Be sure to check out "Attack of the Clones - The Sequel", the latest amp (or should I say amps) project. Also be sure to visit the Glaswerks Forums for questions and answers on the amps.
The clone project came about as I am a very big fan of Robben Ford. I have always been impressed by his overall sound. Now I know that Robben could get a great sound from a Teisco Del Rey guitar and a Kalamazoo amp, but in the real world he uses the famous Dumble amps that have been hand tweaked by Alexander. This project was inspired by the Dumble amps. If you have any suggestions, drop me a note at firstname.lastname@example.org
Amp Sound Clips
Design Specs for the Project
Click on any picture to enlarge.
April 2, 2005
Had a little time and put together of the amp after installing the post OD tone stack. I think I really like the change.
Here is the link
March 27, 2005
I have been revisiting this amp quite a bit since I finished up the last three amps.. I still was never really happy with the response of the amp. Even after extensive work it sounded like a blanket was guarding the high frequencies. So I rewired the amp in the same fashion as I built the last few.
What this means in essence is that I removed ALL of the shielded cable except for three pieces at critical spots. This seriously opened the amp up as far as tone is concerned.
The second area that I worked on was the overdrive section. After much debate I added a small internal tone stack after the over drive stage. WOW what a difference in sound. No more buzz and fuzz on the notes. The bottom end articulates instead of being boomy. Adds a little compression and a LOT of blooming.
Here is a picture of the tone stack ready to install in the amp.
I also reworked the clean tone stack a little. Basically dropping the mid control range to cut some of the lows. This was a big help, especially with humbucking pickups. My tele's still sound very nice as well.
Hopefully I will get a few clips uploaded in the next few days that show the changes.
December 20, 2004
I thought I would post the response graphs that I measured when I was testing the coax versus shielded cable. The first graph below shows the response with the Belden cable that I had originally used in the building of the amp.
Note the drop off above 4khz.
The next graph shows the response after replacing the shielded cable from the input jack to the first preamp stage and replacing the shielded cable from the Gain (Volume) to the tone recovery amp (2nd stage).
Note the shift in peak frequency and the extended high end response.
December 5, 2004
In search of the ultimate tone.... the never ending quest of guitar players and builders everywhere. Well in my search with this amp it was discovered that sometimes "better" isn't always best.
Since this amp was originally completed, I have been searching for that high end "sparkle" that I hear in my '66 Super reverb.
Well I found it yesterday lurking inside of the fancy Belden foil shielded cable that I had used when I built this amp. The bottom line was that amp showed a peak in the response at about 2khz (as measured at the output of the tone recovery amp) and dropped like a ROCK from that point. A spectrum analyzer was used for the measurements.
Replacing the Beldon cable with RG174 coax gave an immediate improvement in amp performance. Why, pretty simple really. The Belden had an extremely large effective capacitance per foot, in the order of 62pf as I measured it here. The RG174/U is a little less than half of that value at 28pf / ft.
Given the improvement, ALL of the Beldon cable was removed and replaced with RG174/U. One step closer to tone MECCA! The response of the amp was extended to beyond 16khz and only down about -2db at 20khz. The Sparkle Factor has now been found!
The next amps I build will be designed to eliminate the need for shielded cable. No matter how you slice, shielded cable is a major league "SUCKER OF TONE".
November 29, 2004 -
New clip added
November 15, 2004
The last couple of months have been spent gigging and making slight tweaks to the amp. Here are a few areas of interest.
In the original configuration of this amp, I used a 500pf in series with a .001uf cap for the treble control. After many trials and tests, the treble cap ended as a single 300pf. This seemed to give the best response with the EVM speakers.
The Mid control started out as a .047uf to a 250K pot. This seemed to have too low of an operating point. The current value for this cap is .022uf. I like the operation and sound with this combination much better.
The bright cap started out as a 250pf across the Volume control, but this was "over the top". This was lowed to a 120pf. Gives a nice sparkle on the top end.
The reverb has went through several changes. Originally I used a small tank in the amp. It now has a 6 spring long tank mounted vertically against the front panel. This required a little reworking of the reverb tank to get the reverb channel floating correctly.
The original configuration had a voltage divider to the reverb tank drive tube. The current configuration is more fender like, using a 500pf to the grid and a 1Meg resistor from grid to ground. This eliminated a loading problem that I found during testing. In additional this rolls off the bottom end a little and gives a better overall sound for the reverb.
This was an area of much testing and component changes. There is something to the 67:1 ratio for Plate / Cathode resistors. By using a 200K/3K combination on the first OD section and a 120K/1.8K combination on the second OD stage the sound opened up quite a bit. Chords are much less compressed and the amp sings like a bird.
I also reduced the coupling caps between the OD stages from .02 to .01uf and .005uf on the second stage. This made a huge improvement in tone and feel.
Capacitors in General
This amp originally was built with all Mallory 150 polyester caps. As an experiment I replaced all caps with Sprague 715P Orange Drops.
The basic comparison of the tone is that the amp was extremely smooth and rich sounding with the Mallory's. They did seem to roll the high end off a bit. The orange drops extended the top end and the bottom as well. I did notice a slight increase in "graininess" in the sound. Not objectionable, but not as clear or rich as the Mallory's. If I were building a pure jazz amp, the Mallory's would be the choice, for rock and blues, I think the orange drops are a little bit better.
As an experiment, several disk ceramics were tested in the Treble cap position, in all cases these caps sounded grainy and lacked the fidelity and articulation of silver micas.
The phase inverter started out using two 100K plate resistor and a 10K balance pot. This did not allow enough range of adjustment. The final values were settled at 82k/100k with a 20K balance pot. Things now work correctly.
August 26, 2004
I finally got around to recording one clip with the new amp. After listening I can also hear that I need to clean up a few noise problems in the room that I record. When you listen closely there is a slight bzzzzz on some notes. This is room noise, small vibrations in the room from "things" laying around on various shelves.
Since completion I have made one change to the amp. This involved changing the reverb pot to a much larger value. This decreased the signal loss (from 9db to .3db) to the power amp section.
So here is the clip, nothing spectacular. This was recorded using an MCA SP1 condensor mic (very cheap) and Electrovoice ND 457 (very old) into a Behringer DDX3216 digital mixer and recorded on a Mackie SDR2496 Recording. Conversion to MP3 was done in Samplitude. The guitar is a 2003 Les Paul R7 gold top.
Clean Les Paul (just noodling)
As soon as I get the house to myself for a few hours I will crank it up and get a recording of the overdrive sounds.
August 1, 2004
The Clone amp project is now officially complete (outside of a little component tweaking). The speaker cabinet was the last piece of the puzzle to be complete. The cabinet dimensions are 20" wide x 29.5" high x 11.5" deep. The cabinet is constructed of 3/4" Baltic birch plywood. The front baffle is of the "floating" type, and is supported only on the sides. The speakers that I used were two Electrovoice EVM - 12L's OEM's that I picked for $80.00 each (sure beat what they were going for on EBay). The rear cover has an oval port.
For you cabinet builders, there are CHEAP sources of usable materials for covering cabinets. I use a local store that handles vinyl products for auto interiors. Most of these places sell what is known as "tonneau" covering trucks. This is the same as tolex and comes in a variety over colors. I will warn you, it does NOT work with 3M 77 Spray adhesive. I have gotten my best results using Weldwood contact cement. Apply to both pieces and wait 20 minutes and apply (carefully). I use a rolling pin (available at any store that has housewares) for smoothing the covering.
Another item that should be easily obtained is the piping for going around panels (that gold stuff you see if my pictures). I bought 10 yards (yes 30 feet) for $3.00. I think Mojotone has it as well, but the price is $3.00 per foot (I think I going to start selling stuff on this site).
Here are a couple pictures of the completed amp and cabinet.
July 22, 2004
The cabinet for the head is complete. The cabinet is covered in black tolex material. Couple of things that I learned during the cabinet construction. It TOO hot in July in Arizona to be doing these kinds of projects in the garage. And the second is don't use 3M 77 on this type of material, it does not stick. I used Weldwood contact cement on the majority of the covering and it worked fine. On the back cover, I was in a hurry and used spray adhesive, a big mistake. It never setup correctly and I ending up making a second panel and redoing with Weldwood.
Hopefully this coming weekend I can get some time to fire up the recording equipment and post a few clips. The amp will probably come back out of the cabinet to make a few tweaks at the same time.
Here is a picture of the amp in the cabinet. The cabinet dimensions are 18.5" Wide * 9.5" High * 11.5" Deep. It looks so tiny.
Now it is on to the matching speaker cabinet. I bought two Electrovoice OEM 12L's for a nice 2x12 cabinet.
July 17, 2004
Finally got to complete the footswitch for the amp and run it through its initial testing. The footswitch shell used is a modified Marshall footswitch that I picked up at Antique Electronic Supply. Well to put it mildly, the amp sounds GREAT!!. I decided to use a short reverb tank on the amp (anyone need a long one?), the one used was an Accutronics 8AB2A1B, this really works well and will fit better in the cabinet. Overall reverb effect is lush without sounding "over the top". I will be posting a few sound clips in the next day or so.....
Here is a picture of the completed footswitch. Click to enlarge.
Here is a link to the schematic of the footswitch (a little different).
July 14, 2004
Eureka!! The first steps in construction are complete. The wiring for the clone project is complete along with some preliminary testing. The best part, so far there have been no wiring mistakes. The only bad part is that my youngest son is starting to take an interest in this amp, kind of like he thinks that he will add it to his arsenal, what a minute buddy this is Dads toy (he already took over my VT-50 and I never get to use it anymore). Here are the testing results.
Power Supply Voltages
|B+ Supply to Output Tubes||
|Bias Supply Range||-20 to -85 Vdc|
These values are pretty close to what I expected. I did see a little lower plate voltage on the first preamp stage than what I expected and will have to investigate this as I "tweak" the amp.
During the initial testing I set the idle current to the output tubes to 30mA each. I measured the power output at 16, 8 and 4 ohms. This was measured into a dummy load that was built a couple of years ago. Unfortunately I didn't write all of the values down, but will update this section when I take measurements. Here are the results at 16ohms, the ones that I did write down. The signal was measured on a scope, the signal was a 2khz sine wave fed into the input jack of the amp. Measurements were made at the maximum "clean" output of the amp.
|Impedance||Measured Voltage (RMS)||Power Output (Calculated)|
The best part the scope show absolutely zero parasitic oscillations.
The final amp construction consisted of doing the final wiring of the amp. I did not like the initial wiring of the controls, so I started over and very happy with the results. The buss bar used is a piece of 3/32" brass rod that I picked up at a local hardware store. Brass is extremely easy to solder and a pretty good conductor. Here is pic of the final wiring on the controls. Click on the picture for a better view.
Below is the complete bottom view of the amp. The worst part of any amp that I have built is the filament wiring. Not sure why, but this is the area where I always have problems. The filaments are lifted about 1.5" above the tube sockets.
The final picture in this installment is that top of the chassis with all tubes installed. The small transformer next to the output tubes is the relay supply transformer. This was obtained at (gulp) Radio Shack, but appears to do a pretty good job. Tubes for testing are Ei 12AX7 for preamp, overdrive and PI. The Reverb driver tube is a JJ 12AT7. The output tubes are Winded "C" / SED.
More progress made in the past few days. All final hardware added to chassis and starting wiring of amp. Below is a series of pictures of the progress. For newest to oldest. I actually got quite a bit further with the wiring, but forgot to take a final picture when I wrapped it up last night.
In the above photo all boards and relays have been mounted. The relays (from left to right) are Effects switching, Boost, Overdrive. The white wire that is "floating" in space from the back panel is the negative feedback wire, it was placed on the 4 ohm tap of the output transformer. The power and output transformers are attached with 1/4-20 hardware, they are not going anywhere. The heavy shielded cable is made by Beldon and has a full foil wrap.
The picture above was taken after the power supply was completed. An initial set of voltage measurements were taken. The no load B+ was 485 volts, bias supply measured -85 volts (to trimmers) and the relay supply measured at 18 volts. If you look closely at the bottom right of the picture you can see the bias board mounted to the back of the chassis. The two radial caps were added to provide additional filtering as well place the bottom of the grid input resistors at AC ground. So far as good (I HOPE!!)
The picture above was taken after completion of the tone stack wiring. The more I look at this, the less that I like it, and it will get rewired tonight (July 12).
The "clone" project continues, albeit a little slower over the holiday weekend due to playing Friday, Saturday and Sunday night.
As I was installing components in the chassis the relay power supply board stuck out as a potential problem area and it needed to be relocated. In addition it was found that filter board had been built mirror imaged, this placed 700 VAC near the presence and reverb control, not good. So to correct all of the potential problems a new filter board layout was designed and built. The new filter board adds the relay power supply section, bias supply and main supplies. Each filter cap will be returned to ground via a separate wire.
The relay supply now will use a chassis mounted transformer with 25.2 VAC output into a full wave rectifier with with 470uf of filtering. This should maintain a constant 16 volts or so with a full load current of approximately 180ma (all relays engaged).
Here is a picture of the updated filter board.
A lot of progress made in the past few days here is a recap of those activities. The bias board, relay power supply and initial chassis work is now complete. It is about to the point where it is time to solder the bits and pieces together.
I used a Hammond 17x10x3 inch chassis for this project. If I build another of these amps, I will probably have a chassis made at a local machine shop. The 10" is a little deep, 8.5 to 9 would have been ideal.
The layout for the chassis was done in Corel Draw. To transfer the layout to the chassis, I used the same technique as on the main board. This technique uses Avery full page label paper. I did leave the protective plastic on the chassis and applied the label paper to the plastic. This eliminates the mess of trying to remove the label after drilling. If you have a chassis and want to try this idea and the chassis is out of the plastic, then cover it with aluminum foil before applying the label paper.
The front and rear panels were also designed in Corel Draw and printed out on 8.3 and 23.4 inch Epson Panoramic paper. After the panels were printed they were sealed with a photographic lacquer (available at all "real" camera shops). These were affixed to the chassis using 3M 77 spray adhesive, the chassis was masked off prior to spraying the adhesive. This technique works pretty well and actually looks pretty good, but I think that for my next project I will have the panels made at a local trophy shop on .040 material. I use the front and rear panels as the actual drilling template, so you have to be extremely careful and not scratch the panel surface. Here are the pictures of the completed front and rear chassis. Now to start the first stages of wiring. I will wire most of the front and rear panels before mounting the transformers and choke. It won't be long now, I am starting to "taste it".......
Front of Chassis
Rear of Chassis
Relay Power Supply
This board was originally built on vector board, but it looked really tacky (reminds me of a song, "Trashy Women"). So I rebuilt the board on a small piece of the G10/FR4 material. The power supply puts out about 20 volts with no load (except bleeder), so I will have to drop the voltage down a bit. The voltage drop will be accomplished by using a few LED's in a series/parallel setup (they are cheap). Each LED will drop about 2.5 volts and will handle about 30ma of current. These items will be built into the footswitch.
When I had originally started this project I was going to use a simply vector style board for the bias adjustments. But after thinking about it seemed a little tacky. I decided to make a small PCB for this section of the amp. Radio Shack (hey they are close) has a simply PCB kit that includes a copy G10 boards, etch, resist pen (Sharpie) and solvent. The entire board take about 90 minutes to build from start to finish. This board measures 2 x1.25 inches and will mount above the bias test points on the rear of the chassis. Note the separate pots for each output tube. Based on my calculations, I should end up with a bias range of -20 to -82 volts.
Worked on loading the parts into the main board. Of course found a mistake in the eyelet layout that had to be corrected. The board is loaded with Mallory M150's for all caps, all resistor (except one) are metal film.
Here is a picture of the completed main board ready for installation.
The relay boards are complete as well (there are three). The relay are 12v DC and have a de-spiking diode (IN4007) and .01uf cap across the coil. The relays are from Radio Shack, but I did some research on the net and found that they are built by Tyco electronics in Japan and seem to be commonly available. Only time will tell how they will hold up. Here is the final relay board after loading parts.
Now it is on to the chassis.
Had some time in the past few days to work on getting the main board redesigned for the eyelets. I also added the Phase inverter trimmer pot to the layout. Have wanted to try this in an amplifier for a long time.
Here is a picture of the main board after drilling and installing the eyelets. The eyelets were obtained at a local scrapbook supply store. The garolite was purchased via McMaster-Carr. (McMaster-Carr has got it all, garolite, hardware, standoffs and can be delivered to you door in a day or two. Beats driving around in the hot AZ sun.)
Here is a picture of one of the relay boards before loading the eyelets.
The relay board measures 1.25" by 2". The relay used is a radio shack part. If if does not work out to be serviceable I will find an alternate source.
The board layouts were drawn in Corel Draw and then printed on Avery full sheet label maker paper. This allow me to "lightly" apply the label to the garolite and then be able to easily (most of the time) remove the label. Here is the Main Board before drilling
Here is the relay board before drilling
The power supply rectifier and filter board are complete at this point.
The filter board was built using my "old" construction method which consists of #16 Brass escutcheon pins that are 5/8 in length (before trimming). This board will probably be replaced at some point in time, but for initial testing will be fine. This board contains both the main supplies and the bias supply. For this amp a Marshall 50 watt replacement transformer was used and the bias must be derived from one half of the high voltage secondary, All caps (except bias) are Sprague Atoms.
I did a lot of research on the net to see what information was available about the Dumble and for the most part it is pretty sketchy. There are a few schematics available for older Dumble amps and a few hybrids, none had everything that I want in the amp.