FX Loop issue

[hesamadman]

Hey All!


I made this fx loop about 6 months ago or so. I put it in my 50 watt build and it was fantastic. I am not very familiar with fx loops beyond this. The only other fx loop I have ever played is in a Lonestar and I didn't even run pedals. I just played with it on and off to see if I could tell a difference. The main thing I remember was just a volume boost. After I realized how beautiful reverb sounds in a loop, I decided to implement it in my builds. I have a build right now that only has one single gain stage before it hits the cathode follower with tone stack and MV then a cathodyne phase inverter. I used a Fender Princeton power amp. This is a very clean sounding amp. There is absolutely zero noise. Since it only has this one gain stage, I like the fact that the fx loop boosts the volume a bit. Its not a loud amp with only one stage and 20 watts. I knew this going in. When I was playing around on the bread board I had a second stage and it was plenty loud but I wanted it cleaner. I felt that with the fx loop and any boost pedals installed before the input would get the extra volume needed. I am making this a combo. Its not a stage amp or anything.


Well everything works fantastic. The fx loop sounds great and the amp sounds great. I only have one issue. When my return pot is turned all the way up, I get a high pitch squeal. If I turn it down to a point it goes away and is fine. Now I know I could cheat and add some resistors to keep it from going above that value and wouldnt have the problem anymore but when I turn it down I lose the extra volume I want out of the fx loop. Its high frequency. I can also turn down the treble pot instead and it goes away. Since I have this same fx loop in a high gain 50 watt with a clean channel and do not have a problem, I wonder if it is a layout issue? When I was in high school starting some kind of report, my handwriting always started out very clean, but some ways through I was getting tired and trying to get through it. So my handwriting went from nice to "ok I just want to finish this crap". Well..... so do my amps. 


So maybe lead dress could have been better towards the end? It just seems odd to have this issue here and not in the other amp. One other thing I did. I have 3 pieces of shielded wire zip tied together. They are also zip tied to the tone stack wiring. I wouldnt have thought this would hurt anything since thy were shielded. I actucally don't know the exact efficiency of shielding wire so maybe I should move those?

[tubenit]

I don't think it's the shielded wiring.

I use 12AY7 or reverse 12DW7 tubes mostly in my active FX loops and I have a tone of gain. You might try a 5751 or 12AT7?

You might consider trying a smoothing cap across the plate resistor of the return triode .......... OR from the plate pin to cathode pin of return triode?

Easy to do carefully with insulated alligator clipped wires and some small value caps.      I'd probably try somewhere between 82p to 220p range?

I would not be surprised if  my amps would oscillate also using a 500ka return pot?   

With respect, Tubenit

[sluckey]

You have the return tube grounds routed through the loop jacks. This means the return circuit relies on the jacks and whatever length of your send/return cables for ground. Wire your return tubes directly to ground instead. Maybe that will help. You should be able to just use a gator clip test lead to ground the return jack directly to the chassis.

[tubenit]

Another thought occurred to me?   Maybe try a 33k grid resistor directly on pin 7 into the return triode?

[hesamadman]

You have the return tube grounds routed through the loop jacks. This means the return circuit relies on the jacks and whatever length of your send/return cables for ground. Wire your return tubes directly to ground instead. Maybe that will help.

Thanks for the input Steve. This is odd I know. The reason I tried it this way was to see if it eliminated possible ground loops from pedal boards. So far this is early in testing and I do not have a conclusion on its success. Though for my current problem, it did not fix it


So heres some updated info. I tried the things mentioned by Tubenit and no success, but here is what I discovered. The FX loop has a bypass switch. Completely removes it from circuit. I have 10 Meg resistors across contacts to reduce popping when switching. First thing is first. With the FX Loop OFF and NO instrument plugged in I can make the amp start to squeal by turning gain and master all the way up and then turning the treble up will make the high pitch dog whistle begin. Heres what confuses me a little bit. If I adjust the send and return of the FX loop, it can adjust the high pitch squeal slightly. Its odd because at this point it is only connected to the circuit via ground and B+. So its getting introduced back in the FX loop somewhere. Again the FX loop is OFF at this point.


Another interesting thing is I put a vibrato pedal in the loop and if the send or return pots are all the way up it makes on hellish of a noise. Turn those pots down some and it goes away and is playable. Sounds nice even. Oh...I replaced the 500k pot with a 250k also. I thought OK this loop design sucks. I need a new loop, but wait a minute this exact loop is in the 50 watt amp I have also. So I plugged it in and installed vibrato. Works perfect. I cranked master, gain, all pots... and no noise.


With the amp in question (20 watt) I used a fender Princeton reverb power amp. I have 2.2k grid resistors on the OT tubes. Im wondering if increasing this will help? The 50 uses 5.6k since its a marshall style. Im also curious about the B+ nodes. The princeton has all 12ax7 on the same node. I dont have reverb or tremolo. I think the only ones I have on the same nodes are the first couple of gain stages. The cathode follower tone stack and the fx loop are separate, but heres how I did those. I did the "bicycle spoke" approach. Like on the AC30 where the B+ comes in and branches in different directions rather than in series. Would this matter?


Lastly, the cathodyne phase inverter only uses half the triode obviously. The other half is unused in this circuit. Should I ground the unused parts?