K, break this down caveman style..

[alphadog808]

Hey guys,
I've been reading up on some of the Mark IV settings but still don't have a clear picture on what they really..do..note, I'm not a tube inclinded person, so if someone could break down these settings in layman terms..I guess more on how the amp should "act"/sound...

Class A vs. Simuclass

Pentode vs. Triode(besides 4 vs. 2 tubes)

 

[mrd]

Both of these have to do with the wiring of the tubes.

I'm not sure on technical terms, but I could explain them to you as best I can...

Class A is basically the least efficient way of running power tubes, I'm about 95% sure it is when all the tubes are run in series. This lowers the effective wattage of the power section, which means you get power tube distortion and saturation at lower volumes. Simul-class means it's simultaneously (hence Simul) running Class A and B. I think I've read somewhere that Simul-class on Mesa amps is not a true Simul-class, but I'm not totally sure on that one. Either way, this way of doing it is more efficient in terms of actual voltage that is turned into sound rather than just plain heat. The effective wattage ceiling is raised, and you need to get it louder for a similar level of power tube saturation.

I don't know too much about pentode vs. triode, all I know is they also affect the wattage. Triode makes the wattage lower, so you get more power saturation earlier, pentode makes it higher.

Generally, if you want a real shreddy, creamy tone, then run Class A in Triode and Tweed power. You will get where you need to be sooner on the dB scale than you will with Full, Pentode and Simul-class.

Hope it helps.

 

[Elpelotero]

this is a great article written by none other than Randall Smith. It can be a little confusing at first, but if you take it in slowly, it makes perfect sense.

http://mesaboogie.com/US/Smith/ClassA.htm

 

[alphadog808]

thanks guys! And mrd, you answered my question...exactly. I should have been more specific in the type of tone I was looking for...

Seems like the bottom line is classA, triode, tweed is the best way to get lots of gain with lower volume...sounds good!

 

[mrd]

thanks guys! And mrd, you answered my question...exactly. I should have been more specific in the type of tone I was looking for...

Seems like the bottom line is classA, triode, tweed is the best way to get lots of gain with lower volume...sounds good!

No prob. Keep in mind this will burn out your power tubes a bit quicker..but hell.. all in the pursuit of tone, right?

 

[phyrexia]

Almost everything MRD said is incorrect in one way or another. I suggest you read Randall's writeup.

 

[devilrob1979]

Almost everything MRD said is incorrect in one way or another. I suggest you read Randall's writeup.

Yep.

 

[mrd]

Almost everything MRD said is incorrect in one way or another. I suggest you read Randall's writeup.

haha, this is why I always write that I'm not 100% sure about it :lol:

 

[metalblitz84]

Almost everything MRD said is incorrect in one way or another. I suggest you read Randall's writeup.

Give a brother a hand for trying to help though!!!!!!!!!!!!!!!!!!!!!

 

[phyrexia]

Nice try. "Luck of the Noob" if you will.

 

[metalblitz84]

lol

 

[msi]

Ok, let me try the class a/ class a/b thing.

Power tubes and amps in general need a bias. The bias is the center point of where the tube can amplify. If the bias is set at 50% of the tubes maximum gain (swing) then the tube can amplify the signal 50% in either direction. Thats because sound moves as a complicated sine wave. The tube can follow the sine wave both up and down. Thats essentially class A.

Class A/B is where rather than setting the tube at the middle of its bias you set it at 0% of its bias. This means it can only amplify one side of the sine wave. But it has 100% of its maximum ability to amplify it with. But to amplify the bottom half of the sine wave you need another tube set to amplify the opposite side of the sine wave. Then when the sine wave passes from positive to negative it goes from one tube to the other. This is why in class a/b amps you needs pairs of tubes, where as with class a you can get away with only one.

Now my understanding of simul-class is that it's a bunch of BS. the tubes are not running in class a and class a/b at the same time. You'd need two output transformers. Whats going on is one pair of tubes is working normally and the other is working in pentode. It's a variation of wiring a tube that offers a different sound and slightly lower output due to the loss of a control grid.

This bias thing is a simplification but as I understand it should be pretty close.

 

[phyrexia]

I think you are confusing push-pull/single-ended and A/AB.

 

[msi]

No your calling the same dog two different names. The class of an amplifier specifies how much of the input cycle the amplifier reproduces. In the case of class A it's the whole cycle. Class B is half the cycle. I simplified class a/b into class B because the specifics were not important. Class A/B doesn't actually reproduce half the cycle, it produces slightly more so that when one tube is shutting down the other tube is starting to power up. Single ended and push pull are simply other ways to describe these. Wiki has a decent explanation of the classes of amplifiers. These class designation have been standard since at least the 40's so the wikipedia entry is reliable. http://en.wikipedia.org/wiki/Class_AB#Amplifier_classes

Now if you take a look at the MK-4 schematics on tube freak pay special attention to the triode/pentode switch on v8 and v9.
http://www.tubefreak.com/mk4-4.gif Thats what Mesa is talking about with simulclass. Wiring the grids of the tubes from pentode to triode. Thus you have two tubes running as a pentode, and a pair running as triodes. But never does any of those tubes in the power amp amplify the whole input cycle. It never goes into Class A operation. Mesa fudges the amplifier class thing to sell amps to people who don't know better. Here is the wiki page for Tetrodes, http://en.wikipedia.org/wiki/Pentode

 

[Buddy]

There is probably more info here than you need .
Page 26 .
http://www.bunkerofdoom.com/tubes/rc25/RCA_RC_25.pdf
RCA_RC_25.pdf (application/pdf Object)
42 megs worth !!