Fender Blackface Bassman AA864 chassis

[Jamlid]

Hi Folks. New poster here, although I've been lurking for a while.

I've built quite a few pedals, 1 x 5f2a from a kit and now I want to embark on a more ambitious project: an AA864, fron scratch.

I'm confident that with some research I can order all of the necessary parts, except I'm struggling to find a pre punched chassis. Therefore, I was wondering if someone here might have any advice as to where to find one (or one that's adaptable) in the UK. So far I have come to the conclusion that a Blackface super reverb chassis would accommodate everything in a AA864:

http://modulusamplification.com/Blackface-Super-Reverb-Chassis-P5742119.aspx

I'm probably going to go down this route, using the already existing tube socket holes and drilling the smaller turret board mounting holes in necessary when it comes to it. Is there anything else I should be aware of or can anyone suggest a better route?

Cheers
Jim

[pdf64]

An SR chassis is rather long and unwieldy, I’d avoid using one unless that’s what you want.

TubeTown sell a blank bassman chassis. It’s in aluminium, so good for putting your own holes and cut outs in, not so good at keeping straight if it gets dropped.

I seen glowing reports from some US builders about a far Eastern chassis and faceplate guy on ebay, so maybe try searching there.

[Willabe]

Welcome! 

Why do you want a AA864 Bassman? There's a lot of different amps to pick from.

Do you want it for bass or guitar?

[Jamlid]

Hi guys, thanks for your replies.

Hmm I did consider the extra length and how cumbersome it might be. Was hoping to avoid drilling anything, but I suppose I will have to manage with my small hobby pillar drill! Any advice on drilling? I'm talking particularly for tube sockets, the big holes and positioning the holes for the screws to attach the socket. Also any advice on cutting out xfrmr holes and AC socket holes?

With regards to my choice of circuit, I first became aware of the AA864 when looking up Steve Albini's gear. Nerd alert! (I'm actually currently working on a somewhat simplified version of an Intersound IVP, if that's of any interest.) However, having read up on the AA864 since then I now get the feeling it would be good for what I want it for: a deep clean sound with a lot of headroom and less of a focus on distortion. I want it for use with guitar by the way.

Cheers

[sluckey]

The Fender 6G6 Bassman chassis would be a much better fit. Plenty of those in the USA. I bet you may be able to find one over there too.

[SILVERGUN]

Also any advice on cutting out xfrmr holes and AC socket holes?

Yeah, don't do it if you don't have to.
If you're not worried about it being a spot on recreation then go ahead and get whatever pre-punched chassis that will fit your project and go with it.


You'd much rather have more room to work and not need it than leave yourself short.

[jordan86]

I’m cheap but I’d definitely pay $50 more for a chassis that required no drilling. Just not worth it to me.

[pdf64]

If you're still looking, I've seen a few builders recommend this site for vintage accurate chassis etc http://www.solandfa.com/Products.asp

[Jamlid]

Hey folks, thanks again.

For those that are interested, I eventually went with a kit from Mojotone for this. Maybe later down the line I will attempt another small build and source individual parts for that instead.

I also bought the chassis pre drilled from an ebay seller in the end.

At this point I am waiting to get paid so I can fork out for the xfrmrs, but in the meantime I have assembled the main, filter cap and rectifier boards. I do have a question though as it seems there's another small board I can't locate on any of the layouts I've seen. The kit I bought comes equipped for both AB165 and AA864 so maybe it's for use when building the former? If anyone has any ideas please let me know.

[mresistor]

Its a bias board for biasing the power tubes. The little board usually is installed up behind the front panel and the PT next to the pilot lamp. The Bassman AA864 and the AB165 also have the rectifier diodes on the board with the bias circuit and it is a larger.  So that isn't the board you need to build either of those bassman designs.


https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864_layout.pdf


https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_ab165_layout.pdf


If you look in Hoffman's store under Fender eyelet boards you will find a Bias #2 board. That's the one you need if one didn't come in your kit.


Is this another Mojo strikes again situation?  Hopefully not.

[mresistor]

How many diodes came with the kit for the rectifier and what is the part # for them?

[sluckey]

Mojo does not sell a kit for the Bassman AB165 or AA864. So which kit did you buy?

The board in the pic is the bias board that is supplied with Mojo's Super Reverb or Deluxe Reverb. It has nothing to do with Bassman amps.

Show us a pic of your rectifier board and main board and cap board.

[Jamlid]

Hey mresistor,

I see! I've actually already assembled that board, with the rectifier diodes and bias circuit on one board as you mentioned... Seems as though this is for a slightly different circuit, probably one that uses tube rectification? Anyway, thank you for clarifying.

Also, if yourself or anyone has any advice on how to attach the filter cap board + doghouse, choke, OT (basically all the stuff on the "outside" of the chassis, then please do tell. I'm sat here trying to plan things out and I'm scratching my head trying to figure out the right way. How do you screw though the outside of the chassis without said screws interfering with the main board? Is it just a case of measuring up properly? I can't find any comprehensive documentation on this online.

Cheers

PS, has Mojo got a bad rep? I thought they were alright? Certainly supplied me with good quality components. Just wish the project came with better docs

[Jamlid]

The kit I bought was the "small parts kit".
https://www.mojotone.com/Blackface-Bassman-Small-Parts-Kit-Caps-Resistors-Screws-Etc-

Pics incoming...

[Jamlid]

Here ya go

[mresistor]

Thats what I was thinking   the small parts kit for BF bassman, and you sourced you chassis elsewhere.   Still AFAIK "ALL" the BF bassman kits had the combo bias.rectifier board. In the pic for the kit I don't see even one bias board of any size but since you said you populated the board for a BF bassman I guess they included bias boards in the kit. That''s nice to know.


The doghouse cap board is screwed down to the outside of the chassis and has a metal cover that covers it. There should be larger holes for the wires to run to the inside and connect to the main board. I think there is info in Hoffmans information   I'll see if I can find it. You can also google pics of how it is assembled.
Hopefully your kit came with the rubber grommets you need for the wire run holes.

[mresistor]

show us pics of your chassis .

[mresistor]

you need one of these  and you'll probably need to drill the holes for mounting it - I don't know because I haven't seen your chassis   or know who you got it from.  The Hunter Brothers maybe.


https://www.mojotone.com/Alternate-Large-Cap-Pan-for-Twin-Super-Bassman 


Also I would tap the holes for #6 or #8 screws so you don't need to use nuts on the inside to mount the doghouse cover.


No Mojo doesn't have a bad rep but people here have discovered a few things that they haven't gotten right, mostly in documentation with kits.

[mresistor]

If this is the chassis you bought then the holes should already be drilled for you to mount the filter cap board and the cover for it.   The holes being tapped is another thing and in my experience they are probably not.  I have had to tap them on the chassis from the Hunters.  I think I used 8/32NC tap for the mounting holes.

https://www.ebay.com/itm/224744614763?hash=item3453d2ab6b:g:FCsAAOSwSeJg66hi

[sluckey]

A few other discrepancies with that kit pic...

3 Octal sockets, but only need 2. 3 noval sockets, but need 4 . 4 RCA phono plugs and jacks, not needed. 6 chicken head knobs should be Fender skirted/numbered knobs. I quit looking. Hope you got everything you need.

[sluckey]

Fender used self tapping screws for the doghouse. Same with the tube sockets.

[mresistor]

BTW if it were me I'd get a multi-tap output transformer with 4,8,16 ohm taps and then in the ext speaker jack I'd install an impedance switch to switch the output impedance to whatever cab I was going to use. I'm sure Hammond has one to fit that bill.   


I'd also go with a Hammond PT and buy from Hawk Electronics or Antique Electronic Supply.

[mresistor]

Fender used self tapping screws for the doghouse. Same with the tube sockets.

Right. And I think Doug has those in his store.  Sluckey it's possible Mojo doesn't have the correct picture for that kit associated with it.

[Jamlid]

The rectifier/bias board was supplied indeed.

Yeah the larger holes on the chassis match up more or less with the one layout I can find from Mojo. As you can see they have grommets and are ready for when xfrmrs arrive.

That is indeed the chassis I bought. Guess I need to find some detailed pics and figure out which holes are for what!

Thank you for your assistance

[mresistor]

I think I'd get one of these to cover the empty octal hole https://www.ebay.com/itm/133694851192?hash=item1f20d58878:g:hxgAAOSwxCVgT4xf


I guess they give you the option to use a tube rectifier, and I suppose most builders might opt for a dual-rectifier build. 

[Jamlid]

Thanks for the transformer recommendations. I'm in the UK however, so will have to source from elsewhere to avoid customs charges.

I've noticed something else however: The filter cap board seems too big. I guess I will drill new holes for the grommets in a better position.

[AmberB]

This is a 1975 made Fender Bassman 50 watt that I have.  You can see how the external parts are laid out here.  Maybe this will help.
The knob that you see by the choke is for the bias control.  I changed the pot when I modified the bias circuit on the amp so that I could adjust the bias voltage.

[mresistor]

Thanks for the transformer recommendations. I'm in the UK however, so will have to source from elsewhere to avoid customs charges.

I've noticed something else however: The filter cap board seems too big. I guess I will drill new holes for the grommets in a better position.

One the filter cap board  those two holes on the corners of the one side should be over the grommets when mounted..  easy enough to drill a hole on the side where the grommet is with no hole on the board for it.
It seems to me that whoever installed the brass eyelets may have had the board upside down when they did it. The board doesn't appear to me to be too wide. Looks like it fits fine with the mounting holes - just need that additional hole over that one grommet. Though looking at Amber's board looks like that one hole is unnecessary What about the ground wire side, are there holes with grommets on that side? I'd iron that out prior to soldering in the components.

[Jamlid]

Thank you for the photo Amber, was indeed helpful!

Also, a little update for you all. I have bought and installed the PT, choke and OT (Hammond 290EEX, 194B and 1760L respectively). The OT needed new holes drilling in the chassis as they did not line up (pictured). Next I was measuring up for mounting the boards: The filter cap board did not line up correctly with the holes (both for screws and grommets) on the chassis, so more drilling required there.

With both of these issues remedied I now have one more qualm. The main board and accompanying insulation don't have mounting holes, neither does the chassis. The small parts kit also only contains enough screws to mount the filter cap board and bias board. I guess I need to measure and drill so that the screws will end up going through the main board, connecting it to the chassis but do not interfere with the OT and filter cap board. Got a say I'd prefer using a board with stand offs compared to this, but I suppose this is more authentic to Fenders original designs.

After that the next step is soldering all wires from off board components (xfrmrs, pots, jacks etc) that need to be connected to the boards completing all eyelet solder joints, before permanently screwing the boards down. After that its wires going from the boards to off board components...

Thanks for everyone's advice on this. Having a tonne of fun!

[mresistor]

YOur main board should have some holes for mounting.  I just mark where they go and if not already drilled then drill the holes..  I've not assembled any Mojo kits though.  Strange  the Hunter Bros didn't have those holes predrilled. Their chassis may be meant more for the hobbiest that will locate things themselves..

[Jamlid]

The board may have them, I can't remember. However, the insulation board did not, which was weird as there were screw holes on the filter cap insu board. The chassis definitely doesn't have them. That's okay though, I guess having to make these considerations now will help when planning/executing later projects!

[Jamlid]

Hey folks. So I'm nearly done with this build it seems!

I have one final issue however, which relates to the primary circuit.

On the wiring diagram I have there are two black wires coming from the PT, one is to be connected to the fuse, the other to the AC switch. However, the Hammond 290EEX I have has a black wire and a black and white wire. On the datasheet the black wire is labelled 0V but the black/white one is labelled 100V... Any ideas what I could do in this situation?

Cheers

[sluckey]

If you are in England wire the primary like this...

[Jamlid]

Great, thanks Sluckey.

Any advice on connecting those two wires? Trim, solder, heat shrink wrap? Maybe even tape or glue down to stop them flapping about?

Tah

[p2pAmps]

http://www.solandfa.com/products-Bassman-Chassis-Kit.asp

Cheers,
Mike

[mresistor]

http://www.solandfa.com/products-Bassman-Chassis-Kit.asp

Cheers,
Mike

The last I heard he wasn't able to use Paypal.  Don't know if that's changed though.

[p2pAmps]

http://www.solandfa.com/products-Bassman-Chassis-Kit.asp

Cheers,
Mike

The last I heard he wasn't able to use Paypal.  Don't know if that's changed though.

That is true but you can wire the money.  Been dealing with songwut for years and he is a solid guy

[mresistor]

Thenks Mike - I think so too and have used his chassis myself.

[mresistor]

Great, thanks Sluckey.

Any advice on connecting those two wires? Trim, solder, heat shrink wrap? Maybe even tape or glue down to stop them flapping about?

Tah,  trim the ends, put some heat shrinkable insulation on the ends, wrap them up, tie strap them, and tuck them away.

[Jamlid]

Hi folks,

I've run into trouble with this thing. This afternoon I carefully checked each connection, checked all grounding (including jack sleeves via all jack sockets etc etc), and generally looked over the building of this amp. After a few hours I finally felt confident that I wouldn't start a fire and/or fry myself to death. The amp turned on fine, no immediately evident problems - it even passed signal and all controls seemed to work as intended.

However, there was quite a loud hum and after about 5 mins of playing I noticed a plume of smoke coming from the xfrmr side of the amp. After a few panicked expletives, I unplugged the amp, discharged the caps, removed the valves and, once the smoke subsided, turned the amp over to reveal melted 6.3V winding wires (green). I think there was a short through a resistor leg that was touching both of the lugs on the lamp fitting. Could this have caused the problem? Unfortunately I started moving stuff about before I verified that there was a short with my multimeter, but I did take the attached photo which I think clearly shows that this is a possibility... There was also a greasy substance on the inside of the chassis, also pictured.

My question now is whether or not I've totally messed up the transformer or whether it is worth trying to salvage it somehow?

EDIT: I just realised that even if I did test continuity before moving stuff around then it would have shown a short anyway due to still being hooked up to the 6.3V winding.

[tubeswell]

... loud hum ... plume of smoke ...melted 6.3V winding wires (green). ... short ...  greasy substance on the inside of the chassis ... messed up the transformer

Yeah probably.

Next time try connecting the lamp to a small terminal tag strip - with the heater winding wires and filament string and balancing resistors attached to that, so as to avoid clutter on the lamp terminals.

[sluckey]

My question now is whether or not I've totally messed up the transformer or whether it is worth trying to salvage it somehow?

I'm pretty sure those green wires are melted inside the PT also. I would pull the PT and remove the bell covers to examine the extent of the melt down. If you are lucky maybe the damage goes no farther than where the green wires solder to the actual filament winding wires. Most likely the thin enamel coating on the filament winding is melted too. If that's the case, the PT is toast. You could solder some fresh green wires onto the filament winding leads and test for 6.3VAC on the new wires. If no joy, then buy another PT.

[Jamlid]

Well, damn. Thanks both. I'm super gutted I messed this up so bad. Oh well. It's a learning curve I guess. I will indeed take a look at the inside of the transformer (just out of curiosity more than anything, doubt its salvagable) and also heed tubeswell's advice for avoiding this in the future.

Is there anything else I should be wary of moving forward? Damaged tube heaters? (Please please no! Not now of all times!)

Thanks again. One day I hope to show you a working, great sounding amp!

[sluckey]

Tube filaments should not be damaged.

[Jamlid]

New xfrmr arrived today! Before I dive straight in, should I replace the 6.3v winding CT resistors? Thanks

[sluckey]

New xfrmr arrived today! Before I dive straight in, should I replace the 6.3v winding CT resistors? Thanks

Only if they are damaged.

[Jamlid]

Cool, will remove them from circuit, inspect visually and measure with DMM. Thanks

[Jamlid]

Hi!

Just an update on this project... its complete! I eventually had it looked at by a tech who was kind enough to go through everything with me. The hum was actually ground loop hum. In my inexperience I had wired grounding points up in the total wrong way. Lesson learned. Other than that (and the initial short that melted the PT) the build was fine. Now I have a lush sounding Bassman! We also installed a hum dinger pot, which is handy. Eventually I'll get it put inside a cab of some kind.

Thanks to you all for the pointers... I'm sure I'll be back soon!

J