Volume control of channel 2 has little or no effect on the hum.Lesson 1. With channel 1 volume control all the way up, the hum doubles. I even tried wrapping the PSU trafo with laminated steel (stolen from the core of a sacrificed toroidal trafo for shielding a PSU trafo on a mic preamp project), without any result. With the ground disconnected everywhere excepted at the PSU cap return, things got even worse, the hum trippled in volume, and with the volume of input 1 even slightly up, I got some nasty HF oscillations.Ĭorrect me if I'm wrong, but I think that if the PSU caps are bad, then I should get some 100Hz buzz instead of my 50Hz hum, right ? Before the cap job the amp already had this hum issue. I checked each step to see if the hum decreases : nope. I tried to disconnect the other circuits from the main PCB, ground the unused inputs in order to not leave them floating, remove the bolts from the main PCB and unbolt the input jacks, so that there's only one connection to the ground, that is the return to the PSU caps (star ground). I checked the solders, they all look just fine. Then I'll be able to star ground the whole circuit and. If the hum is caused by those loops, I guess I have 2 solutions : either live with the hum, or redo the whole grounding of the circuit, and replacing the metal screws by plastic ones to get rid of the chassis connections. ![]() That's a lot of possibilities for stray ground currents to flow ! And every PCB has a ground loop becaiuse the ground plane surrounds it without interruption. Looking at the general grounding network, it appears to me that it's quite a messy one : every PCB corner connects the PCB ground to the chassis, and every PCB is linked together with a piece of wire. The only way to suppress the hum is to ground the grid itself, after the summing resistors, like the "standby" switch does. I tried grounding each input just before the summing resistor and it appeared that each has its own small contribution to the output hum ! With every input grounded (channel 1&2, reverb and tape), there is still some hum left. ![]() Thanx a lot for the schematic ! It is really helpful, as it allowed me to identify the multiple inputs that feed the phase splitter (actually an amplifier before the concertina circuit). If anyone here has the schematic for this amp, I would be really happy to get it !Īny idea to go on the toubleshooting would also be really appreciated. I must admit that I don't know what to try now. I cut the trace to try removing the loop, but the hum got only stronger There is a ground loop on the main circuit, as the PCB is surrounded by an uncut ground trace. I cut those traces on the PCB to keep the AC far away from the PI circuit, and fed the heaters with an external +/- 6,3V DC supply, but the hum is still present, unchanged. I first suspected the heaters, as those aren't fed with twisted wires but directly wired on the PCB. I swapped the PI tube with some new 12AX7, problem still remains. It is still present with the vibrato oscillator tube removed. Most of the hum seems to come from the phase inverter, as it's still present when the input tubes are removed and disappears when th PI tube is removed. It works perfectly, excepted there's a slight but annoying hum (50 Hz) on the output. The reverb drive/receive circuit is transistorised, the rest is all tubes.Īs you can see of the pics, I recapped it completely. The amp seems to have been made in the beginning of the 70, as it is mostly tubed but has some transistors too. The standby switch isn't actually a real one as it grounds the input of the phase inverter instead of cutting B+) ![]() The tubes are : 4x EL84 output tubes (the ines put in a square position) and the rest is all 12AX7. I couldn't find the schematic for this one.
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