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Mesa Boogie 50 Caliber+ Bias Cct. ??
Hi All,

I have the misfortune to have a Mesa Boogie 50 Caliber+ (2 x 6L6) on my bench, with some rather dodgy Bias mods and various other examples of shoddy work.
I reluctantly took this on to help a good friend out and feel obliged to get it working properly for him.

The Bias cct. has been modified to incorporate an adjustment trimpot, which has a faulty track, causing the Bias to go from cold to way too hot with the smallest movement.
I can appreciate the motive behind the Bias mods, but the way they have been done is terrible.
I have removed all the components of this modified Bias supply and now want to make it work properly.
The problem is; I am not up to speed with Mesa amps, due to avoiding them, whenever possible.

Are any of you familiar with the Bias cct. and can you provide an accurate schematic? 
I have downloaded a couple of schematics from the web, but I'm not sure whether they are correct.
Can you explain this unusual (to me) Bias cct. and the Bias connection to the phase inverter cathodes?
What would be a good method of installing an adjustable bias cct., so the Bias can be set for non-Mesa branded (highly expensive) valves?

Thanks for any help.

Hi Noel

The splitter in some Mesa amps uses a negative supply for the tail resistor, which allows the grids to be ground-referenced. This has nothing to do with the bias of the output tubes except that the only negative supply is for the output tube bias so both circuits share it.

Adjustable bias is definite;y something to add - and in this case retain - on any Mesa amp. The pot(s) should be panel types NOT trimmers. Trimpots only have 200-cycle lives where even the cheapest panel pot is good for 15k-cycles. Mount the pot(s) on the tube plane of the chassis so they are accessible and protected like the tubes. Add meter jacks on the rear apron to measure the voltage drop across cathode current-sense resistors that you have to add as well.

Remember to add safety resistors across the bias pot(s) from the pot-0 (cold, negative supply input) to the wiper. Pot-X should be the end grounded via the range resistor. Pot-X is then the correct orientation for 'hot' bias.

Have fun
Hi Noel

If you revert the amp to stock you can measure the voltage across the bias filter cap, since this is used to feed both the output stage and the splitter. This gives a reference for sensible mods.

The cap is unprotected from over-voltage, say if the splitter tube is removed. It would be good to add a zener across the cap that allows there to be somewhat more negative voltage than is present now. This will give you leeway for the bias adjustment portion and allow most tube samples to be turned off, letting you control how 'on' they are at idle.

With more negative voltage on the cap than stock, the tail resistor for the splitter has to be increased in value. It basically withstands the full bias supply voltage,which the schematic shows as 17V or 19V - difficult to read. Anyway, the tail resistor has this plus 2V8 across it and is a pair of parallel 33ks - 16k5 total. Suppose you increase -Vb by 20%, then increase R-tail by 20% and the splitter is happy. Note that you can check the splitter voltages with the power tubes out.

You might also want to add a diode from the cathode end of the tail resistor to ground. Place the anode at ground and the cathode on the resistor. During power-up, the end of the resistor will be held at one diode-drop below ground, protecting the tube. Once the tube warms up, the diode will be reverse biased as the resistor is pulled to its 2V8 level noted on the schematic.

Have fun
Hi Kevin,

Thanks for the info and suggestions.
Do you have a schematic of the 50 Caliber+ that you regard as accurate that you can post, for me to refer to?
I'm not sure whether the ones I have are correct - Schematics attached.


.pdf   mb_50-caliber-plus. The Flying Dutchman.pdf (Size: 71.06 KB / Downloads: 3)
Hi Noel

The basic schematic is correct. However, it depends on whether the amp uses 6L6s or a quad of EL-84s which values are correct for the bis circuit. Mesa's drawings are always backwards with respect to signal input should be at the left and output to the right. I believe Randall drew the schematic as he would see the circuit in an upside-down chassis (Fender style).

Have fun

Attached Files
.pdf   boogie_50cal.pdf (Size: 142.31 KB / Downloads: 4)
Hi Kevin,

As can be seen from the attached photos; the amp is in a sorry state internally. This is how my friend bought it.
I have since cleaned it up a lot, but it will need more to be acceptable.
You can see the charred section of the PCB near the Bias circuitry. 
I don't know the cause of this damage but, after further testing, I have found that the PCB is conductive in this area.

The owner wants to either, fix it and sell it or, use what can be salvaged to make him a new high gain amp.
He is an honest bloke and won't onsell a dodgy amp; but, if I continued to use the PCB, I would also have to replace a lot of the known unreliable components used in these amps. 
This would be a rather costly job and he may not recover the costs considering that he paid $750 for this amp, as it is.
I am now faced with either, isolating the charred board from the local circuits and making a remote Bias board to feed the PI and PA valves directly from the new board and hoping that there won't be other dramas, or to junk the whole PCB and reuse the hardware for a whole new amp. 

Thanks for your time,


Hi Noel

Do you know if the board was damaged before or after the other tech did his mod? In any case, that whole section has to be cut out as burned fibre glass is always conductive. Don't use open trimpots.

Although it seems like more work, it may be most cost effective and quicker to remove the whole board and throw it and everything on it away. Install chassis-mounted sockets and use eyelet boards for a new circuit - not necessarily a copy of the old. That was my solution for a similar Mesa experience. This lets you fix everything wrong with the original - lack of grid-stops; folded signal path; poor quality parts; poor design choices; crazy switching circuit; difficult servicing; lack of proper bias pots and measuring.

In the rebuild, use a standard splitter without the negative rail.

Depending on the supply voltage, you might have to get 500V caps and then you can use singles rather than series-connected ones. Modern snap-mount and radial caps are tiny and inexpensive even if they are 10k-hour types. The old caps are past their useful life regardless of how they might measure.

Of course, it might be even more cost effective to scrap the amp. People make bad choices and in the greater scheme of things whatever the customer paid for this is peanuts in the spending life of most musicians who constantly change gear. (To me mesa is always a bad choice).

Have fun
Hi Kevin,

He doesn't know the history of the amp, except for knowing who the Tech is who did the dodgy Bias mod. I have seen this Tech's work before and repaired some of it. I don't understand why he would have persisted with that burned PCB.
I am curious to know what caused the burn. Maybe the cement resistor hanging off the side is evidence that the original failed and cooked the board, like the Fender amps that do the same thing.

I had a good discussion with the owner yesterday evening and told him much the same as you have said here and he has agreed that we should scrap this PCB and build a new amp. He said that it is better to wear the bad choice of this amp and move on.
To that end, it is now sitting on my bench completely gutted, as a pile of components.

The cab, speaker and all the amp hardware is in good condition and the amp actually sounded really good (surprisingly) before it started misbehaving. I am still surprised that it worked at all and it was always going to be a time bomb, with that shoddy work and the old crap components.

The plan is: to use the cab, chassis & transformers (without any Mesa labelling) and then make a new amp, with quality components, on an eyelet board with the Pre and PI valve sockets mounted on the chassis (the PA valve sockets already are) that can be serviced and sounds great.
The supply voltage runs close to 490V at start-up and settles to 460 at idle, so some good 500V caps will be used.

I agree about Mesa amps; some of them sound good but they cost way too much, for what they are and working on them is a nightmare, which is why I have stayed away from servicing them.

This rebuild may take a while to complete, due to my workload but it will be worth it and he has been using the 40W amp I built him anyway because he says it sounds better than the Boogie did.

Thanks again and I am having fun.

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