Channel mixing/cascading in Fender Bandmaster. Questions
« on: July 22, 2020, 08:38:52 PM »
I have a '69 Bandmaster reverb (AB1069 I think) and have been experimenting with cascading the Normal channel (marshall voiced) into the Vibrato channel to achieve High Gain. It's been largely successful, but I'm having trouble nailing down an ideal way to keep the channels from interfering with one another, without having to switch anything beyond plugging into one channel or the other. (I'm a serious beginner at circuit design. I can follow instructions and use some basic logic, but am admittedly in a bit over my head)

So the "normal" channel input goes through V1A, V1B, and then into the Volume Pot bypassing V2A and the Vibrato Channel's tone stack, cascading into V2B and the rest of the Vib channel. 2 Vol knobs work for gain (also have Post PI vol knob added)

The problem: The normal channel connects at the same point as the vib channel's tone stack, so if I just tie them together, the Vib Treble knob effects the tone of the cascaded signal. I had the idea of moving the 220k Mixing resistors to this position, which mostly works, but the resistor between the vib channel's Treble and Vol pots kind of kills the Highs on the Vibrato channel. I'd like to overcome this, but am not sure where to go from here.

I'm sure the explanation isn't clear enough so I'm working on a layout diagram. Will drop that in a followup post, along with orig schematic.



Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #1 on: July 22, 2020, 09:04:50 PM »
Alright, here's the stock layout and schematic, followed by my current version. All the bright stuff is what I've changed. The area in question is by the Vibrato channel Treble and Volume Pots.

Stock Layout (thanks to robrobinette.com)


Schematic (stock)


Current Layout. (All the bright stuff is what I've changed. Master Vol, Reverb Dwell, re-Voiced "normal" channel, and the cascade)



squarewave

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #2 on: July 22, 2020, 09:35:36 PM »
I have a '69 Bandmaster reverb (AB1069 I think) and have been experimenting with cascading the Normal channel (marshall voiced) into the Vibrato channel to achieve High Gain.
You definitely need to post a schematic of your modified circuit. Otherwise it's just too much work trying to parse your circuit description.

In general, I would just disconnect ch1 mix resistor and send it to the grid of ch2. I guess you could feed into a normalling contact so that you can still plug something into it and disable the cascade.

Personally I think the output section (power tubes and OT) are where the real character of am amp comes from. When you overdrive a transformer, it makes a gong-ish sound. Guitar amps are one of the few places where you can actually hear distortion from the transformer saturating. Of course you need a dummy load to get there without destroying you're nervous system.

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #3 on: July 23, 2020, 11:43:52 AM »
Sorry, I still have a hard time visualizing schematics, so I still think in layout. I'll get there. Anyway. Below is the schematic with the changes relevant to my problem.

I kind of know what's happening, but I don't know how to overcome it. My logic was to move the mixing resistors to where I want the Normal channel to merge into the Vib channel, BUT, that happens to be where the 250K Vib Treble Pot's wiper connects to the Vib Vol pot, so when using the Vib channel that 220k resistor behaves like the Treble pot turned down all the way. When using the "Normal" channel though, this does sufficiently prevent the Vib treble pot from interfering. If I just go back to the jumper between the Vib Treble and vol pots, the treble pot will effect the "normal" channel's tone, but the Vib channel would work fine. I want the Vib tone stack not to interact with the "normal" signal, but I also want to keep the stock tone on the Vib channel.

So is there a way to do both? Different Size Resistor? Bypass cap on the resistor? coupling cap in line with the resistor? If so what value(s)? Something more complex? No way to mix the channels at this spot without unwanted interaction?



squarewave

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #4 on: July 23, 2020, 12:48:33 PM »
You can only mix into a grid. So mix into the grid of V2B. Meaning restore Ch2 the way it was but then just feed the grid of V2B with two 220K, one from the wiper of ch2 volume pot and the other is the sig from ch1. So the only thing connected to V2B grid is two 220K resistors. Then I think it might do what you're looking for.

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #5 on: July 23, 2020, 01:50:51 PM »
I get what you're saying, but I'd still like to keep the Vib channel's Volume pot in circuit for the "normal" channel also. Having both Vol pots makes for greater control of the overdrive, which I find really useful. 

My initial version of this had a DPDT switch in place of the Normal Channel's Bright switch to switch between the two channels right before the Vib Volume pot. It worked perfectly but if I forgot to flip the switch back the Vib channel was unusable.

I guess I either need to give up on the dual Volume pots, get comfortable with the tone stack interaction, or go back to a switch and give up on the AB pedal scenario.

squarewave

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #6 on: July 23, 2020, 02:44:01 PM »
I get what you're saying, but I'd still like to keep the Vib channel's Volume pot in circuit for the "normal" channel also. Having both Vol pots makes for greater control of the overdrive, which I find really useful.
It won't be that useful because you're only adding one gain stage. One gain stage is probably just about right before it starts making farting noises but that vol control for one stage isn't going to do much more than what the other vol control is already doing.

I wouldn't get too crazy with changes. You could easily end up with an amp that's just weird and that would be a shame because it's a nice amp.

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #7 on: July 23, 2020, 07:19:37 PM »
I'll give it a try and see how I like it. You're probably right, though I wouldn't be losing a gain stage, just losing it's attenuator.

I'm not doing anything I can't undo and not putting any new holes in it. That said it was already a little weird when I got it. Previous owner sold the box and put it in a 2x10 cab with some ~80's eminence speakers so it's a nice combo.  Once I get this cascade thing dialed in I'll be really happy with the flexibility. Clean and dirty, both with Vibrato and Reverb. No pedals necessary.

Whoops

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #8 on: July 23, 2020, 10:22:59 PM »
The problem: The normal channel connects at the same point as the vib channel's tone stack, so if I just tie them together, the Vib Treble knob effects the tone of the cascaded signal

And whats the problem of that?
Thats normally considered part of the charm, why do you see it as a problem?

regards

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #9 on: July 23, 2020, 11:12:19 PM »
Oh I have plenty of gear with charm. I don't need more charm in my life. Charm is exhausting.


Whoops

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #10 on: July 23, 2020, 11:38:05 PM »
And whats the problem of that?
why do you see it as a problem?

regards

Re: Channel mixing/cascading in Fender Bandmaster. Questions
« Reply #11 on: July 27, 2020, 08:37:03 PM »
Ok, so I moved the mixing resistors to the V2B grid. Still a little interference from the Vib channel's volume knob, but nothing noteworthy. "Normal" channel is very Marshall. Having moved the mixing resistors to after the Vib channel's Vol pot, it's now as though that pot in the previous configuration is at 10 all the time. Basically less headroom and therefore less flexibility within the lower gain spectrum of it's overdrive. I added a master vol already so it's still functional, but I kinda miss the range of lower gain distortion I had to play with before. The second input brings things down a bit so I think I can work with that. This was also a test with humbuckers but I'm curious how single coils will feel.

New quirk: Reverb hangs like crazy on the high gain channel. I already added a dwell knob, so that is easily tamed down, but I wasn't expecting it to change so drastically. I dig it.

Then I jumpered the two channels together. I expected this to be bad because, if I'm understanding correctly, the two channels should be out of phase, but Ho-Ly Crap. Gain and Tone, but not in a farty way. I've never really done the jumper thing much, but I'm really going to need to explore this because there's serious potential here. I think this might be the final configuration. I still need to replace the RCA jacks for the reverb tank and footswitch, as they're a bit loose, but this is it's new bastardized existence.

I guess I need to order an A/B/Y pedal now.


 

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