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I have included the FET100 schematic that Carvin sent me.It was already on Doug's website:
http://www.el34world.com/charts/Schematics/CARVIN_FET_100.pdf >
The transistors are 2 MTH1520'sI do not know what an MTH1520 is. Like the TIP31 +AND+ TIP32, you gotta get the right part. If MTH is Carvin's house-number, you may have to find out what commercial part is suitable.
First off: EXAMINE for burnt resistors. They are clues.
Also solder a 270 ohm across the ends of P1, the idle current adjust pot. If/WHEN that pot goes bad, FET current goes to infinity, which is bad.
On each big FET, disconnect the Drain wire (you may be able to guess from the connections, or look-up the pinout for the -exact- device you have installed).
Disconnect one end of R13.
Disconnect any speaker load.
In this form, the amp "should" come up to full power-rail voltages (perhaps +50V and -50V DC), and the speaker-jack hot terminal "should" sit at "zero" V (less than 1VDC).
Fix R13.
Now float your voltmeter across R10. You are measuring the voltage which goes to the Q6 FET Gate. It should be small, maybe nearly zero. Un-shunt P1 and adjust it. The voltage across that Gate should rise smoothly to almost 5V, with NO jumps to higher voltage. If it twitches past 5V, the pot is shot and must be replaced. In the end you want to leave the pot set for about 1.0V-1.5V at each Gate. This is "too cool" for best small-signal sound, but that's safest.
If you have ANY doubt that P1 can set a RELIABLE 2V-4V bias, without fail, then replace it with a fresh high-reliability pot. (I would even consider tacking-in a fixed resistor: the exact bias is not critical, but lost-bias is VERY bad.)
Put 100 ohm 10W resistors to connect the MOSFET Drains to where they should go. The idea is to pass "some" current so we can check some more, but not pass BIG current and smoke anything precious.
If a 100 smokes right away, that FET is shorted.
In this form, the amp should bring its output to zero volts DC (<0.1V).
Wire a 100 ohm 10 Watt resistor in series with a speaker, and connect it ("108 ohm speaker"). In this form, the amp should "play", soft, but clean. Soft because most of your power is lost in the several 100 ohm resistors: kills gain and clobbers maximum output. But if something is still wrong (and it often is), you are out a $1 tack-on resistor instead of a $10 screw+goo transistor.
When it appears to work as good as it can with all the choking, take out the added resistors and cross your fingers. (An intermediate step is to try 10 ohm 10W resistors: it will almost play right or it will smoke very fast but cheap.)
A final step is to measure the idle current and trim P1 (or select fixed resistors) to get the suggested 200mA through the FETs (or the whole amp). But IMHO, this may not be necessary in a bass stage amp. It cleans-up the sub-Watt sound, but a bass stage amp never plays sub-Watt, and any slight distortion will be blamed-on/credited-to the guitarist. As long as it does not idle HOT, and doesn't make decays sound like a rock-crusher, it is biased fine.
Topic: Broken Solid State Amp (Read 11587 times)