Mark IV class A & Triode vs. Simulclass and Pentode

[jukesgtr]

I assume my questions on these have been lost between the lines of my earlier posts:

When using 6L6s in all four power amp sockets how the sound and overall behaviour of the amp changes when using Triode and class A compared to Simulclass and Pentode. I hope this is all safe to try...that is one doubt why I have not tried these settings with 6L6s alone, did try with EL34 in outer sockets and did not really like.

Which settings are the best for MAX tube compression?

Fender amps with 2 x 6L6, do they run in class A or class A/B?

ADD: The manual says in class A mode just outer two power tubes work, simulclass has the inner tubes in parallel (or vice versa) with othe other pair. Does "parallel" engaging of inner power tubes make the amp to change from Class A to something else? This does not make sense to me!

Danke!

 

[Boogiebabies]

Parallel is when all four tubes use the same primary to the output transformer. When you switch to Simul-Class, it actually has another primary tap with a different impedance and runs the power section like two different power amps and blends them together through the secondary, hence, simultaneuosly running Class A/B and Class A together. For the most tune compression I would uses Class A with either the 6L6 or EL-34. It can run a nicely compressed 15 watts and give you some output section involvment. The EL-34 will shred and the 6L6 will het even fatter.


Ed

 

[jukesgtr]

Parallel is when all four tubes use the same primary to the output transformer. When you switch to Simul-Class, it actually has another primary tap with a different impedance and runs the power section like two different power amps and blends them together through the secondary, hence, simultaneuosly running Class A/B and Class A together. For the most tune compression I would uses Class A with either the 6L6 or EL-34. It can run a nicely compressed 15 watts and give you some output section involvment. The EL-34 will shred and the 6L6 will het even fatter.


Ed

Thanks boogiebabies, I'll try it! This means that MK4 cannot be run in pure A/B Class since the choices are Simulclass and and Class A.

Old Fenders run in Class A/B?

What about pentode/triode selection, I gues it does not have anything to do with the Class?

 

[brianf]

I won't get into the Class A Class A single Ended or class AB debate because I am not an expert but I can tell you that when I plan on using the lead channnel a lot on my Mark IV I will set it Triode, Harmonics and the Presence pulled out.

Sounds great.

brianf

 

[jukesgtr]

Thanks Brian for your input, I neither want to debate here, just to understand more about my "new" amp...

 

[Boogiebabies]

It can be run in simul class pentode (switch) to have all four tubes in a push pull through the first primary like a regular 100 Watt power section. When you switch to Class A triode it uses the second primary to blend the two classes through the secondary.


Here is an example:

http://www.tubefreak.com/mk4-4.gif


See how when it is in pentode it puts all the tubes through the first primary and if you switch to triode it splits the outer pair to the second primary. It is as close as you are going to get as it switches the outer wiring from triode to pentode and uses the same primary. To me, it looks like a 100 watt style class A/B power section Mesa style.

 

[jukesgtr]

It can be run in simul class pentode (switch) to have all four tubes in a push pull through the first primary like a regular 100 Watt power section. When you switch to Class A triode it uses the second primary to blend the two classes through the secondary.


Here is an example:

http://www.tubefreak.com/mk4-4.gif


See how when it is in pentode it puts all the tubes through the first primary and if you switch to triode it splits the outer pair to the second primary. It is as close as you are going to get as it switches the outer wiring from triode to pentode and uses the same primary. To me, it looks like a 100 watt style class A/B power section Mesa style.

Getting better understanding here, thanks for this info! Just one more thing, were/are blackface Fenders operating in Class A/B or Class A?

 

[brianf]

.

Getting better understanding here, thanks for this info! Just one more thing, were/are blackface Fenders operating in Class A/B or Class A?

Fender Blackface are Class A/B

brianf

 

[jukesgtr]

10-4. Tks Brian, I just wanted to make sure.

 

[joegold]

If what I've been reading on usenet is true then there is lots of confusion out there as to what Mesa means by "Class A".

Evidently all the Simul-Class amps do not really run the outer power tubes in a pure Class A configuation. What they do is to run them in Class AB triode (the norm is Class AB pentode). This makes for less power/headroom and the tubes break up at lower volumes, somewhat like a real Class A power circuit.

When you select "Pentode" on the MKIV's Pentode/Triode switch and SimulClass on the Class A/SimulClass switch it's Class AB in all 4 tubes. That's the most headroom this power section is capable of.

With switches set to Class A and Pentode its just 2 tubes in Class AB pentode, like a 50 or 60 watt amp.

For the most power tube breakup use the following switch settings:
Class A, Triode, Tweed Power (on the front).
Also EL34's in the outer sockets will give you more power tube breakup on any setting.

I assume my questions on these have been lost between the lines of my earlier posts:

When using 6L6s in all four power amp sockets how the sound and overall behaviour of the amp changes when using Triode and class A compared to Simulclass and Pentode. I hope this is all safe to try...that is one doubt why I have not tried these settings with 6L6s alone, did try with EL34 in outer sockets and did not really like.

Which settings are the best for MAX tube compression?

Fender amps with 2 x 6L6, do they run in class A or class A/B?

ADD: The manual says in class A mode just outer two power tubes work, simulclass has the inner tubes in parallel (or vice versa) with othe other pair. Does "parallel" engaging of inner power tubes make the amp to change from Class A to something else? This does not make sense to me!

Danke!

 

[scottywompas]

Do you have a copy of the MKIV owners manual. It goes into freat detail about what these functions do and what the expected results are.

http://mesaboogie.com/manuals/user_manuals.htm

You can download one and print it out.

Scott

 

[joegold]

Do you have a copy of the MKIV owners manual. It goes into freat detail about what these functions do and what the expected results are.

http://mesaboogie.com/manuals/user_manuals.htm

You can download one and print it out.

Scott

I'm not sure if you're replying to me or to the OP.
But yes, I have a manual.

But explanations of the creative ways in which which Mesa uses the terms "Class A" and "Simul-Class" are not covered.

It has been explained to me (by the head tech at Mesa) that the only Simul-Class amps that really had the outer tube pair running in "the Class A range" were the Simul-Cass MKII's and the early Simul-Class MKIII's, both of which had the outer tube pair in triode operation.
[See Randall Smith's explanation how hew gets 6l6's and EL34's to run in Class A here:
<http://mesaboogie.com/US/Smith/ClassA.htm>]
But the tubes available at the time, especially the EL34's, were not reliable in this circuit, so Mesa switched over to pentode operation in the later Simul-Class MKIII's and the in the 2:90. They also stopped recommending the use of EL34's in the outer sockets in favor of the more reliable 6l6's. Evidently running a pentode tube, like a 6l6 or EL34, in "the Class A range" is dependant, in no small degree, on triode operation of the tube. The newer "Simul-Class" amps (later MKIII's, the Simul-Satellite, and the 2:90) have their outer tube pair running in Class A/B pentode. The outer sockets are just biased funny so that the tubes break up earlier, put out less power, and have a different tone. But they're not running in Class A.
[In a Simul-Class map the outer tube pair, the "Class A" pair, has a -47V bias voltage. The inner pair is -52V. And, as far as I can tell, that's the main difference in how the two pairs of tubes operate. There's probably a bit more to it, but not much. I think it's the lower bias voltage in the outer pair that allows for swapping 6l6's and EL34's without needing to re-bias.]

Now, I suspect that when the MKIV is switched to triode mode it does indeed run the outer tube pair in Class A mode, just like the earlier Simul-Class amps operated. But when switched to pentode it is not running those tubes in Class A.

 

[Boogiebabies]

Then how, on the MK IV do they switch out the 2.2M soldered to pins 5 and 8 on the outer sockets ? All I see is the screens getting switched to the 6L6 primary.

 

[joegold]

Then how, on the MK IV do they switch out the 2.2M soldered to pins 5 and 8 on the outer sockets ? All I see is the screens getting switched to the 6L6 primary.

I don't think they have to.
My Simul-Satellite has these 2.2M resistors too, and it's pentode on the outer pair. So says my tech and so says Mike Bendinelli. My understanding is that it's the 2.2M resistors that are responsible for the lower bias voltage across the outer pair compared to the inner pair. I don't think these resistors have anything to do with pentode/triode operation of those tubes. I could be wrong though. I'm not a tech.

The schematic that I have for an early SC MKIII (one of the triode ones) looks like (again, I'm not a tech) it has a different resistor value here. It looks 960k.

If you mean that the 2.2M resistors need to be switched out in order for the MKIV to go *from pentode* to triode then that's another matter. The MKIV schematic makes it look like the triode/pentode switch just switches in/out a 2.7k resistor across the outer tube pair. How that all ties in with the OT I couldn't say.

Regarding our other thread about my Satellite...

My tech said that, in my amp, he found a ground connection, from off of one of those 2.2M resistors, that was intermittent. It was just a little jumper wire going from the resistor's contact on the lug of the tube socket that was supposed to be making contact with the base of the tube socket, a ground point. Sometimes it wasn't making contact and, supposedly, this was what was responsible for the weird bias voltage readings he was getting on my tubes. With the the resistor now making contact with ground the numbers are much closer. He says that the outer pair are now at -47V and -47.9V, and the inner pair are at -52V and -52.9V. Mike Bendinelli says that this is still off from where it should be (he said there should less than 1/10 o a volt difference between paired tubes), even taking into account the cheap Micronta tester my tech was using. But MB doesn't think it's worth it to pursue fixing it. Neither does another tech I talked to also. I.e. It's operating within acceptable tolerances, I guess.

 

[fatboy135]

Danke!

Bitte :wink:

 

[jukesgtr]

Cool additional info, thanks!