AA864 Musings

[sluckey]

You may find this interesting...

     https://el34world.com/charts/toolhowto.htm

[dwinstonwood]

Thanks sluckey, yes, I did watch Hoffman's videos on that page, and I went back and tapered the press (the part in the chuck with the hole) to accommodate closely spaced turrets.

I also found this cabinetmaker who has built Hoffman Stout cabinets before, and has very competitive pricing:
http://www.trmguitarcabs.com

By this time next week I should have the initial chassis wiring well under way! Just waiting on parts delivery.

Hey, thanks for taking an interest in my project sluckey. I haven't seen a Bassman mod exactly like mine, and if it does something well - either lots of "musical" gain, or the ability to take pedals at lower, clean volumes - I'll feel good.

[dwinstonwood]

Got up early and knocked out the turret board.

[sluckey]

Your board looks great. Did you follow my instructions for drilling or did you use some other method?

[dwinstonwood]

Your board looks great. Did you follow my instructions for drilling or did you use some other method?

Thanks sluckey! I Super 77'd your drill guide to the plastic protective covering on the board and used a 3/32" bit in the drill press. It lined up perfectly.

As Hoffman states in his video, it did not take much pressure to swage the turrets. The whole process went smooth.

But, it took a bit longer to enlarge the chassis for the giant PT. The 6L6 on the right is going to be close.

[dwinstonwood]

Like this close:

[sluckey]

That's OK. People are not gonna believe what a monster amp lives in that tiny chassis!   

Your OT and choke will likely need new mounting holes drilled. It's worthwhile to also consider how the board will be placed and slightly adjust the board position, OT position, and choke position to avoid any conflicts. I attached a pic that shows Hoffman's choke and OT position. My board position is adjusted to avoid conflicts. Don't get in a hurry and you'll be fine.

[shooter]

Don't get in a hurry and you'll be fine.

 


I suspect most problems on builds start by not following the above advise

[dwinstonwood]

Thanks. Great advice. I'll temporarily put in the things that can't easily be moved, like the PT, pots and tube sockets. Then I can shift the board, choke and OPT around until nothing is cramped. Double check everything, and then drill.

Yep, it's going to be compact and look mild-mannered, but the weight alone will belie it's appearance. And, if I go with a really efficient speaker (like a 103db Eminence, etc.) look out.

[DummyLoad]

mine started out as 2 x EL34 in P-P - ended up with 2 x 6550 in P-P. nice tones. that little box gets warm. it was a 2018 x-mas gift to my nephew.

--pete

https://www.dropbox.com/s/upuqmxwmwxk7ck0/20181107_223103.jpg?dl=0

https://www.dropbox.com/s/td5b497u434bdrn/Boilermaker_20181021_235128_sm.jpg?dl=0

https://www.dropbox.com/s/fhi3vczzc3r44rb/20181105_173813.jpg?dl=0

[Glenn]

thats Great Pete -

I'm going to be building this AA864 amp soon. i'm using a AMI jukebox chassis that i have. thanks dwinstonwood and Steve

[dwinstonwood]

Thanks DummyLoad. I'll follow your placement of the OPT and choke.

[dwinstonwood]

Hello sluckey, a question, please...

I'm building this project with one input (33k grid resistor, so "HI" only), and one speaker out (I bought a 3-way rotary switch and an OPT with 4-8-16 secondaries). A total of two 1/4" jacks.

Do I want two Switchcraft #12A's, two #11's, or one of each?

I'm thinking that two #11's are what I need - since there won't be another jack to ground when not in use.

Maybe I just answered my own question. Since both the input and speaker jacks will always be in use when the amp is being played, there's no need for #12A's.

Thanks.

[sluckey]

You want two Switchcraft #12A's. Connect the switch lug to the sleeve lug on both jacks. This puts a ground on the input jack tip when nothing is plugged in and this will keep the amp quiet. The speaker jack will also put a short across the OT when nothing is plugged in. The short is much better than an open which could damage the OT and output tubes/socket if eddie starts wailing away on the guitar with no speaker connected.

[dwinstonwood]

Thanks!

[dwinstonwood]

Taking my time, double checking things, reflowing solder joints, etc...

[sluckey]

Checkmarks work OK on a simple board. I prefer to use a highlighter. Works well for me, especially complicated boards.

     

[dwinstonwood]

Cool, I can see it's pretty much impossible to overlook a jumper using a highlighter.

I'm glad now that I didn't use a Fender chassis, and went with the Hoffman Stout that you suggested sluckey. It's probably about the best bargain available for a rigid, pre-punched chassis. It's like 2.5mm thick aluminum. It's easy to drill and cut, but there's no flex, even with all the heavy iron bolted to it.

I'll see if I can get the heater wiring done tonight.

[shooter]

fwiw;
I've built on 3 of those chassis NO regrets!


did you print out the docs BIG?, I usually get something in the 14X17 size.  I use yellow pre-solder layout, then red for soldered, then blue for double-checking/verifying.
 I'm an artist so I have to 

[dwinstonwood]

No, but I'm going to print out the layout at 17" since I hate squinting! 
I like the idea of using different colors. I had to double check every turret to be sure I had a good flow of solder.

shooter, any suggestions for a speaker config? I'll have a 4, 8, and 16 Ohm selector. Two 10's, three 10's, two 12's? six 8's?  Thanks.

[sluckey]

I'm reminded of a chevy vega with a 327 engine.   

[shooter]

chevy vega

when my folks made it to upper poverty Dad bought Mom that version!
3 yrs latter I was going back to Base and my car wouldn't start, Mom loaned me the vega, coming back home the following weekend I got T-Boned by a drunk that blew through a stop at 70mph
I offered my '70 Camero but Mom wouldn't take it.  She ended up with a well done '65 Nova


Ok, Speakers;


roll as many as you can find, begging n borrowing, then decide on how many and with type

[bmccowan]

upper poverty

nice project dw!

[dwinstonwood]

Thanks bmccowan!

I got busy with other stuff, so progress has been slow. I have a tough time wiring up heaters. I usually have to start over at least once. But, it's done!

[dwinstonwood]

OK, I'm stumped. A total brain lapse.

I'm using the 120vac power light that Hoffman sells.

His wiring instructions say:

"One wire goes to the neutral/white power cord wire.
The other wire goes to the hot/black power cord wire after it comes out of your main power switch."

I can't figure this out. I thought the white neutral wire from my PT went to the neutral pin on my power cord socket. I just can't picture how to wire it up.

Thanks!

[sluckey]

Connect the PT white to the IEC neutral lug. Then connect one lug of the lamp to the neutral lug. Now connect the PT black wire to the switch. Also connect the other lamp lug to the switch.

IOW, the lamp is connected parallel to the PT primary.

[dwinstonwood]

IOW, the lamp is connected parallel to the PT primary.

Ah! Now I get it. Thank you sluckey. I couldn't picture it.

[sluckey]

The lamp is a neon bulb, not a LED.

[dwinstonwood]

The lamp is a neon bulb, not a LED.

 

Wow, it took me most of the day to get this far, and I haven't even started on the jacks and pots. Hopefully, I can get that stuff done tomorrow.

[shooter]

I haven't even started on

See reply #56-57   

[dwinstonwood]

Well, one more quick question...

I'm thinking that even though I'm using a speaker Ohm selector switch I still want to connect the NFB to the speaker jack tip, yes? That way it will stay connected regardless of the selector setting (4, 8, or 16).

But, I could be all wrong about that.

Thanks!

[sluckey]

I strongly suggest using four 3/4" fender washers on that PT. If can't find any, then four pennies would be cheaper. 

The NFB wire should connect to the OT 4Ω tap at the Impedance Selector. You want the NFB to remain constant.

EDIT... I don't see any holes drilled for mounting the board. That should have been done at the same time you drilled holes for the OT and choke. The more stuff you put in that chassis, the more difficult it will become to accurately drill the board mounting holes. It's already become quite a challenge.

[dwinstonwood]

Thanks sluckey. I was keeping it in mind, and I knew the relocation of the choke would be an issue. I just needed to move one of the mounting bolts down 1" to clear it.

What I didn't take into account was the large size of the terminal strip. So, I modified it, making it about 1/3 of its original size. It's just as strong, but doesn't interfere with anything now.
 
It's definitely a cramped chassis, but there's enough room; nothing is contacting other parts.

The good thing is that most of the wire runs can be very short. In fact, I see in Hoffman's 6V6 Plexi build that he actually connected the grid resistors directly from the jack to the tube socket. But, I have 20' of RG174, so I can still use lots of shielding.

This photo, in particular, is serving as a good guide for wire choice, etc.:
https://el34world.com/projects/images/Img_9095.jpg

[sluckey]

That relocated screw sure makes me nervous. The screw head is very close to the highest voltage in the amp. And what about the standoff under the board? I bet the standoff is even closer to that turret. If either the screw or standoff touch that turret it's bye bye diode. Puts the PT at danger too.

[dwinstonwood]

That's a good point!

I'll use nylon standoffs and turn the screw head down. Or, I can drill another hole in the board, and move it.

[dwinstonwood]

I think this is right.

I've never wired one of these before...

[dwinstonwood]

This is a good stopping point for today. I still need to add the power and preamp grounding point connections. It's a lot easier for me to deal with that after all of the power wires are routed first.

Tomorrow I'll mount the pots and input jack on a thin board and wire all of that up, attach the ground buss, small parts, etc. I know I can't do that down inside the chassis. Then, install that complete assembly back in the chassis.

I'm following Hoffman's 6V6 Plexi guide, so I'll add the parts to the board after that. Then, I'll add the wires from the board to the pots, just like his guide.

[sluckey]

Tomorrow I'll mount the pots and input jack on a thin board and wire all of that up, attach the ground buss, small parts, etc. I know I can't do that down inside the chassis. Then, install that complete assembly back in the chassis.

Why? You already have the perfect rig to do that. Just install pots, jacks, etc, on the outside of the chassis. Wire it up. Then flip to the inside. Everything will fit perfectly.

[dwinstonwood]

Well, I went with your method sluckey.  It was easy. In fact, the weight of the chassis kept things from moving around on the desk. Without your layout/wiring diagram this would have been a frustrating wiring project. So, thanks again.

I used shielded cable for the three connections going directly to grids. These seemed like the most critical places to use RG174. 

[shooter]

like the wire colors 


shielded wires are great, just make sure no shorts to shields

[sluckey]

It is easy. But you overlooked one small but very important detail. You wired them upside down. Now when you flip the assembly over and mount on the inside, the pot lugs will be pointing away from the open chassis. It will be nearly impossible to get to the lugs later. I highly recommend undoing, rotate everything 180 degrees, and redo.

[bmccowan]

Hmm - this will likely lead to some musing. Sorry, but what the heck, its only a few wires, eh?

[dwinstonwood]

It's fixed. Took a bit over 30 mins.

I got confused by your layout sluckey, and read it wrong. Something about the way I was picturing the folds in the chassis made it look like the tabs were pointing into the chassis.

I can't see an error in my wiring - it matches the layout - but, I probably have something wrong!

[sluckey]

I got confused by your layout sluckey, and read it wrong. Something about the way I was picturing the folds in the chassis made it look like the tabs were pointing into the chassis.

Much better. I can see where it could be confusing. That's the first drawing I've done with the pots on the tube side. All my other layouts have the pots at the top of the page. Less confusing.

BTW, the switch on your volume pot is pull on/push off, right?

[dwinstonwood]

BTW, the switch on your volume pot is pull on/push off, right?

Thanks sluckey.

Yes, it's the one Hoffman sells. Here's his description:
"CTS amp pots with SPST pull switch
Used for master volumes, bright switches and other switching on Fenders.
The switch is on when the pot shaft is pulled out.
Standard 3/8 inch mounting hole."

It's been a mind bender for me. I'm building a Fender amp in a Marshall-style chassis. It's like upside-down and mirror image (if you think about the traditional Marshall control order). Funny, I never thought about that before, but early Marshalls are that way. Maybe so they didn't look anything like Fender Bassman copies?

[dwinstonwood]

Well, somehow I failed to order 100 Ohm resistors (must have been thinking 100K). So, the NFB circuit isn't complete. This also grounds the LTP. All the other parts are in.

Next step is to connect up the two ground points. Speaker jack, input/pot buss, and preamp ground at point "b" in the pic. Everything else, including the bias as well as the 220K 3W cap resistor at point "a".

Then, I'll focus on the wires from the pots.

[sluckey]

Looking good!

Well, somehow I failed to order 100 Ohm resistors (must have been thinking 100K). So, the NFB circuit isn't complete. This also grounds the LTP. All the other parts are in.

If you have a 47Ω use it instead of the 100Ω and move the NFB wire to the 8Ω OT tap.

Next step is to connect up the two ground points. Speaker jack, input/pot buss, and preamp ground at point "b" in the pic. Everything else, including the bias as well as the 220K 3W cap resistor at point "a".

Move point b (preamp ground) to be very near the input jack. The only thing that will connect to it is the pot buss and the preamp ground. Everything else connects to point a (power ground).

DO NOT CONNECT SPEAKER JACK TO PREAMP GROUND!

[dwinstonwood]

Thanks sluckey. This is why I post progress pics. In hopes that you'll catch my goofs. Truth is, grounding is still a confusing subject to me (I read up on it, and forget what I've read).

So, just so I'm 100% clear, the OT secondary has high current (and, if it shorted internally, high voltage), and therefore the speaker jack should be grounded along with the power and B+ grounds and away from the preamp, yes?

As for installing the parts, this time I didn't have any trouble soldering to the turrets. By following Hoffman's suggestion: making an initial solder, and then going back and reflowing a bit more solder, I got nice, shiny domes on top of all the turrets - instead of having solder run out the bottom of the turrets.

[sluckey]

So, just so I'm 100% clear, the OT secondary has high current (and, if it shorted internally, high voltage), and therefore the speaker jack should be grounded along with the power and B+ grounds and away from the preamp, yes?

I'll give you 90% for that answer. Just forget about the voltage. The OT secondary has low voltage but high current. That high current develops a pretty strong electromagnetic field around the wires. This strong field is capable of inducing signal into the low voltage high impedance sensitive preamp stuff. This can cause all sorts of problems for you to chase down later on. Just connect the speaker ground wire to the power ground and don't worry.

Also keep any of the OT secondary wires as far away from the preamp circuit as possible for the same reason.

[dwinstonwood]

100 Ohms from home with this project.  Sorry.

Well, a few other things... washers for the PT, tighten all bolts, trace everything with a highlighter, etc. I did measure each and every resistor before soldering (I can't read color bands).