100w ampeg b-15n (1968 circuit)


after building a successful ampeg b-15n preamp, a customer wanted me to build him a 100 watt version of it. originally, the b-15 was a 20 to 25 watt amp. 6l6 tubes can put out 30 watts each, so four of them would certainly be capable of producing 100 watts. simply doubling the 6l6 count would get me into the 40 to 50 watt territory- to reach 80 to 100 watts i was going to have to implement a fixed, adjustable bias and use a solid state rectifier.

i wanted to build a quality amp that wasn't going to cost $1000. The first issue to tackle was the transformers. b-15 transformers (individually) were in the $250 range, but they certainly wouldn't get me the 100 watts i was looking for. so what other 100 watt amplifiers are out there? fender had some 4 x 6l6 amps that were know to produce 80 watts. i located a schematic for a twin reverb and found that its power transformer was capable of providing the needed power, and at a relatively low cost.

the output transformer was a little tricky. for use as a bass amplifier the output transformer would need to be capable of a wide frequency response. i looked to a reputable source of custom transformers for the answer: edcor. they had a 100 watt model with a 3.3k primary (half of the primary impedance of the original b-15 output transformer) but with a frequency response of 20kz to 20khz. impressive! an order was placed and the transformer arrived 4-6 weeks later at a total cost of around $120. that puts the total cost of the transformers around $250- not bad at all.

mechanical design

the next issue is what i consider the most difficult issue in custom amp building: the chassis. undrilled (blank) chassis are readily available for marshall style amplifiers, but would all the components fit? some quick measurements said that it might be possible, but i put o use my new 3d modeling software to answer this question.

the first thing i needed to do was create the models of all the parts. this takes some time, but makes it much easier to do this in the future since the models can be reused. once i had the models for the tubes, sockets and power and output transformers i populated the chassis model with them and the answer was: yes! everything will fit... barely! (notice the tight clearance between the power tubes and the transformers)

here are some more shots of the model:

notice the tight clearances between the power and standby switches and the transformer. the original 3d model indicated an interference between these components. i had to buy switches with smaller lugs and move the power transformer nearer to the tubes to make it all fit. this and other mistakes would have been terrible to correct if i had already punched the chassis and then discovered the problem.

electrical design

the power supply needed a few changes to get the amp in the 80-100 watt range. a full wave, solid state rectifier was used. a standby switch was added for tube life and convenience, a marshall-style adjustable fix bias was installed, and fuse values were derived from the fender twin reverb.

the board layout was done next. here is what it looked like:

some of the custom features in the amp (as requested by the customer) came from the b-15 preamp that i built as a testbed for this amp- preamp-out/poweramp-in jack, output mute, master volume. another addition was a buffered tuner out jack. however, on this amp the inputs were modified to be a "normal" and -10dB channel. a bright switch was added to the inputs instead of a separate bright channel. output protection circuitry was also added to the amplifier so that it would remain on standby if it was run with a speaker cable plugged into the output jack.

the image below shows most of the features of the amp:


with the mechanical and electrical design nailed down, orders were placed and i handed an undrilled chassis, a pile of parts, drill templates and wiring templates to my co-worker so that he could build his brother an amp. the most difficulty that he had was drilling the octal sockets and doing the cutout for the lay-down style power transformer.

my friend did a great job keeping the wiring neat, escpecially for someone with no previous experience wiring something so complicated. most of the electrical troubles during the construction were to blame on a poor soldering iron. there were some cold solder joints or joint with insufficient solder that were causing intermittents.

here are some pictures of the completed amp:


this amp had some really big issues. the first was hum. 100 ohm resistors across the heater reduced it a bit. after a few weeks of troubleshooting, i located a poor ground path. replacing the ground wire, and an excessively noisy tube fixed the problem.

another difficult problem was the presence of a sputtery type of distortion. this distortion was being introduced at the phase inverter. i had a theory that perhaps one of the components in the
phase inverter was damaged by the malfunctioning soldering iron. i replaced all the components just in case as well as ordering parts for a twin reverb phase inverter.

after replacing the b-15 phase inverter the distortion was still there. i disconnected the negative feedback and the problem was still there. i replaced the phase inverter with the twin reverb phase inverter and now the amp had a beautiful sustained distortion (just what the customer was looking for.

i had read in 'amps: the other half of rock and roll' that ampeg vigorously tested their amps before shipment to ensure that they didn't distort. i think i now know why!

when the amp was finished i ran it over to my friends house and we fired it up (using an SWR LA-15 as a speaker cabinet). it was incredibly versatile! it had a clean, deep bass with beautiful high frequency content. the switches, though some seemed redundant, added some great options to the sound. the bass distortion didn't sound good to me but my friend indicated that his brother loved that sound.

i thought the amp sounded even better as a guitar amplifier, even though it was being tested with a bass speaker. as i stated before the amp is very versatile.


i supplied my friend with some marshall smallbox headcab plans. he designed a custom cabinet, using mdf. it was decorated with bed liner for a spray can and some oxblood grill cloth. i think the amp turned out looking great! (if only i could find a good source for a custom faceplate)


the overall cost of the amp was just over $500. i deem it a great success. it was also the first amplifier that i had made using 3d mechanical design software, something i will definitely be using more in the future.

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