Mesa Simul-class Power Amp

Here is an apparent schematic of (one of two channels) of Mesa Boogie's 'Simul Class' 90 watt power amp.  Their description claims that of the 4 output tubes, two run in class AB and the other two run in class A, hence 'simultaneous class' (both A and AB at the same time):



I can't for the life of me figure out how this is working as advertised.  The inner pair is grounded cathode, with a standard 1.5k grid resistor, and 470ohm screen resistor, connected to the inner taps of an ultra-linear power transformer, with a fixed bias of about -55V.  The outer pair appears to have a resistive divider at the grid that reduces the fixed bias voltage down to about -42V, and the screen grid resistors are 2.7k instead of 470 ohm, and they connect to the outer windings.  Cathodes are grounded like the other pair.

How is this output tube set biased in class A?  As near as I can tell, the bias is certainly 'hotter' in the outer pair by 7-10V, but this would not cause the tubes to conduct for the entire 360 cycle as is required in class A, as a significantly 'higher' bias voltage would be needed to ensure the tubes both conducted for the entire input cycle.

Am I missing something obvious here?

Well from the manufacturers point of view, never let the truth get in the way of a good feature! Also, the outer tubes get less a.c. drive, and have less contribution to the output power.

I've seen the VOXAC30 and clones advertised as "class A"... For hype purposes, cathode bias means class A

https://mesaboogie.zendesk.com/hc/en-us/articles/225099807-What-is-Class-A-and-how-does-it-differ-from-Class-AB-?mobile_site=true

Insight into the Mesa train of thought....

It's not "ultralinear".

My guess is the hot pair sees a 6.6k load, the cold pair sees a 4k load. The 6.6k would be a near-A condition for hot 6L6GC at 20W-30W output. The 4k load would suck ~~50W a pair, but the idle should be cooler and the swing larger. The hot pair would have a little more gain so a small cut-down makes sense to blend well.

If I am right and the two pair work near tubebook loadings, then the tap ratio is 88%, not the 40%-20% which makes sense in UltraLinear.

Ahh I see - the tap point of the transformer isn't specified, but maybe it's drawn in such a way as to show the tap is very close to the outer winding.

However biased at -47v I can't see how this could possibly be conducting appreciable current at a 20V PTP input swing when the input to those tubes drops down to -67V or so.  That doesn't seem to be "near-A", or am I missing something?

EDIT: I didn't consider the 500V supply, and the 6L6 still passes 20mA at even -70v!  So indeed it could be near A at these voltages....

> at a 20V PTP input swing

Where do you see that?

I assume the driver can swing the 67V peak (134V peak to peak) needed to swing the cold tubes to zero-grid. (Barely: it swings ~~74V peak, probably to avoid deep overdrive grid-blocking.)

willing to bet that this came about on a boring day in the lab where somebody says what would happen if you ran el34's and 6L6 tubes together,  cold bias for el34 would equal hot bias for 6L6  , so if you do not provide different bias voltages for the two tube types, you would have a simul-class situation, throw in a tap for pressure relief and there you go.

Mesa did that a ways back>

https://groupdiy.com/index.php?topic=62782.msg795440#msg795440

I tried a similar idea one time , I copied Mesa's grid bias divider idea, but I didnt have the tapped transformer ,so I just connected both the El34 and 6L6 in parralel on each leg of the transformer , It could run either on El34's or 6L6's or a combination of both , set by toggle switches at the back . I arranged to lift the cathodes of the pair not in use .
Of course the o/p transformer wasnt optimised at any setting as it was designed for 4xEL34 .
I think some of the modern Boogie amps provide a 6L6/EL34 bias switch which functions very much like the grid dividing network in the original simulclass , you dont mix tube types across an output stage ,but you can easily swap from US(6L6) to UK/Euro EL34).

I experimented with different  voltages for bias and signal  , where the EL34 pair was set to run red hot class A , the 6L6 biased much cooler maybe up in class B or completely cut off at low signal levels , the integration in sound terms between the two pairs of valves was much more subtle than I had expected ,  I only tested it for guitar purposes of course , when things were set up  right and you cranked it,it packed very serious punch ,

Im glad I experimented around with it , and its something I plan to look at again ,
There is if anyone can filter search Wireless World a few articles relating to similar ideas back in the valve days , 'Extended class A' rings a bell for some reason .

I found this , not a great link ,just about readable ,
http://www.itishifi.com/2017/05/extended-class-amplifier.html

better link on page 72 here ,
https://americanradiohistory.com/Archive-Radio-News/50s/Radio-News-1953-09.pdf