65 bassman bass channel voice mod

[Powerstrokedr]

What would you guys do to revoice the bass side of this bassman? I do not play bass, so it would be cool to have marshall tone on one side, with fender on the other. I have looked through tons of forums concerning this, however most focus on the ab165 circuit. This is an aa864 and a little different. I would like to keep the mod completely reversible for value of the amp. Thoughts?

 

[zzmoore]

Re: 65 bassman bass channel voice mod

Just keep it real simple.
Look at the values for the other tone stack, and go the other way.
Turn the bass switch into a bright cap switch, or use it to switch another Cap/Resistor value into the cathode circuit.
Change the slope resistor for more mids
Increase the plate resistor for more gain
Change the bypass cap to a much lower value than the other channel.
Anyway.....just simple stuff like that
You will be amazed out How Much Difference that will make.

 

[Myaccount876]

Re: 65 bassman bass channel voice mod

Schematic:http://www.thevintagesound.com/ffg/schem/bassman_aa864_schem.gif

You could completely alter the bass channel's tone stack to be more suitable for guitar. Here's a tone stack calculator: http://www.duncanamps.com/tsc/download.html. Just save all capacitors, resistors, and pots. That, or you could just remove the tonestack all together by just lifting the ground wire at the end of the bass potentiometer that goes to the deep switch. You can even make it switchable. That will increase gain, mids, and treble. As it is, the bass and normal channels are out of phase, so jumpering the channels doesn't sound as good. There's two ways to change this: V1 is the bass channel, and V3 is the normal channel, and you can see that the output from the normal channel is after V2's single triode, meaning the bass channel has 3 gain stages while the normal channel has two. If you re-route the normal channel's output to the input of V2, then the normal and bass channels will be in phase so you can jumper the channels and your normal channel will have an additional gain stage be affected by V2. If you (understandably) don't want to change the normal channel at all, but still want the channels to be in phase for jumpering, you can take V2's unused triode and add an additional gain stage to the bass channel, which will give the bass channel more gain while allowing you to jumper the channels with better result. If you jumper the channels, have an additional gain stage, and remove the tone stack, you'll get more gain like a Marshall. It helps if your power tubes are biased hot as well. All this can be reversed if you don't like it, just be careful not to damage anything and save all parts that you remove.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

I might just lift the ground. The reason I asked is in order to get true marshall tone all the pots have to be replaced. Te aa864 uses a 10k, 50k, 250k vs the ab165 250k, 250k, 1m pot line up on the bass side. I really don't want to alter this Amp much. I just would like an extra usable channel that I can switch with an abundance pedal. Lifting the tone stack and sending the signal straight through would give more of a tweed sound, would it not? If so, that's definitely a good tone!

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

I know about the out of phase thing, and I like the idea of adding the gain stage to the guitar channel, and I might even do that as some b hole swiped all the mallory molded caps and replaced those with orange drops on the normal channelife. The bass and treble caps in the tone stack aren't even in the correct orientation as the shielded side of the caps is towards the grid instead of the plate, so they pick up noise.

 

[zzmoore]

Re: 65 bassman bass channel voice mod

How are you ascertaining the shielded side.?
There should not be much audible Noise/Cross-Talk/RF inside the amp for that to matter anyway. I would not worry about it at this point.
Same with cap color. As long as they are the right capacitance and the right voltage.....i would not give it a second thought.
If your amp has Bad/Audible noise issues, the cap orientation would be the last suspect on my list.
That said, i would certainly get it functioning properly in its present condition before i started with any "mods".
good luck

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

I found the orientation when checking with a scope and probe. The shielded lead is backwards. Also I am aware color of a cap is meaningless, however original spec was for a polyester cap, and the 716 OD is polypropylene, so I will replace them with a polyester of the correct value, regardless of what color they are. I say that because I haven't looked on mouse's site just yet for the values I need, so I'm not sure what is available. I have never added a gain stage to an amp, so more info on that would be helpful. If I am reading correctly I can take the output of the normal on pin 3 of v3 and feed it instead to pin 2 on v2, or am I wrong? I have the chassis on the bench for new filter caps, so why not alter the bass channel now instead of later, as it is currently useless to me. The noise of the tone caps incorrectly installed was a sine wave on the scope that I noticed when touching them, not audible. Just doing maintenence and found that. I do like the sound of lifting the tone stack on the bass side and I will try that first, but do give more info on adding that gain stage to put the amp in phase. Thank you

 

[zzmoore]

Re: 65 bassman bass channel voice mod

Look at the schem for the "newer" amp. You just need a couple of mix resistors and feed both channels to that last triodes gird.
I forget what we are looking at here...is that a 64.? Post the schem for the Silver Face 65-66 Bassman, and you will see where both channels pick up that last triode.....probably at the intersection of a couple of 220k resistors to keep them separate.
Out of phase in jumping an amp is not near the disappointment that two equally out of phase speakers can be. Guys jumped the Reverb Amps for years.....:dunno:
Just another one of those things you have to try for yourself to see if it "matters" or not.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

Yeah I hear you on that last part. This is a 65 blackface, October if I crossed the markings correctly. It matches perfectly to the aa864 circuit drawing. All transformers and chassis are 65 as well. I also have some resistors to replace. There are 5 or 6 metal film. Thry will be replaced with carbon comp, again just because I like the original composition parts. Values I'm ok tweaking to get the tone, but I find composition really affects tone a lot.

 

[Securb]

Re: 65 bassman bass channel voice mod

I wouldn't screw with the amp. You can get the right Marshall sound with the right preamp. I used a Chandler Tube Driver with my Fender and got amazing results.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

I'm not really looking for marshall tone. Just something usable on the bass side and due to keeping it simple, I think I'll just lift the tone stack on that side. One wire, no issues. As for the other incorrect parts, I'll be replacing those with the correct pieces.

 

[Securb]

Re: 65 bassman bass channel voice mod

I'm not really looking for marshall tone.

I do not play bass, so it would be cool to have marshall tone on one side, with fender on the other. ...Thoughts?

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

It would be cool to have a Marshall tone on that side, but I am not really looking for that. I'll be happy with something usable. Currently the bass channel sounds like a .047 tone cap on a humbucker with the tone rolled all the way off. Honky, dark and nasally. I don't play bass but if I did, it sure wouldn't be through that either. So, no I am not looking for marshall tone but if I fall into it I wouldn't complain. I feel lifting the tone stack might be the ticket and just might deliver a taste of the old fender tweed tone.

 

[zzmoore]

Re: 65 bassman bass channel voice mod

Yeah I hear you on that last part. This is a 65 blackface, October if I crossed the markings correctly. It matches perfectly to the aa864 circuit drawing. All transformers and chassis are 65 as well. I also have some resistors to replace. There are 5 or 6 metal film. Thry will be replaced with carbon comp, again just because I like the original composition parts. Values I'm ok tweaking to get the tone, but I find composition really affects tone a lot.

Right.....so look at the next schem or two, and you will see how they combined the two channels, which (for better or worse) makes them in phase.
To be honest.....it sounds like you have a pretty good grasp on all of this.
You could try something as simple a TS Defeat and see what you think.....and then take it from there. A few simple changes to the Cathode parts, Slope Resistor and a coupling cap will make a huge difference.
Good Luck with whichever way you go.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

And if I was going to use a preamp for marshall tone I'd plug in my Catalinbread dls, but I just want something usable on the bass side. What's to lose? This Amp isn't original anymore thanks to previous owners and techs, and I refuse to pay 25 bucks apiece for the original caps...

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

Amp is back up and running. Quiet, and has a great blackface tone. The caps definitely need some burn in time, but that will happen soon enough. New caps of all types are typically bright and well..new sounding. The clean channel is put back to 100 percent original specs. The bass channel, I replaced the coupling caps with .022 from the .1. It is much more usable than before and has a great, smooth, bluesy tone that cuts really well. The bass channel was the only one to retain the original caps which I stored away so it can be put back if desired. I am pleased with the resto/mod, and glad another blackface has been brought back from pending doom. It looks much better with the metal film caps yanked out and the carbon comp back in it, and it has that familiar vintage tone drift now when it gets good and hot as the resistors heat up. I brand this a success.

 

[Aceman]

Re: 65 bassman bass channel voice mod

I brand this as foolishness for the sake of personal entertainment. But hey - your amp. Whatever.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

I brand this as foolishness for the sake of personal entertainment. But hey - your amp. Whatever.

Well considering all of the caps, and resistors had been replaced with modern materials, putting original style parts back in it made it sound like a blackface bassman again. It sounds very nice. As for the bass channel, seeing as how I do not play bass....I'll do as I please. If this had been an all original amp...yes that would have been foolish. Since it was not, yes it is mine and I did do what I wanted. Thanks.

 

[MetalManiac]

Re: 65 bassman bass channel voice mod

I brand this as foolishness for the sake of personal entertainment. But hey - your amp. Whatever.

That was my thought. Thee AA864 is the Blackface bassman circuit many people would prefer, in fact, many people mod their AB165's to AA864. Why, IDK, if yo8u want an AA864, get one, because there is a special sound to the AB165 with it's distortion channel unique to that circuit it's just a shame to mess with it.

The amp looked so juked inside I didn't bother to respond as like it's no big deal if it's modded or not.. but I'm sure it sounds excellent either way.

 

[Powerstrokedr]

Re: 65 bassman bass channel voice mod

It wasn't working when I got it, either channel. All the original caps had been yanked, as well as most of the original resistors. I put the normal channel back to aa864 spec....exact spec. Sounds stellar. The bass side...bleh..that's my choice. It actually sounds great. I get a sound much like turning off the bright in the normal. I only changed the value ofnthe coupling caps when i put the correct carbon comps back in it. The pots were original and I didnt change those like some do to get a marshall tone. I used an ab pedal pedal and to be honest it sounds just like switching between bright and normal. Pretty cool to me. Now, if any of you who think this was foolish wants to provide the correct mallory blue molded caps...I'd love to put those in and make it 100 percent stock....I will not pay 30 bucks each on ebay though....especially when they were probably stolen out of amps and replaced with orange drops just like this one was.