Tryimg to understand "Simulclass"

[scotteggshell]

I am trying to understand the Simulclass Power section in my MkIV, if someone has time could they help me out ?

By Looking at the diagram at :
http://www.schematicheaven.com/boogieamps/boogie_mkiv.pdf

Please tell me if I am wrong.
1) It appears to me that one set of tubes is run as Class A and the other in class AB.
2) The 220K(A) res on the screen of one set and the 2.2k(AB) on the other set are used to set the different bias points
3) To by controlling the values of the 220k res on point E you could lower the bias point of both tubes and keep the same difference between them(could I use a rheostat here ?). Thereby biasing one half of the amp with each trimmer
4) OR should the bias voltage be controlled at it's source and keep all 4 tubes ganged together.

Thanx

Scott

 

[avoelp]

... you may want to take a look at

http://www.mesaboogie.com/US/Smith/ClassA-WebVersion.htm

- maybe the answers are in there.

 

[stokes]

When you use the class A switch in the class A position it lifts the cathode of the inner tubes and takes them out of the outptu.Without getting into al the why's and wherefores,it isnt true class A.Neither pair of those tubes are biased anywhere near class A.When in the simul-class position all the tubes are operating,normally 2 6L6's in the inner and EL34's in the outer,they have different bias points by way of the 2.2m and 2.2k resistors,but to alter the bias or make it adjustable you wouldnt put a pot there,you have to do it at the bias supply (not shown on that schem).To get all 4 tubes to bias at the same current draw you could tweak the 220k's and 2.2 k&m resistors,but then you wouldnt have "simul-class".

 

[scotteggshell]

A million questions arise ..
-So, How do you bias these simulclass amps ?
-What idle current should the "class A" tubes be drawing ?
-What about the others ?
-If I just put a control on the bias supply then I will be changing the bias of all tubes at once, but what am I shooting for ?
-Do I set the current in the "class A" tubes and let the others fall where they may ?
-If so what current value ? As you said not high enough for class A, which if I am correct is 100& max Pwr dissipation
-Or should I bias the other tubes for 70% and let the "Class A " fall where it may ?

I'm a bit confused. Thanx

s

 

[avoelp]

A million questions arise ..

... and, just out of curiosity - why would you want to change the fixed bias at all?

 

[stokes]

A million questions arise ..
-So, How do you bias these simulclass amps ?
-What idle current should the "class A" tubes be drawing ?
-What about the others ?
-If I just put a control on the bias supply then I will be changing the bias of all tubes at once, but what am I shooting for ?
-Do I set the current in the "class A" tubes and let the others fall where they may ?
-If so what current value ? As you said not high enough for class A, which if I am correct is 100& max Pwr dissipation
-Or should I bias the other tubes for 70% and let the "Class A " fall where it may ?

I'm a bit confused. Thanx

s

In order to make the bias adjustable independently in each of the pairs,you need two seperate bias supplies,there is not enough room in the chassis of this amp to do so.What you have to do is either set the bias for the hotter pair (inside pair) and let the other fall where they are,or tweak the bias feed circuit to the outer pair so the two pairs match,but like I said,you will then defeat the so called simul-class feature.With the amp as it is,if you were to set the outer tubes to be true class A,when you switch to simul-class the inner pair would melt down.I dont know why Mesa claims the outer pair to be class A,just a marketing ploy,IMO,since I have yet to see one of these amps that had more than 12-15ma's in the outer pair of EL34's,which is not even close to a hot AB.As for the "simul-class",that is a term Mesa made up.All they did was take an ultra-linear output tranny and connect the screen taps to a pair of tubes and tweak the bias feed circuit so you would have a slightly different current draw to one pair of tubes so you could use 6L6/EL34 combo and call it "simul-class".In my experience,it is not that dramatic an effect.If you tweak the circuit so the outer pair draw the same as the inner pair and install all 6L6's and bias it as cold as Mesa does you most likely wouldnt hear a difference,except maybe in clean or rythm mode.Once you switch in the lead drive,the preamp creates so much distortion the power tubes have little affect on the tone.It is nothing revolutionary,others have experimented with mixing output tube types,Mesa was just smart enough to patent and market/hype the idea.
... and, just out of curiosity - why would you want to change the fixed bias at all?

A million questions arise ..

"... and, just out of curiosity - why would you want to change the fixed bias at all?"
The only reason to make an adjustable bias in a Mesa is so you dont have to use the crap current production tubes made today,or to tweak the bias in tubes that are "pre-selected" to match Mesa's specs.In the case of simul-class amps,it is a bit more complicated than in non-simul amps.

 

[phyrexia]

Excellent posts, stokes.

Even if the outer sockets are not true class A, I can still hear a decidedly different sound/feel from my Simul amp, compared to my 60 and the 100/60s I've played.

 

[dodger916]

Excellent posts, stokes.

As usual.. 8)

Even if the outer sockets are not true class A, I can still here a decidedly different sound/feel from my Simul amp, compared to my 60 and the 100/60s I've played.

+1. Sometimes I like it better....sometimes I don't.

Stokes: If I may drift back to the previous thread...when you do the bias pot mod, does one pot control all 4 sockets? In that case, when getting tubes, would I want two sets of matched tubes whose current draw differs by a certain amount? Or, am I just demonstrating publicly my ability to talk myself into trouble, but not out of it?

 

[stokes]

Yes,one pot controls all 4 sockets.As the amp is now,one resistor controls all 4 sockets.The outer sockets grid resistors are tweaked to use EL34's so if you put 4 6L6's in it as it is now the outer pair are going to bias up colder than the inner pair.

 

[scotteggshell]

Thanx stokes, good detailed info , but I am unclear on one thing. Why do you say the inner tubes biased hotter ? Shouldn't the tubes(outside) that are near class A be hotter ? Or am I not getting something very basic. Looking at the diagram it seems to me that the outside tubes have a lower neg voltage applied to the grid than do the inside ones - isn't that considered "hotter" as in more idle current ? Or am I bassackwards?

As to why I want to change the fixed bias, truth is I probably wont change it - I can get Mesa spec tubes. But I want to understand the circuit better and be able to change it if I so desire.. I am a bit fuzzy on power sections.

thanx again

scott

 

[Boogiebabies]

A small issue with Mesa's bias supply is that even if you had two pots, Mesa usually implemented the circuit to bias the power tubes
as left pair and right pair, not inner and outer as on a C+. I would have to pull the MK IV chassis to confirm, but it all may be just whizzing up a rope.

 

[stephen sawall]

Interesting thread. :mrgreen: :arrow: :idea:

 

[scotteggshell]

A small issue with Mesa's bias supply is that even if you had two pots, Mesa usually implemented the circuit to bias the power tubes
as left pair and right pair, not inner and outer as on a C+. I would have to pull the MK IV chassis to confirm, but it all may be just whizzing up a rope.

I think you are right there is a 220K resistor that is shared on both tubes bias circuit on the left and the same on the right. If I am reading the diagram correctly it looks like the Vneg-220K - 220K -2.2M-Gnd forms a voltage divider and the outer tubes come off the 2.2M / 220K junction and the Inner come off the 220k/220k junction. So it's either change one at a time, both left , both right or all at once. Or totally redisgn the circuit. But I am a bit of a novice and may be incorrect on all accounts.

Scott

 

[stokes]

A small issue with Mesa's bias supply is that even if you had two pots, Mesa usually implemented the circuit to bias the power tubes
as left pair and right pair, not inner and outer as on a C+. I would have to pull the MK IV chassis to confirm, but it all may be just whizzing up a rope.

As far as the output tranny is concerned they are paired as "left pair and right pair",but if you look at the schem the bias is supplied to have the inner pair having the same bias as each other and the outer pair biased up the same.When you switch to "class A",you remove the inner pair from the circuit,with the amp in simul mode you have two tubes on each side of the OT that have a different current draw,so that side of the OT "sees" an avg. of the two.
When I refer to the outer pair being cold,I am going by the numbers when you install 6L6/El34,a big problem with the adjustable bias mod in simul-class amps is that if you try to bias the outer pair even as high as class AB,forget class A,the inner pair get too hot.So it is a balancing act.I did help one poster here tweak the outer pair to get it closer to the inner pair,but in our combined opinion,it wasnt worth the effort.Better to find a pair of EL34's that will bias up hotter at the bias setting you settle on for the inner pair.Another guy I did the mod for had Doug at Dougs tubes get him the hottest EL34's he could and it still fell short of matching the inner pair,and didnt even approach class A.

 

[90wreck]

I had no idea(I did have a clue...but this goes into all the detail I really wanted to know).....Thanks for all the info.