Why no bias select switch on MK IV?

[boola]

Hi,

I was wondering, how does the Mark IV allow you to change between EL34's and 5881's without needing a bias select switch like other Mesa amps?

Why don't you need to change the screen resistance or is it somehow done automatically?

Thanks!

 

[chris_EIT]

I don't know much about amps but I'll have a stab!
Class A and Simul-class allow this transition
Class A's applications with be the only one allowed to use EL34's, and everyone says that mesa are biased very cold, I'm probably wrong here but maybe these allow for the differences between 6l6 and El34 in the outer tubes!

Correct me guys, teach me XD

 

[Elpelotero]

nothing is done "automatically." Mesa's have a fixed bias. This means that the amps have an internal bias that is set cold. You can not put any random tube in there and think it will be ok. Mesa suggests a range of tubes that you can plug in and be safe. Each of these tubes fall within a range that is acceptable for the fixed bias. If you put a tube that falls outside of this range, it will either blow up and get burned quickly.

I suggest you call the factory before Thursday afternoon and ask them. They will tell you technical details specific to your tubes, how to check them, and how to know if they'll match with the mark4.

P.S. Using other manufacturer's tubes besides Groove Tubes or Mesa will void the 5 year warranty on your amp.

 

[6L6C]

I don't know much about amps but I'll have a stab!
Class A and Simul-class allow this transition
Class A's applications with be the only one allowed to use EL34's, and everyone says that mesa are biased very cold, I'm probably wrong here but maybe these allow for the differences between 6l6 and El34 in the outer tubes!

Correct me guys, teach me XD

Chris your right!

Also another reason you don’t see a bias select switch on the Mark IV like some other Boogies. With Boogies with select switches it allows you to change out the whole power section say 6L6 to EL34. Not supposed to do this with a Mark IV outer tubes only can be changed as Chris already stated. To cool part is the mix of the 6L6 and the EL34 in simul-class gives the amp a real nice flavor.

 

[boola]

Yes, it must be something to do with being a class A circuit. I believe that other Mesa amps which have the bias select swith are AB.

 

[Monsta-Tone]

I really get tired of this 'Class A' BS. It's all marketing.


The truth of the matter is that the outer 2 tubes are Cathode Biased, which means that the Cathode is not tied directly to ground. It has a Bias resistor between the Cathode and ground.

This is a sort-of 'Self Bias.' This is the same thing that the Vox AC30 uses.

I have done this mod to my DC-3. It does not operate in Class A, which means that the tubes are constantly being pushed to the limit. It does mean that I can use any brand of EL-84 and the amp will Bias itself.


The whole 'Class A' thing is blown way out of proportion. Randall Smith is a great marketing agent. His paper on Class A explains nothing, but is a good read. :lol: Maybe he should run for President, he's got my vote.



The truth about the Mark IV is this:
Outer tubes = Cathode Bias, which means that they can self bias with either 6L6 or EL34
Inner tubes = Fixed bias, which means that the Power Supply circuitry will allow for tubes that are in the general range of the Mesa tubes.
Tweed Power allows the use of 6V6's because it lowers the B+ voltage and allows the amp to operate properly with this type of tube.

It's a really brilliant scheme. Sell the amps with an un-adjustable bias, and then sell the owners the tubes that are within your specs. :lol: Wish I had thought of it.

 

[Boogiebabies]

I just plug my tube in and let them rip. The worst thing you can do is try to correlate how your amp make good tone. I mean really, the outer pair is not running at 300V with a -11 bias. It's class A moniker is a stretch, but I really dont care. It sounds amazing.

 

[6L6C]

The whole 'Class A' thing is blown way out of proportion. Randall Smith is a great marketing agent. His paper on Class A explains nothing, but is a good read. :lol: Maybe he should run for President, he's got my vote.
It's a really brilliant scheme. Sell the amps with an un-adjustable bias, and then sell the owners the tubes that are within your specs. :lol: Wish I had thought of it.

There is someone else on this board that you would get along with. (Don’t want to mention names) He did some work for me and we got to talking and pretty much said the same thing although he used more colorful terms for the white papers. :shock:

But did say the thing works and likes the sound of Boogies

 

[Monsta-Tone]

The worst thing you can do is try to correlate how your amp make good tone.

:lol: :lol: :lol: :lol: :lol: :lol:



That's how I feel about theory. I used to get really caught up in what I was playing. Now I figure, if it sounds good, it's good. Makes playing much more fun when you just do it and enjoy.

 

[boola]

Yes I agree but unfortunately some of us NEED to know how things work

Thanks for your explanation!!

 

[devilrob1979]

Get yourself some Winged C or Svetlana tubes and don't worry about it. Everything will be fine. These things are biased so cold that it'd take a whole lot to fry your transformer.