Bassman AA864...

[TubeGeek]

I just received this bassman amp in for service.  It has been severely modified and probably not in a good way.  It craps out after 5 or 10 mins.  I want to restore it to it's near stock value for the owner but first I am going to try and figure out what the heck the mod is all about.  From what I can see the intent was to turn the amp into a gain monster.  

I think the amp is better off restored to original stock or close with a few minor mods but keep it in the fender realm.

Based on what you can see, does this mod look familiar to anyone?  It's worth a shot, maybe this is some special mod that I know nothing about.

[plexi50]

It looks to me like coupling caps and 100K plates were put right on the tubes them selves instead of the board and is wired for a gain and volume pot like a high gain amp

[TubeGeek]

I just finished working on this amp tonight.  I ended up rebuilding it to pretty much stock aa864.

I am going to voice the amp tomorrow and finalize it.  The owner asked for the bass channel to be used for bass guitar and the second channel for guitar.  I am going to experiment with the normal things like different cathode bypass caps, coupling cap values etc.

I have noticed from the few mins I have tested it that this amp has way more gain than I thought it would.  I have a bandmaster amp from the same era and it sure doesn't have the snarl that this bassman has.  This changes what I was going to do for tuning the amp to the owner's likes.  The cab that will be used is a 2x12 loaded with greenback's.

Do you guys have any recommended tweaks for this circuit that I can experiment with ???  I always get good ideas from you guys!

[TubeGeek]

It looks to me like coupling caps and 100K plates were put right on the tubes them selves instead of the board and is wired for a gain and volume pot like a high gain amp

Yeah...what I figured.  I found three eyelets that had not even been soldered.  The previous tech that serviced/modified the amp did not pay much attention to detail.

[TubeGeek]

Couple things going on with this amp...

The normal channel is nice and quiet.  It's the bass channel that needs a little attention.

The bass channel has a ton of gain.  I've honestly never worked on a bf fender that has this much gain.  It's not a bad thing IMO, i think i can make a nice lead channel out of it. Originally it was going to be a bass gtr channel but I don't really feel the design is suited for that.  Correct me if I am wrong.

I have been experimenting with different cap values in the tone stack of the bass channel.  With the stock 0.1uF's I get a motorboating oscillation when the bass control and vol is turned up past 5.  I have temporarily relieved the motorboating by changing the cap value on the bass pot to 0.022uF.  I may leave this in because I kinda like how it tames the bottom end a bit.  I am still experimenting with options.  I also like the grid input cap of the PI to be 0.022 but I had to reduce it back to stock 500pF to get rid of low freq motorboating.

I am hearing pink noise coming from this bass channel.  I have replaced the 100k plate resistors and tried different caps including the 250pF treble cap all with no improvements.  Any idea where the noise may be coming from?

I have tried different tubes in v1 and v2 and PI positions too, none of these helped eliminate the noise.  I am beginning to think that it could just be the design of this channel with the two gain stages.

Any help is appreciated.

[plexi50]

Is the noise still there if you pull the PI tube?

[TubeGeek]

Is the noise still there if you pull the PI tube?

No.  It's not in the power section.

I'm sure it's in v1 or v2.  The noise disappears when i pull either v1 or v2.

Here is a link to the schematic and layout for anyone inclined to help   http://ampwares.com/schematics/bassman_aa864.pdf

[VMS]

Have you tried chopsticking?

I would try moving the normal channel input wire further away from the red wire that comes from the third stage of the bass channel.

[TubeGeek]

Have you tried chopsticking?

I would try moving the normal channel input wire further away from the red wire that comes from the third stage of the bass channel.

Yep, chopsticked the heck out of it.  Very microphonic around v1 and v2 components.

I have decided to do a little more work to the bass channel and mod it to a trainwreck express style.  Hopefully after it's converted the noise issue will be resolved.

[PRR]

> The normal channel is nice
> The bass channel has a ton of gain.  
> I am hearing pink noise coming from this bass channel.

That may be the way it is.

Normal has two stages before the mixer; Bass has THREE stages.

Because that would be absurd excess gain, the treble-pot has a top-stopper, there's no bottom-stopper ("mid" pot/resistor), and there is a 2:1 divider between 2nd and 3rd stage; also two heavy treble-cuts (one 0.001u cap on the 2-3 divider, 0.003u cap on 3rd stage plate).

Both are rolling-off above 1,500 Hz. You want guitar to extend past 3KHz; this may be Leo's idea of how to scale for bass guitar. Other values (see Deep switch) suggest a HEAVY low-tone bass guitar sound.

I think it is for soft-plucking a low-output bass pickup, in an environment (cowboy bar) where some mellow hiss is acceptable. That may not be what modern bassists want. It probably aint quite right for guitar neither.

That's assuming as-drawn. You found screw-ups and mods, so who knows what it really is?

Try un-modding the mods, fixing what's screwt, maybe going to a more standard Blackface tonestack, then do this to get less excess gain and extended treble.

[TubeGeek]

I started over with the bass channel.  I couldn't get rid of the noise.

I got a good idea for topbrent to implement a trainwreck style voicing to this channel.  Great idea.  Thanks Brent.

Here is what I have done:

v1 cathode pin 3 has a .68 and 1k5 resistor with the deep switch now adding a 25uF in parallel for a tweed/plexi style switch.

v1 cathode pin 8 has a 25uF and 2k7 arrangement.

I used 0.022 caps in the tone stack.

Slope resistor is 100k

500pF treble cap

100k plate resistors on v1

v1 pin 6 plate goes to a 0.022 cap then to150k to gnd.

v2 cathode pin 8 has a 10k resistor, non-bypassed.

Lastly I used a 0.1uF cap to couple this channel to the 220k mixing resistors.


Question...technically speaking am I doing anything wrong with this setup?  I love the way it sounds but want to make sure I'm not overlooking anything important.

I have attached the voltages I am measuring too and the trainwreck layout I have been referencing.

[spacelabstudio]

I started over with the bass channel.  I couldn't get rid of the noise.

I got a good idea for topbrent to implement a trainwreck style voicing to this channel.  Great idea.  Thanks Brent.

Here is what I have done:

v1 cathode pin 3 has a .68 and 1k5 resistor with the deep switch now adding a 25uF in parallel for a tweed/plexi style switch.

v1 cathode pin 8 has a 25uF and 2k7 arrangement.

I used 0.022 caps in the tone stack.

Slope resistor is 100k

500pF treble cap

100k plate resistors on v1

v1 pin 6 plate goes to a 0.022 cap then to150k to gnd.

v2 cathode pin 8 has a 10k resistor, non-bypassed.

Lastly I used a 0.1uF cap to couple this channel to the 220k mixing resistors.


Question...technically speaking am I doing anything wrong with this setup?  I love the way it sounds but want to make sure I'm not overlooking anything important.

I have attached the voltages I am measuring too and the trainwreck layout I have been referencing.

I did some work on a friend's AB165.  The normal channel was really hissy.  Don't know if it's the same noise you're hearing or not.  But I just replaced pretty much all of the resistors in that section and it got really quiet.  I used metal films everywhere except for the plate load resistors where I used new carbon comps, following RG Keene's advice.  My friend was pretty happy with it.

Chris

[topbrent]

Right on!  Great job James!

Glad I was able to help you out on that one.  Now we need a full tone report.

What value did you use as the fixed mid resistor on the new circuit?

I started over with the bass channel.  I couldn't get rid of the noise.

I got a good idea for topbrent to implement a trainwreck style voicing to this channel.  Great idea.  Thanks Brent.

Here is what I have done:

v1 cathode pin 3 has a .68 and 1k5 resistor with the deep switch now adding a 25uF in parallel for a tweed/plexi style switch.

v1 cathode pin 8 has a 25uF and 2k7 arrangement.

I used 0.022 caps in the tone stack.

Slope resistor is 100k

500pF treble cap

100k plate resistors on v1

v1 pin 6 plate goes to a 0.022 cap then to150k to gnd.

v2 cathode pin 8 has a 10k resistor, non-bypassed.

Lastly I used a 0.1uF cap to couple this channel to the 220k mixing resistors.


Question...technically speaking am I doing anything wrong with this setup?  I love the way it sounds but want to make sure I'm not overlooking anything important.

I have attached the voltages I am measuring too and the trainwreck layout I have been referencing.

[TubeGeek]

We went with 22k for the fixed value of mids.  Pretty much full on mids.

The tone is very pleasing. The normal channel has the typical fender normal tone and the bass channel is basically a lead channel.  It breaks up nicely and doesn't sound too far over the top.  It has a nice bite to it with the customers custom beck strat with noiseless pickups in it.  I would have reduced the gain a bit if his guitar was different.  Paired up with his strat it sounds awesome.  The owner came over last night to complete the amp voicing to his preferences and he was elated.  He couldn't believe that I resurrected this amp from being non-useable.  A happy customer means a happy me.  I finished this amp just in time for the guys first cd release party this Saturday night, it will be used all night long so I will get a good chance to listen closely.  I worked with this guy on his cd recording too, we recorded the bed tracks at my place with my equipment.  Here is a link to a few of the songs...http://www.reverbnation.com/zojoblack

This is a fun amp to modify, I hope I'll see another one come across my bench someday.

[topbrent]

That is excellent man!  I am pumped that it turned out so well on this one. 
I have a 68 Bassman sitting here that will probably get the exact same treatment.


Did the aa864 amps owner dig the .68/25uf cathode cap switch?

We went with 22k for the fixed value of mids.  Pretty much full on mids.

The tone is very pleasing. The normal channel has the typical fender normal tone and the bass channel is basically a lead channel.  It breaks up nicely and doesn't sound too far over the top.  It has a nice bite to it with the customers custom beck strat with noiseless pickups in it.  I would have reduced the gain a bit if his guitar was different.  Paired up with his strat it sounds awesome.  The owner came over last night to complete the amp voicing to his preferences and he was elated.  He couldn't believe that I resurrected this amp from being non-useable.  A happy customer means a happy me.  I finished this amp just in time for the guys first cd release party this Saturday night, it will be used all night long so I will get a good chance to listen closely.  I worked with this guy on his cd recording too, we recorded the bed tracks at my place with my equipment.  Here is a link to a few of the songs...http://www.reverbnation.com/zojoblack

This is a fun amp to modify, I hope I'll see another one come across my bench someday.

[TubeGeek]

Heck yeah he digs the tweed/plexi switch!

So do I, this won't be the last time I implement that little idea.

[RobBozic]

Hey 'Tubegeek' I just noticed you used a 22n for the coupling cap between V1 (6) & V2, the TW uses a 2.2n.

Rob

[TubeGeek]

Hey 'Tubegeek' I just noticed you used a 22n for the coupling cap between V1 (6) & V2, the TW uses a 2.2n.

Rob

Good eye!  Yep looks like I did make that mistake.  I guess I am passing more low end through that point with the 0.022uF.  It didn't sound like a problem, is it technically?

[RobBozic]

No not really, if it sounds good... it is good. However you should experiment here with different cap sizes especially in conjunction with the input cap to the phase inverter.

Rob

[TubeGeek]

No not really, if it sounds good... it is good. However you should experiment here with different cap sizes especially in conjunction with the input cap to the phase inverter.

Rob

Yeah thats what i figured also.

[phsyconoodler]

Happy customer means more jiggle in your jeans. Good on ya!

[mac0611]

you could make the bass channel into a tw express type circuit. Pull the #2 input jack and make that the volume control then you can have bass/mid/treble. from v2 run a .0022 cap/150k high pass filter to v3, then to the 220k mix resistors. You may have to tweak the gain by either playing with the 150k resistor or adding a pot as a gain control.