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Garland,Mesa-Boogie don't make EL84's. They use the same stock that hi-fi amps use: current production Sovteks, Ei, and JJ.
So, what's good for the Goose is good for the Gander. Just be sure to re-bias the amp, if it's a Fixed Bias type, when you change out power tubes.
Noting your comment, Steve, I have a question about power tubes and Mesa amps. My guitarist just scored a Mesa Stereo 50/50 power amp.
While the tubes are still good (unit is only three months old), there is no user adjustable bias for the power tubes (a quartet of 6L6). The local music shop (and even my tech buddy who works there, the "Tube Guru") don't have a clue about the amp. They are not a Mesa dealer either. Of course Mesa says "use our tubes only". Should he just put in a matched set of what ever sounds best to him, or does he have to dump the coin on "Mesa" labeled glass?
Just curious for his future!
Dman
Dave,Not familiar with the stereo version. But, most MB run a FIXED BIAS, with no pot for adjustments. That's why their admonishment to buy "matched" Mesa-Boogie approved tubes. Most of their 6L6GC amps have a separate bias supply which puts on anywhere from -51 to -56 volts, for 6L6GC amps.
This setup is a lot like the old Black Faced Princetons. You can add a pot to the bias supply. The first resistor in parallel to the filter caps may be a 10-K ohm resistor. Replace it with a 10-K-ohm linear pot, but be sure to keep the other fixed 10 K-ohm (or it can be a 68 K-ohm) resistor in place.
If the resistor is only one fixed 27 K-ohm, for instance, replace this resistor with a 10-ohm pot and a 22 K-ohm fixed, in series.
You will still need to buy a match quad of 6L6GC---as the pot can only adjust the negative voltage to all power tubes. But, at least you'll be able to compensate for weaker or stronger sets of tubes.
One note: If this is one of those "Simul-Class" amps which can take either 6L6GC or EL34---I believe that these have some type of auto-biasing circuitry. Plus, a hybrid cathode biased/fixed biased setup. Adjustable bias would not be as simple.
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