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AB165->AA864 mods (lvery long!!!)


 
2/11/1999 3:11 PM
Ted
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AB165->AA864 mods (lvery long!!!)
It has been suggested that a very good mod for Bassman AB165 is a sort of "half-way back to AA864" mod. Following is a description of this mod. I had a few questions about this which are listed at the very end. As well I would be interested in any comments/opinions about this suggested mod. Many thanks and sorry its so damn long!  
 
________________________________  
 
[By Andy Ruhl] The AA864 Bassman vs. the AB165 Bassman I will have the schematics and layouts for both of these amps  
up soon.  
 
: : Fender had two versions of the blackface Bassman head, the short lived AA864 and the AB165. There is also the 6G6-B  
model which has a "blackface" but is really "brown" or "white" in the nature of the circuit (had a presence control), and had white  
barrel knobs so it is really not a part of the "blackface" family. I'll also discuss why I think the AB165 is not worthy of being in the  
same family as other blackface amps.  
: : First, the AA864. This amp looks like a blackface iteration of the circuits that came before. It has the standard blackface  
power amp section, with the standard feedback loop. The normal channel looks and sounds like the normal channel on other  
blackface amps (relatively speaking). The bass channel adds a gain stage and a different sort of tone stack which is similar to what  
Fender did with the 6G6, 6G6-A, and 6G6-B versions of the bassman head.  
 
: : The AB165 looks like nothing Fender ever did before, and indeed is a prelude to some of the idiocy that occurred in the later  
silverface era (tonewise of course).  
: : Power amp - The power amp has some very strange things added for a Fender. For instance, there are 220k feedback  
resistors from the plates of the power tubes back to the grids of the power tubes. The blocking caps between the plates of the  
phase inverter and the grids of the power tubes are now .022 instead of the normal Fender .1. The feedback loop now gets  
connected to the other side of the phase inverter via a .1 cap. This cap limits the frequency spectrum of the feedback (this may be  
something you want to experiment with in a project amp, NOT a valuable blackface amp!). Why doesn't the amp go into positive  
feedback runaway you tech heads ask? They reversed the leads on the secondary of the output transformer to reverse the  
reference of the feedback. The coupling cap to the phase inverter is a .1, which is far larger than anything that we saw before from  
a Fender amp. They probably did this in tandem with the afforementioned .022 caps later in the circuit to limit the bass content. In  
earlier years, Fender did the opposite thing and used a smaller coupling cap to the phase inverter (like a .001 or a 500pf in the  
case of the AA864) and used the larger blocking caps later on.  
: : Pre amp - The preamp on the AB165 does look fairly similar to the AA864 but does differ in a few key ways. The normal  
channel, which looks the same as before, is now connected to the circuit before the third gain stage. The third gain stage now has  
a 470k interstage feedback resistor that makes this tube work like both a compressor and distortion stage (even though that's not  
what Fender intended, but they didn't think we'd turn it up that loud anyway...). This is actually a cool thing, and is a good way to  
get a nice singing amp tone. Fender did add some caps across the 100k plate load resistors, which is not a good thing for tone.  
These caps send some of the high frequencies to ground, which doesn't help tone any. One of these caps is a 500pf, the other is a  
.001.  
: : Power supply - Fender did add some filtering to the AB165 power supply, which is a good thing. It probably helps tighten up  
the tone and reduce the possibility of motorboading (like some tweeds did). The schematic calls for an 8uf-450v cap at the end,  
but I've always seen a 16uf-450v cap there.  
: : Bias supply - Fender really screwed the pooch on this one. They set one tube up as standard non-adjustable fixed bias and  
then you adjust the other one to match the fixed one with the pot. Seems like a good idea right? Wrong. These amps almost  
always hum when they are adjusted so each tube draws the same amount of current. I don't know why, maybe an engineer can  
explain it to me. So when you adjust the pot to make the hum go away, the tubes are not idling the same and you get nasty  
distortion when you turn it up.  
 
:  
: : --------------------------------------------------------------------------------  
 
: : Modding the AB165 to sound better via some ideas from the AA864 amp.  
 
: : --------------------------------------------------------------------------------  
 
: : I don't consider the AB165 Bassman a proper blackface amp because of the nuttiness that is going on with the circuit. Sure,  
from the outside it looks like a standard blackface amp, but the changes to the circuit don't make a whole lot of sense when it  
comes to decent guitar tone. These amps never were much good for bass, so I won't even try to consider what the tonal  
difference between the AA864 and AB165 for bass because it's practically a moot point. So back to guitar tone. The AA864 has  
more of a blackface tone, but doesn't provide too much excitement. The AB165 has that extra compression/gain stage....  
Hmmm.... Not too hard to figure out this one is it? Mod the AB165 halfway back to AA864 specs for better tone!  
 
: : --------------------------------------------------------------------------------  
 
: : What to do to the AB165 to make it sound better.  
: : Pre amp - Take out the 500pf and .001 caps across the 100k plate load resistors. That will open up the high end. There's  
nothing else to do until you get to the phase inverter!  
 
: : Power amp - Change the coupling cap to the phase inverter to a .001. Make the feedback loop look like the AA864 by  
connecting it to the high side of the 100 ohm resistor. Change the .022 caps to .1. I actually use .047's here because I think it  
tightens up the bass a little bit. Don't forget to reverse the speaker side of the output transformer by connecting the green to  
positive and the black to ground.  
 
: : Bias suply - Make it look like any other blackface amp. I normally use a 50uf-50v filter cap, but anything higher than that up to  
about 100uf is fine. Use 220k grid resistors instead of the 100k's.  
 
: : Power supply - Change all the electrolytics with original values. The extra 16uf in the power supply of the AB165 (but labeled  
as an 8uf on the schematic) is a welcome addition and should stay. It would be a good idea to change the carbon comp resistors  
under the cap pan with some metal oxide or metal film ones to reduce noise and popping (or the chance of having it happen later  
on).  
 
: : Some notes: This mod takes a bit of creative thinking to get the layout to work. Fender had a nutty looking layout with the  
AB165. Some of the eyelets that you think you want to use could be grounded from underneath by a hidden wire. Look at the  
schematcs and layouts so you can figure out what you are going to do. I'd draw it out, but I don't remember exactly how I do it. It  
only takes a minute or two of thinking to figure it out, but you do have to look at it a bit.  
 
:  
: : --------------------------------------------------------------------------------  
 
: : That's it. Now you have an AA864/AB165 hybrid that sounds better than either. The amp compresses and distorts great, but  
you still get a good clean tone. Please resist the temptation to be a jerk and do this to an AA864 model amp. Those amps are  
rare, and I think most of them were built in late '64 which makes them Leo era amps. Don't mess with them! You can usually find  
any blackface (AB165) or silverface Bassman head for fairly cheap to do this to. AA864 heads are off limits for modding as far  
as I'm concerned.  
 
________________________  
 
I have a couple of questions about Andy's recommendations for modifying the AB165 towards an AA864 ciruit.  
 
Q1. He suggests connecting the feed back loop in the power amp via "the high side of the 100k resistor" The schematic for the  
AB165 doesnt show a 100k resistor to ground the way a AA864 does. Shall I add? Shall I leave the 47k and the 0.1 cap as is  
(rather than replace with 820k) so I will be connecting the existing cap to this new 100k resistor which goes to gnd?  
 
Q2. His suggestion is to replace grid resistors in the bias supply with 220k (instead of the 100k). Doesnt the AB165 schematic  
already show 220k's same as AA864?  
 
Q3. In the preamp the schematic shows a 0.01 cap across the bass channel plate resistor not a 0.001 as Andy suggested) but I  
guess it doesnt matter anyway since I'm to get rid of those tone suckers anyway)?  
 
Q4. Could I couple to the PI with a 0.01 uf instead of a 0.001 (currently there is a 0.1 cap in the AB165)? Only because I dont  
have any 0.001's lying around. Or, could I use the 500pf which I will remove from the Normal channel plate load resistor?  
 
Thanks for any suggestions. [Ted]
 
2/14/1999 10:31 AM
SpeedRacer
email

RE: >>modifying the AB165 towards an AA864 ciruit.<<  
 
>>Q1. He suggests connecting the feed back loop in the power amp via "the high side of the 100k resistor" The schematic for the AB165 doesnt show a 100k resistor to ground the way a AA864 does. Shall I add?<<  
 
The resistor in question is 100 OHMS, not 100K. The AB165 doesn't have it, so you'll need to add it.  
 
>> Shall I leave the 47k and the 0.1 cap as is  
(rather than replace with 820k) so I will be connecting the existing cap to this new 100k resistor which goes to gnd? <<  
 
Replace the 47K with an 820 OHM (again, not 820K..) if you're going with the AA164 circuit. Remove the .1uF cap altogether.  
 
>>Q2. His suggestion is to replace grid resistors in the bias supply with 220k (instead of the 100k). Doesnt the AB165  
schematic already show 220k's same as AA864?<<  
 
Yes. The AB165 uses 220K, they're just in the wrong locations (well, one is anyhow). Here's a tip: look at ANY BF fender output circuit and compare the layout. They are all basically the same. Dupe an amp with a 4-ohm output (Bassman, Twin, Pro, Vibrolux etc) and you're all set. Copy their bias supply. It will work for your amp.  
 
>>Q3. In the preamp the schematic shows a 0.01 cap across the bass channel plate resistor not a 0.001 as Andy suggested) but I guess it doesnt matter anyway since I'm to get rid of those tone suckers anyway)?<<  
 
Not tone suckers, just LPF..  
Lose the caps IMHO. Add the two channels the way the other BF amps do - via a 220K summing node at the input to the Phase inverter.(The eyelets are all there to do this.) This frees up a whole 12AX7 for some experimenting!)  
Output section: You can use a 56K feedback resistor (instead of 820) and a 5K presence pot (instead of the 100-ohm resistor) to give the amp a more "tweed" feel. Use the extra speaker out for the presence pot.)  
 
>>Q4. Could I couple to the PI with a 0.01 uf instead of a 0.001 (currently there is a 0.1 cap in the AB165)? Only because I dont have any 0.001's lying around. Or, could I use the 500pf which I will remove from the Normal channel plate load resistor?<<  
 
You can use nearly any value you want. It just changes the bass response of the amp. You'll need to play with the value to get what you want anyhow. Try anything from 500pF to .1uF.  
 
Have fun!  
 
ps: I can send you any/all of the layout diags  
in zipped in BMP form if you want. Email  
me if you need 'em. AA164, AA165 & AB165, etc.
 
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