D05 is a vintage hybrid guitar amplifier. The preamplifier stage is driven by a vacuum tube, the power stage by the a class AB solid state IC.
It's built on top of an old portable turntable found in the dumpster. This time I've found just a few parts of the turntable. In particular the low part of the wood case, and the electronics.
Unfortunately I've not taken pictures of the system when I've found this, you have to believe if I say it was in horrible conditions. The full turntable mechanism is missing. The amplifier tube is broken, but I've notice the preamp tube was not.
WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.
I've search for the datasheet of this tube, the 6AT6 (https://www.radiomuseum.org/tubes/tube_6at6.html), it turn out it has enough gain to be used as a guitar pre amplifier. Indeed it is used in the Fender "Harvard" 5F10, that's a vacuum tube amplifier made by fender from 1955 to 1963, you can find more info about his amplifier here: https://en.wikipedia.org/wiki/Fender_Harvard.
The preamplifier tube I'm using is manufactured from the Italian company Europhon.
For this design I've decide to keep things clean, I do not want this amplifier to be equipped with an overdrive circuit, all the distorted sounds should came out from the tube saturation.
I've put the Tillman FET preamp (http://www.till.com/articles/GuitarPreamp/) in front of the tube, to prepare the signal for the tube gain. This preamp has a 3dB gain.
After the first preamplifier stage there's the single tone controller, which is derived indeed from the Big Muff tone stage. I like the way this tone stage boost high and bass sounds, also it drops the signal just a little.
Here it comes the heart of this amplifier, the 6AT6 valve preamp stage. Even it is used by the Fender 5G10, I've not just copy and paste this circuit here, I have to bias the valve for the voltage provided by the power stage I've built. This stage has to be clean enough even with full volume coming from the input.
The last stage is based on the TDA2003 solid state class AB amp. This is driven in single rail supply, +12V, in a pretty common configuration, bang on the datasheet application circuit.
Up to here the amplifier sound clean, but I would like to add a bit of saturation if needed. For this reason I've added a booster stage between the tone and the valve pre amplifier. The booster is based on the LBP-1 booster, with just a few values changes.
After the tube preamp there must be voltage divider that arrange the signal for the amplifier, if the signal is too high the poor TDA2003 will clip, and I just want the distorted sound comes from the preamplifier, not the amplifier.
I've finally add a 4ohm 6" speaker I've around, not that big cause the cause is pretty small, for this reason there's also a mono jack speaker output that disable the internal speaker output, this way one can use an external cabinet.
The power stage for this amplifier use a trick to drive the valve with the correct voltage. As you know tubes are driven with high voltage power, indeed other IC use low voltage. I've use a standard 230V to 12V with primary and secondary connected in reverse to pull the voltage up to +170V, used by the valve preamp. Keep in mind that using a transformed in this configuration induct some loos on signal, usually 20%, after all we are using a transformer in a way it was not built for.
Power stage also provide +6.3V for the valve filament, this is done by using a 7805 with the ground biased using two 1N4148 transistors. +12V is provided for the TDA2003 stage and finally +9V for the solid state preamp stage.
As you may notice, there's almost more of power stage than preamplifier and amplifier stage.
All the electronic have been mounted on the original iron holder of the turntable amplifier. I've just added a few new holes for the new board. It has been spray painted black.
Final stage was to build a decent case using the original one. I've cut from the original case the bottom part, the one that holds the turntable mechanism. On top of the case I've pierce the wood to make the space for the controls, Jack input, Volume, Tone control, Gain control, Saturation engage switch, On/Off button. I've pasted and prepared the wood. Then I've used vinyl glue to cover with fabric the top of the wood, like it's done on vintage amplifiers.
The front mask is the one from the original turntable. The hole which contains the voltage selector has been replaced with acrylic glass, behind the tube can be seen. The tube is secured using also a 3D printed holder.
A 230V light bulb it's used for the on/off detection.
At my ears: it sounds good, especially if used with an external case. I've used with a couple of pedals I've built, I must admit it makes the pedals sound really good.
At my eyes: it's a joy! Each time I see this amplifier I'm happy for having built it.
Notes
- read risk disclaimer
- excuse my bad english