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Topic: The Fender 5F6-A Bassman Circuit: A 21st Century Adaptation |
Bryan Martin
From:
Quebec, Canada
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Posted 3 Oct 2017 7:07 am |
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A few years ago I decided to design a vacuum tube amplifier for pedal steel. I ended up writing a paper for the Audio Engineering Society about the amplifier. I hope some of you find this interesting.
Cheers, Bryan
Here is a link to the paper:
https://www.dropbox.com/s/tvjgd76s1hvhktf/Bryan%20Martin%2069%205F6A%20bassman.pdf?dl=0
Academic writing can be mind-numbing, so just be warned. I will quickly summarize what I was going for below, sorry for the long text.
This began with the search for a guitar amplifier that would be conducive for use with the pedal steel guitar, but in low-volume situations, such as the home studio. The requirements were:
1. Increased headroom of the input stage.
2. Clean yet exceptional ‘tone’ at low volume.
3. Relatively flat frequency response in the context of classic American amplifiers.
This lead to the adaptation of the Fender 5F6-A circuit.
The Fender 5F6-A electric guitar amplifier is one of the most highly regarded, prized, and sought-after amplifiers in history. It is also the design that Jim Marshall appropriated as the basis for the amplifier line that bears his name. It is known as the 4-hole Bassman, owing to two Normal Channel and two Bright Channel inputs, the ’59 Bassman (moniker used by current Fender Musical Instrument Corporation), and the 4-by-10 Bassman, as the 5F6-A is a combo amplifier with four blue, Jensen ten-inch loudspeakers with Alnico 5 magnets in an open backed cabinet.
The exact origin of the circuit is murky. Gerald Weber in A Desktop Reference of Hip Vintage Guitar Amps states that “Leo Fender did not design the Bassman circuit. The circuit was . . . patented by AT&T and Western Electric (WECO) in 1948 and later licensed to Fender . . .†It is known that the early Fender designs were gleaned from the RCA and other vacuum tube manuals of the day. The only factual reference to a WECO connection is that the first amplifier produced in 1947 bore a label stating: "licensed under US Patents of American Telephone and Telegraph Company". The WECO business model was one of licensing and leasing technology. It had a great many patents for circuits used within a complete amplifier. This may have been the reason for the crediting on the label.
The Bassman Circuits
The first iteration of the Bassman circuit appeared in 1955 as the 5D6, followed immediately by the 5E6; the 5E6-A (manufactured from 1955-57); and the 5F6 in 195 The 5F6-A version appeared in 1958 and was manufactured through 1960.
The first goal of the circuit adaptation (increased headroom) was already inherent in the original design of the 5F6-A due to the use of the 12AY7 instead of the usual 12AX7. The adaptation for ‘great tone at low volume’ was achieved by replacing the 6L6 power tubes (rated at ~50 watts/pair) with a pair of 6V6 vacuum tubes (rated at 25 watts/pair).
This adaptation required a careful choice of output transformer, as the 6V6 vacuum tubes have a much higher anode-to-anode resistance than the 6L6 tubes. The output impedance at the secondary of the output transformer for a pair of 6V6 tubes is approximately twice that of a pair of 6L6s.
Flexibility in output tube choice can be achieved by designing with an output transformer with a power rating of 50 watts, and a output impedance to accommodate a pair of 6L6 vacuum tubes into 4, 8, and 16Ω loads. Therefore, if 6V6 vacuum tubes are inserted, then the 4, 8, and 16Ω (6L6) will correspond to 8, 16, and 32Ω (for the 6V6s). The amplifier can then be used for low-volume situations, but also can be used for higher volume demands such as live performance (with an adjustment of output tube bias.).
To achieve the final design goal--relatively flat frequency compared to traditional tone controls -- a switch was inserted for the bypass of the tone control circuitry. This removed the bass and treble boost of the tone circuit. When the tone circuit is inserted, there is an added pot that acts as a ‘blend’ control between the flat frequency response (direct) signal and that which is manipulated by the tone circuitry.
Last edited by Bryan Martin on 3 Oct 2017 7:19 am; edited 3 times in total |
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Bryan Martin
From:
Quebec, Canada
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Posted 3 Oct 2017 7:17 am Some pictures of the build
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The parts to build this amplifier can be had in kit form, except you will need to order an output transformer with the 3 output impedances. What is great is that you can get the correct chassis. Laying out, drilling, and punching chassis can be challenging.
Here are pictures of the build.
Cheers, Bryan
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Tim Whitlock
From:
Colorado, USA
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Posted 3 Oct 2017 12:41 pm
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A right nice looking rig Bryan! I'm a low grade moron when it comes to electronics. Am I reading your description correctly to say that you can interchange 6V6s and 6L6s with your design?
Last edited by Tim Whitlock on 3 Oct 2017 8:11 pm; edited 1 time in total |
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Bryan Martin
From:
Quebec, Canada
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Posted 3 Oct 2017 8:09 pm
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Tim,
You are correct. They can be interchanged.
But the amp needs to be re-biased when you change tubes.
Also, the output transformer needs to have 3 impedance taps as the output impedance of each pair is different. But you can basically get an 8ohm output from each set of tubes.
All this stuff can be a bit confusing.
Cheers, Bryan |
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Bryan Martin
From:
Quebec, Canada
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Posted 3 Oct 2017 8:44 pm
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Tim,
Just to add some information to your question. Any amplifier that has 6L6 output tubes can have them replaced with 6V6 output tubes. You simply cut the wattage in half, down to about 22 watts.
But the amp needs to have adjustable bias to set the 6V6s correctly.
Also the output at the speaker jack will be doubled. 4ohms for 6L6s will be 8 ohms for 6V6s. So you will need an output transformer that has an output of 4 ohms for the 6L6s (unless you have a 16ohm speaker cabinet).
Hope this answers your question.
Cheers, Bryan |
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Bill Terry
From:
Bastrop, TX
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Posted 4 Oct 2017 6:42 am
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| Very cool Bryan! I owned an original '59 5F6-A until it was lost in a house fire. I never knew what the buzz was all about until I owned one. They are really great amps.. and a kit is about as close as I'll ever be to owning another one. |
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John McClung
From:
Olympia WA, USA
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Posted 4 Oct 2017 9:54 am
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How good are the various stompboxes that emulate the Fender Bassman tone? There's a BOSS pedal, and an HAO Sole Pressure, and a SansAmp pedal. Probably more than that, too.
_________________
E9 INSTRUCTION
▪️ If you want to have an ongoing discussion, please email me, don't use the Forum messaging which I detest! steelguitarlessons@earthlink.net |
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Robert Parent
From:
Gillette, WY
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Posted 4 Oct 2017 12:25 pm
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Interesting article... Thanks for posting.
Robert |
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Bryan Martin
From:
Quebec, Canada
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Posted 4 Oct 2017 3:08 pm Emulation
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John,
I don't have much direct experience with the emulation boxes, but I will try and address this as best I can.
Firstly, I'm a tube amp guy, and I don't like the solid-state stuff for guitar amplification, so I am biased right off. I was a record producer and engineer in NYC for a few decades, and I have worked with, and own a great many of the amps. So I am coming at this with the benefit of having worked extensively with these things. I was lucky.
A great many of my friends who I have made records with (so they too have used the real things) find it very inconvenient to take large, multi-amp rigs on tour. So many of them turn to emulation as a way to be able to reproduce the sound of the record in a live situation. What they have told me is that the emulation gets them about 80-85% there.
Another thing to take into consideration is how the emulation is being implemented. There are two ways: digital modelling, or analog circuit modelling. In both cases there is a solid-state (transistor or opamp) at the input and output of the unit. In the case of digital there is also a analog-digital (input) and digital-analog (output) happening in addition to the emulation.
This is far more circuitry than in any tube amp. Every stage of an amplifier circuit adds distortion and effects tone. There is always a cost for adding a gain stage or processing.
Personally, when I play steel through a solid state circuit (evening in a volume pedal), I feel less connected to my amp. I can tell, I can hear it and the effect it has on my tone. So I am a minimalist. I am sure most players could tell just by switching back and forth. But each player's set up is personal. Different players are looking for different things from their setup.
In reference to this amplifier, there are techniques I employed in the physical build, such as star grounding, shielding of signal wiring, careful placement of AC heater wiring, and physical layout. This greatly reduced noise, and improved the sound of an amp. I also tuned some of the coupling caps by ear.
These methods can be very time consuming and lengths few manufacturers would got to. I had a lot of players try this amp, and it performed beyond expectations.
So another question that would arise in modelling is: did they model a really great version of said amp? A model is only as good as the specimen used as a reference.
All this being said, the modelling out there has been going on for a lot of years now, and is fairly advanced. There are likely some high performing units. And if it sounds good to the player, then it is good. And it puts a lot of options for a reasonable price in one's hands.
But for me, I can hear the difference, and I think we have a long way to go before the emulations (if ever) will be as good as the real thing.
Sorry for the long musings, Bryan |
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Mark McCay-Moran
From:
Lake Tahoe
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Posted 5 Oct 2017 12:18 pm
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Thanks so much for this Bryan!
Electronics/old tube amps one of my passions-- may have just found my next build! |
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Michael Butler
From:
California, USA
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Posted 5 Oct 2017 1:06 pm
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great build, bryan. thanks for the details as i just recently finished a 58' 5e3 deluxe clone that sounds great.
very interesting to see michael and bryan enclosing info about their builds.
play music!
_________________
please see my Snakeskin's Virtual Music Museum below.
http://muscmp.wordpress.com/ |
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Steven Paris
From:
Los Angeles
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Bryan Martin
From:
Quebec, Canada
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Posted 7 Oct 2017 3:41 pm
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Steven,
I have checked out the power scaling. The way it works is that it basically reduces the plate voltages on all the tubes in the amp.
In the case of PSG and clean sound, you lose all the headroom. I doubt the tone would be maintained. 20 watts is definitely a bit above bedroom level.
But the master volume pretty much gets this amp there.
After this amp I built a "bedroom" amp. It is a suped up "champ" approach with 2 input channels: EF86 and 12AY7, going to a 6C45C driving a single KT88 output tube. No tone controls.
Maybe I should post that one as well.
Cheers, Bryan |
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David Mason
From:
Cambridge, MD, USA
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Posted 7 Oct 2017 8:00 pm
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This is risking eternal damnation, or at least, the snapping-turtle-of-purity getting too darn close to my appendages for comfort, but, I must leap to a few musings of my own - hopefully NOT just the usual. I AGREE with a lot here, but there are also some points I wish to... make pointier? Like:
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| Every stage of an amplifier circuit adds distortion and effects tone. There is always a cost for adding a gain stage or processing. |
Having a parametric midrange circuit allowing me to control the center point, width, and boost or cut can only be improved upon by... having TWO of them, upper-and-lower mids! If the one-volume knob-only tube amp doesn't have them, do "we" know how often a recording engineer or soundman is applying these later? If the "cost" of adding this is, it sounds better - I'll pay it.
And, SPEAKER CHOICE is perhaps the least-acknowledged, yet most important "tone control" available.
A: Run your stuff throught a "flat" P.A. channel.
B: GAAH! Poke out yer ears! Or at least, cover your ears.
C: If, like the vasties of vasty, you use 10," 12" or 15" speaker(s) with no horn or tweeter, you are slaughtering every frequency above 2.2K or 2.5K or 2.35K in an incredibly important range. I can't even begin to compare amp to amp unless I know what's after the power-stage - size, roll-off, number of speakers, closed, open cab, ported, cone type, ohms etc. Some speakers sound awful one way and not another, some speakers just sound awful - but slaughter those frequencies we MUST, one way or another.
Due (I think) to being stingy, and the child of genuine depression-era parents (1930's dep., not Dr. Phil-ish dep.) I find "disposable amps" to be mentally offensive, just as I prefer the feeling of "real" cast-and-ground scissors to those flat stamped Fiskars-type scissors. But they sure do cut good, and... as we keep traipsing down the road to disposable-everything, it's quite clear that Fender Blues Jrs, newer "Supersonics", Roland Cubes, etc etc etc. can sound GREAT, not just good. You can't hear they're disposable until they suicide themselves, and it's BECAUSE:
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| ...techniques I employed in the physical build, such as star grounding, shielding of signal wiring, careful placement of AC heater wiring, and physical layout. |
- they only have to get their lengths, balances, proportions correct ONCE then the cookie machine stamps them out by the hundreds, each just as perfect as the last (perfectly-CHEEP too).
Let me say why I DO really like "modeling" certain effects and parameters, and that is control, even at MINIMAL levels. Like putting a chorus effect on but at maybe 15 or 17 out of 100 - just enough to stereo-tize the signal without overt "I'm-Andy-Summers-and-it's-the-Eighties!" Or, I now consider delay with-OUT control over the ducking parameters to be, frankly, inferior, less useful, less musical. And, when slapping on multiple effects, I'm not running into multiplying gain stages like with single stompers, where every effect has it's own pre-and-post gain! More than three of these are usually messy, noisy... Rather than add a noise gate, I'd rather subtract preamps. And I can experiment with levels, the order of effects, if anything it lets me rule out things faster, not rule them all in. I can decide to chuck one out, or, say, dig out a higher-quality stand-alone "tremolo" effect because the cheapster pointed the way for me.
O.K., highs-and-lows:
Is it safe, or fair, to put these statements in the same paragraph?
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A) Firstly, I'm a tube amp guy, and I don't like the solid-state stuff for guitar amplification, so I am biased right off.
B) I don't have much direct experience with the emulation boxes,
C) But... I can hear the difference, and I think we have a long way to go before the emulations (if ever) will be as good as the real thing.
D rep.) I don't have much direct experience with the emulation boxes. |
Well. O.K. So let me say what I DO really dislike: First, the reviews, and apparent quality of the boogers seems to be about how authentic they are, rather than how good they sound - I can certainly make ANYthing sound pretty awful, and the more choices I get (I guess?) the more I can do so! My current bestie Digitech RP360 has... lemme see... 15, yes, FIFTEEN speaker cabs with 4 x 12" speakers. What does that even mean? Sounds like some tweakies of reverb "resonance", depth, volume, EQ curve, in other words the same stuff I get lotsa other knobs for... plus single & double 12's, 15's, a few 10's AW COME ON. I don't even CARE if it sounds real, I just want it to sound good, but we surely are into the realm of "feature creep" where I have to make choices for, and about, nothing useful. Leading up to the next major snivel, like:
So far, I guess, the Fractal Arts "Fx-Ax II XL+" is their premie... Remember the Fx-Ax I? Howza bout, the Fx Ax (plain) II, before it got all XL'd out, and certainly not Plus'd... how long before the Fx-Ax III? And the III+, or do we go "XL" and THEN Plus....
For Pete's sakes, we've been listening to Line6 PODs on American Idol for the last 15 years, what really matters here? (Xually, I've been TV-free for a decade), but: the reasons that stuff is often so awful-sounding is because the musicians don't get time to set up for great tone, and/or they don't care (do we?), and/or the biggest cause in MY opinion is because FOX is famous for stepping down their visual and audio signal quality, to even worse than Mp3-grade. And, Line 6 is my least fave of all I've tried, though they've all been dutifully & grimly incestuous and thieving for many years. The "best" Fender FUSE is likely to be better than an older Digitech, the new Digitech is likely to be better than an old POD, Vox, now the companies that make the software for others are selling their own - "Ampli-fire", anyone? It's not just yer dog's modeler, but your dog's flea's modeler, your dog's flea's dustmite's modeler too. I DO understand how this can drive "one" screaming back into the arms of the trusty 1971 50w no-master Marshall in the back of "one's" closet... 
(Even trade it all in for a dobro & go live in the bushes, I DO get that part.... Say, is dat the new improved Bushes Mk II+ ya got there...(?) |
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Bryan Martin
From:
Quebec, Canada
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Posted 8 Oct 2017 6:25 pm
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David,
It was brave to risk your eternal soul, but I think you are safe. The forum should always be a place where members can exchange their viewpoints. Even if they think it may not be that of the majority.
My post was limited to the performance of a single amplifier. Of course, in live or broadcast settings when the variables of PA, sound-men, etc. come into play, this takes the sound of the steel out of the hands of the player. But I am concerned with the sound of one amp. And for argument's sake, we can say that the player plugs into a cabinet that they like. So what I am talking about is a closed system: one amp, one cabinet . And if comparisons were to be made, the cabinet would not be changed. So what the discussion is about it how this amp would sound through a single cabinet, no other variables are involved in this investigation.
I do not gig. I work in recording situations, where the listening environment is accurate, precise and reveals subtle differences in sound quality. I have also done quite a bit of research into design and construction of a lot of equipment: recording, mastering, and amplification.
The best amplifier is a straight wire with gain. Unfortunately, mankind has not yet conquered this obstacle. Instead we have developed many wonderful devices, that all have imperfections. Some pleasing, some not. The history of amplification in the 20th-21st century has been a quest for devices of ever increasing gain and smaller size, with little regard for inherent sound quality.
Each subsequent generation (beginning in the 20's with the triode tube) has been smaller, had more gain, more distortion, less linearity and worse sound quality than the previous one. Which means that greater corrective measures were needed to fix their poor performance. Triode and pentode tubes are magnitudes superior in linearity and have much lower inherent distortion than solid state devices, but lower gain, and larger size requirements. These circuits are also generally much simpler. There are other reasons they sound pleasing to humans, but I'll stop here.
As for as modelling goes, I don't have much experience, that is not to say I have not examined these units, found them lacking (in my opinion), in sound quality compared to tubes amps. That is not to say that others will not find them useful and good sounding. Just not for me.
I plug my 6139 into my Dekley low boy, go to a Fender reverb, then to the amp, then cabinet. Personally, if I need more equipment to sound good, I feel that I had better spend more time practicing, as other effects, etc. are just covering up my bad playing and intonation. But the world would be boring if we all did just that. That is why the guitar world is rich with all kinds of stuff to make different sounds.
Just because someone made a PC board does not mean the person designing it cut no corners, had no constraints, or was striving for sonic nirvana.They were trying to make it inexpensive. And designing a PC board to the levels of high performance audio is really expensive. You need a triple layer board with isolated through holes and a star ground on each board. Few do this because it costs a fortune.
High performance audio in very inconvenient on many levels. And if you are going to design at this level, you need to know where all the limitations and benefits are at every level of circuit and physical design, how to optimize all systems, and then to get it right you need to listen to it during the prototype stage, and at the build stage. The ears are always the tell.
So you need a lot of knowledge, and either a lot of money or a lot of time. Modelling is too complex, has too much circuity to optimize, and by the nature of the physics, does not allow connection between the fingertips and the ear to be as minimal as that of a tube amp.
Keep it real out there, Bryan |
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Steven Paris
From:
Los Angeles
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Posted 10 Oct 2017 12:22 am Re: The Fender 5F6-A Bassman Circuit: A 21st Century Adaptat
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| Bryan Martin wrote: |
This adaptation required a careful choice of output transformer, as the 6V6 vacuum tubes have a much higher anode-to-anode resistance than the 6L6 tubes. The output impedance at the secondary of the output transformer for a pair of 6V6 tubes is approximately twice that of a pair of 6L6s. |
Just how did you determine that?
_________________
Emmons & Peavey |
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Jack Hanson
From:
San Luis Valley, USA
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Posted 10 Oct 2017 4:08 am
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This has been an interesting thread.
Best sounding home builds I ever played were a 5E3 and an 18W where the builder installed a switch to eliminate the tone stacks. Bypassing the tone control(s) really made 'em sing. |
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Bryan Martin
From:
Quebec, Canada
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Posted 12 Oct 2017 3:49 pm
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Steven,
I believe you are referring to how I determined the output impedance of the pairs of power tubes. The easiest way is to consult the tube manuals or tube data sheets. They will list the essential operating point for push pull class AB pair of tubes. This will tell you the impedance of the tubes. Then you look at the output transformer data. Most manufacturers will suggest which tube pairs are suited for the transformer along with a wattage rating. The impedance transformation of the transformer is the square of the winding ratio of the transformer. For example a 4:1 winding ratio transforms the impedance by 16. So if a pair of power tubes had an impedance ratio of 64 ohms the impedance at the other side would be 4 ohms. You can easily do the math if you know the impedance for each pair of tubes and the winding ratio.
If you know all the operating points of a circuit you can more accurately predict all the impedance parameters. But the data sheets and transformer specs are really good enough.
The best book I have ever found that explains tube electronics is THE ESSENTIALS OF RADIO by Slurzberg and Osterheld. 1947, I think. If you are a builder just buy it. Search Abebooks. The shipping will be more than the book.
I hope I answered this question. If I have not, get a little more specific and I will try and get to it.
Cheers, Bryan |
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Bryan Martin
From:
Quebec, Canada
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Posted 12 Oct 2017 4:21 pm
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Jack,
I agree with your observation on bypassing the tone stack. There are two ways the the sound is effected:
1. The tone stack is a passive equalization circuit, and you lose about 16dB, or half the gain of the first gain stage, when it is there. See the pic below. It shows all the gain and loss in the stages of the circuit used in most Fender amps. So bypassing the tone stack gives you a lot more overall gain in the amp, and a flatter frequency response (pic 2).
2. For my taste, the curve imparted by the tone circuit is a bit too extreme for me in pedal steel. You cannot get a flat response if the tone circuit is there. It is always doing something. The second picture shows the output frequency curves of a Fender Vibrolux set at 10, 5, and 0. The input signal is in blue, so you can see the tone stack is always doing something.
Cheers, Bryan
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Bryan Martin
From:
Quebec, Canada
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Posted 13 Oct 2017 12:20 pm Output tube impedance
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Steven,
On the output impedance determination. If you go to the my first post and download the paper, it is explained in section 3, and specifically 3.2. But I will also post some of it here. I measured the impedances directly and detail the setup here.
The output impedances were measured with the setups shown below.
The output impedance (Zout) was measured with the “On/Off Methodâ€. The voltage is first noted with the switch open, and when closed.
Eqn. 1: Zout = (ZL * (Vopen – Vclosed))/Vclosed
--where ZL is the dummy load resistor. As predicted, the Zout of the 6L6 pair is ~½ that of the 6V6 pair.
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