so i'm currently looking at a Marshall re-issue plexi from 1987 that has been hacked by someone attempting to make mods and remove the effects loop, needless to say i've had to rewire much of the pre-amp and replace a tube socket (someone cut a plate pin from the V1 tube socket) and replace missing components in order to get back to original but encountered something that i can't account for...now with the pre-amp volume controls fully counterclockwise there's still signal....i've lost my mind investigating grounds to no avail so am left wondering if this is normal??
Check cross talk beteeen tubes and tube sections, wiring and mainly if the wiper of the pot takes a low resistance to ground when fully counterclockwise viewing it from the control shaft.
in fully counterclockwise positions the wipers are low resistance (no appreciable resistance) that i can detect so i am left wondering why there's any signal left...i can appreciate that the high impedance and high gain can be problematic but am at a loss...
to add to this problem there seems to be some RF rectification occurring as i hear faint radio.
to add to this problem there seems to be some RF rectification occurring as i hear faint radio.
Take a 1uF 400V capacitor and bypass to ground any plate and any grid to listen where the problem starts. Discharge the cap between one point test and the next.
It isn't an RF bypass, it's an audio bypass. If you shortcircuit the signal to ground in a specific point, and audio signal dissapears from the speaker, then the undesired audio signal is mixing at or in a previous stage and you reduce the area for investigate.
If my memory is right, when I was a kid, an old radio-tv technician explained me how he used a capacitor to do the following:
1. Measure the voltage. He touched the point with an unknown voltage and after removing the cap, shorted it to ground. The lenght of the spark would give him approx. the value of that voltage. Very crude, but what to do if you have no volt meter.
2. Short the signal to ground to isolate the following stages, just like Osvaldeo said.
But as I wrote, I was a kid. I'm stepping in the seventh decade of my life and it wouldn't surprise anybody if I'm not 100% acurate.
Worth a try and let us know how it worked out.
The younger generation may learn something...
Ah those were the days my friends...
1. Measure the voltage. He touched the point with an unknown voltage and after removing the cap, shorted it to ground. The lenght of the spark would give him approx. the value of that voltage. Very crude, but what to do if you have no volt meter.
2. Short the signal to ground to isolate the following stages, just like Osvaldeo said.
But as I wrote, I was a kid. I'm stepping in the seventh decade of my life and it wouldn't surprise anybody if I'm not 100% acurate.
Worth a try and let us know how it worked out.
The younger generation may learn something...
Ah those were the days my friends...
Here this system was also used. It was called chipómetro or "sparkmeter" in a roughly translation.
well as this is happening at the very first pre-amp tube and other then the hi low attenuator of the input jacks there's not much that could be at play...still wondering if this is just a normal thing for this amp.
I am not musician not intrument player, but Electronic Engineer and no amp must have audio output at loudspeaker with volume control set to zero. Otherwise, which is the sense of this control?.
i'm also of the mind that "0" should mean no output from the speaker, but with all the "mods" done to this anything is possible...i just can't wrap my head around why it's happening, grounds are good so somehow the signal is coupled or leaking into other stages and making it to the output.
I have a 50w plexi from 1967. (JTM50, if it matters.) It has always had a bit of signal just audible when both volumes are zeroed.
As I have never had any use for it with the volumes zeroed, I've never worried about it.
Cheers, and regards,
Ant.
As I have never had any use for it with the volumes zeroed, I've never worried about it.
Cheers, and regards,
Ant.
Almost certainly. Though "fixing" that would involve changes that I do not want to make on such an amplifier.
Cheers, and regards,
Ant.
Cheers, and regards,
Ant.
PRR would that be filter caps??...maybe?
the tone seems ok but bass notes are flubbing and they seem to fluctuate on decay.
the tone seems ok but bass notes are flubbing and they seem to fluctuate on decay.