Mesa Boogie Simul-Class Operation ...

[hasserl]

Anyone familiar with how this is achieved?

I read thru the patent description, but could not get the graphics to display, so I had to imagine the circuits as I read thru it. But it appears to me that the basics are: you've got two pairs of tubes running in parallel push-pull operation. One pair of tubes (besides having the screens tied to the plates for triode operation) has a different bias voltage applied to it so that the tubes run hotter than the other pair (which are wired up for regular pentode operation). Is that it? Is there any means of reducing the voltage of the "class A" pair of tubes? If not, do they truly operate in class A?

It is my understanding that there is more to class A operation than merely adjusting the bias voltage. That if you try to adjust the bias voltage of a pair of tubes in a class AB amp so that they operate in class A, they will probably over dissipate, that the voltage is typically too high to allow this and that you need to get the voltage down to a level that will support class A operation without over dissipating the tubes. So I'm struggling to understand how MB is able to do this without some means of reducing the voltage to that pair of tubes.

[Joey Voltage]

Anyone familiar with how this is achieved?

I read thru the patent description, but could not get the graphics to display, so I had to imagine the circuits as I read thru it. But it appears to me that the basics are: you've got two pairs of tubes running in parallel push-pull operation. One pair of tubes (besides having the screens tied to the plates for triode operation) has a different bias voltage applied to it so that the tubes run hotter than the other pair (which are wired up for regular pentode operation). Is that it? Is there any means of reducing the voltage of the "class A" pair of tubes? If not, do they truly operate in class A?

It is my understanding that there is more to class A operation than merely adjusting the bias voltage. That if you try to adjust the bias voltage of a pair of tubes in a class AB amp so that they operate in class A, they will probably over dissipate, that the voltage is typically too high to allow this and that you need to get the voltage down to a level that will support class A operation without over dissipating the tubes. So I'm struggling to understand how MB is able to do this without some means of reducing the voltage to that pair of tubes.

To simply answer your question:

It is a hoax! Mesa doesn't operate any tube in their simulclass models anywhere near Class A first of all, and second there is nothing to the transformer either (the "Class A" primary taps are really directly connected internally to the Class AB ones) it's all an elaborate rouse, and all they do is knock down the bias voltage, and input signal for the outer EL-34 pair.

so to answer your question how do they do it? They don't!

[Bruce / Mission Amps]

To simply answer your question:

It is a hoax! Mesa doesn't operate any tube in their simulclass models anywhere near Class A first of all, and second there is nothing to the transformer either (the "Class A" primary taps are really directly connected internally to the Class AB ones) it's all an elaborate rouse, and all they do is knock down the bias voltage, and input signal for the outer EL-34 pair.

so to answer your question how do the do it? They don't!

Hmmm The OT is like an ultra-linear one with the 6l6 tubes running off the lower zed UL taps and the El34s at the end windings of the OT.
The EL34s are set up in faux triode mode and the 6L6s are operating in regular old fixed bias, pentode mode.
The El34s run on the same bias supply as the 6L6s but there is a voltage divider on the tube socket at the grid lugs to lower the bias voltage for EL34s and to run a hotter idle current while in triode mode.
The 6L6s get shut off when in "Class A" by lifting the cathodes from ground... how the hell that makes the El34s run in Class A is beyond me.
The El34s, running in faux triode mode with that high B+ voltage, still makes about 20-25 watts but I doubt that there is any Class A thing going on.
That is "it" and all there is to it.
I really think this is classic MB BS....
However, the amps sound really good in the faux triode mode at 15-25 watts output regardless.

[Joey Voltage]

Hmmm The OT is like an ultra-linear one with the 6l6 tubes running off the lower zed UL taps and the El34s at the end windings of the OT.

Thats the thing, I mentioned this above, there is no tapping point in the primary winding of the OT, It all an elaborate rouse to fool you, and create mojo. I know some people that have dissected these OT's, and weren't shocked by what they found. you can get a similar vibe with a plexi type OT, and a 2.2K primary.

And yes, the whole trick is that it sounds GOOD to boot because it is a decent design. thats the magic, the rest is smoke and mirrors

[teemuk]

I threw the thing to a circuit simulator and actually this "simul-class" thing does seem to work.

The "class-A" pair, as was already mentioned, is not really running in class-A at all. With these tubes you have the basic class-AB with hot bias -setup and this is how the amp also runs in the "class-A" mode. So, these amps being true class-A is indeed a myth.

The simul-class operation, on the other hand is a bit more interesting: Because this one tube pair has a hotter bias and an attenuated input they actually never stop conducting current during the full cycle of the amp producing it's rated output power in simul-class mode. However, as their bias is not a true class-A bias the output current waveform is not a perfect sine: The current conduction is limited making the waveform rather flattened from the other half-wave. So in simul-class mode this pair of tubes is running in sort of a "pseudo-class A" mode where conduction never stops but the waveform is not amplified accurately either.

Because in this mode of operation the "pseudo class-A biased tube" still keeps conducting current while the tube running in class-AB does not, the pair of tubes keeps providing additional current close to crossover point, thus minimising the crossover distortion.

Interesting.

...And indeed, another simulation - in which I removed the “pseudo class-A” tube pair - shows a significant increase in crossover distortion when those tubes are not in the circuit. So they DO have an effect. In essence, the class-AB pair can run with a colder bias because it is compensated by the other pair with hotter bias.

---

So, as far as I see it, this circuit is not entirely bogus. One could wonder though, what merits does it have over properly biased ordinary class-AB amp. IMO, the simul-class circuit is a solution to a problem that doesn't exist. But one can never underestimate how nicely that "class-A" term sells - no matter how bogus it is.

[Merlinb]

Simul-class operation is described in RDH4, Mesa's patent is void.

The original purpose of the system was to get the favourable low-distortion of the class A triodes at low signal levels, which is where most of the music is. You also get the low output impedance of the triodes too.

Also, in AudioXpress (November I think) someone published a new design for a class A triode + Class AB pentode hifi amp, with showed promise. You can also read about it here: Octode Topology

[teemuk]

Simul-class operation is described in RDH4, Mesa's patent is void.

Wouldn't be the first thing they claimed to have invented and managed to patent despite all prior art. Anyway, I think the patents for Simul-Class have expired already so discussion about their validity is nowadays pretty moot.

---

BTW, now that TubeCad was mentioned, I was more impressed with the class AC amplifier that the site discussed: Above certain levels of input signal amplitude class A biased devices switch off and class C biased devices kick in. That at least makes some sense efficiency-wise compared to Mesa's two parallel class-AB amps with different bias setting. It was a transistor circuit, though.

[Gregg]

Hmmm The OT is like an ultra-linear one with the 6l6 tubes running off the lower zed UL taps and the El34s at the end windings of the OT.

According to people who have disassembled one it's seems to be a regular 50 Watt OT:

I recently disassembled a fried simul-class OT from a 2:90.
It was marked: 562004R-1 EIA606-540

The windings were (I may have missed a turn or two since the two primaries don't match):
74 turns in two layers (4ohm)
831 turns in seven layers (Primary part 1)
42 turns in one layer (8ohm) (in series with 4 ohm)
832 turns in seven layers (Primary part 2)
74 turns in two layers (4ohm)

The two 4ohm windings were paralleled.

There were no taps on the primary, except for the center tap.

I got 0.315 and 0.8mm wire for the rewind, but both these seem to be about 0.01mm thicker than what mesa used.
I hope it fits anyway...

Measured over entire primary:
DCR 86ohm
~9H inductance
~16mH leakage inductance when shorting 4ohm tap
17.5kHz upper -3dB frequency response with 4ohm load

108 laminations of each kind measures 95.2x79.7x37.2mm assembled.

[Bruce / Mission Amps]

Thats the thing, I mentioned this above, there is no tapping point in the primary winding of the OT, It all an elaborate rouse to fool you, and create mojo. I know some people that have dissected these OT's, and weren't shocked by what they found. you can get a similar vibe with a plexi type OT, and a 2.2K primary.

And yes, the whole trick is that it sounds GOOD to boot because it is a decent design. thats the magic, the rest is smoke and mirrors

Whoa!
Are you guys saying that the MB MKIII OT does not actually have a 40%-60% tap plus the final overall tap on the primary... as a real ultra linear OT would?
It's complete BS!!??
Bruce

[Gregg]

Are you guys saying that the MB MKIII OT does not actually have a 40%-60% tap plus the final overall tap on the primary... as a real ultra linear OT would?

According to the witnesses...

[Kagliostro]

Gregg

are you sure you disassembled a Simul Class OT

or you disassembled a Simul Link OT ?

Kagliostro

[Gregg]

Gregg

are you sure you disassembled a Simul Class OT

or you disassembled a Simul Link OT ?

It was somebody else. In the post he mentions the OT was from a Mesa Stereo 290 amp. The exact part number is also there. The topic is from the sloclone forum.

[Kagliostro]

Opps ...

SORRY Gregg

Kagliostro

[Joey Voltage]

It was somebody else. In the post he mentions the OT was from a Mesa Stereo 290 amp. The exact part number is also there. The topic is from the sloclone forum.

Yeah, I know of two people on there that have dissected Simul Class OT's, One of them was my buddy Tom, who owns an MKIV. He reported exactly the same thing, and what many others have suspected..... Complete BS!

[Kagliostro]

Today in an italian forum I learned that the sloclone user


was an italian, Robi

see this thread

Mesa Boogie e Simul Class = qui i dati di un trasfo disassemblato

the world is small

Kagliostro