The MESA BOOGIE© Express 5:25 is a 2-channel- 4-Mode 25 Watt Guitar Amplifier with Dyna-Watt technology. The 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 25 Watt rated output. 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.

There are [2] separate 5:25 amplifiers that have been modified to the following specifications… these amplifier modifications make the amps easier to service and operate…

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BIAS-Mod

I drilled the amp chassis and installed two Bias-Test-points, 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.)

To facilitate measuring the cathode current, On the power tube circuit board, I broke the connection between pin-3 on the EL-84 tube sockets and ground and installed two 1-Ohm resistors in-line.
There is a resistor network to reduce the B+ voltage to a 200 mV range and there is a rear panel 50k-Ohm bias control pot. That bias control pot allows you to set the bias for any manufacturers tubes, (including Mesa Tubes) for optimum power.

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Hi-Frequency Snubber

The snubber is a network of resistors and capacitors that filter very high frequencies. These high frequencies are not in the audio range and must consequentially be managed before the output transformer's secondary; lest possibly driving the transformer into saturation before it really needs to. The snubber circuitry connects between the two primary windings on the output transformer.

* Guitar Player Magazine Article – (August 2011) - “Smooth Your Sonic Ride” - by Dave Zimmerman
* Radiotron Designer’s Handbook – Fourth Edition (1953)

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Tube Shield

The tube shield was constructed from a 0.060” square of aluminum and perforated with ½” holes and attached to the aluminum bar on the rear of the amp. I felt that the stock aluminum bar did not provide enough protection for the tubes. The new shield protects the tubes while still allowing air-flow.