Slaving Amps and Preventing Hum

[cheeb]

I'm trying to smoothly slave my Fender Hot Rod Deville 410 to my early-70's Fender Bronco. The obvious way to do this is to connect the speaker out on the Bronco to the Deville's "Preamp In" jack. This works, but it hums like a beehive. I also direly need the ability to switch between the Bronco/Deville setup to going directly into the Deville's input. Therein lies a problem: When the Bronco is plugged into the Deville's "Preamp In", the regular instrument input jack on the Deville is silenced. Any ideas?

[MusicAudio]

I haven't looked up the schematics for these amps, but I do have some thoughts for you.

First, I'd be very careful with plugging the speaker output of one amp into the preamp in of another. The input of the power section on your Deville isn't expecting the kind of voltage swing that a speaker output gives you.

Second, I you're going to need to do some modding on these amps to get what you want.

So here's what you need to do in a nutshell:
- Add a preamp out to your Bronco (before the PI if it's a push/pull amp)
- Change the switching jack on your Deville (preamp in jack) to a footswitchable circuit that either routes the deville preamp OR the preamp in jack to the power section

If I have time I'll try to dig up the schemos and maybe give you some more detailed/better suggestions.

Hope that helps some.

-mike

[Ben N]

Are both amps powered through the same circuit?

Taking a quick look at the factory schem (http://www.blueguitar.org/new/schem/fender/hotrod_deville.pdf), you have a preamp out jack that is not affected by your power-amp in--in fact they are designed to work as a buffered effects loop. So it would seem that any old A/B switch should do what you need--just run the A input to the Bronco, the B input to the HRD preamp out jack, and the output to the HRP power amp in. I once did something similar between the Line/recording jack of a Deluxe Reverb and the fx loop of a Fender Champ25se. (That hummed, too!  :icon_biggrin But it sounded good.)

If you keep the two amps close together, then it would probably be best practice to keep the cord between them as short as possible, so a remote A/B box with a relay would probably be best.

I would also be concerned about driving the HRD's power amp with the speaker out of the Bronco. Rather than modding it for a preamp out, though, I would just attenuate the speaker out--whether with a commercial attenuator, an L-pad or something like Fender put in my DR, which is just a single resistor and cap, IIRC. That way you have more of the sound of the amp. It could even be a separate box, so you don't need to make any mods to the Bronco.

As for hum, what you most likely have there is the dreaded ground loop, and those are a bitch to solve without compromising safety (by lifting grounds). This has been discussed, particularly at Ampage, so check that out. If your A/B is one that isolates grounds as well as hots, then at least when you are using the HRD preamp you should not have the hum.

[cheeb]

Thanks for all the help! I'll try the A/B idea and see how that works. I need to build a speaker attenuator for the Deville anyway so i can just do one for each while I'm at it. It looks like what I'll need is R.G. Keen's Amp Switching A/B that uses transformers to isolate the amps, preventing the ground loop hum.