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FX loop with 6112 tube on Epiphone Blue custom : hum noise

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kermit44:
Hallo Leute!

Ich bin begeistert von Audio und Musik, spiele E-Gitarre und Kontrabass in einem klassischen Orchester.
Ich bin Französin und weiß ein wenig über Deutschland, weil ich mein Diplomarbeit an der ULM-Universität abgeschlossen habe - 6 Monate im Jahr 1996.


I've bought one of these FX loop with 6112 tube to install in a Epiphone Custom Blues amp. I've inserted the loop prior and after R32 (just before the PI), which is connected to the cap of the phase invertor . I've tried the green insert and the red insert - see schematics.
The heater is feed with the AC6,3V (5,7AC mesured) of the power tubes - via twisted wire.
The loop is working but it introduces some hum noise (I can see it on the 1st and even more on 2nd anode). The HT from E is quite good. I suspect the noise is comming from the AC6,3V.
I tried to add a 2µF cap on the AC6,3V => no help.
I v' tried also to regulate the 6,3v with 4 diodes + 4 700 µF cap for the 6112 only => got 6V DC.
The hum is still there.
I've tried also to get the 6,3V from the 12,5V of the preamp for the 12AX7, by adding a serial resistor, the noise is still there + noise from the fan.

Maybe I'm on the wrong way. Do you have some advice to use this fx loop?

MfG / regards



Yvan

iefes:
Hi Yvan and welcome!

You said you've tried to heat the tube with DC and the hum persisted, so I guess it's not heater hum you're getting here.
I think you might have a problem with the grounding scheme of the FX-Loop. How and where did you connect ground coming from the FX-Loop with the rest of the circuit?

kermit44:
Hi Iefes,

The FX loop is powered with B+ ( E point in my case) and ground. In addition, I've used schielded wire to connect the fx loop to the existing PCB of the amp. The schield of the wires are connected to the ground of the PCB also (and not to the loop pcb to avoid "ground loop" effect.
Also, I've added a basic swith to by-pass the fx-loop when not use close to the loop, and when the loop is by-passed, no hum/noise.

I'll add a picture of it.

Yvan

Dirk:
Please add another RC Filter behind the Point E for B+ of the Loop. Try with different values for the R, start at ca. 8k and lower the resistance if it works. If this filter does not help then you have a bad ground design.

Best regards, Dirk

kermit44:
Hi Dirk,

Viele Danke für deine Hilfe!

Unfortunatly, the extra resitor doesn't help. :-[
I've ran some some tests and realised that this amp has 2 modes : "Class A" (15W) and "class AB" (30W).
In Class AB, the some is very small (quite good in fact).

The class A is not a real class A, but another way to set the G2 tension with lower voltage (using B, after the choke, instead of A, after the diods), I don't really understand why this mode is more noisy with the loop only.

Here're some pictures:



Regards, mfG,

Yvan

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