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This is a MESA BOOGIE© 20/20 Dyna-watt Stereo Power Amplifier. This amplifier stores energy to release on the attack of notes that you play through the amplifier...this dynamic boost on the attack of the notes makes the amp seem louder than the rated 20 Watt output per channel. This is a great little power amp that packs 2 x 20 Watts of EL-84 Power into 1 Rack-unit. The Mesa Boogie design topology calls for a fixed bias and you need to buy tubes that are in the same power range as from the factory... That means buying tubes from Mesa Boogie© or buying tubes from a 3rd party and trusting that the tubes that they sell you are in the proper power range for a specific current.

Here are the modifications that make the amp more serviceable....



I installed four Bias-Test-points in a small internal box that are accessed by a top panel cut-out, One test point for each of [4] tubes, a Hi-Voltage Test-point for the B+ voltage and a ground test point (There are no dangerous voltages on these test points so you can use a standard voltmeter set to the 200mV Scale to make the measurements.) I installed four 1-Ohm Cathode resistors, and modified the circuit board to allow measurement of Cathode Current on the above test-points. There is also a custom circuit board with the new Bias Supply resistors and Capacitors. (This is required to split the stock bias supply to feed each channel separately allowing you to set the bias separately for channel A & B) In the same box as the test points, there are two 50k-Ohm bias control pots, one for each channel. Those controls allow you to set the bias for any manufacturers tubes, (including Mesa Tubes) for optimum power. The Break-out box also includes a 9-pin D-SUB connector with all of the above voltages to feed an external metering panel.

bias1 ohmbias board



In order to facilitate the monitoring of the power tube bias and the B+ VDC while the amp is operating without having to de-rack the amplifier, I produced a 1 RU Rack Panel that integrates Front panel input & output jacks with 2 Voltmeters that can verify the bias for four tubes and the Amplifier's B+. The voltmeters are fed from the 9-pin D-SUB connector on the amplifier's Break-out box. Each voltmeter has a three position switch that can switch between the following...

  • Tube A Bias for each 1/2 of the amplifier,
  • Tube B Bias for each 1/2 of the amplifier and the
  • B+ VDC.

This Custom Panel allows quick diagnosis and vitals monitoring while the amplifier is operating during a live show. You can set either METER to display the B+ VDC and then you can switch between each power tube's bias for one channel on the other METER. (vice-versa to check the power tube's bias on the other channel.)




On this Vintage of 20:20 power-amp there was hard-wired, un-removable power cord... I removed the original power cord and installed a standard EIA power outlet so that you can de-rack the amplifier for service without messing with the power cord which is usually cable-tied in the rack.




The way that I do this mod is with a 3-way switch for three different frequency responses...Stock, Punch & Deep. Stock is with the switch up, the middle position is the Punch setting and allows a little more bass to be amplified than in the Stock mode and Deep is with the switch down. In my deep mod, I drilled two small holes in the front panel for the 3-way switch. The switches fit between the Volume and Presence knobs of each channel and are easy to access from the front so that you can instantly hear the difference in the settings as you play. (All of this looks stock as you can see in the pix.)

Because of the bias mod, the 20:20 replaced 2 Mesa Boogie© Express 5:25 heads and has become my main power amplifier (weighs less too!)....