end rolloff of 18Hz, I'm sure it would have solved the problem of any 'E' string flapping and then some! (It would even handle a low 'B' from a 5-string bass today without blinking!) Thankfully, Fender did not go this route, and today we can find examples to demonstrate how good a guitar amplifier the Bassman is.

The output transformer from our tweed Bassman has attained far too much hype. Fender selected their transformer by virtue of the fact it was probably the only 2-ohm transformer to be had! (2-ohms is a very unusual output impedance.) It just happened to have an interleaved winding, which increases high frequency response; real important in a bass amplifier! See ELECTRONIC PARTS for how transformer specs influence the tone response. Why did Fender decide to use a 2-ohm output impedance? We may never know for certain, so we'll just have to speculate. Some BS 'guru' may chant about branch inductance, but this seems unlikely. It is possible Fender decided that in a parallel configuration three speakers will still function just fine should a single speaker 'blow', and be matched close enough to the rated secondary impedance. Remember the tweed Bandmaster? It utilized three 10" speakers, and used the Bassman output transformer. In a series-parallel configuration, blowing one speaker also removes the 'mate' from the chain, so the load doubles in effective impedance value.

Specifications given on catalog inserts tell a story that is hard to believe and/or explain. Many of Fender's early classic tweed amplifier advertisements are represented in the book Fender, The Inside Story (Forrest White). While Mr. White has limited electronic knowledge and shies away from stories about the technical development behind the Bassman amplifier (or any other Fender product), the catalog insert reveals more than he ever could. Submitted for your consideration, under the heading 'Technical Characteristics';

• The Fender Bassman Amp is a compact, portable musical instrument conservatively rated, with all necessary controls for handling complete High Fidelity music reproduction.
• Power Output: 60 watts at less than 5% distortion.
• Frequency Response: With optimum tone control settings +1 dB 20 to 20,000 cycles per second at 50 watts.

The 'General Description' of the Bassman amplifier has its' own mysteries;

• ...features the latest in electronic design, inasmuch as the inputs are so arranged that tremendous surges of voltage can be accommodated without overloading the first tubes, thereby eliminating a great source of distortion which is normal in all older type amplifiers.
• In addition to these many features is the fact that this amplifier has an over-size power supply employing an extremely heavy duty power transformer ...
• The output transformer is also much heavier than is usually found in amplifiers of this size.

Thankfully, this is all 100% untrue. Ignoring the obvious exaggeration about High Fidelity music reproduction, first of all you'd never get 60 watts out of a Bassman, even at 'peak' power. This makes the 'conservatively rated' angle a complete falsehood. Secondly, the frequency response is BS and meaningless to boot, as the speakers might have (at best) a response of 90Hz-4kHz. The 'over-size power supply' is anything but, leaving the output transformer the only 'feature' advertised that can't be argued by the average guitar player or technician. Of course, 'potted' audio transformers of the day were much larger and heavier than any Bassman transformer, but I'll let that tall tale go, as it is repeated for many Fender models during this period of advertising.

Reissue 'tweed' Bassman amplifiers have paid the mortgage for more than one tube amp 'guru'. What started the whole 'reissue' series of amplifiers from Fender also prompted many eccentric TV repairmen to put out their own tweed-style 'clone'. And it started a cottage industry of 'upgrade' kits claiming to make your modern counterpoint sound more akin to its' ancestor. I have read (with a grin) many magazine articles stating that by merely replacing the Russian tubes with some Yankee Doodle Dandies and substituting the regular baffle board for one made of pine will be the ticket to tone heaven. Other opportunists state that by buying their expensive kit, (which comes complete with the 'proper' capacitors mounted on a 'tag' board!) you will have the sound. Let's think about this prospect for just a minute, and realize that the people selling these 'upgrade' kits are overlooking a few key points (or, are hoping you aren't smart enough to ask these questions).

• Are you trying to capture the sound of a brand new 1959 Bassman or a well-played 1959 Bassman as it might sound in 2004? This makes a huge difference in the way you design your amplifier.
• Either way, your B+ is about 60 volts too high. It is one reason why the reissues are 50 watts as opposed to the original 40 watts. Tubes also tend to exhibit a lower plate resistance with higher plate voltages¹. This will effect the frequency response as the 'Thevinine' equivalent time constant is altered. (One tube amp guru-written book mistakenly states that 'tubes pass high frequencies more easily at higher plate voltages'. WRONG!) This is why you can't just blindly follow the schematic for component values and expect identical tones unless everything else is exactly the same. So, how will one fiberglass board sound better than another fiberglass board? It cannot. STRIKE ONE!
• A 12AX7 is used in the reissue for the first preamplifier tube. The original 12AY7 gave less gain, and had a much lower plate resistance for the same plate voltage. Also, the 5U4 in the reissue is not even close to being the same as the original GZ34. Once again, the circuit board doesn't even enter into the picture. FOUL BALL!
• You have too much power supply filtering to give the traditional 'sag' when the amplifier is driven hard. The extra filtering will also tighten up the bass frequency response. At the risk of sounding like a broken record; how does the circuit board material compensate for this? STEE-RIKE TWO!
• Does the reissue Bassman output transformer have exactly the same primary impedance? Does the reissue output transformer utilize the exact same type of iron in the core? Does the reissue output transformer use the same wire gauge? How can a reproduction tag board compensate for this? FOUL BALL!
• Letting up on the circuit board criticisms; the speakers in the reissue Bassman have all the 'wrong' ingredients to yield the same tones as the original recipe. This led to the folklore of some people 'beating up' their speakers to soften them up a little. While very possible to do, I don't really recommend it at all. After all is said and done..........

To be fair, the reissue Bassman is not a bad amplifier; they simply don't sound anything like an original. If you own a reissue, play it for what it is (a very good guitar amplifier), as opposed to complaining about what it isn't (an exact replica of its ancestor). And if you are thinking an 'improved' fiberglass board can drastically alter the tone of your reissue Bassman; please strike your head against any available wall sharply, and continue to do so until some sense has replaced these wishful ideas.

There is no reading list for this lesson. You should, however, take to the task of inspecting as many original and reissue tweed Bassman amplifiers as you can to verify what I have just told you. Make sure you play a bass through one!

 1) Basic Theory and Application of Electron Tubes; Department of The Army Technical Manual.

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