Tube FX-Loop add-on for guitar amps

[gtudoran]

This is a new project.
An FX-Loop add-on that can be used on any guitar amp that doesn't have one. Plug and play, standard 6.3V heaters / From Pre / To Power and B+ pads.

It's using a 6N17B submini triode.


Schematic:

Best regards,
Gabriel Tudoran

[deadastronaut]

another great project, nice one gabriel.

[MetalGuy]

Nice project. I've seen similar in an online shop.

3 remarks on the schematic:

1/ 6N17B max anode voltage is 250V so be careful where you connect it or just decouple it properly from the respective B+ node
2/ Most guitar preamps can develop up to 30V after the volume pot so if you're shooting for a max of 0dB send signal the cathode follower voltage divider should look more like 100k/2k2 or 100k/4k7.
3/ 100k send pot defeats to a great extent the purpose of the send stage namely the low output impedance. For low output impedance this pot should be max 20-25k .

[gtudoran]

@MetalGuy thank you for your pertinent comments. This was the first of it's kind The version 2 will  have a separate voltage drop/filtering cell with a resistor that you can change according to your anode voltage. This module was intended to be added before master volume (never understood why some amps use an FX loop after the master volume - the signal level will vary according to you master volume position... ) And the 100k pot value was taken from the marshall standard FX-Loop, but yes you're right a 20-25k pot would be more suitable for this.

Best regards,
Gabriel Tudoran   

[MetalGuy]

...(never understood why some amps use an FX loop after the master volume - the signal level will vary according to you master volume position... )...

There are different ways to organize an effects loop. If we're talking Marshall type 5-6 knob amps placing it after the Master volume is also an option. This way when the loop is on (I'm assuming it's switchable via relay and is out of the signal chain when off) the Master volume will be also your Send level. The trick is to design the CF voltage divider in a way that it will decrease the send level (no Send level pot,for example for a maximum output level of 0dB or -10dB) to the same extent as the return stage gain level . This way you'll have no signal loss compared to the one you have at the Master pot taper and switching the loop on/off will no effect on the amp total level.
Considering the +/- 150V cathode to heater max voltage of this tube I would also recommend you to check Soldano Decatone, ENGL and Framus Cobra types of effect loops.

[thelonious]

Thanks for sharing this, Gabriel! Good stuff.

[mrsmash23]

Hello, this project is exactly what I was looking for! Are there any updates for this FX loop or is it good to go as it is (I noticed 25k pot recommendation). Thank you in advance!

[gtudoran]

@mrsmash23: the fx-loop is already used by us in this configuration (we build aprox. 10-12 pcs) and is good as it is.

Best regards,
Gabriel Tudoran

[Ben N]

Like a Vox V941 or Dumbleator, but small--I like it!

[gtudoran]

And the presentation brochure is ready:

[gtudoran]

As it's already fitted in a lots of amps we've made some PCBs for the this module: