fender super concert repair

ogeecheeman · Post by **ogeecheeman** » Sun Jul 06, 2014 9:22 pm

1993 60 watt fender concert tube amp. o-scope image is normal and symmetrical through all stages until output of phase inverter when it look like 2nd order harmonics.see pic. anyone see this before or have an opinion.maybe negative feedback loop gone awry? Tubes have been replaced.all DC voltages in spec. original problem-failed output tube possibly from liquid spill.screen resistor had been replaced(someone else-unqualified)
I replaced screen resistors and CR 55 /CR56 = R3000 flyback diodes.output tranny measures good. bias is right on it.

jjman · Post by **jjman** » Mon Jul 07, 2014 2:27 am

If the grids of the output tubes are approaching/reaching zero on their peaks the scoping of the PI output(s) would approach "clamping." What is the peak output grid voltage during this particular test scenario?

ogeecheeman · Post by **ogeecheeman** » Mon Jul 07, 2014 2:39 am

thanks for the attention.Almost as soon as I posted I solved it.open plate resistor on plate-pin6 of driver tube
.no b+

Stevem · Post by **Stevem** » Mon Jul 07, 2014 11:31 am

with the amps level of B+ voltage at that point those PI plate resistors need to be 1 watt, not the 1/2 watt they are built with, in fact any where the B+ is feeding a plate resistor is over 325 there is a need for 1 watt resistors.
Also even if there is a lower voltage, the amp will have less hiss if a larger wattage resistor is put in place.

Tube Amp Archive