We just got a very cool Webcore 4903 outboard amplified speaker in for repair and upgrade and lucky for us the folks at Webcore put a schematic inside the unit to make repairs a snap. this is a simple amp that has very little gain. It uses two el84 tubes and one 12ax7 tube.
I converted it to be used as a guitar amp, by swapping out the RCA input to a switched 1/4 inch jack and I put a 1m resistor to ground to help control oscillation, other than that I just replaced all the leaky caps…. and it sounds great, I played my Takamine acoustic through it and wow it sounded sweet. the only drawback is volume since there is only 1/2 of a 12ax7 for pre-amp gain it was not very loud.
here is the schematic (click to enlarge)
Webcore-4903 Schematic / 4904 (click to enlarge)
inside the Webcore 4904 amplified speaker 2 el84 tubes and 1 12ax7
Exterior front of Webcore 4904 amplified speaker
James DeRosso makes really cool monster sculptures of all types, when he was visiting me one day a few months ago he saw my tube collection and he asked for a few old tubes, well here is the result if you would like to see some of his other monsters or if you want to commission your very own custom tube monster you can check out his website or blog here .
James DeRosso tube monster and a Miniwatt ECL 80
In case you are wondering – the amp is an 18watt el84 / 6bq5 Marshall Plexi clone with a ppmv and the guitar is a 1968 Gibson es335
here is a more normal monster without the tube 🙂
A Tubeless Monster by James DeRosso - awesome
Here are a few photos of my latest guitar project, a black face Princeton reverb. This amp will have a few upgrades from the original; it will have the following added features. Individual bias for each of the power tubes, and test points on the back of the amp to measure the bias current; since I didn’t have a power transformer with a 47v ac bias tap, I did with Jim Marshall did and am taking the b+ and dropping it to 47 volts before the ½ wave rectifier. This will give me the bias current needed to run the vibrato and keep the power tubes happy. The reason I can get away with this is because I have used a very large power transformer, this should reduce any sag in the bias voltage during high volume transients.
I will post a few sound clips when it is done.
Here are a few schematics and layouts incase you want to build your own
Princeton Reverb Schematic with a GZ34 – 5AR4 Rectifier
Princeton Reverb Schematic with a 5U4 Rectifier, this is the amp that I am building since I have quite a few 5U4 tubes laying around.