MK111 Heater resistors

[user 41014]

I´ll check as soon as I open the amp up, to change the main electrolytics, hopefully in a few days. If I remember correctly (wasn't now able to see in the pictures I took) there was one overall pot (likely replacing R117), and one for each side which I think both balanced the outer/inner tubes (I have simulclass) and also affected the balance between the sides.

I'll see what the pots do and draw a schematic of where the three pots are connected and post here. Will be interesting to see if there's any comments if this is a good arrangement, or if there are any drawbacks in the way it's implemented. Anyway, it was also hard to understand what I actually would aim for as idle plate current for the inner/outer pairs, there seemed to be different thoughts on that.

Much obliged

 

[user 41014]

Boogie tends to cold bias tubes, inner tubes are for Class A/B and outer are for Class A. A typical Class A amp has tubes biased at 100% of plate dissipation, you can see Boogie is around 70%. A typical Class A/B tube amp will be biased around 70%, you can see Boogie is around 50%.

If nothing else, the way Boogie biases certainly helps extend tube life!

Based on my love of EVM's and my old marks I would add (for good or bad based on what you like) colder biasing in general boosts the headroom I seem to be a fan of when you cranks it up. I like my saturation from the pre's and thunder from the power tubes of 6L6GC's / 7581A.

But it's not like Mesa didn't learn their lesson pushing the engine of their ported and polished caliber experiment.

Idr if it was the .50, .50+, 50 cal, etc but their is a 4xEL84 version which is the epitome of why you should always pull a used chassis when acquiring one. It was biased so hot iirc it burned up the circuit boards enough to get a reputation before switching to 6L6 like the big brother marks. I got one from GC that had the burnt smell and sure enough that amp went right back whence it came.

Mesa followed it up by knocking one out of the park with the venerable DC5 tho stock it's not so bedroom friendly.