New Circuit Board and Chassis
Douglas as Guitars and Amps
Sep|28|2009
I began work on a new amp last month and it is almost to the first power-up. Here are some shots of it taking shape. It is a refinement of the Ga40 circuit I build last summer. I changed the preamp only using the tremolo channel this time and removing the RC filter to give it a less anemic tone. The power section is new with a larger OT and 7591 power tubes for that Ampeg/Stones sound. So this amp is very American sounding but not very Fender. I started by drilling the circuit board, and punching the chassis. This was very time consuming, partially because I redid the layout so there were not capacitors on the underside of the board like the Ga40. This was my first totally original layout and I think it will work well. I am using a capcan on this amp and had to make a trip to Harbor Freight to get a cheap chassis punch that was big enough. Now I know why their chassis punch sets are so cheap. Because they ARE so cheap. Cheaply made that is.
Wattage Testing
Douglas as Guitars and Amps
Aug|1|2009
So I was getting used to my new audio generator today by test the output power of my various amp builds. The generator is much nicer than hooking up a laptop and balancing it on top of one’s scope.
I used a 1k sine wave and a 100 watt 8 ohm dummy load with a true RMS multimeter across the load. I also measured the resistance of the dummy load while it was warm after the test. The heat causes the resistance to rise slightly
Here are my findings.
Spitfire build – 12 watts (P/P EL-84, cathode bias, EZ81 recto)
B-18 build – 40 watts (P/P 7027, fixed bias, 5AR4 recto)
Micro AA764 – 2 watts (SE 6AQ5, cathode bias, SS recto)
Now my numbers are pretty conservative because I was looking for the clean output. I didn’t allow the crossover notch to appear in the sine wave when I did my test.
Maximum output was,
Spitfire – 21 watts
B-18 – 105 watts?!
AA764 – 3.5 watts
This was the point where things were horribly distorted and the scope had a grotesque looking square wave. Interestingly, the AA764 produced a beautiful square wave with the right input signal. It really is a high gain amp for your bedroom if you like the smooth overdrive of a 6V6-family tube.
Negative Feedback
Douglas as Guitars and Amps
Jun|22|2009
I added a negative feedback wire to the chocolate champ to increase the headroom. I never took a picture of the insides so there you go. I tidy layout and pretty blue tag board. Some of my best work.
The Chocolate Champ
Douglas as Guitars and Amps
Aug|5|2008
After a frenzy of amp modding and building, two new projects saw the light of day.
The first is my Chocolate Champ which is a variation on the Microchamp by Adam Alpern. It uses a single-ended 6AQ5 as the output tube and a Read the rest of this entry »
The Latest Cab
Douglas as Guitars and Amps
Jul|5|2008
I finished another cab to match the one I made for the GA-40. This one has a 12″ Jensen Neo in place of the 2×10. The baffle was a tight fit as you can see from the grill cloth rippling. It sounds good, Read the rest of this entry »
Ga-40 Clippage
Douglas as Guitars and Amps
Jun|16|2008
Here is a clip montage of different settings on the GA-40 amp. All recorded with my Reverend Roundhouse (LP copy) straight in and miked with an SM-57. No EQ or compression used.
Gibson GA-40
Douglas as Guitars and Amps
Jun|12|2008
So I set out to find what all the fuss was over these amps. They are supposed Tweed Deluxe killers but the circuit seems unlike any fender I’ve seen. Instead of a 12AY7 pre tube on the Deluxe, the GA-40 uses a 5879 pentode, much like an EF86. The tremolo is unlike anything Fender had, using a 6SQ7 as the oscillator.
Fixing the Leslie’s passive crossover
Douglas as Guitars and Amps
Jun|12|2008
I did an overhaul of my leslie amp and thought that replacing the motor capacitor in the crossover would improve the sound as well has saving the upper driver from failure. The problem was the crossover used some very specific values to reach it’s 800hz crossover point. One side of the cap was 7.8uf and the other was 12.5uf. This was a primitive device. Could I restore it using modern parts. Read the rest of this entry »
Leslie pedal part II
Douglas as Guitars and Amps
Jun|12|2008
Sorry for the delay, I’ve been really busy now that summer has arrived. Here are the final shots of the DIY Leslie preamp pedal.
Here are all the parts in. This is before I blew up the transformer by connecting the center-tap while using a bridge rectifier.
DIY Leslie Combo Pedal part 1
Douglas as Guitars and Amps
May|19|2008
In my never-ending quest for new projects. I am building a pedal to run keyboards, guitars and what ever else I can think of into my Leslie speaker. This is a smaller project but involves some new techniques I will be using on my next amp. Above you can see the tube I’m building the preamp around. A 12AU7 that I mounted on a piece of my old Xbox CD reader. Read the rest of this entry »