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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

HELP! B+ voltage problem/burning power resistor

hey, im new to the fourm, been a fan for quite awhile though, i have a problem and this looks like a great place to start!!

OK

heres the story:


i have a peavey VTM 120 guitar amp, about a year ago, i was playing and i started smelling smoke, i turned it off and took a look to investigate, found that the power resistor thats in line with the b+ voltage got burnt. (its a 400ohm, 10W btw), after not much findings besides the burn gash, i checked the resistance value with my meter, everything was fine.. i turned it back on never had any more problems... nothing solved.. since then, its been down waiting on filter caps, i got them recently and i've replaced them, today i was again playing, cranking it up, only now with the chasis revealing the components, and lo and behold, it started smoking again, this time i threw a meter on it, and found that the voltage drop on the amp load side drops considerably when cranked (about 72 volts!) and the resistor got too hot to touch, but ONLY when i have the guitar turned up, and the amp master volume turned up, mind you, and in the hi gain jack (there are 2 jacks, low gain and high gain) BUT the tranny side only drops about 20 volts when cranked from 473 to about 452-3. with the volume turned down on the guitar, reguardless of where i have the master volume set at, and the amp out of standby, the sag accross the resistor is about 8 volts. and if i have the master volume low enough, and the guitar up, it wont sag, but as i turn the master up, it startes reallly sagging and engs up at the voltage different on the load side of like i said about, about 72 volts, and the resistor starts to burn.. i know sag is normal when an amp is driven into clipping, but this is too strange... any suggestions on this wierd anomaly??

thanks guys and gals, i have schematics. but i'm not sure if they would be much help at this point..

Joe
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
Well, perhaps the 400 ohm resistor is too low power. Let's see, with a 72 volt drop, you must be drawing something like 180 mA at full crank. The resistor will dissipate .18x72 = 13W. A 20 watter might be more appropriate and make sure that it's well spaced from the circuit board and has some ventilation. Alternatively, you could use a 20 or 25 watt aluminum-clad resistor that would bolt to the chassis to get rid of some heat.