Single-channel AB763 Build(s)

[joulupukki]

I guess this is finally gonna happen! Time for a new build thread…

It warmed up to 40°F outside so I spent the afternoon with some power saws out on the driveway getting the hard part taken care of. Actually, gluing up the boxes is probably a bit more stressful than the stuff today, but at least the boxes are nice and tight even without glue.

Right now my plan is:

Build a combo and an amp head for an AB763 single-channel amp, both the width of a Princeton Reverb. The combo amp is a couple inches taller (18” high) than a Princeton Reverb just to give plenty of room for a 12” speaker. The front is slightly slanted just like many Fender amps.

I think I’m gonna use the voltage dropping resistor values from Rob’s RR763V Blackvibe (it’ll 6V6 build) but maybe the layout board from Hoffman’s single channel AB763. Planning to use a 40/20/20/20 cap can for the filter caps and that should free up some space in the amp and make it so I won’t have to have a doghouse.

The chassis I built from a repurposed front panel of a tankless water heater. I didn’t build it with a slant in the front, to keep it easier to build so I’ll be cutting out the front top of the cabinets to make the knobs easier to access and see.

I’ll build the full on AB763 circuit with reverb and tremolo first and be able to test it in either cabinet. If it turns out to work well, I’ll build another one for the empty box. If the reverb circuit gives me too many fits, I’ll make the amp head be a Rob’s Blackvibe 6V6 (no trem or reverb) and call it a day. Then one or a few of my other amps will be on the chopping block (I don’t really need more than … maybe two?).

 

[King Fan]

I like your woodworking. Nice.

... maybe the layout board from Hoffman’s single channel AB763.

This may be obvious, but do you mean the Rob-style Blackvibe eyelet board Hoffman builds/sells, or the Doug-style turret board he also sells? If the latter, you'll be aware of the different layout.

I admire Doug's space-age layouts, but as an earth person I find it easier to follow Rob's....

 

[joulupukki]

I like your woodworking. Nice.

Thanks.

This may be obvious, but do you mean the Rob-style Blackvibe eyelet board Hoffman builds/sells, or the Doug-style turret board he also sells? If the latter, you'll be aware of the different layout.

View attachment 1058167

I admire Doug's space-age layouts, but as an earth person I find it easier to follow Rob's....

Well … Rob’s layouts are definitely WAY more understandable, I agree. But, his Blackvibe doesn’t have Reverb & Trem, which I’d like to have in the combo amp. I like how Hoffman’s board has the bias circuit on the board because … There just isn’t gonna be that much room in there. I’ve gotta fit everything inside the width of a Princeton chassis. My PT is the stand-up kind and so it doesn’t have to take up space inside the chassis.

After getting this used DRRI to check it out, I really don’t like the optical trem, so I like the fact that Hoffman’s 1-Channel amp uses bias tremolo.

I’m still undecided on this and may just order a blank board and drill all the turrets myself (it’ll be a turret board and not eyelets). But, it’ll have to be a hybrid of Hoffman / Blackvibe. I need to figure out how to run the DIYLC on my Mac and then start playing around with layout ideas.

Some mods/features I’m planning:

- 3-way bright switch (using an ON-OFF-ON switch) like I did in my PR. Down will be a little brighter, middle is no bright cap, and up is most bright
- Mid-Raw knob
- Likely no standby switch - not really needed and just takes up space

Some mods/features I’m still pondering:
- Maybe use two LND150 FETs for the tremolo stage (to reduce total # of tubes needed)
- Lar-Mar/Type-2 Master Volume - though I’d like to see/hear some examples of whether this is really worth it on an AB763
- Power amp input - still need to study this one out. Sometimes I like to plug in a modeling device and just amplify that sound. It’d be nice to have this amp be more versatile in that way (and I could get rid of one of my other SS amps). I think I could do this similarly to how the FX loop works on the Bassman Micro (except without the send).

What else am I forgetting?

 

[joulupukki]

I've discovered my first mistake...

I learned to cut the fingers of the box joint just barely longer than the material. I think I may have gone 1/64" deeper so that the fingers just barely stick out – with the plan to sand them down smooth.

Unfortunately, this means now that the chassis I built just *barely* fits in there with no extra play. That's not gonna jive when tolex is applied so ... I guess I'll have to trim the edges of the chassis just barely. Shouldn't be a problem long term, but just kind of annoying. Haha. Oh well. I'll have to remember this for next time.

 

[joulupukki]

On the plus side, the box is nice and square! Man, that's a lot of clamps!

 

[BigDaddy23]

That's a bummer but if you can trim the chassis then all good!

FWIW I build my cabs to have 3mm clearance each side of the chassis. I've found that this allows for tolex on the sides and tolex/grill cloth clearance top/bottom with a tiny gap. The little bit of extra gap is handy when sliding the chassis in/out during final assembly or for maintenance. The same goes for baffle boards. I've got x4 3mm shims that I use when setting up the baffle so everything lines up with appropriate clearances everywhere.

 

[joulupukki]

That's a bummer but if you can trim the chassis then all good!

FWIW I build my cabs to have 3mm clearance each side of the chassis. I've found that this allows for tolex on the sides and tolex/grill cloth clearance top/bottom with a tiny gap. The little bit of extra gap is handy when sliding the chassis in/out during final assembly or for maintenance. The same goes for baffle boards. I've got x4 3mm shims that I use when setting up the baffle so everything lines up with appropriate clearances everywhere.

Shims, smart! Yeah, it's not going to be fun to trim it, but at least it'll be possible since it's just a c-channel bend with some end caps. I think I'll have to trim down my baffles a tiny bit more than I already had because I am going to put some piping around the edges as well.

I got sidetracked crafting a front panel with Autodesk Fusion 360, Inkscape, and KiCad. I sent off the file to JLCPCB, paid around $20 (for five of them) and hopefully they'll turn out. I'll have to wait to cut any holes in the chassis 'til they arrive, maybe in a couple weeks. For $20 I figure I can always send off for a different design if it doesn't work out.

 

[King Fan]

I need to figure out how to run the DIYLC on my Mac and then start playing around with layout ideas.

It's pretty easy these days, tho on first launch you do have to cancel the warning, then click the Security & Privacy panel's permission to download an app from an unknown developer, then click 'download anyway' on second launch. Let me know if any problems there.

 

[joulupukki]

It warmed up to 38°F so I spent the afternoon making the cutouts at the top of the amp cabs and routing the corners. Next up will be to putty any and all holes (my finger joints weren't perfect, but these definitely aren't gonna come apart) and then sand them so they're ready for tolex and grill cloth.

 

[joulupukki]

Interesting to see how the size compares with the DR:

 

[joulupukki]

Chassis crisis averted…

 

[frederikdr]

That woodwork is fantastic. Not only super solid looking, but also just esthetically pleasing!

 

[tele_savales]

Did you build your own sheet metal brake?

 

[joulupukki]

Did you build your own sheet metal brake?

No. I used the one at my dad's HVAC shop.

 

[joulupukki]

This is a work in progress but I'm happy to see that all the components will fit on the board (whew) and be inside my princeton-reverb-sized chassis. I still have a LOT of wires to still hook up. I'm surprised how long it takes to do this! Hats off to you guys who've done a full one from scratch!

This is a mixture of the one-channel AB763 Hoffman build (for the bias tremolo circuit) and the layout DIYLC file from one of Rob Robinette's AB763 pages.

I'd like to get this all finished up so that I can place an order and make sure I've got all the parts before Christmas break. I think I'm going to use the polypropylene caps for the tremolo circuit. I still need to figure out what kind of capacitors to use on the rest of the amp – maybe orange drop or more expensive jupiter or blue sozo? No idea there. I think most of the resistors will be carbon film.

 

[King Fan]

I like all the planning you're doing and have done. Nice DIYLC, too.

 

[joulupukki]

I like all the planning you're doing and have done. Nice DIYLC, too.

Thanks. Still learning. I'm definitely not going as fast as my previous builds. Those were fun whirlwind builds!

So, do you think it'd be a problem grouping up the cathode bypasses like this (so that the tone stack and plate wires are shorter on V1)? Would that cause any weirdness by having the leads to the cathode be longer like that? For that matter, I could even move the cathode circuits from V3 over to that same area too – would that also improve anything? I guess it'd make the plate leads be shorter on V3.

 

[Lowerleftcoast]

I think I'm going to use the polypropylene caps for the tremolo circuit. I still need to figure out what kind of capacitors to use on the rest of the amp – maybe orange drop or more expensive jupiter or blue sozo? No idea there.

As a rule, we try to make a circuit that is quiet and stable. We break that rule with the tremolo circuit. We want it to oscillate. One way to start the circuit to oscillate is to encourage noise. How about some microphonic caps? The circuit works with a low frequency oscillation. The AB763 circuit calls for a 25uF cathode bypass cap. We *say* 25uF provides full frequency, and to our ear it does, but some subsonic frequencies are rolled off. A 50uF or higher value cathode bypass cap helps to get the party started in the tremolo circuit.

As for the other caps... Fender used plain old polyester. If you want it to be like the original circuit, use polyester. If you want a particular color, you get to pay more. The orange ones have a polyester cap but you have to hunt down the right part number to get it.

 

[joulupukki]

As a rule, we try to make a circuit that is quiet and stable. We break that rule with the tremolo circuit. We want it to oscillate. One way to start the circuit to oscillate is to encourage noise. How about some microphonic caps? The circuit works with a low frequency oscillation. The AB763 circuit calls for a 25uF cathode bypass cap. We *say* 25uF provides full frequency, and to our ear it does, but some subsonic frequencies are rolled off. A 50uF or higher value cathode bypass cap helps to get the party started in the tremolo circuit.

Interesting. This particular trem circuit is from the Hoffman AB763 which follows closely the Model 6G16 Vibroverb. This should hopefully be a much more smooth bias tremolo vs. what would normally come with the opto trem (which I'm not personally a fan of). Looks like Hoffman suggests a 22uF 50V cap there (C29 in his schematic).

As for the other caps... Fender used plain old polyester. If you want it to be like the original circuit, use polyester. If you want a particular color, you get to pay more. The orange ones have a polyester cap but you have to hunt down the right part number to get it.

I think I'm gonna probably go with the cheap poly ones for the trem circuit and Mallory 150 for the rest.

 

[joulupukki]

Ok, I think this is nearing readiness.

I removed the bias board and instead squeezed the rest of the board a little closer together and moved the bias circuitry onto the left-hand side of the board. Hopefully that's good.

Questions I still have (anyone wanna take a stab at these?):

1. [DONE - see later post] I'm not sure exactly how to hook up the Lar/Mar (Type 2) Master Volume (because I switched the tremolo to be bias tremolo)
2. A normal AB763's reverb recovery tube has a shared cathode bypass cap & resistor so that's what I did here. Hoffman's one-channel AB763 makes it so each side of the 12AX7 has its own. Can it be shared like this?
3. If I added a bleeder resistor, I think I'd add that to the B+ terminal of the can capacitor to ground terminal, but not sure what value it ought to be (guessing a 2w metal oxide?)
4. Should I use a thermistor on the neutral power wire? Fender's schematics show using one. I used one in my 18W TMB build when I was using a Toroid PT and it stopped the inrush current that was blowing the main fuse.
5. Spot any other errors?

Oh, I still need to see if I can figure out how to add a power amp input from the back panel. I'll think on that tomorrow.