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MiniMax - Fleapower Guitar Amplifier Project(updated 11-11-13)
I've got this flea-power amp idea from many guitar player forums where guys were seeking a small low power DIY tube amp for bedroom practice. Another reason was to have a low power amp for saturated power-tube recording and stage use if miked to a PA. The goal was to build a good sounding amp on the cheap without sound quality compromises. Below are few design examples of low power amps where one can choose the output power(-tube) and features. All biasing resistors are calculated for 200VDC plate supply.
Also, check out this little Guitar Amp Project THE TINY 3 WATT
The MiniMax Amp
The basic MiniMax amp features two-stage pre-amp and one power stage. One double triode 12AX7/ECC83 and one power pentode is used. It has no tone controls, just a volume control. Lack of tone control doesn't mean it has limited sound reproducton. On the contrary, the whole music spectrum is amplified within its design and component limits. The output power can be set by choosing a particular output tube. See table I below. Choose the appropriate cathode resistor Rk and screen resistor Rs
The MiniMax Ex
The MiniMax Ex amp features in addition to above, tone control and three pre-amp stages using one 12AX7/ECC83 double-triode tube and one triode-pentode tube. This amp will go into overdrive with no effort if cranked up. A clean reproduction is also possible when used with lower volume and low-gain input [IN1]. The output power can be set by choosing a particular output tube in table II below. Choose the appropriate cathode resistor Rk and screen resistorRs for the power pentode and cathode resistor Rk and plate resistor Rp for the triode section.
The MiniMax Fury
The MiniMax Fury is a full featured four gain stage amp with tone controls. It employs two double-triode 12AX7/ECC83 tubes and one triode-power pentode tube. This amp has two separate input channels that can be mixed independently. One input channel [IN1] is a high gain overdrive channel and the other is a normal gain channel. Other features are a boost switch for even higher gain, tremolo effect, effects loop In/Out and a master volume control. Note that the boost switch and FX loop In/Out are optional and may be omitted. Just connect the coupling capacitor directly to the volume pot as shown in the bubble.
NOTE: the russian 6P1P is a Beam Tetrode, basically identical to 6AQ5/EL90 but in a 9 pin tube.
NOTE: 6P1P; 6F3P and 6F5P are russian tubes.
NOTE: For 6BM8/ECL82/6F3P change Rf resistor to 39k
There are 3 PSU circuit versions. Choose your version from table III depending what power tube is used. PSU1 is the simplest version. PSU1 and PSU2 supply only 6.3V heater voltage whereas PSU3 supplies both, 6.3V and 12.6V. So a power amp with a 12CU5 tube will need PSU3 circuit. PSU2 circuit requires more parts but supplies 6V DC heater voltage. All versions are easy on the budget and will cost no more then $40.00 for the power transformers.
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