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Photo 1—Photo courtesy of Eric Taylor (gearhounds.com)
I have a Mesa/Boogie Stereo Simul-Class 2:90 (circa 1994), which I play as quietly as humanly possible. It’s way more amp than I need, though I do like the sound. This amp uses eight 6L6 power tubes, which is way expensive to re-tube, and the tubes produce a lot of heat. There are two rows of four 6L6 power tubes, one on top of the other.
My questions: Which tubes are turned off when the half-power switch is engaged? Can I rotate them to make wear more even, so the tubes last longer? Would these “disconnected” tubes still wear out? Is it possible to remove four of the tubes to save some money and reduce heat? If so, which ones?
Thanks for your many informative columns.
I’ll use four pictures to answer your question as clearly as possible. In Photo 1, I’ve divided the tubes into two sections. The group on the left consists of the four output tubes for channel B, and the tubes on the right are for channel A. The two tubes on the top row of each channel are used as half of the push/pull output stage, and the two at the bottom are the other half.
Photo 2—Photo courtesy of Eric Taylor (gearhounds.com)
This output stage is what Mesa calls “Simul-Class,” and the architecture can be broken down even further. One tube from each half of the output stage runs in standard class A/B mode, while the other is biased to run in a class A-type mode. Exactly which output tubes perform each task has varied over time due to production changes. This can be important if you want to run different types of tubes for class A and class A/B operation, so let’s look at both layouts.
Photo 2 shows the layout for the older style Simul 2:90, which has a small blue pilot lamp on the front panel. Here we can see that the class A-biased tubes are located on the far ends of the layout, with the A/B counterparts occupying the middle four sockets.
Photo 3—Photo courtesy of Eric Taylor (gearhounds.com)
Photo 3 shows the layout for the newer-style Simul 2:90. This model has a large red pilot lamp on the front panel. Things have changed a bit: While channel B’s class A tubes remain the outside pair, channel A’s class A pair has moved to the inside location. (In the older 2:90, the output tube sockets are handwired. In the newer 2:90, the output tube sockets are mounted on PC board, so this change may have been to accommodate identical PC boards for each channel.)
You asked which of these tubes is turned off in half-power mode. As you told me in a subsequent email, you’ve already learned this amp doesn’t actually have a half-power mode—it has what Mesa calls “half-drive” mode. This is accessible only from the rear-panel jack indicated in Photo 4. This feature does not disengage any output tubes, but alters the driver circuit to supply far less signal to the output tubes. It may also cause the driver circuit to distort earlier, mimicking some output tube distortion. This means no tubes are turned off in this mode. Bummer.
Photo 4—Photo courtesy of Eric Taylor (gearhounds.com)
Okay, here’s where I thought I’d be saying you can simply remove the class A/B tubes and run the amp with the Class A sets, substantially reducing output power. But something told me to give my friend Mike at Mesa a call, and I’m glad I did. He strongly recommends against removing any tubes, and here’s why: While electrically removing tubes from operation is exactly what happens in the Mesa Mark amps, the internal voltages in the 2:90 are higher. Though operation at these higher voltages under standard conditions is fine, removing four of the eight output tubes causes voltages to rise even higher, possibly leading to premature tube failure—especially with today’s inconsistent tubes!
So unfortunately, you cannot remove any tubes from your amp. The best help I can offer has to do with heat. Since the class A sockets can support either 6L6 or EL34 tubes, I recommend using the latter. Because the fixed-bias voltage remains constant—and a 6L6 tube requires a bit more bias voltage than an EL34—this should slightly over-bias the EL34 tubes, making them run a bit cooler. Sorry I couldn’t make things quieter, but at least we can cool off your tubes a bit.