| Author | Message | ||
     Michael Crimmins |
I am the proud owner of an immaculate condition (10+) MM 410. Steve verified the model with me about three years ago. Here is what has happened. The amp sat for the last 2 and 1/2 years. I had a new custom Carvin guitar built for me and when I went to plug my amp in, it sounded like sh--!! Absolutely muddy and no "depth." I am an auto mechanic by trade, not an electrician, but I would also like to figure out how all of this works. I took to a tech and it needs new tubes. I wanted to install them myself and try to learn how to do the biasing thing correctly and safely. I am going to go with the JJ E34L tubes and I was wondering if there is a place to go to learn to do biasing. I have fluke meter, and I have built myself, from another board, a capacitor discharge "tool." I love this amp, and I want it around for a long time. Any help is appreicated. I don't even know which circuit board it is, so that would be helpful also..thanks in advance. Michael | ||
| michael kaus |
OK. How comfortable are you working around high voltages? If confortable, it's really not that difficult, depending on how you do it. Does your amp have a 12ax7 tube for a phase inverter? If so, it's biased in the standard way with negative bias voltage and the cathode current is measured and either read directly or by means of a resitor in the circuit. If it has no 12ax7, it's cathode limiting by way of a transistor network and it's pretty much plug and play. Let me know which amp you have and I'll(or someone here) will try to help. Mike. | ||
| Michael Crimmins |
Yes it has a 12ax7, although I am not sure what the actual "board number" it is. Is is as easy as outlined in the MM shop article # 1 second step? (after discharging the caps of course!) thanks for your help!! | ||
| michael kaus |
Well, it can be that easy if you are a betting man. I believe that's the one about measuring voltage at point Y and adjusting to get half a volt crap. I'm not a real fan of that as it give you a total idle current value for both tubes of 50mA(divided by two, that's 25Ma per tube). Thats right up there at the top and perfectly ok if that's what your hottest tube is pulling. Measuring the idle current per tube is much better and more accurate. For that, you will need to get either one of the available current adatper thingy's that you plug into your amp and plug the tube into it and read the bias current directly(or the bias voltage if you get the one with the resistor in it) or you will have to solder a 1 ohm resistor on pin eight before it goes to the ballast resistor and measure the voltage drop across it. That is the preferred method for me. I use one of the bias adapters also but they can sometimes be pain in the ass if you have a twin with four power tubes and you have to shut it off each time and switch the adapter to another hot tube. I would personally(especially if it's a 130) order up some 1 ohm resistors of the metal oxide high heat kind(just my personal kind), measure them for accuracy , install them when you want to bias. Hell, if you do a nice clean job, you can leave them in there and have it ready for the next time if you wish. It won't hurt a thing. Just measure the resistors and mark each socket as the difference between 1 ohm and 1.3 ohms does screw up the calculation a little. Mike. |