[Michael Wiebusch] found the leftovers of a wrecked vintage tube radio in a pile of electronics junk. Unfortunately, he could not recover any vacuum tubes in it. And to his dismay, it didn’t even have the output transformer, which he figured would have been useful in a guitar amplifier project. The output transformer is not easy to come by nowadays, so he was hoping to at least score that item for his future build. All he could dig out from his dumpster find was a pair of speakers and he ended up building nice Output-Transformer-Less Tube Guitar Amplifier around them.
Valve output stages are generally high-impedance which means they cannot be directly interfaced to low impedance speakers. An impedance matching output transformer is thus used to interface the two. Back in the day when valves were still the mainstay of audio electronics, many cheap amplifier designs would skimp on the output transformer to save cost, and instead use high impedance speakers connected directly to the amplifier output.
[Michael] found a nice reference design of an OTL amplifier for a 620 ohm single speaker. He decided to use the same design but because these speakers were about 300 ohm each, he would have to wire his two speakers in series. At this point, he decided to make his build useful as a proper guitar amplifier by adding a preamplifier stage replicated from another design that he came across. A regular halogen lamp 12V transformer takes care of the heater power supply for all the tubes, and a second, smaller 12V transformer is wired backwards to provide the 300V needed for the plate supply.
The final result is pretty satisfactory, considering that it all started with just a pair of junked speakers. Check out the result in the video after the break.
Continue reading “Dumpster Dive Speaker Results In Tube Amplifier”
[Dano] builds a lot of guitar pedals and amps. He needed a speaker cabinet dedicated to this task in order to be a consistent reference when checking out his electronic creations. He ordered a couple of 10″ guitar speakers…. and they sat around for a while.
Then one day at the craft store, he stumbled on an inexpensive wooden trash can. It had a tapered design and came with a lid. As would any normal person, [Dano] immediately thought these would make a perfect speaker cabinet so he bought two of them.
The trash cans would be used in an upside-down orientation. The intended lid makes for a well fitting bottom of the cabinet. Holes were cut for the speaker and two terminal blocks. Since these cabinets would be used for testing a bunch of different amps, two different terminal blocks were used to permanently have multiple connector types available.
A pair of modern kitchen cabinet handles were used as carrying handles for each of the two cabinets. If a speaker cabinet one speaker tall is cool, a cabinet two speakers tall must be twice as cool. To get there, the two cabinets were bolted together using electrical conduit as an industrial looking spacer. Those brackets bolted to the sides of the bottom cabinet are actually Ikea shelf brackets that [Dano] had bought and never used. The Ikea brackets support casters making for easy moving around the studio.
Overall, [Dano] is happy with how his cabinets sound. They are very unique and interesting at the least. We’d be happy to play some riffs through them!
Check out this
odd different looking guitar practice amp. It looks like a professionally manufactured product but it certainly is not. [Bradley] made it himself, not just a little bit of it either, all of it.
One of the first things you notice is the quilted maple wood grain of the case. There is no veneer here, this started out as a solid maple block. The front radius was shaped and the recesses for the control knobs and input jack were bored out using a forstner bit. The case was sanded smooth and several coats of high gloss tung oil was rubbed on to give the wood a perfect finish. A small piece of grill cloth protects the speaker while adding a little more class to the amp. The bottom of the case is actually a cover for a computer hard drive. A rectangular hole cut in the hard drive cover makes way for a 9 volt battery compartment.
There are two control potentiometers, one for volume and one for gain. Any old knobs wouldn’t do for this project. [Bradley] knurled and turned his own aluminum knobs and they look awesome! The units power is turned on when the guitar cord is plugged in. An LED not only indicates that the power is on but it also gets brighter with the volume input from the guitar. The LED also pulses if two strings are out of tune with each other giving the guitarist an opportunity to tune one of the strings until the LED stops pulsing. When it is time for some private jamming headphones can be plugged into the amp and doing so cuts power to the speaker.
The electronic circuitry is [Bradley’s] design also, but unfortunately he doesn’t share the schematic. I suppose he wants to keep his amp one-of-a-kind.
[Atdiy and Whisker], the team behind [The Tymkrs] YouTube channel, are at it again with a tombstone guitar amp project.(YouTube playlist link) Their amp began life as a Philco Tombstone radio which had seen better days. By the time [Tymkrs] got their hands on it, it was just a shell of its former self, as someone had already stripped all the electronics.
The amplifier itself is a disused Leslie tube amp [Tymkrs] had on hand. An LM386 serves as the pre-amp, making this a hybrid solid and vacuum state machine.
The tombstone speaker is especially interesting. [Tymkrs] went with an electrodynamic field coil speaker. Field coil speakers have no magnets, instead using a high voltage (approx 90V DC) coil to create a magnetic field for the voice coil to push against. This sort of speaker was commonplace in the 1930’s, as large magnets couldn’t be made lightweight enough to be used in a speaker. As magnet technology improved, permanent magnets became a staple in speakers.
[Tymkrs] paid special attention to the finish of the amplifier. They brought the tired old radio back to a high shine, then added a Metropolis inspired overlay from aged copper-clad board. The result is an amp that looks great and sounds great!
Continue reading “[Tymkrs] Tombstone Guitar Amplifier”