Lighthouse Electric Vacuum Tube Project

MiniMax - Fleapower Guitar Amplifier Project

(updated 11-11-13)

MiniMax Amp Features:

  • Low Cost
  • Off the Shelf Parts
  • Basic & Extended Versions
  • Choose Output Power Tube

This project is under development and future upgrades and/or changes may be implemented.

Introduction

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


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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.


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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.


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TABLE I SINGLE POWER PENTODES
TUBE TYPE CATHODE RESISTOR Rk SCREEN RESISTOR Rs BASE OUTPUT POWER TUBE DATASHEET
6AK6 510 Ohm 1 W 1k 1/2 W 7 PIN B7G 1 WATT PDF Datasheet
6AQ5/EL90 250 Ohm 1W 220 Ohm 1 W 7 PIN B7G 2 WATT PDF Datasheet
6P1P 360 Ohm 1W 220 Ohm 1W 9PIN B9A 2 WATT PDF Datasheet
6M5/EL80 160 Ohm 1 W 100 Ohm 1/2 W 9PIN B9A 1.3 Watt PDF Datasheet
6BW6 270 Ohm 1W 100 Ohm 1/2 W 9PIN B9A 2 Watt PDF Datasheet
6BN5/EL85 360 Ohm 1W 100 Ohm 1/2 W 9PIN B9A 2 Watt PDF Datasheet

NOTE: the russian 6P1P is a Beam Tetrode, basically identical to 6AQ5/EL90 but in a 9 pin tube.

TABLE II TRIODE-PENTODES
TUBE TYPE TUBE SECTION Cathode RES. Rk Bias Res. Rb Plate RES. RP BASE
6AB8/ECL80 Triode common w. Pentode NA 68k 1/2 W 9PIN B9A
6JL8 Triode 150 Ohm 1/2 W 150 Ohm 1/2 W 33k 1/2 W 9PIN B9A
6GV8/ECL85/6F5P Triode 270 Ohm 1/2 W 270 Ohm 1/2 W 47k Ohm 1/2 W 9PIN B9A
6BM8/ECL82/6F3P Triode 2.2k Ohm 1/2 W 2.2k Ohm 1/2 W 220k Ohm 1/2 W 9PIN B9A

NOTE: 6P1P; 6F3P and 6F5P are russian tubes.

TABLE II TRIODE-PENTODES
TUBE TYPE TUBE SECTION Cathode RES. Rk SCREEN RES. Rs OUTPUT POWER DATASHEETS
6AB8/ECL80 Pentode 390 Ohm 1/2 W 3.3k 1/2 W 2 WATT PDF Datasheet
6JL8 Pentode 120 Ohm 1/2 W 2.2k Ohm 1/2 W 2 WATT PDF Datasheet
6GV8/ECL85/6F5P Pentode 360 Ohm 1/2 W 220 Ohm 1/2 W 2 WATT PDF Datasheet
6BM8/ECL82/6F3P Pentode 470 Ohm 1 W 220 Ohm 1/2 W 3 WATT PDF Datasheet

NOTE: For 6BM8/ECL82/6F3P change Rf resistor to 39k

Power Supplies

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.

PSU1 Schematic  PSU2 Schematic  PSU3 Schematic

TABLE III POWER SUPPLY VERSIONS
TUBE TYPE PSU VERSION TRANSFORMER PN: SUPPLIER
6AK6 PSU1 or PSU2 XPWR024-120 or P-T292 EDCOR or  Antique Electronic Supply
6AQ5/EL90 PSU1 or PSU2 XPWR024-120 or P-T292 as above
6M5 PSU1 or PSU2 XPWR024-120 or P-T292 as above
6CU5 PSU1 or PSU2 XPWR024-120 or P-T292 as above
12CU5 PSU3 553-VPS230-110 and 553-F5-12 MOUSER
6AB8/ECL80 PSU1 or PSU2 XPWR024-120 or P-T292 as above
6BM8/ECL85/6F5P PSU1 or PSU2 XPWR024-120 or P-T292 as above
6BM8/ECL82 PSU1 or PSU2 XPWR024-120 or P-T292 as above
OUTPUT TRANSFORMER FOR ALL VERSIONS  Antique Electronic Supply - P-T31

Feedback Or Suggestions Are Welcome.

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