How are the Tubes arranged in a Simul-Class 2:90 - I vs. II?


Stereo Simul-Class 2:Ninety™ Power Tube Arrangement - Versions I and II

Whether replacing an entire set of power tubes or swapping out a matched pair due to a tube failure, it is important to know the power tube arrangement for each pair of tubes working together in a push-pull configuration of the Stereo Simul-Class 2:Ninety™. 

There is actually an earlier & later version of the Simul 2:90 and the easiest way to distinguish one from another would be to look at the rear panel of the amplifier for the positioning of the jacks and switches (see images below). You'll also note that fans are located on opposite sides of the chassis for each version of the 2:90.

The images below indicate how the pairs of tubes are arranged in each amplifier for both the A and B channels. The tubes located in the sockets positions highlighted in Red are the Class A sockets, while those highlighted in Blue are its Class AB sockets. The Stereo Simul-Class 2:Ninety power amplifier ships from the factory loaded with 6L6 power tubes.

Tube pairings for the Simul 2:90 Versions I and II. 


Stereo Simul-Class 2:Ninety™ Preamp Tube Arrangement & Functions

Looking at a 2:90 from the rear, there are three 12AX7 preamp tubes (positions V1, V2, & V3).
V1 is located nearest the input jacks on left of the rear panel.
V1a - (left tube) is the Input Stage for Channel A.
V1b - (left tube) is the Input stage for Channel B.
V2 - (center tube) is the Phase Inverter for Channel B.
V3 - (right tube) is the Phase Inverter for Channel A.
This arrangement is the same for both Version I and Version II of the Simul 2:90.

This article applies to MESA power amps commonly referred to as a Simul-Class 2:Ninety, Simul-Class 2:90, Simul 2:Ninety, Simul 2:90, Simul 290 or simply the 290.

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