I have a solid state guitar amplifier with a reverb tank. I have done a bit of research as to how these work. I now want to replace the reverb tank with an FX loop for normal pedals (instrument level). Therefore, the system in the amp that drives the reverb tank needs to be modified so it sends and returns correct levels. My guitar outputs about 800mV max. So the FX send needs to output about 800mV without additional gain from the preamp. The return signal of the reverb tank is obviously very weak and it is (just like the send signal) amplified with a 4558 amplifier ic. I have done a bit of reverse engineering and I have found out which part on the schematic refers to which part on the pcb. Reverb tank schematic

Now my question is: Can anyone help me with modifying the reverb section so that the incoming signal and the signal that goes back to the rest of the system doesn't change, but just the send and return values?

Let me know which terminals I have to check with my multimeter when I have the amp on clean. This way I'm sure I won't give you useless information.

PS: The coils at send and return will be gone. Those refer to the reverb tank's systems. I will basically replace them with 1/4" jacks so J6 and J7 are positive and negative terminals (tip and sleeve). Same for J8 and J9.


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    \$\begingroup\$ I strongly recommend changing the question title to be pertinent to your question. i.e. "Can I replace reverb circuit with effects loop on guitar amplifier?" or something like that. \$\endgroup\$ – JYelton Jun 19 '18 at 18:18
  • \$\begingroup\$ But I aleady know that it can be done... But thanks for the tip. I'll find a better title. \$\endgroup\$ – Hendriks3D Jun 19 '18 at 18:22
  • \$\begingroup\$ I'm actually quite interested to see how this can be done, as a new guitar player and electronics nerd. I'm just suggesting the title be more narrow. Perhaps "How to maintain audio level when replacing effect circuit on guitar amp?" \$\endgroup\$ – JYelton Jun 19 '18 at 18:24
  • \$\begingroup\$ What does point C connect to? That goes through R52 to the gate of Q5. \$\endgroup\$ – JRE Jun 19 '18 at 18:29
  • \$\begingroup\$ I think that if you put your effect in place of the reverb tank, then you will hear the effect mixed with the signal that doesn't have the effect. The reverb circuit is such that the signal goes through U5A, then the reverb is mixed in with it (also in U5A.) \$\endgroup\$ – JRE Jun 19 '18 at 18:33

Point X thru a 1k resistor and 10uF cap would do for an effects send I suspect, and if you lift the end of C41 connected to IC6A, that would do as a point to connect the FX return.

I would try that before doing anything more sophisticated.

The reverb mixing is done by IC5B, with the return muted by Q6 (when the gate is at ground), elsewhere there will be arrangements to pull the gate negative to enable the reverb doings. Point A,B switch the lead/rhythm channels in essentially the same way, these are mixed by IC5A.

IC6B causes the reverb tank to be driven current mode, you could use it to buffer the FX send, but would need to change the resistor values.

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  • \$\begingroup\$ Do you need any information on what voltage readings I get at certain points? Maybe you could help me redraw the reverb section. \$\endgroup\$ – Hendriks3D Jun 19 '18 at 20:30
  • \$\begingroup\$ Having just had a look at the full schematic, you are aware that there is a perfectly good Line Out & Line In socket pair on the back that I would expect to do just fine for inserting some pedals. Seriously, level just is not that critical, and guitars are generally close enough to line level anyway (Just not line level impedance) that I would expect this to do just fine, no modifications needed. \$\endgroup\$ – Dan Mills Jun 20 '18 at 10:12
  • \$\begingroup\$ Just saying that those are already in use for another thing. I just need an independent FX loop. \$\endgroup\$ – Hendriks3D Jun 21 '18 at 1:44
  • \$\begingroup\$ I'm not looking for an alternative. Could you maybe help me finding the right modifications to the circuit? I don't have much knowledge on electrical engineering in audio myself. \$\endgroup\$ – Hendriks3D Jun 24 '18 at 17:56
  • \$\begingroup\$ @Hendriks3D What does 'independent effects loop' mean to you? I can think of about half a dozen places in that diagram to place a couple of sockets that would do something that might fit the bill. Are you looking for something variably mixed in with the direct path (which is what the reverb does), something in series with the existing loop (Easy!), something just in the clean channel? I am not drawing all of them (And there are several other possibilities)! Give us a clue. \$\endgroup\$ – Dan Mills Jun 24 '18 at 20:08

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