Manual is incorrect; 45W mode is Class A/B

[phyrexia]

Ok everybody, I just spent some time on the phone with my rep because the Mark V manual has some pretty glaring inconsistencies. I will clarify them here.

90W - All Four Tubes. Simul-Class.

45W - Inner Tubes. Class A/B. I Swear. I Promise.

10W - uses the two tubes closest to the Rectifier tube. Single Ended. One tube is used for actual output and the other tube is used to balance the load on the transformer.

The manual is just flat out incorrect.

For instance:

page 3:
45 WATTS shuts down the middle pair of output tubes wired in CLASS AB and allows you to operate the output section in just the CLASS A mode for a perfect lower wattage range that clips sweeter and earlier, and yet is still plenty powerful enough to play out in medium sized venues.

page 31:
45 WATT (CLASS A, PUSH-PULL):
In this setting two of the output tubes (second and third from right – facing rear) are used in a Class A style of wiring to create 45 watts of vintage warmth and clip-ability. Class A power is less efficient than Class AB, producing more heat and less wattage, but is much sought after these days for its smooth clip characteristics – especially its ability to transition across the threshold of clip without “gacking” or sounding harsh.

 

[DaveDaveDave]

I don't doubt at all that you heard what you said. I also don't doubt that that's exactly how it operates. I'm really not sure what to believe anymore, but I did test this out for myself, to some degree.

If you flip between 90 and 45, the middle set of tubes stays on, and the outer pair is the set that is controlled by the switch, as evidence by the blue glow of elections. The manual is definitely wrong about which set of tubes is on or off. Anyone can test this for themselves and see.

Of course, who's to say which pair is running in A and which in A/B, (if in fact any tubes are running in class A at all).

So, I tried the same test on my Mark IV. When you set it to class A, the middle tubes are on standby, just like in the manual for either amp. Hmmm.

The only thing I'm going to trust for sure is an oscilloscope. Anyone around have one handy? (Didn't think so.. )

Just a little more food for thought - On the one hand, it seems unlikely to me that the remaining set of tubes with all that plate voltage would be the more A/B than A because the remaining pair is taking up some of the slack from the other set. The bias supply circuit doesn't change at all, from what I've seen of earlier Mark Series schematics, so the remaining pair is actually going to get a higher bias voltage if the other two has their plate voltage bypassed. Who knows - there could be a lot more wierdness in this amp than any before. For all I know it's cathode biased.
On the other hand, there's the wattage that boogie says it produces. While you can get 60 watts out of a pair of 6L6s, 45watts of plate dissipation is just at the edge for most 6l6 tubes out there to run in Class A. I'd think it'd be more realistic to get 35-40 watts out of class A, instead of running the tubes out at max spec.


Just my 4 cents.

Dave

 

[rabies]

1) if the manual is wrong regarding this, that's a total disaster in my opinion (others will surely argue it's not)

2) what amp that's class A actually gets *more* than 30 watts out of a pair of EL34/6L6 tubes running in pentode? class A runs less efficiently and hotter b/c the power tubes don't cycle in the typical push/pull cycles that class AB do (i.e. class A doesn't give the tubes a "break").

My Mark IV had class A sox in outer pair positions and the inner pair "shut off" when I put the amp in class A mode (as evidenced by my weber bias rite).

So I imagine that the class A sox are still the outer ones but how can you really prove this (other than listening with a keen ear?)

So if this is really true, has anybody confirmed the error in the manual with Mesa?

Also, plz record some clips with class AB and 4xEL34 (phyrexia!) with a LP guitar. I'd love to hear that in channel 2 especially.

 

[rabies]

so is the 10 watt mode class A or AB or what?

is class A really available as well as class AB and simul-class in this amp? and if so, what wattages?

now I'm confused...

 

[phyrexia]

first off: rabies, I agree. disaster. HUGE error. My rep was not pleased when I pointed all this out to him.

Also, you're right. The math is not there to get 45w in Class A.

As for 10w mode: It is interesting and I am not an electrical engineer, but I'll try to explain it as best as it's been explained to me.

In 10w mode, one power tube operates in single-ended class A. I think this is the most honest Mesa's ever been regarding a Class A amp. It's not cathode biased class A, it's real honest to god class A single ended action. AFAIK.

Now under normal circumstances, the output transformer would need an even load on each tap (for instance in a 100w amp two tubes "push" and two "pull", hence "Class AB Push-Pull"). So Randy figured out a way to use a second tube (this on the other tap of the OT) to balance the load.

So that is why the outer or inner tubes are not used together for 10W mode. Instead one tube from the Push pair and one tube from the Pull pair are used. This is why the two power tubes closest to the rectifier tube are used.

 

[phyrexia]

The manual is definitely wrong about which set of tubes is on or off. Anyone can test this for themselves and see. Just my 4 cents.

Dave

What's worse is that the manual contradicts itself in regard to which set is turned off. Early on it says one thing and later in the manual it says the opposite.

The Mark V power section is a Mark IV power section with Class AB instead of Class A, with the addition of 10W SE mode from the Lonestar Classic revision and the Express amps.

 

[MrMarkIII]

So... Switch to 45 watts, remove the outer pair, flip it off standby... any sound?
No?
Now we know which pair is off in 45 watt mode.
QED.

And here's a philosophical question:

"If the manual is wrong, does that mean the amp sounds bad?"
Discuss...