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

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?

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

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

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

Did you try to move some of the surrounding wires around using a wooden stick (chopsticking)? The three wires going to the output transformer (red, blue, brown) seem to come very close to the effects loop circuitry, especially the recovery stage which probably has significant gain to bring the effect-signal back up to amp-levels and could therefore pick up some noise from the close wires. Apart from that, where exactly does the ground-connection go? A good option would be if it would run to the negative lead of the supplying filter capacitor. The negative lead of the filter cap supplying the following stage might be an option as well. Good luck ! :topjob:

Hi Iefes,

Thanks for your answer and sorry for my late reply.
The ground for the schield wires comes from R39, which is close to R32 (on the left of R32) : this schield is not connected on the loop side to avoid ground loop.
The loop itself is connected to the ground of the speaker connector - I've try to connect it to a minus of a HT capacitor : no helps.

I've try to move the loop away from the HT wires of the output transformer : no help.

REgards

YVan

Dear Yvan,i am no expert but to me it looks that either the reverb tranny or the van are quite near and could induce hum? May be you could try some kind of metall shield inbetween them and the loop to check this.Cheers Gerhard

Hi Gehard,

With longer wires, I've set the loop outside the chassis and the noise is still there.
Next try will be with another power transformer (from the ones I've in spare) to provide the AC 6,3V for the loop only.
Will let you know.

Regards

Yvan

first step: test the loop "in front of" the amp, to make sure it is working (like a the Dumbleator)  If the loop passed the test then you can install it after the first gain stage (first tube-section) for the next test and so on. So you can check for the best position of the loop but first make sure that the loop itself is fully working.

Best regards, Dirk

HI Dirk,

A quick question, I've assumed that the Anode resistor of the return triod is R9=100K (I can't find any R9 written on the PCB).
Am I right? Also, on one (B+) side on this resistor, I can measure 118V, with is close to the 120V you mention on the document, but on the other side , I've only 66V, not about 112V as you mention.

To me this is normal to have such value as an anode resistor and 66V looks fine to me in a common cathode follower amp.

So is the anode resistor is 100K?

Regards

Yvan

OK, I've made the full  schematic of the loop and I've inverted value of R6 (was 10K) with R9 (was 100K) and the result is much much better, the noise has almost fully disappeared.


Do you agree with this modification?

Regards
Yvan

Dear Kermit, if you read this: I know the topic is old, but I encountered the same problems but I left the loop in my B-flex unused, until the amp died. The amp was repaired, it head a winding short in the power transformer. I received a new transformer, brought the amp back to function, and now the tube fx loop problem remained. The loop worked, but I hat to fully crank the send-potentiometer and instead hat too much output, so the return pot I had to set to very low values. Your input is very valuable, because I also had figured, the 59 V ant the anode of the send is hard to believe and explain, why I had the fully open the send pot. I also account the hum issues to that, at least to a part. When I first tried, I found it annoying, because a well set up B-flex is almost dead quiet for about 0 ...70% of ,,Blast", which is gain. I also figured, that the 6112 is a very silent tube and rather pretty linear which makes it an excellent choice. If I do not get it quiet enough, I'll try the dumbleator circuit as a loop. I also tried to trace back the tube fx loop. I would find it interesting, if we got the same result. Any way, we could make a contact?