My bassman is either an AA371 or an AA270

[Gr8Scott]

Checked out the schematics and layouts. Mine matches an AA371 pretty well, but it looks a little different on the output stage. I don't see any small caps running to ground off the tube sockets on this one like the AA371 and AA270 diagrams show. I'll keep looking, but I haven't seen them yet (probably should use a flashlight as my livingroom is pretty dim). I guess that explains the intensely clean nature of this amp.

If I'm going to "blackface" this puppy, I'm going to need to do a buttload of work on it. The basic structure of the pre amps and the phase inverter are very different from this and the AB165 or AA864. If I'm gonna heat up a soldering iron, I want to make it as close to the AA864 circuit as I can. I have to say that I thought the bass channel was the good sounding one, but I was mistaken due to the faceplate being scrubbed into oblivion. It's the normal channel that sounds best on this amp. The Bass channel has a very grainy distortion that isn't very pleasant sounding. This amp needs some work to sound it's best. It has all the right transformers etc, so it's possible, but the road will be long on this project.

http://www.schematicheaven.com/fenderamps/bassman_aa864_schem.pdf

http://www.schematicheaven.com/fenderamps/bassman_aa270_schem.pdf

http://www.schematicheaven.com/fenderamps/bassman_aa371_schem.pdf

 

[Stratman]

Re: My bassman is either an AA371 or an AA270

My Bassman uses the AB 165 circuit, so I asked Jeff Seal to blackface it to spec and although I am not sure of the actual circuit design he used - it could be a AB165 or AB864. I will have to check with him to find out which one of the two he used.

BTW - The AB 864 is the better sounding circuit, but it's not still not as good as the 5E6 circuit.

 

[Gr8Scott]

Re: My bassman is either an AA371 or an AA270

I'm shooting for the AA864 circuit myself. I'm gonna get it as close as I can with this amp. Seems like it's going to really be a long road to get it just right. I'll do it in baby steps and hopefully I can come up with the right parts etc to make it sound good. I need to swap the blue and brown leads running from the 6l6 tubes to the output tranny as they need to be reversed to match the blackface circuit.

Found the snubber caps on the output stage. They were hiding in plain sight on top of all the resistors on the socket. The diagram shows them leading off one of the pins and then to ground, but they put this .02uf ceramic cap between the two pins that go to ground and then run one pin to ground instead. Took these puppies completely out of the circuit so the output tube sockets precisely match the old blackface output tube sockets. Next step is to swap the current 47K resistor for an 820 ohm feedback resistor and remove another ceramic disk capacitor that is piggybacked with it. Then I'll have to do some component changes on the phase inverter setup and change it to a real bias adjustment instead of a fake one. I'll also have to add a couple of mallory 25mfd/25v caps to the input stage on the bass channel (that is what's making the distortion sound so crappy on that side) plus a resistor change or two on the last gain stage before the phase inverter. Man I can't believe these folks willingly did this to what was such a fine amp design. They sure did make it cleaner, but they did so much more to it that makes it hard to reverse. :nono:

Though the road may be long, I'm looking forward to working on this amp. It's a challenge to get everything right, but it's teaching me so much working on it. I'm learning amp anatomy as I go along. My lesson for the day was what a feedback loop looks like and how the Presence knob on amps works (increases the resistor value between the phase inverter and the speaker outs). More resistance equals less feedback.

 

[kmcguitars]

Re: My bassman is either an AA371 or an AA270

Here's a great link on Bassman circuits .http://www.tone-lizard.com/Bassman_Modifications.htm

BTW, I was given a 6G6-B Bassman in my youth . Someone had painted it Black? So I had no clue what I had. I took it to be repaired. The repairman knew what it was right away,And offered me a pile of goodie's (amounting to a bucket of spit) for it. I should have known. If only there was a Internet back then whit great forum's like this.

 

[Gr8Scott]

Re: My bassman is either an AA371 or an AA270

Thanks for the link. I've got that puppy printed and on my coffee table as we speak. The info contained on that site is invaluable for what I'm doing. Gives a great overview of what needs to be done (especially for the AB165 circuit). Lots of it transfers over to my circuit too.