Hi all,
Getting horrible squeals / feedback through zero loss fx loop on my 2 channel plexi.
With fx loop off and all my pedals through amp no feedback, with cable as link with fx loop on no feedback, only seems to feedback with quad cortex .
I’m using 4 cable method.
Here is video of problem, any advice is welcome.
Regards
Spence
The FX loop of the QC boosts the signal. I usually have the send of the QC down about 4.5-5db. This will help prevent it from boosting the signal and making it feed back. There’s a thread on here where someone figured out the amount the QC boosts the signal with its fx loop.
There’s isn’t a way to shut that off, other than saving this adjustment as a default to the fx loop block or adjusting the level of the fx loop send/return in the IO panel.
I don’t think it is this sorry, as I’ve adjusted the send and return levels on the QC and it just seems to lower the overall volume and change the actual sound?
I think it maybe a wiring error ?
The feedback sounds out of phase ? It maybe an impedance issue of some sort?
I’m going to take out the NFB and the depth control and see if that help?
Did you adjust the ground lifts in the QC? I’ve had the QC cause squealing if the ground lifts aren’t adjusted.
No it’s not the gnd lifts but good shout, already tried that.
I have a 220k resistor in parallel with 220k pot to give 100k for NFB into 27k then into depth pot with 470pf on it then to 8 ohm winding.
I’ve removed the NFB pot and the depth pot and just have 100k NFB resistor to 8 ohm tap and feedback is now gone.
I also changed the EL34 tubes as i smelt a starting to get too hot smell!!
so it could have been the output tubes failing, they were quite an old set with the writing on the glass which was very faded!!!
Time to get your tube amp checked out–don’t just swap power amp tubes, you will need to have them biased in the amp by a tech… Unless you are a tube amp tech, in which case I don’t need to tell you that.
Back when I had a 100W tube amp (Fryette Pittbull 100CL), I got the same kind of crazy microphonic hiss moments before a power tube went out.
This all makes sense. None of the stuff you adjusted on the amp to reduce the noise should have been fine as is. When an amp is about to go there’s lots of little quirks like this that happen unfortunately.
Would recommend getting the whole amp looked at since it’s totally possible it needs some other parts looked at aside from the power tubes, or a proper bias. Assuming it hasn’t been serviced in a while if the tubes are pretty old.
A friends Marshall plexi that’s similar to yours caught fire once due to someone just changing the power tubes and not checking other components. It started smoking while it was in a studio
Yeah this is t my first rodeo with valve amps, I have built and serviced several but I do lack a fair bit of know how when it comes to strange problems as this problem just didn’t seem to fit with installing a fx loop and changing the presence control.
The tubes I had were total on their way out, writing on them faded, starting to smell etc!!
I always check bias and all B+ voltages along with fx loop b+ voltages, I’m not a just chuck it in sort of person.
The amp is behaving much more like it should now.
It’s a two channel plexi ( EVH stylie) I’m trying to get that metroplex mk2 emulation section done and the opto masters to see how that sounds.