buzz All tubes but the 6l6's removed. History: bought this amp as a repair project. replaced all caps. originaly the amp was quiet but but had very little volume and was some what distorted. now has volume (slightly distorted) but buzzes like crazy. Both power. and output transformers do not look original. The rectifier tube had been removed and a diode rectifier was intalled. I beleive it was because the transfomer was not correct for a rectifier tube. voltage seems good
Music - Fender - Custom Vibrolux Reverb Tube Electric Guitar Amplifier -...
Answers & Comments
Here is the schematic:
http://elektrotanya.com/fender_vibrolux_6g11_schem.pdf/download.html
Check the quality of the DC bias as being free of ripple.across teh 25 mfd/50v cap filtering the bias.
If much ripple there, replace the cap. Also the .05 bypass on the swinger of the intensity control.
This should be an easy fix, but you might need an oscilloscope to diagnose it quickly... or just parallel caps across these to see if it kills the hum.
The main high voltage filter caps are also suspect as well as the bypass of power to the ealier inverter stage a 16mfd/450v
There are lethal voltages in this so use great caution when troubleshooting... the caps hold a nasty charge.
Yes, notice that the plate resistors for the inverter come from a bypassed B source. If that source had a lot of ripple, then some could go through the coupling capacitors and into the grids of the 6L6's. NORMALLY this would be balanced and you would not get much hum out of the speaker though.
An important test is to check the bias voltage on the 6L6 grids... You should see minus 45 volts or thereabouts on those grids. Also check the screen voltage on the two 6L6's and you should find it very closely matched.
I assume you are hearing the hum via the speaker... PLEASE NOTE that often when transformers are changed the mounting may be such that magnetic fields can rattle chassis and worse other transformers and chokes in the vicinity. I have seen some transformers that were not varnished and the windings rattled very loud.
One other thing is to verify that the heater supply is fed from a CENTER tapped transformer winding. IF they don't have a centertap on the traansformer for the filament, take two 33 ohm resistors, one from each side of the heater winding and join them both connected to ground. This makes a fake centertap.
The 33 ohm resistors should be 2 watt type for the fake centertap.