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The GrampAmp

The GrampAmp Vacuum Tube Amplifier


While visiting my grandparents I found some old tubes in a box which used to belong to my grandfather. Apparently they were part of an old "Radionette" radio, which is now long gone. Having no prior experience with tubes other than a VFD display, I thought it would be pretty cool to recycle the tubes by using them in an audio amplifier. The tubes I found were 5Y3GT, 7C5, 7R7, ECH21 and an unknown tube which was never identified. After some research I found the 5Y3GT is a full-wave rectifier and the 7C5 is a decent beam amplifier. I would have used the 7R7 as a pre-amp, but I later discovered the filament was out of order. Since sockets had been purchased and installed in the loudspeaker cabinet at this point I bought a NOS 7B6 on ebay to replace the 7R7.

Tubes

During my dumpster-diving adventures some summers ago I found a good many speakers, which I still have stashed in a closet. A 4-ohm, brown, 70's speaker caught my eye and was chosen to be the cabinet for this project. When first using tubes I might as well get rid of anything else that's old. The only only real setback remaining was finding the required transformers. The power transformer required a center-tapped 400V output, a 6.3V filament, and 5V filament. On top of that tube amps require impedance matching, which is easiest done with a transformer. That adds up to a lot of expenses for something which was only meant as a novelty. However I had a MOT with a shorted secondary, and a still functioning primary. Not my first choice, MOTs run hot and noisy, but I took it. As for the impedance matching transformer, the datasheet for the 7C5 (output tube) specifies a plate load of 5k ohms. Since I'm using a 4-ohm speaker that's a ratio of 1250:1, and an impedance ratio of sqrt(1250):1 or 35:1. The output power of the 7C5 is 5W, roughly the same as your standard wall-wart. The voltage I planned on using in my amplifier was 250V, close to the 220V mains used here. A transformer with 35:1 ratio would have an output voltage close to 9V peak, so I dug through my junk box and eventually found a cell phone charger with a transformer which seemed about right. That settled the transformer problems! When modifying the MOT I didn't want to cut it apart otherwise the buzzing would make it useless (not that it's quiet otherwise) for audio applications. To be able to wind a 200+200 turn winding on a closed core I had to cut two 5 meter lengths of wire, intertwine them, and wind them on a pencil. After a 5 meter length was transferred to the core I repeated the process, and kept soldering the two lengths together to get two continuous pieces. I hope the picture and some intuition make this clear.

MOT rewinding

With the rare components secured I could start working out a circuit. Basically I had to look at other designs and read about tubes until I understood their basic functioning. With that done I checked out the tube datasheets which provide sample circuits or simply show which voltages need to be applied where. After stitching some of the images together I had my design. :-) I found the tube datasheets at either; Radiomuseum.org, NJ7P Tube Database or The National Valve Museum.

Schematic

The transformer voltages are AC values. Tubes are wonderfully simple to use once you get to know them. The DC voltage is supplied by a dual diode, the 5Y3GT, and filtered by an RC network. Hardly efficient, but I wanted to use all tubes in this amplifier so the near 100V volt drop from the transformer output to B+ (the 250V line) is just something I'll need to put up with. Compare that with the 0.6V drop of a silicon rectifier! The 5Y3GT can only support a limited filter capacitor size, so don't try putting too much before the filter resistor (680R). There's not much that can be said about this amplifier that is unique from any other tube design out there. If parts of the schematic are unclear read about tube amplifiers on the net. This page discusses triode amplifiers (which can be applied to the pentode, 7C5, as well) and this one the output transformer. In the picture below you can see i even used ancient resistors and capacitors. True vintage sound! I'll admit there's a lot of humming, but remember that nothing has been optimized for fidelity, and there are plenty of things I could have done differently to attenuate humming. Using modern silicon diodes to provide 250V and larger filter capacitors would do wonders. Paying better attention to the layout of the circuit would also improve signal quality. Remember if you use silicon diodes and a proper filter, the voltage drop will be much less so the transformer voltage can be 100V lower. 180+180V would do nicely.

inside view  top of amp  Completed amp




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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.


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