Mesa "Duo-Class" Power

[OrganicEffects]

Howdy folks.

Forgive if this has been covered already, but a couple searches didn't turn up anything.

I haven't been too active with tube amps lately due to catching a few other bugs, but I recently played a Mesa California Tweed combo amp that I thought sounded really great. There is a power attenuation rotary switch that selects between 40W/30W/20W/10W/2W using 4x 6L6's that I thought sounded particularly good, although different, at each setting. I went through the info they have around this particular switching system, which they apparently call "Multi-Watt" and "Duo-Class", the latter being because the 10W-40W options are PP A/B, while the 2W is SE Class A (according to their literature). Here is the excerpt describing each settings:

"Mesa’s Patented Incremental Multi-Watt power amp, featuring Duo-Class and Dyna-Watt technologies, provides five power, two operating-class, and three wiring options. And you can access everything via a single 5-way power switch!

40 watts — 4 x 6V6 Class A/B Pentode
30 watts — 2 x 6V6 Class A/B Triode + 2x6V6 Class A/B Pentode
20 watts — 2 x 6V6 Class A/B Pentode
10 watts — 2 x 6V6 Class A/B Triode
2   watts — 1 x 6V6 Triode + 1 x6 V6 Pentode, Single-ended Class A Parallel"

Searching outside of this forum, I came across this amp books link, commenting on the "Duo-Class" patent that Mesa has.

https://www.ampbooks.com/mobile/amp-technology/amp-patent-7173488/

It may become obvious looking at this schematic below, but I don't fully understand patent. I get the wording, but the translation to applying it to the California Tweed description is confusing. The way I see it, it could go two ways. One, the 6V6 supplying the AC and DC to the power transformer is the 1x pentode or triode, and the other, 'inactive' remaining pentode or triode is the 6V6 supplying just the DC.

For me, this raises the question of why would they use one pentode and one triode in this configuration, and not two of the same?

The other option, is that they use all 4 tubes, a pentode and a triode driving the transformer with another pentode and triode being inactive and supplying the DC, which is what I attempted to sketch out below. This seems like it would be way more than 2W, but what do I know.



In the class 'A' 2W SE configuration, as described in the amp books like, the switches would be:
SW1A: Triode
SW1B: Triode
SW2: Closed
SW3A: Open
SW3B: Open
SW3C: Closed
SW3D: Closed

What do yas think?

[PRR]

> why would they use one pentode and one triode in this configuration, and not two of the same?

99% of players would not understand that question. I grok it technically, but what do it sound like??

Look at the Mark IV. Different tubes, but almost as much switching as your california burger.
https://www.kobra.hr/Schematics/Mesa%20Boogie/mk4-4.gif

IF the product has any musical merit (beyond wowing the buyer), I bet it was developed in all-nite testing, not in theory or in chatroom.

[HotBluePlates]

... Mesa California Tweed
...
2   watts — 1 x 6V6 Triode + 1 x6 V6 Pentode, Single-ended Class A Parallel"

... why would they use one pentode and one triode in this configuration, and not two of the same? ...

You have options of 40w down to 10w, and then a big jump down to 2w.  It winds up being only 2 watts due to a combination of one of the 6V6s being in triode mode, and the "mis-loading" of the output transformer not giving all the power output one might otherwise get from a pair of single-ended 6V6s.

Two things come to mind:

• On a different forum populated by players mores than techs, you'll find guys complaining that playing a 4w Champ at home (through a semi-sensitive speaker) is really loud before distortion.  They're hoping/imagining something in the 70-80dB SPL range, and being surprised to find out "only 4w" makes 100+dB of racket before it distorts.  So less-than-4w is desired.
• When Triode Mode first appeared in guitar amps, it was pitched as a "half-power" option.  Except triodes sound cleaner & darker than pentodes (or pentode-mode of the same tube).  The guy who builds Komet amps stated directly that triode mode sounds like garbage (he has some strong opinions about guitar tone, though I find I often agree with him).

So my guess is Mesa figured 2x parallel pentode-mode 6V6s might be too loud for their target market, and 2x parallel triode-mode 6V6s would be too tame.

Or it could have been an arbitrary choice: One of each waves off the folks who might gripe that the lowest-power mode should have been triode or should have been pentode.

[OrganicEffects]

Thanks for the replies, gents! Much appreciated.