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Bypass Preamp?

Started by goldtop, November 22, 2007, 10:39:47 PM

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I have an idea which hopefully makes since to someone on here. I own an old Peavey Deuce guitar amp that has a SS preamp and tube power amp. The clean tones are excellent (for me), the overdrive lacks crunch, as the gain is increased the distortion is soft and fuzzy (common for these amps). I've been using this http://www.artecsound.com/qdd.html onboard distortion gadget installed in my guitar and getting excellent results.

I now have a new guitar and the onboard unit will not fit, I'd like to install the unit inside my amp if possible, either run in front of the preamp, bypass the preamp or hardwire it into the effects loop. Any help or ideas would be appreciated.  Andy


Well, it doesn't look overly complicated...

Just to get this out of the way: You are sure this is the right schematic? Peavey has introduced quite a few "Deuce" amps. You have one of VT Series, right?

Then, let's get to the most obvious issue: The 9V power will be a problem. You must either construct a separate regulated 9V power supply for replacement (with a proper circuit you can even take power from the existing supply) or just use a battery, which you will have to replace now and then. Personally - before making anything permanent - I would just fit the distortion inside a suitable case and make it a stomp box  - but that's just me.

1. In front of the preamp:

The simplest configuration, I guess. Clip off the amp's input jack wires then wire the existing input jack of the amp to "Input" and "gnd" pins of the distortion circuit. Clip off the output jack of the distortion unit and connect it to the wires that connected the input jack of the amp. Problem is that you have three channels with different inputs and the circuit can be fit into only one of them. Unfortunately there are no easy solutions to get around this problem.

Yeah, you could just wire a new jack to "Input" and "gnd" pins of the distortion unit and house the thing inside a proper box. This is a lot simpler and reversible configuration: No need to drill mounting holes to amp chassis etc. I'm pretty sure it would even fit into one of those small boxes you see attached to guitar straps.

2. Bypass the preamp:

I am not so sure you want to do this: The preamp introduces some gain and frequency response alteration. Bypassing will most likely prove to be a disappointment. Again, the simplest way to bypass is just to fit the distortion circuit inside an external box. Plugging this box into the "power amp in" then bypasses the preamp. I am quite sure you will need some additional gain in this configuration, though. This means that you would have to build some gain stages that follow the distortion unit. It's not complicated if you know what to do.

For hardwiring... Basically, you need a DPDT switch. This is a dual switch. One of the switches goes to one of the input jacks to divide the signal path. The other switch goes to either "preamp out" or "power amp in" jack where it selects one from two alternative signal paths. The signal path must be AC coupled since there is a DC offset present. If you ever worked with any effect circuits the configuration should be clear to you.

3. Hardwire into the effects loop:

You have to options here, which are to wire the unit to either send or to return side of the loop. Either way you decide to do it, it is once again a very simple configuration (at least in theory): You ground the units "gnd" pin to amp's ground, disconnect the loop jack and wire the distortion unit in between very similarly as you would have done in the case of "In front of the preamp". You may run into problems with signal levels, which in the FX loop are likely too high for the circuit. If so, you need to attenuate the FX loop's signal with a resistor divider and then make up the lost gain after the unit with a gain stage.

In my opinion, the easier solution is – once again – to have the distortion inside a separate box that you just connect to FX loop if needed. I recommend that you experiment with the possibilities of this configuration at first. You can do the same things with this setups - it's just not as compact. If it seems to work you may wish to consider the hardwiring as a next step but if it has issues you need to map them out before you can successfully hardwire anything.


Temuk: Thanks a million, You've given me allot to consider. In front of the preamp seems best, as long as I can still control channels with the foot switch. It will take some research on my part before beginning anything, I first need to learn about draining power...  Thanks again, Andy