Weber 6A20 Deluxe Reverb Amplifier Build

[MarkE]

This article is also posted on: http://www.diy-music.com (http://www.diy-music.com/)

okay guys...Im a nut case I know, but I have been so enchanted with vintage tube guitar amp circuits that I decided to build a 6A20...aka AB763...aka Fender Deluxe Reverb amp (circa 1965) from scratch. I just ordered all the stuff and I should have it in a few weeks. Im totally stoked!!!

here is a pic from someone else who recently completed one

http://www.thehawkstudio.com/picts/DSCF0292-1.jpg



Guts and glory!


http://www.thehawkstudio.com/picts/DSCF0282.jpg



http://www.thehawkstudio.com/picts/power6.jpg


It will have a single 12" speaker like the original, but mine will be alnico instead of the original ceramic.The circuit layout is identical to the original but Im already planning a few mods like replacing all of the capacitors with high end stuff etc.

How kool is that? A REAL reverb tank...ya know...the kind that makes thunder when you kick the amp ROFL!!!
Wow...I get to hand wire this baby from the ground up. Im gonna take my time and do it right. I want it to be my cleanest project ever.

I was interested to find out that Fender has reissued this amp recently. It sells for about a grand. With all the upgrades I should be able to pull this one in for under 700$. I plan to write article...pictures etc etc to bore you guys to death with along the way.

GAS ATTACK!!!!!!!!

[babayaga]

Wow...go for it matey....! Will be really interested to see how you get on with this one...

[MarkE]

Thanks man. hey...I just started a second one. This time it's a scratch build, my own design. Check out the thread I just started on it if you are interested.

[MarkE]

8/21/09

Welcome to my 6A20 build chronicle. I will attempt to do a detailed blow by blow on building this great amplifier sold by Weber. I hope that the information is useful for anyone looking to build, or is building one. I'll try to get as many photos as I can.

Before I begin with the first installment I would like to mention Ted Weber for a moment. Without him this would not be possible. Sadly, Ted passed away last week.
Ted Weber is well known throughout the amp building community as a teacher, musician, electronics guru and overall really great guy. In addition to helping countless amp building noobs like me, his company (Weber VST) sells a variety of affordable amp kits, speakers, attenuators, tube, parts, you name it. Everyone in the amp building community will simply miss the hell out of this incredible human being. My biggest sorrow is not having known him better. Rock on in Heaven Ted....you will be sorely missed.

Okay, on to happier tasks...

The 6A20 kit finally arrived today. There isnt much to write about as yet so I will let the pictures tell the story.


The group shot right after unpacking. The kit was packed extremely well and there was no damage to anything.

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The speaker is a Weber 12" Alnico. They ship the speaker mounted (smart).

http://www.thehawkstudio.com/picts/6a20/IMG_1540.JPG

The chassis is laser cut. I examined it closely. It appears extremely clean as far as burs and sharp edges go, which is a relief as I hate metal work. Well, I don't hate it, Im just not very well equipped for it.

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The reverb tank. Im really looking forward to getting a "real" reverb up and running after years of dealing with the digitally synthesized variety.

http://www.thehawkstudio.com/picts/6a20/IMG_1556.JPG

My first impression is extremely positive. The quality seems excellent given the price of the kit. I can see right away this build will be a lot tighter than my last one. There are a lot more components fitting into a much smaller space. I'm up for the challenge and am really looking forward to getting started. I'm thinking of building the kit stock, as is, but I'm considering an orange drop cap upgrade. I'm on the fence on this decision. I don't really care about the expense I'm just wondering if you can hear a significant difference in the sound. Its times like this when my lack of experience with tube amps shows. I'm leaning toward a stock build.

I guess the first step is to inventory everything and get everything separated out by category. If there is something missing I want to get it on order now.

Edit: Nothing missing. Go Weber! I decided that it couldn't hurt installing orange drops. They are on order. Well, they look pretty cool if nothing else.

[MarkE]

08/23/09
I finally got to work on the chassis this morning. I started by stripping the film and installing 4 long standoffs. I love it when things are nice and shiny. It wont be long before my dirty grubby fingerprints are completely covering this thing.

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I drilled all the holes that I needed, and enlarged a few. Interesting, there was no hole for the bias pot so I chose a location and drilled one. A quick cleanup and we can start fitting everything.

http://www.thehawkstudio.com/picts/6a20/IMG_1568.JPG

Here we are, all of the basic guts are in place except one input jack which I have coming. Here are views from various angles.

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I was waiting over six weeks for this kit to arrive so Nate was kind enough to substitute a Heyboer HY 025130 transformer instead of Weber's which is on back order. I like the transformer but had to scratch my head a bit because it required a different mounting. Instead of the transformer belly going through the chassis I had to vertically mount it. I test fitted it into the amp case to make sure there wasnt any interference anywhere. Everything fit like a glove so I committed to the verticle mounting. Actually, I like it better. There is a little more wiggle room inside the chassis.


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The chassis is ready except for the ground bus which I will install next time. The grounding scheme I chose requires that the four input jacks and 4 RCA jacks be isolated from the chassis. I tested them after tightening them down. All okay. Once I install a ground bus I'll be ready to start wiring. I will most likely start with the power wiring and then on to heater wiring.

[MarkE]

Things are proceeding normally. I now have all of the power wiring done as well as the heater runs.


I did pull a stupid which someone on another forum straightened me out on. With the power transformer and output transformer in close proximity, it is standard to orient the transformers 90 degrees apart to minimize magnet field coupling. Makes sense....duh!! I rotated the transformer easily. If there is coupling once the amp is alive I will switch to a belly mounted power transformer.

Here is a shot of the completed heater run. I am happy to have that behind me.

http://www.thehawkstudio.com/picts/6a20/IMG_1598.JPG


Here are some details on the ground bus. I'm using a 12ga square tinned copper up on standoffs. The tie down point is at the chassis near the input jacks. The grounding scheme worked so well on my last amp that I decided to do it on this one as well.

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A closeup of the four isolated RCA jacks and their mini-bus.


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Here is a shot of the eyelet board under construction.

http://www.thehawkstudio.com/picts/6a20/IMG_1627.JPG

[MarkE]

Work continues on the eyelet board. I elected to attach the flying wires last. A bit unconventional but that's me.

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[MarkE]

Okay, All of the flying wires are attached to the board. In addition, the wires from the dog house and other areas have all been completed. I will be tackling this mess in the near future.

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I decided to construct a dual bias for this build as I wont be matching tubes. This involved drilling a hole for a second bias pot (still missing) and building the bias supply on a terminal strip. The terminal strip takes less area than the supplied board and I was able to tuck the entire thing away neatly in the upper corner.

The location of the pots are not ideal as they will be in the far left corner of the amp once its constructed, and will be blocked from easy access by the power transformer. I figured that I could live with that as they wont need to be adjusted that often.

http://www.thehawkstudio.com/picts/6a20/0906091-IMG_0016.JPG


I split the bias right on the board by drilling a small hole with a pin vise and separating the resistors. The yellow and green wires connect to them individually and will run to the bias wipers.

http://www.thehawkstudio.com/picts/6a20/0906091-IMG_0011.JPG

[MarkE]

Okay, time to tackle the wire mess. I started by landing the wires to all of the front controls. I gave a lot of thought (and research) as to how to actually route these. Most 6a20 build photos seem a bit messy to me. I chose to create three bundles and route them individually to the extreme front of the chassis under the pots. Then, I can bring up each wire into it's needed position from underneath the pot. I think this is a lot cleaner, and is a whole lot cleaner than it looks in the photos.
I think this routing will be okay, but I wont know for sure until I fire this thing up.

http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0027.JPG

The first bundle drives the volume, bass and treble controls for the normal channel.

I chose to use shielded cables for the two inputs. Instead of passing them through the board, I'm routing the normal input cable around the right side of the chassis. The cable for the vibrato channel is routed under the board instead of through it.


http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0004.JPG

The second bundle runs under the ground bus and drives the vibrato channel controls.

http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0010.JPG


The third bundle runs to the remaining controls, reverb, vibrato etc.

http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0012.JPG

The first bundle breakout to the normal channel pots.

http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0015.JPG

The second bundle breakout to the vibrato channel pots.

http://www.thehawkstudio.com/picts/6a20/0906092_IMG_0016.JPG

The third bundle breakout to the vibrato / reverb controls.

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That completes all of the front panel control wiring. Now its on the the rear of the chassis.

[MarkE]

Rear chassis wiring

Here is the dog house completed. I'll leave the cover off for now until I know that everything works.

http://www.thehawkstudio.com/picts/6a20/090709_IMG_0001.JPG
Edit: Oops...I missed soldering the negative side of one of the A filters. I went back and got it later.

The rear wiring went off quickly and easily. There were only two places where I had a little bit of trouble because I chose to wire the heaters first. It was a choice I made and expected it. Not too bad though.

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Here is the wiring for the four RCA connectors and their bus.


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The only thing left to do is wire the bias pots, construct the bias monitor jack and connect it to the cathodes of the output tubes. Then I'll be ready for the smoke test.

Up until now this has been an enjoyable build. There are a lot of connections in this kit, and also a bit of head scratching while planning it. I'm not a heavily experienced amp builder but I have considerable experience in building electronics. I would not recommend this kit as a first build.

It remains to be seen if I will have any difficulties with the way that I chose to lay this thing out. Examining the circuit, I can see that lead dress is very important. I think I have everything arranged for easy change if it becomes necessary. While planning this I was mindful to keep higher current/high voltage wires well away from signal lines. In the one or two spots where they needed to cross, I crossed them at right angles. I'm hoping to avoid motor boating, farting or other weird problems. And, of course, I'm hoping for low hum and hiss.

[MarkE]

First Power Up

With the amp build complete except for the bias wiring, I decided to power up and test it. I hooked it up to a light bulb limiter, got the meters setup and hit the switch. The A supply came up to the proper voltage. No ripple on the scope. I then took it off standby and the B, C and D supplies came up. All looked normal. I poked around the board a bit measuring voltages at all the key points. Again, normal. First step complete.

I shut it down and bled it out. I then plugged in all of the tubes except the output tubes. I powered up again. I checked B, C, and D supplies. All normal. All heaters lit.

I then injected a sine wave into the input and scoped the phase inverter. All normal. Great!! Here is the waveform.


http://www.thehawkstudio.com/picts/6a20/0907092_IMG_0006.JPG


All of the controls seemed to work. I checked the vibrato channel with the same results. It all seems good. I shut it down and bled it out again.

I was dying to find out if I had any noise in the amp so I picked a signal off the PI input and fed it to the board. I also hooked up the reverb tank and foot switch. I powered up again and I had the sweet tone of the sine coming through the studio system. Looking good.

I grabbed a guitar and plugged it in. It all worked perfectly. All of the controls, reverb and vibrato all worked. It sounded nice. But the biggest thing is....NO NOISE!! What I hear after getting the power tubes in place is another story but it really looks promising. Its dead quiet, even at full volume.

I shut down, bled it out and decided to call it a day. Here is a shot after the bleed out.


http://www.thehawkstudio.com/picts/6a20/0907092_IMG_0001.JPG

All in all...very exciting. This is my second build, and my second with no problems. I'm a happy camper.

[MarkE]

It's finally completed. Now I have to clean up the mess. Cut wire is everywhere lol

http://www.thehawkstudio.com/picts/6a20/090909_IMG_0001.JPG

It sounds really good. It is the nice, clean, crisp sound that I remember from the old days. The reverb and vibrato work really well too. Actually, just to mention, vibrato and tremolo are often confused. What this amp has (and had) is really tremolo as it varies the volume of the signal and not the pitch. Fender labeled it vibrato which I'm sure is partly the reason for the wide spread confusion. In any case it works really well and sounds amazing. Totally classic.

The reverb tank is mounted on the cabinet floor in a tolex bag. This helps to isolate it from sonic vibration.


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The transformer on this amp is mounted vertically instead of the usual belly mount. This turned out to be a non issue and I actually like it more. In earlier photos you can see that it leaves a lot of extra room in the chassis.

http://www.thehawkstudio.com/picts/6a20/090909_IMG_0012.JPG

The back covers in place and I'm ready to rock. I gave it its first shake down with the Ibanez six string.

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I am like totally happy with this. It sounds amazing. I managed to get a line out signal from it into the board and the recorded sound is great too. I'm sure that I will use it somewhere down the line. I'm actually more into the technical aspect of doing this, the detailed construction of the electronics rather than actually playing through it. Recreating these classic amps is a real charge.

The next kit is on its way. Its a 300W bass amp head patterned after one of Ampeg's SVT series amps from the 60s. Following that will be a Marshall like 100W lead head. Stay tuned.

MarkE