Difference between AA864 normal channel and BF Bandmaster normal

[sheeba8w5]

I'm working on a AB165 Bassman. Its currently set up as an AA864 on the normal channel and PI and power amp. The bass channel has a Train Wreck preamp of sorts. The Bassman normal channel is a good clean channel but not as warm as I'm used to from my old BF Bandmasters I looked over some old AB763's and the only difference is a coupling cap that is inserted between the last tube stage and the 220k mix resistors. I added a .047 UF coupling cap in between those on my baseman and it sounds just like my BF bandmaster. It also knocks down the power a bit as well which makes the amp a bit more usable for me. Is this in any way going to cause an issue? My second question is can I switch it in and out of the circuit? Thanks in advance for the help!

[Jazz P Bass]

Interesting observation on the tone difference.

I think that 0.1uf capacitor is to keep Vdc off of the Intensity control. (Vibrato)

I would think that a straight out 'switching' of that capacitor in & out would be problematic.
Besides there being 275 Vdc on one end, it will probably 'pop' when you switch it.


I'm not saying it cannot be done.
Just that it will take a little more thought, circuit wise.

[sheeba8w5]

Yes the 0.1uf cap is on all of the AB763 Vibrato channels in that same position. All of the Normal channels have the .047. With the cap installed I notice the amp is less fatiguing. I think it must be cropping out the very low frequencies as well as softening the highs just a little?? I was going to wire a switched jumper in parallel to the cap. Then I could AB the two sounds a bit more easily. I wouldn't need to use that live... just for auditioning the the differences.

[sheeba8w5]

Any more thoughts on this? I'm going to swap a .1UF in that spot tonight to see what that will do in comparison. I'd still like to be able to access the sound without the cap installed. Maybe I could just put the jumper on the bright switch for now??

Thanks

[52 Bill]

Any more thoughts on this? I'm going to swap a .1UF in that spot tonight to see what that will do in comparison. I'd still like to be able to access the sound without the cap installed. Maybe I could just put the jumper on the bright switch for now??

Thanks

I would try adding a second 0.1uF in series with the original 0.1uF cap.

Leave the original one connected to the tube plate and add the second cap from the original cap to the 220K input mixing resistors.

The two caps in series should give you a total capacitance of 0.05uF, and if you jumper across the second cap the total value will go back to 0.1uF.

Just a thought.

[sheeba8w5]

Thanks Bill. I will give that a go.

[sheeba8w5]

I Installed the dual .1uf caps in series. I definitely like the old Blue .047uf cap more than the two 6PS .1ufs's in series. I also like the sound without the cap installed.