Mesa Boogie Simul-Class Operation ...

[Joey Voltage]

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

Yes, Roberto Mezzano. He's a very good friend of mine

[AE5881]

Hello All,

first, I'm from Germany and new here. I hope my english ( surely not the best :-) ) is fairly understandable.
I own a Mesa Boogie Mark IV A Combo since already 20 years now and I'm still very satisfied with the amp.Last week I bought a new pair Mesa Boogie STR 425 5881 6L6 power tubes, match group GRY, which I put in the outer Class A sockets. So far so good, the measured idle bias current was in a expected range of around 30 mA in Tweed Mode and 40 mA in Full Power. I must say, that was not always the case in the past, rather less or cold if you want so, especially with the inner tube pair (14-20mA).
But over the years, I did also learn, that my MK IV does not improve so much soundwise with a hotter bias. By accident I read this interesting post, especially regarding the Simul Class OT and of course I was wondering, if the OT of my MK IV also would not have the expected two add. tapps. I must say, until this week,I never had to take the chassis out of the combo housing. No techn. problems, beside tube changing from time to time. But this new info regarding the Simul Class OT was the reason for me to do it now. And beside this, so I have the opportunity after some tests perhaps to mod the bias circuit by installing a bias pot.

But to make the story short, the OT in my MK IV has 5 primary taps, so as it actually should be. According to the color of the primary wires it could be the 562004R-1 Typ, according to a topic of "The Boogie Board".
please look here:

http://forum.grailtone.com/viewtopic....+100W#p101074

the middle OT in the picture looks like the type in my MK IV.

In the mean time I did read also the US Patent from 1985 R. Smith about his Simul Class Power Amp concept, just for interest and understanding :-)


Kind regards

[Gregg]

But to make the story short, the OT in my MK IV has 5 primary taps, so as it actually should be.

It may have 5 primary taps at the outside but as it was already mentioned (actually reported by people who have disassembled some of these OTs) inside transformer there are only 3 taps.

[AE5881]

Hi Gregg,

thanks for your answer. OK, of course I can't look inside the OT and at time I have no reason to dissasemble it. But I did several measurements to get the approx. values ot the OT. First, if the primary taps would be connected together inside the OT as mentioned, then the DC Ohms should be zero or close to zero Ohms between them, isn't it?. But they aren't, at least with mine OT. DC value between the inner tube pair tapps is around 112 Ohms and between the two outer tapps around 90 Ohms, which results to a remaining DC Ohm value of 112 - 90= 22 Ohms. That means around ( also measured ) 11 Ohms for each add. tail winding.
According to this also the measured AC-Ohms values are different. I got values of approx. 3,3k at the inner tapp pair and 2 k at the outer pair.
BTW, I believe that the real wiring of the power tubes and the OT is according to the US Patent, drawing Fig. 1 .
There the plates of outer tube pair(ClassA ??) are connected to the inner tapps and the plates of the inner tube pair (ClassAB or B) to the outer tapps.
This should be correct, also according to my measurements (the Ohms values of the inner tapps would be higher then the outer tapps, at least as drawn in the available schematic) It seems that the tapps are simply drawn inverted, perhaps for better reading only.
Please consider, that the AC values surely not exact, due to my limited measurement possibilities. But what counts IMHO is that they are clearly different.

To clear things, I surely still like my MK IV but also other brands of tube amplifiers. I'm dealing with tube technics / amps, also old radios btw. for a long time.
My intention is clearly not to start controverse discussions between M/B haters and lovers.
Its all for techn. interest, information and understanding

For example, at time I'm testing, if it is worth to mod the MK IV's Bias circuit for installing one or two bias pots, especially regarding the Simul Class design with it's different bias values for the inner and outer tube pairs.
But this is another topic for later :-)

In this sence
Kind regards
Arno

[teemuk]

I'm quite convinced that they may have built the Simul-Class amps with both transformer styles, initially with those UL-styled ones, and - after figuring out those really did not add anything essential to the circuit's operation - with ordinary ones. It's not an ultra-linear connection in the first place, merely just a plate voltage reduction scheme and a way to alter the impedance matching a bit, but likely they just figured the amps work better without all that. Manufacturers do retain rights to change designs the way they like and designs have a habit to evolve.

Why they kept it "secret" and just did an internal connection could be for several reasons, such as:

- They did not want folks to start all kinds of rumours why some of the amps have tapped transformers and others don't, possibly they also wanted to avoid such issue when providing replacement parts. eg: "No, I want a tapped transformer because that has the MOJO!" - "Sorry we don't have them in our inventory anymore." - "Well f*k you then! I'll tell everyone that Mesa sucks!"

- Or... They just wanted to keep the repair procedure simple: same colour codings and connection styles prevail and no need to issue confusing corrections to service notes telling how to install the replacement transformers that would have been different style. Instead the new replacement transformer fits in just the same way as the older one.

[AE5881]

Ok, that makes sense and could of course be the case either way. BTW, for a long time MK-IV schematics from 92 seemed to be available with a normal 3 - Tap OT drawed. I had luck and got this version 1993 complete from the former German Mesa Boogie distributor. A later revision then had the 5-tap OT drawed. I bought my MK IV-Combo Jan. 1991. According to the hand signature on the chassis the amp was built 10 / 1990 :-)

Kind regards

[Bruce / Mission Amps]

To be clear... are you saying that, based on your DC resistance measurements, the outer taps are actually connected to the 6L6s and the inner ones are connected to the EL34s?

[g1]

I'm quite convinced that they may have built the Simul-Class amps with both transformer styles, initially with those UL-styled ones, and - after figuring out those really did not add anything essential to the circuit's operation - with ordinary ones.
...Why they kept it "secret" and just did an internal connection could be for several reasons, such as:

I would agree they probably realized it didn't make much difference, maybe by accident, maybe by testing.
As for keeping it a secret, the lack of the extra taps would invalidate the patent wouldn't it?

[AE5881]

To be clear... are you saying that, based on your DC resistance measurements, the outer taps are actually connected to the 6L6s and the inner ones are connected to the EL34s?

Hi Bruce,

yes, I also was rather confused in the first moment after my measurements, but the tap wiring seems to be like the schematic Fig. 1 with description of the M/B US Patents Nr. 4,532,476 + 4,593,251, at least with my MK IV.

One moment, I'll look after the link for the patents

ok, here is the link
http://patimg2.uspto.gov/.piw?Docid=...iew+first+page

if you go to page 2, you will see the mentioned schematic

Regards
Arno

[teemuk]

As for keeping it a secret, the lack of the extra taps would invalidate the patent wouldn't it?

It's likely up to attourney's / jury's to settle that. ;-)

Anyway, I'd make a strong case that the entire operating principle of the circuit is prior art. In fact, such prior art is even referenced in the patent itself, and you can also find similar circuits described in the Radiotron Handbook.