1 channel AA864 bassman with typical blackface verb

[goldstache]

Getting ready to build just the normal channel of a blackface bassman but with typical AB763-esque reverb.  Anybody got tips on what topology I should go with?  Driving fixed bias 6l6 or 5881.

I've got the iron so I'm chomping at the bit to get started!!!!
Thanks

[p2pAmps]

Getting ready to build just the normal channel of a blackface bassman but with typical AB763-esque reverb.  Anybody got tips on what topology I should go with?  Driving fixed bias 6l6 or 5881.

I've got the iron so I'm chomping at the bit to get started!!!!
Thanks

Do you plan to make your own custom eyelet board?  Have you figured out where you are going to stuff two more tubes and a reverb tranny?  I would like to see your plan if you have it drawn up...

[tubenit]

The normal on AA864 is two gain channels.  Here is how I would attempt this.  Ignore the values in the schematic and look at the topology.

You can use a one tube reverb which I personally like as well as the standard Fender reverb ........... OR you can use a 12AT7 reverb driver like Fender does. 

There is an abundance of information about "one tube reverbs" here:  http://el34world.com/Forum/index.php?topic=7957.0

I actually have a layout that goes with this schematic.   (I also have a SCH editable version of both the schematic and layout)

With respect, Tubenit

[tubenit]

IF you wanted a more typical Fenderish style 12AT7, then you could do this.  However, this leaves you with an unused triode on the 12Ax7 recovery tube.

You could use that as a "boost" gain channel that is relay switched.

With respect, Tubenit

[sluckey]

Getting ready to build just the normal channel of a blackface bassman but with typical AB763-esque reverb.  Anybody got tips on what topology I should go with?  Driving fixed bias 6l6 or 5881.

When you remove the Bass preamp from the AA864 what's left is exactly like an AB763. May as well add that AB763 reverb circuit.

[goldstache]

Great info!  Looking at the AA165 bassman/pro reverb. Seems pretty similar to what I'm after.  I am punching my chassis and staking boards.  I have all the iron choke, OT, PT, reverb XFMR, and matching tank.  I think I'd like to try the AB763 reverb thang.  That's what was in my head all along, and I have all the parts.  I'll just try it and tweak as I go.

Preamp- (send to ab763 verb) 2nd stage(EQ)- driver stage (reverb return and dry sum)- PI- fixed bias 5881.

Thanks tubenit!  Lots to review there and I have used one incantation of the one tube reverb!  Liked it a lot!

More to come.

[tubenit]

Just for clarity,  when I stated to ignore the values and look at the topology,  I was referring specifically to the reverb topology and it's insertion points not the preamp or tone stack.

Building the AB763 makes sense to me just as Sluckey suggested.

With respect, Tubenit

[goldstache]

I have all my Iron and Board layouts.  Putting together the Chassis.  Im going with AA864 Normal Channel and AB763 Reverb. 


I'm getting everything laid out on the chassis and was checking Transformer orientation for noise.  Interesting findings and wanted some insight based on what I'm hearing in some headphones while "candeling" the OT.  In most fenders with reverb from end to end, the chassis is laydown PT far to one side by AC input. (normal) choke bell facing short side of chassis close to PT.  Then OT (same orientation as Choke) close to the choke.  Then cap pan under bulk of circuit on bottom of chassis. 


When my PT primary had AC on it, with headphones on the OT secondary, I noticed a fair amount of hum until I was around 6-8" away from the PT.  cool move it, no problem.  Then I hooked the headphones up to the secondary of the Reverb XFMR.  Lots of noise until i went 45 degrees to PT. (normal fender config) 
Here's my quandary:
Moving filter cap pan between Choke and OT's new location. Will it cause issues? 
With the OT moved further away from the PT (where it was silent) will it incite noise between the now close proximity to the reverb XFMR?


Is there a way to check flux lines between the moved OT and the Reverb XFMR?


Thanks.  I've fought this noise in reverb circuits before and would love to eliminate most of the woes before the chassis gets "HOLY"
 
 

[goldstache]

I had the PT long side adjacent to the short side of the chassis.  (Not how Fender did it)  Moved it 90 degrees problem solved.


Duh!

[EL34]

I make single channel AB763 boards
They are on this page
http://hoffmanamps.com/MyStore/catalog/TurretBoard.htm


Information, layouts, etc are on the library page

[goldstache]

Thanks EL34!!!   For everything really.  Sharing your designs, which some I've built, and making this forum what it is.......
Mainly for me, a place to get help when I'm stumped!


Got chassis bent and drilled.  Rough assembly complete, and board ready for population!
Here's some pics!


And away I go!

[p2pAmps]

Thanks EL34!!!   For everything really.  Sharing your designs, which some I've built, and making this forum what it is.......
Mainly for me, a place to get help when I'm stumped!


Got chassis bent and drilled.  Rough assembly complete, and board ready for population!
Here's some pics!


And away I go!

Looking super nice and I LOVE those Classic Tone transformers.  Can't wait to hear her...

Are you doing anything special with the left side?

[macula56]

I make single channel AB763 boards
They are on this page
http://hoffmanamps.com/MyStore/catalog/TurretBoard.htm


Information, layouts, etc are on the library page

do you make a single channel board with just reverb and no tremolo circuit? I just made one but would definitely have bought one from you if I had seen one on your list of boards.

[sluckey]

I don't think Hoffman currently offers a single channel AB763 with reverb but no tremolo. Probably would not take much time to offer it though. Send a pm to Doug.

[goldstache]

Nope just standard 5881 PP.  I was waiting for tube retainers at time of photo.  Hole on far left is power cable, and it needed some dremel work for a snug fit!

[goldstache]

Well it's all wired up!  Worked from the the get-go.  Love when that happens. 


I built it with the AA864 Normal Channel (with some tweaks) and the AB763  Reverb circuit out of V1 and summing with dry before PI.  I don't have the extra gain of the Bass channels extra triode stage. Just the Normal Channel. 


I have an original AA864 that I love, but am finding that my version with reverb is not as loud as the original.  I am wondering, since I have another 1/2 of a triode, would you recommend a voltage divider into another gain stage before the PI.  I don't have all my voltages down written down yet, but I do know that my B+ is about 25V higher than schematic. 
Should I bias this "extra stage" like classic fender, or experiment with a train wreck-esque stage to Jam the PI for some sweet sing?  I will probably try both but wanted some input.  Thanks so much!

[goldstache]

Well, here is what I ended up with.  It's kinda AB763 and AA864 mashed together, driver for P.I. is a trainwreck borrow. Pretty clean overall and gets a little hair past 6 on volume.  It's a great sounding amp and the low/high switch is perfect for studio/stage environments.  Noise floor is real low and there is no EMI problems with the iron orientation.  Just thought I'd share the schemo's sorry I havent written in voltages yet.  My buddy Dann Bonds is gonna love it!

Thanks.

[sluckey]

Nice to see neat hand drawn schematics. We need more of that. Kinda adds a bit of realism to this hobby.

I combined your 3 files into one pdf in case you like that idea.

[goldstache]

Thanks Sluckey!!!  I'm still working on making my layouts clean.  Unfortunately file management has never been my strong suit.   

[goldstache]

gut shot