i got a fender bassman 100 head recently and after much adding and removing of effects i still can't decide whether i like the "bass" or "normal" channel best.
so i was thinking i should get an a/b box to switch back and forth between them. but... if i had that, why not get an a/b/y switch and be able to combine the channels?
the problem, as far as i understand it, is that the two channels are phase reversed so that the a standard y-type switch would not be of much use.

has anyone had any experience with this kind of thing? are there any a/b/y switches that have a phase switch on one channel?

Hello,

I don't know of any AB boxes with phase selection, but you could probably build one pretty easily.

However, I just looked at a bassman 100 schematic. the channels seem to be in phase.

http://www1.korksoft.com/~schem/fenderheaven.htm

fender amps with reverb on one channel are the ones where the two channels are out of phase. The reverb channel has an extra gain stage to amplify the reverb return, and every gain stage flips the phase of the signal. So on a fender reverb amp one channel has two gain stages, and the other has three. The then combine electrically. On the bassman one hundred, each channel has two gain stages before combination, maintaining phase coherency.

give it a try.

ben adrian
oakland now, san francisco soon.

ah, ok. so now that's what i get for never havin learned to read schematics. thanks for your help, i just tried it and, indeed, no phasing problems appeared.


on the other hand i found out that the radial switchbones are a/b's that have phase selection, should you ever need one of those...

If you want to learn more about tube amps, and specifically fender tube amps, pick up "a desktop guide to hip, vintage amps" by Gerald Weber. He's a texan who loves his "vintage tone monsters" and holds some tone-myth beliefs as law, but the book explains quite a bit about fender amps and has an almost complete library of pre 1970 fender amp schematics.

Check out the Tweed Champ schematic. Studying that can teach you how to read schematics and the basics of guitar amp electronics. With a high school physics textbook and a couple tube amp books and manuals, you can have a lot of fun, and really make your amp sound exactly how you want it to sound.

have fun.

ben adrian
san francisco

i've never tried configuring two fender amp channels like this in a Y configuration, but I have done A/B'ing with both two channels and two amps using a simple morely a/b/y switch.. there always seems to be a huge pop when switching though. im curious if anyone knows a fix for this?

i am sure someone in this forum will post a more profound answer to this - but the basic problem are the passive switches that moreley employs. you need faster (and (or?)) active switches to avoid that problem. and, yeah, that means buying a more expensive switch.

so, that's what i know.

Sorry to jump in here without some kind of formal introduction, but I've been wondering about the phase issue on 50 watt Bassman heads. Are they also phase correct?

Meanwhile, to address the original post: Barber Electronics now has a clean boost pedal with two outputs that correct the phase on amps with Fender-style preamps. I can't remember the name of it, but it's powder blue and has some kind of kitschy, retro-50s look to it.

http://www.barberelectronics.com

It depends on the era of head. Some bassman heads have a third gain stage on one of the channels. You can find schematics online. Do you have a specific model for which you want to check phase coherency?

ben adrian
san froakland, ca