| Author | Message | ||
     Mark Terry (fm711) Username: fm711 Registered: 02-2012 |
Music Man 112 RD- 100. I had broken a 6L6GC and bought replacement tubes (Electro-Harmonix) and an AMP-HEAD MPD bias tester to to check the bias and plate voltage. The plate voltage showed 630 volts but the tubes only showed 5mV (actual 5mA, since the Amp-head bias checker uses a 10 Megohm high-voltage resistor). Is such a low reading adjustable with the bias adjustment or indicative of another problem? The amp does seem to work. (AMP-Head thought there might be a burned out screen grid or no cathode current due to the high plate voltage. But, from what I've read here, the plate voltage sounds in the ballpark or even a bit low?) | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
those amps have the SS driver board so you should be measuring the cathode current across the resistors on the driver board as outlined in the SS driver section of the adjustments section on this site. | ||
| Lars Verholt (lmv) Username: lmv Registered: 11-2009 |
fwiw, the bias current is supposed to be minimum 6 and change mA (25mV across 3.9ohms). With a scope you can adjust it until crossover disappears, then check what the idle voltage actually is. In the 'Hi' position, the plate voltage should be 715V-60V=655V Sincerely, Lars Verholt |