AB763 Build on 16x8in chassis: is this a workable layout?

[mcamelo]

Hello amp builders.

I'm finally getting into my 3rd amp build. I'm aiming for a AB763 combo. My previous builds were a stock 5E3 and Plexi6v6, both in head format. This one being a combo with reverb and vibrato, it is my most ambitious build yet.

As if that wasn't hard enough already, I've picked a non-traditional chassis size. It's a 12x6x3in chassis blank that I will have to drill myself (another first but I want a portrait form factor, as opposed to DR's landscape). I'm also planning to add a 4-step reactive attenuator to the circuit (unless someone here talks me out of it).

Now to my request for help: can you please advise if you see any flaws or pitfalls on the layout I am planning to go with (see attached photo*). I am especially concerned with access during assembly/service, length of wiring and EM interference between components, but all feedback will be much appreciated.

(*) The turret board will obviously reside on the inside of the chassis and has been placed there for reference only.

[Stuff4bikes]

Looks like it might be tight. Can some of those resistors be mounted on the side walls? Any probably turn those transformers so the laminations don't effect each other. Looks like a good build.

[PRR]

Where are those power resistors going? (Why so many of them?)

If outside the chassis, and electrically "live", someone could get a shock. (Plastic insulation would burn-off in a major resistor fire.)

If inside the chassis they block or cook the turret board?

[PRR]

I just realized those power resistor values suggest a speaker attenuator. So shock is not a real risk. Heat won't be with 2*6V6, could be with2*6L6.

[Willabe]

No, I think that chassis for what you want to build is way too small.

Just asking for trouble. 

[sluckey]

I agree. I think you will have a lot of difficulty stuffing all that into that shoebox. Wiring is likely to be a nightmare. I've built several simpler amps in a 12.5 x 6.5 chassis. My Plexi6V6 is about as much stuff as I care to put in a small chassis and still be able to work on it later. I can't imagine adding verb and trem.

[acheld]

A couple of thoughts:   

1) While it is certainly possible to build a DR using this general layout, it will not be easy.   This is a setup for reverb hum, among other things.
2)  So this will be a combo.  What will be the orientation of the chassis to the cabinet?  Blackface style? Tweed? 
3) Where are your big electrolytics going?  Doghouse?  Cap cans?
4) Do you think your lightweight chassis will support your iron? 
5) I understand the need/desire for an attenuator, but suggest that adding this to your build will be adding to your layout challenges.  The circuit is simple, but the layout is not within the context of your project. Maybe building it with a separate chassis?
6) As already pointed out, your OT should be rotated. 

All of the above is NOT to say your project can't be done.  It certainly can -- there was a fantastic small build within an old car radio this spring without hum -- really packed build (and apologies that I can't recall the builder's name).   But it will be a challenge.

It's worth building this in your head, or on paper, completely.  Where will each fastener go?  How will you support the chassis? Knob layout.  Location of your fuses, grounding scheme, pilot light.   Each component interacts with the whole.     

And that is why this hobby is fun!   

[dude]

I'm sure you "might" get a nice working AB763 but from my experience with jamming a 6V6 Plexi in an AO-43 Hammond Chassis, 16" x 5" X 2" depth, ain't worth the effort. I was determined, works great but a ton of work, and like Sluckey's mentioned, replacing components is a nightmare. After "several layout designs", a "few builds", I finally figured a way to make it work and be able to replace stuff. No way would I do it all over again. Good luck.   

[mcamelo]

Thank you, Gentleman, for chiming in.

I will explore the points you bring up and evaluate my options, namely, opting for a larger chassis and/or ditching the on-board attenuator.

I will draw a 1:1 wiring diagram to see how tight it is going to be. I certainly do not want to find myself in a position equivalent to trying to fit a Plexi6v6 into a 5x12 box (ouch) and I'm willing to take the time to get it right.

I do appreciate you all sharing your painfully acquired wisdom :-)

I will share more when I am able to make progress again.

[Willabe]

......, opting for a larger chassis and/or ditching the on-board attenuation.

It wont fit in that chassis even without the on-board attenuation.

You wont be able the wire it up inside, too many tubes for that chassis.

[vampwizzard]

Itll be super noisy. I make my single channel ab763s in similar dimensioned boxes and the builds are TIGHHT. Add in the extras and youre asking for trouble.

If you do make it though, ill venmo you $5 for a beer.

[mcamelo]

I'm so glad I asked for guidance before continuing. The extra $35 for a new chassis* is negligible next to the hassle and frustration of a humming building. I'll go with the landscape form factor which will allow for ample real estate inside the box.

* do I see a class A build in my future?

[Joel]

I've worked on F111 valve radios (many moons ago!) and I know that you can fit an awful lot of valve circuitry into a very small volume.  I also know, from long and bitter experience, that any fault finding you will need to do in such a small volume will be a nightmare.

If you plan very carefully, you can fit everything in. But, your life will be much easier if you build in a bigger enclosure.

[mcamelo]

 

I've noticed a huge mistake in my original post that may or may not change the opinions expressed so far: I've listed the enclosure as measuring 12x6in when in fact it is 16x8in. I wish I could blame my lack of familiarity with the imperial system, but the honest truth is that I relied on my memory instead of referring back to the specs in the invoice of measuring it myself. So sorry about the head fake.

Does this change anyone's opinion?

I understand this is will not be a comfortable build but I've also haven't been able to find a suitably larger chassis. I sure can find larger boxes, but they are all too large. I can't go past 16 inches long without also moving to 10 inches deep or deeper, which will make the open-back cab too deep (I do not want the cab deeper than 10 inches).

I'm attaching two photos where I have the layout (now without the attenuator) side-by-side with my Plexi6v6 build (12x6.5in). Judging from the photos, it does not look worse in terms of component density. But I am aware that complexity grows exponentially with the component count so I am definitely interested in knowing if the consensus still is that the 16x8in chassis is too small.

[vampwizzard]

a lot easier than the previous set of dimensions but youre looking at princeton reverb sizes now. still interesting. May be doable.

[acheld]

I'm familiar with that particular chassis, so didn't really pay much attention to your numbers before I posted earlier.

I think it is doable, but it will be a challenge.  There are a lot of components to the build.

Your layout is much better now. 

My bias would be to build the amp, get it settled, and then build a separate attenuator box.   Lot's less complexity that way, even if you mount the box in the combo cab somewhere.

[mcamelo]

I will go with a marginally larger chassis (17x8). That's the largest I can source that will still fit into a combo cab no deeper than 12in. The extra 1 inch is not much but will give me some extra room near both ends for the rails needed to support the chassis inside the cab.

I will build the attenuator in a separate chassis and see if I can mount it onto the walls of the cabin. If not, I will just use it as a standalone unit.

I will update this post with the new layout once I get my hands on a larger chassis.

[Willabe]

My bias would be to build the amp, get it settled, and then build a separate attenuator box. Lot's less complexity that way, even if you mount the box in the combo cab somewhere.

Yes.

[mcamelo]

The new, 1 inch longer, chassis has arrived.

Please look at the proposed rough layout and point out anything you don't like - other than the small chassis :)

I'll try to make this small alum chassis into a doghouse for the electrolytic caps.

I'm specially interested in any advice about the orientation of transformers and choke.

[pdf64]

The OT and reverb transformer wiring may be entering the chassis in close proximity to, and running alongside, early preamp circuits. That’s not a good recipe for maintaining an adequate margin of stability.
I suggest to keep audio transformers close to their associated valve socket close to their associated components.

[sluckey]

Copy this layout as closely as you can...

[mcamelo]

Copy this layout as closely as you can...

Close enough?

[pdf64]

Are you omitting the normal channel or the trem?

[mcamelo]

Are you omitting the normal channel or the trem?

The normal channel. I like one trick ponies :-)

[pdf64]

The reverb OT would be better moved along one step then, to avoid the anode wiring to V2 driver being any more convoluted than necessary.

[Ed_Chambley]

I use 2 size chassis. 17 x 8 x 3 and 20 x 8 x 3.


I use the Marshall layout because it has never failed, and yeas I have built a Slo 100 in the same way.  I do not use a wide circuit board, I cut them to 2.25" and the length needed.  Most often, I simply use terminal strips

[mcamelo]

The reverb OT would be better moved along one step then, to avoid the anode wiring to V2 driver being any more convoluted than necessary.

So move the reverb transformer to sit between V2 and and v3? Makes sense. Consider it done. Thank you!

[mcamelo]

I use the Marshall layout because it has never failed, and yeas I have built a Slo 100 in the same way.  I do not use a wide circuit board, I cut them to 2.25" and the length needed.  Most often, I simply use terminal strips

  Beginner questions: what is the Marshal layout? What other layouts are out there?

[sluckey]

Marshall layout looks about the same as the Fender chassis photo I posted, but usually not as many tubes.

[pdf64]

Well, the OT and choke positions are usually swapped over, which helps to keep the OT primary wiring shorter.
And the mains / fusing / PT primary wiring are usually confined to the far end of the amp, well away from signal circuits.
And of course the much deeper chassis and ‘tidied away in the corner’ heater wiring can make the back panel layout somewhat less constrained.

[Willabe]

New amp build, you should read this if you haven't already.

Even if you go with a proven layout, you should read this link on grounding.

http://www.valvewizard.co.uk/Grounding.html

[mcamelo]

Thank you all for the info. I'll be reading it all up. I want to take the time to get this one right.

Quick questions on the reverb OT: does the rotation matter? How about the distance to the two nearby tubes?

[dram56]

I built an AB763 in an existing Gibson GA-20RVT chassis.  The chassis was 17 x 8 x 2.5.

I used Doug Hoffman's AB763 "short" turret board layout for the main board.  Keep in mind you need to also make a seperate Bias turret board.  I found that was impossible to do the "doghouse" filter cap boad on my chassis.  So I used can caps.

It's best to keep the power section all together and seperate from the pre-amp sections.

PT - Filter Caps - Choke - Rectifier Tube and Bias Board.


Then your OT needs to come next so it's as close to your output tubes as possible and rout the output wires to the rear of the chassis between the output tubes and the Phase Inverter.

The Reverb Transformer should be located between your reverb driver 12at7 tube and your reverb recovery 12ax7 tube.  Orientation rotation wise is not critical.


I was using existing tube socket holes and regrettably crammed in the v5 tremolo tube between v4 and v6 in a triangle arrangement.  Like your first layout.  Take it from me, don't do that.  It caused numerous headaches.


Don't forget about all your rear panel stuff.  Power cord, Fuse Holder, Speaker Output Jacks, Reverb Tank RCA input / Output Jacks and Reverb & Tremolo Foot Switch Jacks.  I used a 5mm LED for my Power light to save some space.  Still, everything was very crowded.

So try to keep all of your interconnect wiring as short as possible.  If you eliminate the Normal channel, you will be OK with 5 preamp tubes vs 6.  That will save you from trying to cram that extra tube in there.


Check out some my chassis pics to see how crammed mine was.  Good luck.

Dave

[mresistor]

I would recommend a single channel as well.

[mcamelo]

Thanks for sharing your experience, Dave. I'm halfway through the build and I think I will be alright as far as room goes. I am building the single channel version, so that has definitely helped.

I am looking at the grounding scheme now and one thing has got me puzzled: Blencowe's chapter on grouding schemes states one should avoid ground loops. But looking at Hoffman's schematic, the circuit is supposed to be connected to the chassis at more than one place, causing ground loops. Most notably, to the very left of the board, via the input jack, and the very right, via the "the transformer ground lug". Is Hoffman's wiring design just ignoring Blencowe's grounding principles and I should feel free to modify it to eliminate the ground loop, or is this an important design element that I'm better not trying to mess with?

[tubenit]

Here is Hoffman's grounding scheme. More information in his excellent  Library of Information.

[mcamelo]

Here is Hoffman's grounding scheme. More information in his excellent  Library of Information.

This is perfect. Thank you!

[dram56]

I followed the Hoffman Grounding Layout and had good results.  His AB763 design, his grounding layout. 

I used the LED mod that slucky pointed me to, for the Tremolo which boosted it's Intensity (I mounted the Yellow LED in the front of the chassis).  The LED flashes with the Speed of the Tremolo, looks cool.  Watch the routing of the LED wiring, keep it as short as possible and away from other wiring. 

The Bias Board and connection from that and the Tremolo Intensity pot is also a source for noise to be picked up, so be careful with that routing as well.  You may have to experiment a bit to find the best, quietest, routing.  Same goes for the Reverb Recovery & Driver tube wiring.  Shorter is better.  I found these areas to be the source of more noise issues that the grounding scheme. 

The main thing with grounding is to keep the Power section grounded to a different area (near the Power Transformer) than the Preamp section, including the Input Jack.  I have read, that it is better NOT to use one of the Power Transformer mounting bolts as a ground, simply because of the weight of the transformer flexing the chassis when the amp is moved around, which COULD cause a grounding issue sometime in the future.  They recommend making a separate dedicated Ground lug somewhere near the power transformer.  That's how I have done my recent amp builds.  But, to be honest, I have not had any issues with my older builds that were grounded to the transformer mounting bolt.  Always solder wires to the terminal lugs, never crimp them.  Bottom line, the AB763 circuit has more places that can cause noise issues than the grounding scheme.

BTW, I used the Jensen 12" Blackbird speaker in my build.  Has that awesome Jensen chimey sound at lower volumes, but doesn't fall apart when pushed.  I highly recommend that speaker.

[dram56]

I forgot to add...  If you have enough room to use the "Standard" Hoffman board layout instead of the short board layout, you will be better off than having to use the separate Bias board arrangement, like I did.  It's cleaner and easier and less chance of introducing noise into the Tremolo circuit. 

There is one odd thing in Doug's circuit that I changed.  In the tremolo circuit, there are (2) .1 mfd caps in series.  The first one that is wired to the output of the Intensity pot is per Fender's brownface spec, but the second .1 mfd cap (located to the left of the first one) is an addition.  I don't know why it was added, but I found the Tremolo worked better by changing that one to .047mfd.  And the 470K resistor that runs horizontally and connects to the top of the .1 mfd cap, I dropped to 220K to help increase the Intensity some. 

That, and the LED mod for the Tremolo made the effect much more intense when dialed up.  I also did the common mod to slow down the Speed of the Tremolo by just using .02mfd caps instead of the .01/ .01 / .02 configuration.  And plain old ceramic caps actually seem to work fine in this application (maybe even better than film type).  I am kind of a Tremolo snob, and to me, this IS one of the best sounding tremolos I have heard in a Fender amp.  I was going for the Magnatone amp style sound.  It's pretty close.

[sluckey]

There is one odd thing in Doug's circuit that I changed.  In the tremolo circuit, there are (2) .1 mfd caps in series.  The first one that is wired to the output of the Intensity pot is per Fender's brownface spec, but the second .1 mfd cap (located to the left of the first one) is an addition.  I don't know why it was added

Nothing odd about the two .1µF caps. Doug used two because he copied the 6G16 schematic exactly and it also has two .1µF caps. Those caps are not in series. One cap couples the tremolo signal to the INT pot while blocking DC voltage from the tube. The other cap is a shunt to ground, just like a filter cap. It smoothes the tremolo signal but it also reduces the amplitude a bit too. That's why using the .047µF instead increases the tremolo signal (but less smoothing.
 
      https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_6g16_schem.pdf

[dram56]

I stand corrected.  Sluckey is correct as always.  I must have been looking at a different fender schematic where they only used (1) .1mfd cap.  The 6G16 does indeed use (2).

In my case, the tremolo intensity was weak as built per original spec. 

I see where the second cap in this configuration on the "Princeton Reverb" amp is a .022mfd, instead of the .1mfd.  So, apparently, even Fender realized .1mfd cut too much of the intensity. 

Sluckey helped me to get better results out of my build, so, I defer to him.   

[mcamelo]

Anything I should be looking out for with regards to routing the OT secondary and reverb out wires? Routing them next to each other looks cleaner but should I route them far apart, instead, to avoid interference? Routing between PI and 6V6s still an option.

[Willabe]

Anything I should be looking out for with regards to routing the OT secondary and reverb out wires? Routing them next to each other looks cleaner but should I route them far apart, instead, to avoid interference?

Yes. Do not put them together. The OT secondary wires have the most current in the amp going through them. The reverb pans out put has the least signal voltage and current in the amp.

Keep the verb out wires away from the OT secondary's.

[pdf64]

… reverb out wires...

Not sure if that means the amp’s output to the reverb tank, or the reverb tank’s output back to the amp.
If the former, it probably won’t be an issue. If the latter, it may be ok if screened cable is used.

[dram56]

I physically mount the Reverb Xformer between the Reverb Driver and Recovery Tubes on the chassis. 
That way the wiring from the Reverb Transformer Input side has a short run to the 12AT7 Driver Tube & to B+. 

And the Reverb Transformer Output wiring to the Reverb "Tank" Input Jack & Ground can also be kept short. 

The Reverb Recovery 12AX7 tube connects to the Reverb Tank Output Jack.  So, all of this is at the PreAmp side of the chassis (far away from the Power Amp side).  That puts the Reverb Tank In & Out Jacks and also the Reverb & Tremolo Footswitch jacks at the right side of the rear of the chassis (when viewed from the rear)

As for the Output transformer, I mount it so the input side can easily access the (2) 6V6 tubes and I run the Output wire(s) out somewhere between the Phase Inverter tube and the 6V6's.  Usually that puts the Speaker Output jacks just left of center (when viewed from the rear).
That's how Leo Fender did it, and it seems to still be the best layout.  If you plan your layout for all of the transformers, and keep the wiring as short as possible and not cross over other wiring you will have a better chance of less interference noise.

[mcamelo]

I finished the build a couple of weeks ago and it is working well.

Physically, the layout was quite manageable and it didn't feel too crammed.

I still haven't made my mind up about how quiet it is. There is some hum present but I haven't yet spent anytime investigating where its coming from or not even benchmarked against any of my other amps. I may start a thread about that when I get there.

For now, I just wanted to thank you all for the invaluable input and share a photo of the (almost) finished circuit. I'm not the neatest of builders but this might be the most organized build of mine so far (three in total).

I am quite happy with the sounds I get for the amp, so thank you all for so selflessly sharing your wisdom with me.

[dram56]

Glad to hear you got your amp up and running.

I decided to finally make some clips of my hoffman dlx rvb build the other day.  I uploaded them to Soundcloud.

Please excuse my playing.  These clips are for demonstrating the sound of the combo amp and a cheap Firefly LP Special style double cutaway guitar with P90 pickups.  No pedals, just the amp's internal reverb and tremolo. 
I recorded live with a Tascam DR-40 digital recorder.  I used Audacity to convert to Mp3.  No post effects other fade outs.
https://soundcloud.com/dave-ramsey-949064652/sets/ram-z-amplifiers-20-watt-combo-amp-and-firefly-lp-special-style-guitar?si=bc2beea8019547cd89d4173d78496a4b
Dave

[mcamelo]

That's sounding pretty good. How did you record it? I am always a bit weary of posting my own recording because I feel my recording technique doesn't make justice to what I am hearing coming out of the speaker.

[PRR]

...How did you record it?...

I thought he explained it well.

....I recorded live with a Tascam DR-40 digital recorder.  I used Audacity to convert to Mp3.  No post effects other fade outs....

The DR-40 is an old model, $140-$80 used. New thing is the DR-40X, $200

[dram56]

I just placed the Tascam DR 40 about 6" from the speaker, which is a 12" Jensen Blackbird.
I only used the built-in internal mics of the recorder.


No fancy SM57s or mixers.


Volume on the amp was set to 3.
Treble at 7
Bass at 5
Reverb at 3
Treble Intensity 5
Treble Speed at 4