the reviews i had read of the marshall 1974x had me dying to try one. then, of course, there is always the the net's 18W hype. unfortunately, there are no guitar shops near me that stock this sort of thing regularly. the only thing left to do was to build my own!!
the entire point of this build was to keep costs as low as possible. it is not an exact replica, but it should be at least 90% accurate.
chassis
budget cut number one. why pay $50-$100 for a metal box? but what else is there? I'm not exactly sure where the idea came from but the thought occurred to me- a cakepan! hence this amp was originally called the "cakepan amp." a used cakepan can cost you nothing or maybe $1 at a yardsale or thrift shop, a new one from $3-$7, not to shabby. so is everything gonna fit? let's have a look....
it will fit! one of the nice things about the cakepan is its easy to drill through, sometimes too easy.. the drill bits can grab and deform the metal, so be careful. now that i know everything is going to fit i laid it all out on graph paper, actually engineering paper. i used my paper template to mark the holes and drilled the cakepan. to my surprise the knobs were knocking into each other! lesson learned: engineering paper is not spaced by 1/4" per grid, so don't use it to do layouts. time for a new cakepan....
i should mention that this chassis is set up 'combo style' with the tubes mounted on the side ( or back) of the chassis, opposite the controls.
power supply
budget cut number two. the power transformer can be one of the most expensive parts of an amp. why do they have to be so expensive? what else could i use? i decided i would use two separate transformers to stand in place of one. one transformer will supply the 6.3 volts for the heater and the other will supply the high voltage supply for the plates of the tubes. finding a high voltage transformer is a bit difficult. I could use a step-up transformer to supply 240 volts to the rectifier, but they were a little pricey. so how do i get a sufficient high voltage supply for the plates?
looking at my tube power supply diagrams, i found one labeled 'voltage doubler.' now i can use a simple 120V isolation transformer ( less expensive ) to provide the high voltage supply. of course, now the tube rectifier is out (a voltage double is possible with a tube recitifier.) after rectification i should see about 320 volts, surprisingly close to the voltages coming off of the tube rectifier on the stock circuit.
here's the voltage doubler circuit that i used:
later, i realized that i could have accomplished the same thing with a bridge rectifier. it also provides approximately a rectified dc voltage of 2.8 times the ac voltage applied to it - and you don't need capacitors.
turret board
the turret board layout had to be modified a bit from the classic 1974 layout, or it wouldn't have fit in the cakepan! i had to mount the voltage doubler, filter caps and power resistors on a separate board. here's what it looked like:
initial tests
when i first fired up the amp, i heard a wonderful buzzing noise. this was followed by smoke coming from the isolation transformer. I quickly turned the amp off and rechecked the wiring to the power supply. I had the center tap to the isolation transformers secondary grounded. I disconnected it and it powered up fine.
the amp was also plagued by parasitic oscillation. many attempts were made to reduce this and finally using shielded wire on the inputs fixed it permanently!
i mounted the cakepan in an old stereo speaker cabinet, using a 12" speaker i bought for $1 at a surplus sale. here is a pic of the speaker:
and now its ugly cabinet:
eventually, i decided to convert the chassis to a head style, so that it could be mounted more securely in a cabinet. I also rearranged the layout of the front panel and added a master volume to make it more family friendly. this incarnation of the 1974 became known as the cakepan mkii.
cakepan mkii panel layout
in order to make room for the master volume, the trem and normal channel's single jacks were stacked on top of each other. a nicer looking indicator was also used. by the way, the knobs are all from an old scientific instrument i purchased for $1 at a surplus sale.
cakepan mkii wiring layout
this time around i added a little more space between the power supply board and the turret board. i also gave myself a little more room around the tube sockets and moved the turret board closer to the pots. notice the shielded wiring from the inputs. a pentode/triode switch is also mounted on the rear panel.
mkii chassis from above
the transformers are indeed mounted in an unusual arrangement! since I could mount all the laminations perpendicular to each other, i mounted them as close to perpendicular as possible.
the tone
the amp always sounded a little harsh and thin. in diagnosing this problem i read the B+ off the voltage doubler and found that it was dropping by a very large amount at full volume. i later replaced the isolation power transformer which fixed this problem.
the normal channel didn't sound that great to me, especially at maximum volume. i eventually made an attempt to put the normal channel to marshall 20 watt specs, which improved the sound in my opinion.
the trem channel sounded beautiful. it had a little more bite than the normal channel and a little more brightness to it. the tremelo circuit itself also worked quite well.
overall, the amp was a little too brash for my ears. i had wondered if it was the solidstate rectifier or something else from my build. one day i finally saw an official marshall 1974X at guitar center. it sounded very familiar as i played it. i came to the same conclusion that the 1974 was not the holy grail.... at least not to me. this amp has since been disassembled and its parts are in use in many other amps.
sound clips
the following sound clips were recorded with the damaged power transformer at low master volume settings. i did not record any clips after this problem was fixed. the setup for these files was a Gibson Les Paul Faded DC (P-90's) into the cakepan amp and then into a weber alnico blue dog.
normal channel max bridge pickup
normal channel max neck pickup
normal channel rolled off
trem channel
normal channel maxed (through oaktron speaker)
conclusion
i must first say that using surplus parts had an ill effect on this amp. the values of the pots drifted as the amp was operating. the master volume would work well, until the amp warmed up, then the amp's output would diminish into almost nothing. also the normal channel volume pot dropped from 500K to 250K somehow and needed to be replaced.
( i later discovered that the type of master volume i used was part of this amps problem. the 'crossphase' master volume, as I like to call it, mixes more of the out of phase signals from the phase inverter together as you turn the volume down. This type was used by matchless in the dc30. it did not work well in this amplifier because the phase inverter was not balanced! the phase inverter tube sections have different plate load resistors values and therefore different outputs. as the amp warmed up, the signals would increase and become more unbalanced reducing the volume.)
the use of the cakepan as a chassis was a good way to prototype the amp, but it did have its ill effects also. the thin metal did not offer much protection from mechanical vibration. it also was very easy to bend the metal when drilling when the bits would catch. the chassis also got very hot -especially the combo (mki) style chassis.
overall, the build was a success. i learned a great deal and was able to build a classic guitar amp for less than $140, much better than the marshall reissues price tag of over $2,000!!
29.3.08
marshall 18W model 1974
cheap and easy chassis
a good chassis can be another costly component of an amp build. luckily, sometimes we don't need or want a good chassis! if you are in this situation, the easy solution is to build your amp inside a cakepan.
advantages:
cakepans are cheap ( ~ $3 - $8 new.) they are also easy to machine.
sometimes you can get them for free. they are a good temporary solution for experimental builds that may not last long. they're almost disposable!
disadvantages:
easily bent by catching drill bits. poor dampening of vibrations. difficult to mount in a cabinet. get hotter than standard chassis. non-stick coating needs to be sanded off of grounding points
here are a few pictures of my "cakepan amps"
making a turret staking tool
pictured above is the turret staking tool i have used for my past 3 builds. it has made staking turret boards considerably easier and cost far less then $10
using the staking tool
1. layout and drill the appropriate holes in your turret board
2. place the hole intended for the turret directly above the anvil ( pointy part) of the staking tool. ( do this visually, until you can see the anvil through the hole )
3. hold the board firmly in place and insert the turret into the hole
4. place the receiving end of the staking tool over the turret
5. give the staking tool about 4 good solid taps with a hammer
6. turn the turret board over and ensure the bottom of the turret has been sufficiently flared and is seated tight against the turret board
making the tool
supplies
the first item you will need is the actual turret staking anvil and base. hoffman sells a good one for $5. this page mostly applies to this tool, but may be adapted to others.
the next item you will need is a scrap of wood. i used part of an old 2x4. it is not quite thick enough but it works for now.
then you will need some thin foam with adhesive backing. this comes in quite handy
for many things around the house. you can find it anywhere that sells craft supplies.
you will also need the following tools:
drill, drill bits, hammer, blade screwdriver or small chisel
construction
1. drill a hole through the wood base large enough to accommodate shaft of the turret tool.
2. thread a nut onto the shaft of the turret tool and insert the assembly into the hole
3. give the shaft of the turret tool a good solid hit. you should see an indentation made by the nut
4. using a small chisel and hammer ( or just a blade screwdriver ) remove the
wood on the inside of the indentation, until the nut itself will fit
inside it and flush against the wood.
5. repeat step 2 thru 4 on the other side of the wood base
6. thread the anvil part of the turret tool through top and bottom nuts and into the base.
7. place the sticky-back foam on the top and bottom faces of the wood base
this will give you a smooth sutface that will give a little when you stake the turrets
8. adjust the the height of the anvil base by threading it using the
screwdriver slot on the end opposite the anvil end
9. start staking!!
punching large holes in a chassis
doing the metal work on your own amp is one of the most difficult parts of building an amp from scratch! unfortunately, the tools to make the large holes in your chassis can also be expensive. here are just a few options.
chassis punches
these are what you really want, if you can afford them! they are a great investment but can be very expensive. a chassis punch mostly consists of a bolt a cutter and a nut. you simply drill a hole to accommodate the bolt, place the bolt through the hole, attach the cutter and nut, then tighten the bolt. the cutter cuts through the metal as the bolt is tightened. you get a very nice looking hole! try to find a standard size to accommodate your most used 8 and 9 pin sockets.
drill bits
you can purchase drill bits for large holes, but this is probably not your best option. the bits can be expensive and they will need to be sharpened quite often. luckily they can be sharpened quite easily.
stepped drill bits
these drill bits are like 5+ bits in one. these also can be expensive. one other drawback is that each time you use the bit you will wear down smaller hole sizes. my stepped drill bit is pretty dull. i can only use it on cake pans.
hole saws
i have never tried this method, nor have i ever heard about anyone else trying it, but i'm sure it can be done! you may get only a few holes per bit, however
spade bits
these bits are made for boring large holes in wood. i had heard of others using them on thin aluminum chassis (like the hammond chassis.) i have tested it out on cakepans and hammond cassis and it indeed worked. This could be a very cost effective option since these bits are almost disposable at their low cost. i wouldn't try this on a thicker chassis however, as you bit probably won't last long. so far on smaller chassis i have got about 7 holes drilled from one $3 bit.
general punching tips
when ever you are cutting metal, use a lubricant! it will cut down heat, make the cutting process easier and extend the life of your tools! use wd40 or even 3-in-one oil if it is all you have. I have not been able to source a proper cutting oil yet. Please comment if you know where to get some.
cut surfaces are sharp and can have burs and barbs on them! use a deburring tool of roundish file to disarm your chassis holes. these buts really hurt!
what about holes that aren't round?
great question! often a feat that requires some ingenuity. square chassis punches do exist put run in the hundreds of dollars. here are some other options:
drill bits and saw
drill out the corners and saw the length using a metal saw such as a hacksaw or jigsaw
router
many people make a template for their hole, then drill a hole to accommodate a router bit, pop in the router and cut away the material they don't need. i have never tried this, but it sure sounds fun!
nibbler
these little tools "nibble" away material a piece at a time. slow and steady does the job. it would probably be a little more efficient to drill a hole to remove most of the material and then nibble away the rest. i use a nibbler to make mounting holes for EIC sockets. I start with a 3/4" hole, then nibble away what's left. not bad tool to have around.
non-standard transformer mounting
transformers are some of the most costly components in building an amplifier, and a good place to shave off some costs from an amp build. in reality, transformers are simply comprised of windings ratios and power ratings! often electrical equivelent transformers are available for half the cost of "replacement" transformers, but sometimes they don't fit mechanically. if this is your situation, here are some options:
mounting a standup ( x mounting) transformer in a laydown ( z mounting ) chassis
i was faced with this situation when putting my marshall build into a new chassis. how would I mount my stand up transformer in there? after a trip to the hardware store the answer became clear - at a cost of less than $1!
1. at your hardware or electrical supply store find a cover for a 4" square electrical box like this
2. place the cover over the chassis transformer mounting hole
3. mark where holes need to be drilled and then mount the cover using appropriate hardware.
4. place the standup transformer on the newly mounted cover plate. mark where wholes need to be drilled and mount the transformer to the cover plate. don't forget that you'll need to drill holes for the
wires to come through!
here is how it looks on my marshall 1987 build
mounting a laydown ( z mounting ) transformer on a standup ( x mounting ) chassis
for my Vox build, i had a friend of a friend roll me a chassis to fit an existing cabinet. the cheapest transformer i could find was a laydown type and i didn't have the resources to cut a huge hole in the chassis. here is the solution:
1. transfer the mounting pattern to the chassis, including the holes for routing the wires through the chassis. drill or cut the appropriate holes.
2. measure the length between the where the mounting bolt starts to leave the transformer and where the wires start to leave the hole in the end bell. add about 1/2" to this measurement and find the closest length (go for a bit longer)
of threaded standoffs in the appropriate threading and install screw them onto the mounting bolts.
3. now thread 3/8" -1/2" mounting bolts of the same threading through the chassis and into the threaded standoffs.if the mounting bolts are longer than the end bells you may be in luck and can mount the transformer as if it was a standup type.
here is how my vox build turned out:
circuit board building materials
garolite materials (such as FR4/G10)
there are many choices when it comes to board building material. the classic choice is FR4/G-10 also known as epoxyglass, among other names. one of the names for this material is garolite. FR-4/G-10 is simply one grade of garolite. the following information on garolite grades was taken from mcmaster.com:
there are two main attributes we want with this material: electrical insulator and flame retardant.
the following grades of garolite meet both of these specifications
XX
LE
CE
G-9
G-10/FR4
here is a list of properties for each material:
grade CE garolite: compared to grade XX garolite, grade CE offers higher impact strength. it's a cotton-cloth laminate with a phenolic resin binder.
grade G-9 garolite: this woven-glass fabric laminate has a melamine resin binder for superior strength. it retains its shape and size, plus is good for use in wet conditions. rated for flammability.
grade G-10/FR4 garolite: a glass-cloth laminate with epoxy resin binder, this material is the flame-retardant version of standard G-10 garolite. it offers excellent strength and low water absorption. rated for flammability.
grade LE garolite: similar to grade XX and CE garolite, grade LE is lightweight and strong. it offers lower water absorption than Grade CE so it can hold its shape better for tighter tolerances. it's a fine weave cotton fabric with a phenolic resin binder.
grade XX garolite: like all garolite, grade XX is lighter than metals but dense and strong. it's a paper-based laminate with a phenolic resin binder that absorbs less moisture than grade CE or LE Garolite. good for use as a template board or for gasketing and gears.
garolite is often available in convienent 3" wide pieces for those who don't have the equipment to cut large sheets. it is also available in many colors.
GP03 ( red swirl )
this has become a favorite in the marshall building community. it is an electrical grade fiberglass and it often referred to as "red swirl" material. this is also available in 3" wide pieces and is only available in red. there is no mention of its flammability rating. if you choose to use this kind of board, protect your self with some rubber gloves, etc. as the fiberglass causes slivers easily and they don't feel good and are difficult to get rid of.
fiber board (vulcanized fiber)
this is the kind of material used in the old fender amps. It is very thin and flexible and is usually fitted with eyelets rather than turrets. It is usually mounted to the chassis with a same-sized sheet of fish paper between the eyelet board and the chassis.
choosing mounting hardware
eyelets
eyelets are are a simple and very cost effective way to make connections on your circuit board. they mount nearly flush with the surface and are hollowed out. simply place all the connections in the hole and flow in some solder. buy these in bulk (100+) to bring their cost down to pennies each, if possible.
turrets
turrets are available in many varieties. a basic turret mounts firmly into circuit board material and has a post for mounting components and wires to. sometimes this post is hollowed out and sometimes it is not. some turrets have vertical slots, some have double or even triple stacked posts. turrets provide a lot of mounting space and make it easier to unsolder components. buy these in bulk (100+), if possible, to bring down their cost. you'll be glad you have some extras around.
laying out a turret board
there is much information about designing layouts to realize your circuit, but i'll let the other sites cover that topic for now.
this page will focus on how to transfer an existing layout onto your own turret board.
supplies needed
circuit board material, turrets or eyelets, staking tool, drill, hammer (or drill press )
pencil, marker, architects triangular scale, saw and miter box (if board is not to proper length)
graphics program (optional)
steps:
1. count the number of turrets needed to complete your circuit board. make sure you have enough on hand
2. decide how much spacing you want between the top and bottom rows of turrets. often this is determined by the size of your components. some resistors will not span a 3" board, so choose a general spacing that will accommodate your parts. you can always increase of decrease this spacing for individual components, if necessary.
3. draw top and bottom guidelines across the length of your board for the top, bottom and center rows of turrets with a pencil. this will be easier by using your triangular scale. mark the center line first, then using the 1/2 scale ruler measure and mark your vertical turret spacing. do this for the top and bottom rows and draw your guidelines along the board for the top and bottom rows.
4. now choose a horizontal spacing for the turrets. this is usually 3/8" - in this case you will use the 3/8 scale ruler. place the zero mark at the left edge of the board and make tick marks with a pencil at each "inch." no math needed! just count across the amount of turrets needed for the top row on the ruler marking a few extra
just in case you miscounted the first time.
4b. (optional) if you have a diagram or picture of a layout available for your circuit, import it into a graphics program on your computer. flip the image vertically and then print it out. you will use this modified version of the circuit in the next step. the benefit of doing this is that you will be marking your layout on the back of the turret board! this will leave all your messy guidelines and mistakes were no one will notice them!
5. carefully transfer the layout to the turret board using the guidelines and tick marks. using your marker, place a dot over the tick mark where a turret will be placed, place an dash over the tick mark where there is a space in the layout and place an x over the tick mark where mounting holes will be drilled. carefully marking each tick mark will help eliminate errors. include all mounting holes!
6. double check, even triple check the layout on your turret board with your layout template. you'll want to correct any errors before you start drilling!
6b. mark the right edge of the board, then place it in the miter box and cut to length with saw (if needed )
7. now that your layout is verified it's time to start drilling! using the the correct size drill bit for your turrets, drill out all of the dotted marks on your turret board.
8. using the correct size drill bit for you mounting hardware, drill out all of the x marks on your turret board.
8b. some like to drill holes to feed wires through the turret board. if your layout did not accomodate this, mark the places you will need to bring wires through the board with X's and then drill them out using the proper drill size for your wire.
9. if you skipped step 4b, clean the board up using a large eraser, get rid of all of those pencil marks! if you completed step 4b, simply flip the board over!
10. mount turrets using staking tool and hammer (or drill press )- looks good doesn't it?
tube amp calculator
Set Signal Generator for 1KHz at about 1VAC
Generator Output:VAC
Volts across secondary: VAC
Secondary Tap Value / Speaker ImpedenceOhms
Turns Ratio
Primary Impedence Ohms
Bleeder Resistor Calcs
Supply Voltage: Volts DCCapacitor value: microFarads
Resistor value: in Kilo Ohms
| Time | Volts | Watts |
|---|---|---|
| 100ms | ||
| 500ms | ||
| 1 Sec | ||
| 2 Sec | ||
| 5 Sec | ||
| 10 Sec | ||
| 15 Sec | ||
| 20 Sec | ||
| 30 Sec |
welcome
welcome to the enginerd diy tube amp blog. the information on this blog is to share some of my tube amp project experiences with others. this would especially come in handy for those who are interested in building their tube amp projects on a budget. this blog will serve as a 'temporary' site until I get my own website up and running. enjoy!
-------------------------------------------
This stuff was added to host these images after my original hosting site went down
