# Fender Bassman Circuit Query



## dtsaudio (Apr 15, 2009)

I have a customers silver face Bassman head here. It has had some mods done to it and the owner wanted it put back to factory but there are a couple of circuit differences that don't match any of the schematics I could find. 
The amp is labelled as a AB165 chassis, but I know from this era, that could be wrong. I have attached a photo of the board layout and aside from the mods I have corrected this is what I have found so far.
Board layout is definitely AA165 (ignore the small white cap to the left)
The coupling cap, and resistors between the 2nd and 3rd stages on the bass channel are AA864 (the values were changed in a mod)
The feedback from the output transformer is AA864
The cathode resistor and cap on the Bass first stage are positioned like the AB165 (big white one on the left)
but the small white cap which is the Deep was missing and should have been there.
Here's where it gets a bit strange. 
The tone stack caps in all the versions I've seen should be 0.1ufd on the Bass channel. They are 0.022ufd. This appears factory.
One cap on the phase splitter is the wrong value. Schematics show 0.1u, it is a 0.047u

So this thing looks like a combination of the AA165, AB165 and the AA864 chassis with a little something else thrown in. It works fine, and the tone controls operate as they should. 
Anybody seen this kind of thing before?
I have it as close to factory as I can with the mods removed, but the differences have me a bit baffled.


----------



## cboutilier (Jan 12, 2016)

Calling @keithb7


----------



## keithb7 (Dec 28, 2006)

I am not too familiar with the SF Bassman circuits. Previous experience tells me that on some days at the factory, it would appear that the only consistent thing, was inconsistency.

I recommend you join the SF Amps Facebook group and post your query there. There is a grand master there. Nic Grabien. He knows more I think about what was going on, on the gactory floor than Leo or Forrest did. Lol.

Call Nic out in your post. He'll answer and probably tell you the story. Good luck.


----------



## WCGill (Mar 27, 2009)

I've seen a few AA165 and AA864 mashups that were definitely factory and one had the wrong value bass caps as well. I'd call this one a AA165 (could be wrong here). The feedback circuit from the AA864 and AA165 seem to be identical. The boards from the AA and AB165 seem to be identical as well.
What I'm not getting from the board shot is the 27K (?) 1w resistor at the top off the 470k resistor-nothing in any of the schematics from what I see. Definitely interesting but maybe not in a good way.


----------



## dtsaudio (Apr 15, 2009)

I thought it might be an AA165 as well, but there are some differences. You're right the AA and AB165 boards appear to be the same. The circuit is slightly different though. I actually hadn't noticed that. The tone caps being different is what threw me. I am inclined to call this an AA165 as well despite the caps. Although one bypass cap is missing from the third 12AX7 like in the AA864.
The 27k/470k thing confused me for a bit as there is a 27k shown on the schematic coming off the power supply. However that's not it. That 27k is going to ground and being fed by the 470k. This turned out to be some king of mod to reduce the output from the Bass channel's second 12AX7. It should be two 220k, with one paralleled by a .001ufd cap. That part is from AA864.


----------



## mhammer (Nov 30, 2007)

From the description, it appears somebody wanted it optimized for guitar, rather than bass.


----------



## dtsaudio (Apr 15, 2009)

That's how it appears. The new owner wants it as stock as possible, however, which is what he is getting. That's the reason for this thread. I wanted to give him as much info as possible as to what he has.
FYI, this thing is in fabulous condition 8/10. 

Keith, I'll try the FB page too.


----------



## dtsaudio (Apr 15, 2009)

I forgot to mention the strangest mod. 
If you look at the very right you see a large white cap. This is the factory 25ufd cathode bypass cap for the first stage. It's present location is correct and where I put it.
What was there, was a 0.68ufd cap. That 25ufd was connected to the Deep switch and at the cathode. The .02ufd to the deep switch was taken out. It worked more like a boost than the deep control it was supposed to be.


----------



## nonreverb (Sep 19, 2006)

I think the Bassman circuit is possibly one of the most revised tube amp circuits in history. I've seen several versions....


----------



## loudtubeamps (Feb 2, 2012)

FWIW Dan, I would rebuild the board using the the version stamped on the tube chart label. You could go crazy trying to second guess a transitional build from the factory. I had a B.F. Pro in recently with a similar situation. This one had been modded extensively. Alot of the original parts were gone and a total strip and rebuild was necessary.Thankfully the owner had the original front panel plate. I would also suggest removing enough wiring to allow the board to be flipped for inspection. Chances are there will be heavy solder on some of the eyelets, some of the 'blobs' may be ready to let go or in close proximity to an adjacent eyelet.If the owner wants the amp restored to 'original as possible', it would allow access to route and connect any (under board) wiring originally done at the factory. Have fun...Cheers, D


----------



## toby2 (Dec 2, 2006)

AA165 - phase inverter has the same value original capacitors . The AB uses .022's and a different feedback network . The preamp has been messed with but it is no big deal to set back to AA


----------



## dtsaudio (Apr 15, 2009)

loudtubeamps said:


> FWIW Dan, I would rebuild the board using the the version stamped on the tube chart label


That ship has sailed. I thought of that, too much $$$. I had to work pretty much with what was there and managed to get a good compromise. It is pretty much an AA864 circuit even though labelled AB165. It never was an AB165, too many incorrect parts and the feedback is wrong. The only strange thing is the 0.022ufd in the tone stack. They are factory and no one has messed with.
The amp now works fine with all the strange mods taken out.


----------



## KeyserSoze (Jan 8, 2015)

Sorry to bump this, but just for curiosity's sake...

AB165's came with 2x .022's 'tone' caps in the phase inverter, and 3x 0.1's 'tone' caps in the bass channel. A common mod (for decades now) has been to take the 3x 0.1's from the bass channel and use two of them to rewire the phase inverter to AA864 specs. Then rewiring the Negative feedback section (beside the Phase inverter) to AA864 specs with the remaining 0.1 cap stolen from the bass channel.
Then the spare .022's (stolen from the AB165 Phase Inverter), are used in the bass channel which is reconfigured for a Tweed/JTM45 voice.

I'd seriously doubt it's a factory AA165, as they were actually only made for a few months before switching to the AB165 schematic, and because it's a SF Bassman...
I'm assuming the mod was maybe done a long time ago, or has just been in a smokey room/or played a lot to dull/oxidize the solder joints from when this mod was done.

BF and SF Bassman's are famous for being modded, and being a very basic and great sounding circuit laid out on a eyelet board PTP style ...it also makes them easy _to _mod.

It's hard to make these sound bad, I'm sure the owner is still in the honeymoon phase enjoying it!


----------



## Brett Pearson (Apr 26, 2016)

I once had a SF Bassman come in where someone wired a distortion pedal into the circuit! The pedal was stuffed in the bottom with wires running through a hole into the chassis...lol. I too have seen a lot of amps from the 60's whose circuits don't necessarily follow the tagged model number. I have an old Ampeg Reverberocket whose tube chart has the earlier octal pre-amp tubes crossed out and 12ax7 penciled in under them. They just kept using the old charts even after transitioning to 12ax7 tubes until they ran out of them....waste nothing I guess.


----------



## sammyr (May 7, 2013)

bassmans are frustrating like that. My bassman is a 68 drip edge and it has the ab165 tube chart but the circuit inside was actually the aa371 or aa270 I can't remember which.


----------



## nonreverb (Sep 19, 2006)

The engineers went wild on the Bassman which is mystifying.... the irony being the Showman was a far better "bass" amp then the Bassman ever was during that era.


----------

