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A Mini-Bassman ? (Amp Build)

 
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Gorecki
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Joined: 24 Feb 2007
Posts: 238
Location: Glenwood, MD

PostPosted: Tue May 15, 2007 1:58 am    Post subject: A Mini-Bassman ? (Amp Build) Reply with quote

Hey All,

I got the schematic for the following amp build from a friend and also said I could post it here.

Thought some might find it interesting. Wink
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Gorecki
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Joined: 24 Feb 2007
Posts: 238
Location: Glenwood, MD

PostPosted: Tue May 15, 2007 1:59 am    Post subject: Reply with quote

A Mini-Bassman ?

Well, pretty darned close ... here's the result of a month of my labor, perched atop a real Bassman, front and rear views.

The "Mini" is about 40% the size of the original and 60% of the weight. It produces 48 watts output power at 5% distortion compared to the Bassman which produces 50 watts output power at 5% distortion.

Oh yes, it is indeed powered by vacuum tubes !

I'll write more about this little gem now that I know how to get photographs attached to my posts.

cheers,
randyc





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Gorecki
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PostPosted: Tue May 15, 2007 2:00 am    Post subject: Reply with quote

Why Did I Build It ?

About six weeks ago, my little brother asked me for amplifier recommendations - he wanted to replace a small Fender (solid state) amplifier with which he was dissatisfied. He provided me with a budget number and I gave him a short list of candidates.

After thinking about it for a while, I e-mailed him and told him not to buy an amplifier, that I would build him one. My motives were multiple:

1. He had a birthday coming up.
2. I felt that, ignoring my time, I could design/build him a high performance, high quality amp for a couple hundred dollars.
3. I was in between projects.
4. I wanted to try a couple of ideas that have occurred to me over the years, a medium power amplifier would be a good test vehicle.

OK, so we agreed that I'd build him a custom amp, the next step was determining the specifications.

I have always loved Fender Bassman amplifiers, they have an excellent sound, good reliability, simple design and adequate power to accomodate most jazz venues. (They are no-frills amplifiers, less things to break, maybe that's why there are still so many of them in service.)

But I felt that there was a definite gap in tube amplifier availability, from the small combos in the 12 to 20 watt range then a jump to around 50 watts. I thought that an amplifier producing around 35 watts would be more versatile and not too bulky.

I didn't want to just copy an existing circuit, good as existing ones may be, for several reasons. One reason was that most production amplifiers use the same (EXPENSIVE) tube line-up ... and why not, most existing designs are just copies of older designs that are copies of even older ones ... and so on. I wanted a "new" approach, if "new" is the right adjective to apply to a technology that has been obsolete for a half-century.

Here's an example of my point: if I were to replace the tube suite in one channel of my old Bassman, I'd be buying a 12AX7, a 12AT7 and a matched pair of 6L6GC's at a cost of about $45. In comparison, the tubes that I eventually selected for my "mini" cost $20.45.

But designing something different than the original just to be different is not good engineering. Remembering all of the things that I liked about the original Bassman (performance, reliability), I came up with a short list of what I wanted to include in the new design:

1. Same (or equivalent) power transformer as the original.
2. Same (or equivalent) output transformer as the original.
3. Same tone control stack as the original.
4. Same preamp tube as the original

Some of the things that I definitely wanted to change, besides the tubes, from the original design were:

1. The ability to adjust the power level DOWN from fifty watts to something more modest, extending tube life by decreasing current/power dissipation.
2. My brother was uncertain about speaker configuration - a single 12 cabinet would be nice and light but he was leaning toward dual 10's ... so I needed to provide the ability to switch between 4 ohm and 8 ohm output operation.
3. Make the amplifier smaller and lighter.

My brother had his own requests:

1. He asked for a slight overdrive capability, so I included some excess gain in the preamplifier circuits and added an additional level control potentiometer.
2. He requested something "pretty" (not partical board covered with contact paper) made from a readily available hardwood, a wood that he could find easily when he makes the speaker enclosure (note: my brother lives in Maryland and I live in California).

Without detailing the technical and economical decisions that were made during this project, I eventually ended up with a circuit that I thought had a good chance of meeting all of our design goals. Even in this day of computer simulations, however, one can never be entirely sure of final performance until one has built the hardware and powered it up !

One of the first problems that I faced was that my lab power supplies did not have the high voltage capability needed to test individual circuits. I looked on the internet but found that high voltage, high current vacuum tube laboratory supplies just aren't available any more !! There isn't a need.

Detour 1: stop work on the amplifier, design and build two high voltage power supplies. Since I didn't want to spend a lot of money on items that I might not use again, I relied on parts from my junk box to build these supplies. I ended up with one positive supply, +380 volts at 150 mA, completely adjustable and one dual polarity supply (+300 volts at 100 mA and -150 volts at 50 mA, both completely adjustable). (Frankly, building these supplies was much more of an engineering challenge than the guitar amplifier.)

Because I didn't want to subject myself, my wife, my animals and my neighbors to the blast of full-power audio testing, I needed a "dummy load" which is just a high-power resistor network that substitutes for the speaker. I found two 16 ohm 25 watt resistors and wired them in parallel but quickly found that they became WAY too hot during testing.

Detour 2: stop work on the amplifier and figure out a better high-power speaker load. I got an empty 1 quart paint can from the hardware store, drilled a couple of holes in the top of the thing and mounted two insulated binding posts to which I soldered the pair of 25 watt resistors. Curiosity compelled me to drill another small hole in the top and install a tiny rubber grommet, through the center of which, I forced the shaft of a meat thermometer. I filled up the can with cooking oil and lowered the resistors and the thermometer into the can and sealed it. I painted the outside of the can flat black to radiate the heat and my high-power load problem was solved. (As a matter of interest, after several hours duration at 50 watts, the meat thermometer indicated that the interior temperature of the cooking oil exceeded the boiling point of water.)

After building prototype circuits to prove the concept, I was confident enough to start ordering parts, many of which came from Antique Electronic Supply in Arizona. I have no affiliation with these folks but wanted to mention them because they have a good selection of parts and provide prompt delivery service.

What I ended up with as a tube lineup was the following:

preamp 1/2 12AX7
postamp 1/2 12AX7
driver 1/2 6BR8A (pentode)
phase splitter 1/2 6BR8A (triode)
power amplifiers 6JN6 (2)

Once I had all of the parts, I was able to make a detailed layout (AutoCAD) from which the chassis could be machined. After that came assembly, starting from the output and working backwards, performance testing each circuit as I assembled it. Once the entire thing was together and working, small refinements were made until performance was satisfactory.

One of the refinements made apparent during testing the power amplifier stage was that the power level of the amplifier could be changed within reasonable limits by varying the screen grid bias voltage. The addition (on the rear panel) of a control that varies the screen grid bias voltage of the two output tubes provides performance flexibility. The variation is around 40 volts and the result is that the output power level of the amplifier is adjustable between approximately 25 watts and the full power of 48 watts.

Finally came the woodworking part of the project. I selected Ash for the cabinet material, it's strong, machines easily and looks just like oak with a coat of polyurethane on it ... Traditional cabinets require that the joinery be made with finger joints and glue - no nails or screws. So that's the way that I did it, later photos show the details of the joints. A couple of coats of poly and then set the cabinet aside for a day or two.

Finally everything was ready, small improvements had been made to the electronic circuits and the cabinet was drilled for chassis mounting screws and rubber feet. The result is what you see in the initial photographs.

In summary, I'd like to make a cautionary point. One might get the impression from this narrative that it's possible to mate up power transformer, output transformer and output tubes haphazardly and come up with good, working hardware. It's not. Hours of thought, calculation and computer simulation went into the determination and resolution of the various design/performance trade-offs. My task was made somewhat simpler because components of proven performance and reliability were readily available.

I'm going to include a few more photos, showing the various stages of progress of this project. I hope this has been interesting, it sure was a lot of fun for me !

cheers,
randyc
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Gorecki
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Joined: 24 Feb 2007
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Location: Glenwood, MD

PostPosted: Tue May 15, 2007 2:00 am    Post subject: Reply with quote

Machining The Chassis

Here's the blank, powder-coated chassis in the vertical milling machine. It's still covered with protective plastic in order to minimize scratching during the machining process.



After milling the rectangular cut-out to accomodate the power transformer, starting to cut the holes for tube sockets.



The completed chassis before touching up bare metal with black paint.


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Gorecki
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PostPosted: Tue May 15, 2007 2:01 am    Post subject: Reply with quote

Assembling The Chassis

Several photos assembling various parts ...






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Gorecki
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PostPosted: Tue May 15, 2007 2:01 am    Post subject: Reply with quote

Testing And Refining

This is a breadboard that I constructed to evaluate, characterize and test the tubes that I selected for the output power stage. My home-made high voltage power supplies weren't up to the task of driving these tubes at full power so I had to pulse-test them, I drove them at full power for a few milliseconds and then "rested" the power supply for about six times that duration. The net effect is that the power supply thinks that it is working 1/6 as hard as it really is Smile



Here is the entire amplifier, during initial testing. There are voltmeters all over the place on my lab bench because I am carefully monitoring bias currents as a function of input drive levels. At this stage of testing, confidence in the design has not yet been established. In the background, the two home-made power supplies can be seen - they are the only instruments on the workbench with wooden chassis, for obvious reasons. Just behind the amplifier chassis, my home-made paint can high power load is visible.



And although the amplifier still has lots of small refinements left to be made, I just could not resist taking it for a test drive. I "drove" it with the L-4CES, shown leaning against the workbench. The speaker (out of sight in the picture) was an 18 inch JBL in a Fender Showman cabinet. How did it sound ? AWESOME !


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Gorecki
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PostPosted: Tue May 15, 2007 2:02 am    Post subject: Reply with quote

And Now For Some Woodworking

After selecting Western Ash as the cabinet wood, I have ripped and crosscut the material then planed it to thickness. Shown here is the progress at the end of the first day with the finger joints about half completed ...



The following day, I cut the remaining finger joints, routed the handholds and glued the cabinet together, here it is before applying polyurethane.


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PostPosted: Tue May 15, 2007 2:02 am    Post subject: Reply with quote

Afterthoughts

Reviewing this project at completion, there are a couple of things that I would have done differently and one performance parameter with which I am not completely satisfied - the two are related.

The amplifier sounds really good, a full, rich sound through the three very different speaker configurations to which I connected it. BUT there is a nagging little power supply buzz that I was unable to eliminate, despite working on it for several days.

I have concluded that this is a layout issue, the fact that the chassis is considerably smaller than the original Bassman places all of the parts closer to one another. The first preamplifier tube is only two inches away from the power transformer and coupling between them cannot be eliminated unless the entire layout is changed - something I am certainly not going to do at this point.

Several sketches in my lab book, done at night over a glass of wine, depict different layouts that might eliminate the hum. If I ever build another one of these, at least I'll have my documented thoughts regarding the problem on record.

For now, my brother won't care, that buzzing sound is only a nagging problem when one plays quietly, something that my brother has never done in his life Smile

Cheers, I hope that this has been entertaining !
randyc
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Dean



Joined: 19 Apr 2007
Posts: 287
Location: California

PostPosted: Tue Jun 05, 2007 9:39 pm    Post subject: Reply with quote

MAN THAT IS COOL !!!! You sure have the skills ,now all you need is a 48 hour day.Hearing you talk of your brother ,I see myself when it comes to my sons....They are doing very well in the line of work they choose.One is a manager in a bank in Alaska and the other works with the ceo and vise president of Drum Works or better known as DW Drums.As a parent your kids doing better then you did is everything ,and doing something they enjoy to boot.
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Gorecki
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PostPosted: Tue Jun 05, 2007 11:18 pm    Post subject: Reply with quote

Oh Dean, don't think you saw the first message, I didn't do this build. I expect to do one like it (actually I expect to do a stereo rack version) whenever life allows (oh..and the other 4 strats I haven't finished). Crying or Very sad

The guy that did it is Randy and it was such an awesome build and he did such a great job at describing the process, I asked him if I could steal it to post here. Mr. Green

I have all the documentations, schematics...everything for this build so if you want it I can hook you up! Wink

It's funny you mentioned kids...at this point they (mine) are going to be great contributors to the world..I'm so very proud! In a weird way, I'm glad they didn't take an interest in music because it's the only thing left I can do better than them! Laughing my buns off!

Peace!
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Dean



Joined: 19 Apr 2007
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Location: California

PostPosted: Wed Jun 06, 2007 12:15 am    Post subject: Reply with quote

WOOPS ,did I tell you I was old ,half blind,and on meds......yhea my kids can out do me anyday....Love it.I'am starting my venture into jazz.I have all the time ,but after my last injury....I have to over come depression ,and pain in my neck and back ...my shoulders hurt when I play ,too.I set most of the time when I play.I am not complaining ,when you choose hard labor for a living , sometimes you pay for it.But with the life I've had I would not change a thing.
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