MMB test drive tips wanted - PASSED the TEST!

[Sidney Vicious]

I have a MMB project which I think is finally ready for launch. New old stock transformers, new caps, - original pots, tube sockets and other chassis components.

I soldered everything into place last night and have double checked to make sure the wiring is correct as per the layout drawing and reference board I have. All solders appear to be solid.

Have been told that before I fire it up with tubes I should do so without tubes in it - what will that tell me if anything? What should I look for in the untubed but turned on state?

I do not want to burn the trannies or anything stupid.

Or should I simply put the tubes in hit the switch and smell?

What say you?

Edited to add:

My search tell me it is to check voltages therein?

Which pins/what voltages/any tips and tricks welcome.

[Tremo]

I'd fire it up first without tubes, and check all the voltages to make sure they're at all the right places. Measure the tube socket voltages to make sure there are no mis-wires. and of course, look for smoke.

If you pass that test, plug in the preamp tube and do more measuring, make sure it's drawing the correct current, and inject a signal and make sure you have grid drive going to the outputs.

Lastly, plug in the outputs and do more measurements.

[Sidney Vicious]

Tremo:

Thanks for the input but I am very new at this and need more detailed info - any place you can direct me? Besides sending it to a tech? Isn't it something I can check with a multimeter?

[Dacious]

If you get your DMM and set it on DC in a scale of up to 400 volts, you should be able to touch it to points in the wiring and read voltages.

On the MMB schematic off Ampwares, off the rectifier diode junction you should get 295 volts. On pin 3 of either power tube socket you should get 290 volts.

At the output of the second 20uf filter cap you should get 200v.

On the wire between pin 6 on the 12AX7 and the phase-inverter transformer you should get 246 volts. On pin 3 you should get 165 volts.

Now, these figures will be higher, by 20-50 volts because the tubes aren't in. Also, I think you indicated you were using a different transformer. If you get no voltage it indicates a bum connection

But if you get these figures +20-50 volts and no smoke, you can risk sticking the tubes in as Tremo says, preamp then power amp. No smoke - all is probably good.

Clip your negative lead to the chassis, and keep one had in your pocket, use the +ve lead to check the points carefully - don't earth on another component or the chassis.


MMB Schematic

[Sidney Vicious]

Thanks so much - will do - and will keep hand in pocket!!!!! - and will report.

[Sidney Vicious]

Did as suggested and carefully went through some preliminary testing of circuit without smoke, fire, shock, or melting.

Put it all back together, plugged in a guitar, and SWEET!!!

Thanks to all.

Sidney

[Rizo]

Congrats, sounds like good one. Are you planning (or have you made) any mods?

[Sidney Vicious]

I may install a fuse - it already has a non-original Weber Sig Alnico.

As it is, it sounds perfect to me so I will do nothing to change the signal path.

[Rizo]

Yeah, I think a fuse should definately be added, just in case. For the life of me, I don't know why Fender never put 'em in.

[Tim Armstrong]

Maybe they were con-fused.


Thankyouthankyouverymuch, I'll be here through Thursday. Remember to tip your waitress!


Tim

[jonkay1]

Pardon me for asking, but what is an MMB?

[Spidercaster]

MMB = Fender Music Master Bass amp, which converts very nicely to a guitar amp.

[Sidney Vicious]

And there was one modification done the amp - a .047 cap at the input was removed and the connection jumpered.

(Spidercaster - my little boy would love that axe of yours. :) )

[Spidercaster]

Thanks Sid ':D'