Mesa Boogie Triple Rectifier red plate special

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CJ

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9 power tubes, what a beast, but wait, 3 of them are 5U4's, wtf. over?

ahh,extra current needed over the dual rectifier which has 4 power tubes,

leave this space heater on for too long and you lose bias on the EL34's,

what to do?
 

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well, you can do somethings like adjust negative bias voltage, but we need to swap resistors for that,

you can get new tubes which might be more stable than the old ones, but that looks expensive and these old tubes test pretty strong,

what about the James Brown trick of combining fixed and cathode bias so if the tubes start to run away the self bias clamps them down?

could be a hassle with this chassis so we will adjust bias after a good warm up and a fan might be a nice touch,
 

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no schematics out there for the triple yet, but word has it that it is exactly like the dual, only different,  :eek:

so here is the pwr supply for the dual which is where we need to operate,

dual rectifier just means you have your choice of either solid state or tube diodes,
 

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here is the audio part of dual rectifier,

there is a bias switch on the triple for 6L6 or EL34,

Mesa wants you to use their tubes, that way they can leave out pots for the bias and keep people from fiddlin, but with the advent of new tube companies now days, there becomes a need for bias adjustment capabilities, especially with JJ 6L6 tubes which run on a lower bias than old school tubes,
 

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> what to do?

Rip it all out, put in a tag-board and 6550s.

Me I'd lose the bottle rectifiers. Eight 6550 is a good number, even for 150W out. They'd loaf.
 
we are going with 9 KT 120's, should be good for 500 watts with a stompbox,
mis matched DC will give it some funk,  :D

we stuck a 18 K in place of the 15K bias divider, went from 39 volts to 41, and mixed and matched the tubes for better balance as there was 20 ma difference,

should be good for another 1000 hours,

 
back on this Mesa Triple, power transformer was getting hot enough to fry an egg on, this amp has the option for either 6L6 or EL34 tubes,

RCA receiving manual says heater current for 6L6 is .9 amps, 1.5 amps for EL34

so if the EL34 option was an afterthought by Mesa, and they did not use a bigger transformer,

well, 6 * .9 = 5.4 amps heater current for 6L6
6 * 1.5 = 9 amps for EL34 heaters

this has been an age old problem for rockers wanting to hot rod their Fenders by adding crunch with EL34's,
easy to re-bias but not much you can do for the pwr xfmr,

so we needed to help the pwr xfmr shed some burden , but no room for another 120/6.3 transformer

so we rip out the choke and install a Hammond 166S6, rated for 115/6.3 at 10 amps, should do the job, but...

see that 115 VAC? big trouble, we get 7 volts now on the EL34's,

so we strip off 2 turns from the 28 turn sec, bake and dip, and get 6 volts now, which  is a good fix as it keeps this hot chassis a little cooler,

pwr xfmr is now running  as cool as a cucumber and the heater xfmr is a bit warmer but not much,

we theorize that the pwr xfmr got so hot that the bias voltage begin to droop enough to cause thermal runaway of one of the tube banks (either side of a P=P circuit will red plate, but usually not both as once the tubes get hot on one side, they hog the current)

problem is now we do not have a screen choke,

solution is use a resistor, this puts control of plate current back inn the hands of a resistor which can be changed to further tame this beast,

and we can use Fender cold bias specs(60% plate disp. bias) to keep things even cooler during set breaks,

here is the amp with the aux xfmr added,

hopefully this will cure the amp, if not we have other issues, like ultra sonic osc. from a channel or two,

if so, we get out the scope and the pico farad caps and get after that also,

ripped out 12 Henry choke with 115 ohms DCR shown next to xfmr


 

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