Ab763 reverb circuit in separate mini chassis

[Toxophilite]

Hey
I was struck by a thought (ouch)
I have 2 AB763 type deluxes that i built in hammond AO-43 chassis
Space was limited so I couldn't include reverb
It would be cool to have reverb in one
Would it be possible to build an AB763 reverb circuit (complete with tubes and transformer) inside a mini chassis
Mount it close to the main amp chassis and then run B+, heater wires and audio lines to it
Too much potential for noise??
Has anybody done this sort of thing
I guess another idea would be to piggy back a mini chassis right onto a bare spot on the main chassis, kind of like a cap barn dimensionally except with 2 tube sockets mounted on top..That would keep all the lines inside chassis?


Too crazy?

[jjasilli]

Yes, go for it!


In the old days, sometimes the preamp chassis was up top, and the power amp chassis down at the bottom of a combo cab, connected by an "umbilical chord", with heater, B+ and signal wires, often plugged into an octal socket.  Note the the pins on the Octal Plug were LIVE.


Sluckey posted on this and may have info on his website.

[Willabe]

Tubenit has built tube stand alone reverb units to be inserted in a passive Fx loop, just a pair of 1/4" jacks, in an amp. They have their own power supply.

I'm sure he posted the circuit drawings and layouts with pictures in the schematics section.


              Brad   

[eleventeen]

If it were me, I would lean more towards building a 6G15 standalone reverb unit or some sort. I guess it would be an additional "thing" you have to drag around, and that's one reason why I've owned a few of them but never really fell in love with them. I once bought a '64 white-covered one for $40.

[Toxophilite]

$40 is a pretty good deal considering the piles of gold they sell for now!

I have a 6G15 i built and it works well and i have a good way to use it with my amps without losing the top end like they're prone too

However if i could get away with just adding a few lbs to one of my 30 lbs deluxe builds that would mean one less trip and item to remember!
And make my main amp a little more versatile.

[EL34]

better yet, build a Re-Vibe


I would use the Mojo779 power tranny that I used on all my reeverb and Re-vibe projects
http://hoffmanamps.com/MyStore/catalog/Transformers.htm


The 6G15 reverb unit and Revibe build info is here
http://el34world.com/schematics.htm

[Toxophilite]

THE revibe is a very cool unit, as is the 6G15
I built a 6G15 into an old garnet reverb unit chassis but I was hoping I could add to my existing amp and thus have one less item to carry


How essential is it to the AB763 deluxe reverb circuit is it to have the extra gain stage(1/2 12ax7) after reverb circuit?
Does it do anything other than add gain?

[Willabe]

It mixes the dry preamp signal with the wet reverb signal.


                 Brad     

[Toxophilite]

hmm
So is it necessary to the reverb circuit to have that extra gain stage prior to the PI?..Will I need to have a 6av6 (or another 12ax7 and use 1/2)in my little reverb mini chassis as well as a 12ax7 and a 12at7 ?


It looks to me like the signals are mixed just prior to the grid of that gain stage.
Wrong? right?

[sluckey]

So is it necessary to the reverb circuit to have that extra gain stage prior to the PI?..Will I need to have a 6av6 (or another 12ax7 and use 1/2)in my little reverb mini chassis as well as a 12ax7 and a 12at7 ?

All you need is a 12AT7 and a 12AX7, just like the original AB763.

It looks to me like the signals are mixed just prior to the grid of that gain stage. Wrong? right?

Right.

[Toxophilite]

Thanks!
Off to pick up the reverb driver transformer I ordered some weeks back when I imagined these chassis might have enough room
This will be an interesting experiment
I do have a 7 pin socket with a corresponding plug that should give me just enough leads to run heaters, B+ and audio in and out.
Though maybe 2 lines would be better, one with all the power and the audio ones separate

[shooter]

the audio ones separate

I've been pondering an umbilical build, would you still do the ground signal source braid for the coax or would both sides of the coax be grounded for a 2 chassis setup?
thx

[Toxophilite]

I see about 9 ground points in the reverb schematic
I would probably tie them all together on a lifted ground and lead it back to the appropriate point on the main chassis ground, Maybe tie the two chassis together too


I would tie one end of the audio coax to the appropriate ground on the main chassis


I could be wrong but that's how I'd start...and then trouble shoot if anything was amiss

[shooter]

Thanks, I was thinking the 2 chassis would be tied together for shock, just not sure on signal.

[sluckey]

Shooter, here's how I did the connection between the preamp and power amp chassis on my stuffed Vox project. I used RG-174 for the signal and two STP cables for heaters and B+.

     http://home.comcast.net/~seluckey/amps/vox/Vox.pdf
 

[Toxophilite]

Hi Sluckey
I was looking at that link too
I don't think I have an 8 pin jack and I need to go to and fro to the other chassis for the audio for my idea


Can I get away with NOT using the shield on the B+  and the heater lines

[shooter]

thanks Sluckey, I stole that pdf a couple months back, but for the pcb layout, now I can use some more!  Thanks for all you've done here.

[sluckey]

@ Toxophilite, the 8 pin jack is an octal tube socket. If you want one umbilical cord like I did, then using STP for the heaters is a good idea, but may not be necessary. Definitely don't need STP for the B+ and ground connections. Since you need two signal paths you may want to just do a separate dual RCA jack and stereo RCA patch cord. Or just mount your mini-box directly to your main chassis and eliminate the interconnection headache all together.

@ shooter, just today I updated that pdf file to better show exactly how my umbilical cord is wired. You may want to get the updated file. I treated the signal cable just like a patch cord between stereo components, ie, the shield is connected on each end.

[Toxophilite]

Excellent I had just finished bending up my rough and ready minibox  to piggyback on the chassis when I read your post this morn. (it seemed like the easiest route and best fit too)
Since then I planned some layout, installed some sockets, the 100kL pot, and some terminal strips..should be fun wiring it up in the tiny space. It'll remind me of recapping the echolettes I had..like brain surgery!

[kagliostro]

Nice add on choice

only one thing, if one of the long sides of the add-on chassis was very short (3/8 ") you had a way to do a more comfortable soldering job

then you can close the missing side using the 3/8" part as support for screws and  add a sheet of appropiate measure

maybe you're still in time to do something like this

Franco

[Toxophilite]

THanks for the tip!
I'm already well into soldering the components in
Almost finished in fact.
Still have to mount the transformer and run the leads into the add on and install the phono plugs and a 1/4 jack for the footswitch. The add a couple of overlong coaxial in and out cables and then install on chassis and see if it all works.

[Toxophilite]

So I have this thing mostly wired up
Originally I was going to mount the reverb driver transformer on the main chassis but it will be easier if it is on the piggyback chassis(which will be bolted to the main chassis)


Keep in mind it's position on the chassis in the preceding pictures
 I was wondering if this orientation would cause  a problem?
(You kind of have to mentally flip this thing up side down, the transformer would be almost lying on the main chassis pointing down the chassis towards the PT end)

[Willabe]

That reverb OT is fully shielded, so it should be pretty good. You already mounted it but it should be away from the 1st preamp tube as much as possible.

You'll find out soon.


                  Brad   

[Willabe]

Looking at your pics, it might have been better to mount the OT on the other/far side of the mini chassis, closest to the cabinet side wall. Or, on the back (far) side of the mini chassis opposite the verb pot.


                  Brad   

[Toxophilite]

I hadn't mounted it when I posted (it was just sitting there) however, I impetuously went ahead and mounted it there before reading your posts
It'd be easy to move and remount and i already took it off to examine the options
there is room to mount it on the cabinet wall side but no room at all to mount it opposite the reverb pot(which is actually at the back of the amp)


My phono plugs for the reverb tank we're also going to be on the side nearest the cabinet side wall, kind of close to the front of the amp. I could mount the transformer on the cabinet wall side right at the back, there's just enough room
Is it a big deal if the phono plugs and the transformer are mounted on the same side and close to each other??

[Toxophilite]

Time to install
Crossing my fingers as there's a lot crammed in there though I tried to keep nasty things on one side and audio things on the other

[Willabe]

I bet it'll work fine.

The verb OT is shielded and you have it away (5"-6"s?) from the 1st preamp tube (?) and you have a shield on the tube.

If you had mounted the OT on the other side of the mini chassis it would have been right next to that 1st preamp tube.


                   Brad   

[Toxophilite]

Actually the reverb OT is about 2-3" away from the preamp tube with the mini chassis in between


I powered the unit up
And I get reverb,,,but I also get a loud buzz/hum
Not tonnes of reverb, not what one would expect and buzz


With the reverb turned off the amp is pretty well how it was before though maybe a little louder (it has an extra gain stage now doesn't it??)


turn reverb up and the humm/buzz goes up accordingly


So the inevitable troubleshooting begins!
Note to self..leave an extra inch on all cables running to piggyback chassis to make life easier when doing the *^%(&^%(& inevitable troubleshooting

[Toxophilite]

 


Well I've been trouble shooting and trouble shooting!!(and *^%$*%^ troubleshooting!!!!!!)
All the right components are there and supposedly in the right positions
I pulled the 12at7 and I still get the hum/buzz
I disconnected the reverb tank and I still get the hum/buzz!!
And if I tun down the reverb the hum/buzz goes away
So that seems to be saying the hum/buzz is in the reverb recovery stage, the 1/2 12ax7


Not sure why though. I might disassemble and rebuild that section, or wire it up with longer leads to the main chassis so I can measure voltages with everything plugged in
Which I can't do now


Anybody have any other suggestions??

[sluckey]

So that seems to be saying the hum/buzz is in the reverb recovery stage, the 1/2 12ax7

Sounds logical.

I wonder if it's wired correctly? I see an orange drop cap connected to a 100K and a red wire. I assume the red wire is one of the B+ inputs to your chassis? There should be no coupling cap connected to the B+ in an AB763 reverb circuit.

This reverb circuit expects a 4AB3C1B reverb assembly. A tank from an AO-43 will not work.

Did you draw a schematic? It should look very similar to this...

     http://home.comcast.net/~seluckey/amps/tdr/tdr.pdf

[Toxophilite]

I didn't draw a schematic but I did follow the ab763 schematic and layout
Humourously I have it pretty well memorized from going over it last night! and it's identical to yours I believe
Though of course i could be very mistaken


The orange drop is the .003 cap that comes off the plate of the recovery stage of the 12ax7 and goes to the 100KL pot, there is also a 100k resistor on the same plate that goes to the B+ for the plate
THe red wire is on of the B+ inputs as you suggested


Both the orange drop and the other plate 100k resistor have red sheathing on their legs as they're a little longer


the plate resistor going to the plate of that has the .003 cap has no red sheathing
It's a little confusing looking there to be sure!

[Toxophilite]

Well i desoldered a bunch of the connections to the chassis, lengthened some things and alligator clipped others so i could at least test voltages


The result is quite mad looking and a royal pain in the @$$!!! (see below)


I picked up a couple local radio stations and was brought back to my community centre guitar teaching days when i heard the entirety of "Smoke On The Water" while testing.....


THe nasty buzz/hum was still there at the same volumes when the reverb was turned, the reverb was working but the buzz made it all unusable.


I measured :


12at7


pin 1 and 6 - 405 vdc
pin 2 and 7 -  3.5 ma
pin 3 and 8 - 3.6 vdc  (shoud be 8.7)


12ax7


pin 1 - 159 vdc
pin 2  - 0
pin 3  and 8 - 1.4 vdc
 pin 6 - 159 vdc
pin 7 (couldn't measure as it's late here and there was a godawful loud buzz when I put the probe on it, tried a few times..didn't wan to PO my neighbours)


Went over the schematic a couple more times and things look right


I measured the transformer to check it and got 2 ohms on the secondary and 1 k on the primary


I beginning to think a hammer might fix it really good!!!!


...Has anybody ever noticed that alligator clips are like guitar picks..and socks??

[kagliostro]

Are you comfortable with those power tubes so close ?


Franco

[Toxophilite]

I appreciate your concern, but that would be a sidetrack and it hasn't yet proved to be a problem


The amp itself works great
The reverb addition doesn't so far


I tested the reverb driver circuit by removing the amps power tubes and plugging the reverb input into a speaker and it seemed to work fine
Had to crank it to get some volume but it is a 12AT7


One thing that is NOT original to the AB763 vibrato channel is that I have a .047 cap coming off the plate of the 12ax7 prior to the reverb rather than the .02 on the schematic. I didn't think it would be a problem


I also swapped many tubes and tried 3 different gibbs pans and even another small 3 spring pan I have from a modern fender tube amp)
same results with all of them. Reverb...but BUzzz/hum

[Toxophilite]

Further experimentation would seem to indicate that a large amount of buzz comes from the 2 chassis being connected


And the hum actually has a bit of oscillation to it



THe phono plugs going to reverb are grounded to the chassis


My guess is this is a ground loop of some sort as the mini chassis is connected to the main chassis right by the input section of the amp
It is a guess though


When I hook the mini chassis via alligator clip to the ground node that I'm using for the reverb circuit ground (the same one as the PI) the buzz/hum is considerable reduced almost to levels that could be accounted for by the spaghetti of alligator clips connecting the whole unit up


time to go to bed!!

[kagliostro]

May be is worthwhile to try something like this on your added circuit



see here, Bridge + resistor + capacitor



Franco

[Toxophilite]

I think I'm getting better at holding my own hand!!
So grounding was the main issue
I had the phono jacks being grounded at the mini chassis and everything  grounded at one of the main ground points on the  main chassis(where the PI grounds)


Right now just to get the unit to work and stop being frustrated...


I've grounded everything in the reverb circuit to the mini chassis right at the reverb input phono jack(to tank)


reverb works when it's turn up I still get a bit of a hum but not make-it-totally-useless humm
i also get a tiny bit of RF...no smoke on the water though..


I was curious if I isolated the phone jacks..and the reverb footswitch jacks and then ran all the grounds to the main point would this remove all the hum??

[jjasilli]

Where are the filter caps for the reverb tubes?  If they are far from the reverb tubes, that may be an issue.  Maybe put them near the reverb tubes in the supplemental chassis.

[Toxophilite]

Hmm I'm not sure what you mean as there are no filter caps for the reverb tubes


On the AB763 schematic the reverb takes it power from point B for the 12AT7 (same as the power tubes)
and point D for the 12ax7( same as the other preamp tubes)


No specific filter caps
Only electrolytics are on the cathodes of the tubes

[Toxophilite]

Well I tried it with all the grounds lifted and tied to the PI node...worse buzzz


So i left it how it was and installed and it works great with very little noise
Very full reverb with less bright splash than the super..seems to make the amp brighter too
Kind of looks like a parasitic reverb mistletoe and it's a little cock-eyed but what the hell


I have to build a naugahyde bag for the tank and lie it in the bottom of the cabinet like my super
It tried bolting it upright and lying flat (it's a gibbs tank from a hammond organ same as my 6G15) but it squealed like a pig!
Wrapped it in a towel and lay it on the bottom and it works fine


The hammond tanks are open and have mounting bars(for upright mounting) and rubber suspension mounts but they're old and it's be better with a bottom on it and naugahyde bag...luckliy I have tonnes of naugahyde and a sewing machine....hahah (really!)

[Toxophilite]

3 hours of sewing machine trouble shooting and repair (maintenance really) and 5 min of sewing results in a fender style reverb tank bag

[eleventeen]

Missing from your saga is your estimation of what caused the hum to go away! Whaddya think?

[Toxophilite]

Oh , it's a couple of posts back. blah blah blah blah blah, probably hard to find!
I should've probably started a blog instead of a design tweaking thread!



I grounded everything in the reverb circuit to the reverb tank phono input (to tank)where it mounts on the mini chassisI ground point at the power end of the main chassis


Made a huge difference, The unit gets it's ground to the amp through mechanical connection to the main chassis
There is still some quiet hum when the reverb is cranked with a tiny bit of RF in there two. However the one or two times I use the reverb cranked it's in combination with echo ("Shot in the dark" and one of my tunes "Frankincense") so I'd defy anyone (or myself) to hear it amidst the reverberation echo chaos


Perhaps isolating the phono jacks and then grounding to the mini chassis through a 10 ohm resistor could possibly eliminate that last bit of hum (I did that on the SF style line out on the I installed on this amp and it eliminated hum in the stereo amp) but I got tired of troubleshooting the unit and mangling my careful wiring job in the process. Seemed like diminishing returns

[sluckey]

I think you should pull the tank from your SR and try it in this build. I think you will be pleasantly surprised at the difference it will make.

[Toxophilite]

 
You know what's funny, you're very right!


Is I have 3 gibbs tanks..three! beside the tank in my super and the other gibbs tank in my 6G15


I thought they were all the same as they were from hammond organs(I didn't kill them all...)


But the one I happened to randomly pick for testing and trouble shooting was the only one that had the input grounded and the output isolated
and it's a much quieter yet fuller and less splashy reverb (more realistic in my opinion)...like you'd want with an organ or a vocal but not necessarily for the fender amp crazy reverb sound
Likely it's impedances might have been different too(I haven't measured it)..otherwise it was physically identical


When I tried one of the other tanks after noticing the difference inputs and outputs ...BOOM! instant  fender big splashy reverb sound and PLENTY of it. Now I have that tank mounted on some heavy foam and cosy inside it's tank bag which is screwed to the bottom like my super.
 The unit sounds great and I'm am quiet happy to have one less piece of gear to carry and connect(the 6G15) as well as a single amp(for tiny gigs) that's a little more versatile. YAY! I really appreciate all the hep and direction I've received

[Willabe]

When I tried one of the other tanks after noticing the difference inputs and outputs ...BOOM! instant  fender big splashy reverb sound and PLENTY of it.

Ok, which tank? # please, we're all trying to learn here from each others builds.   


               Brad     
 

[Toxophilite]

Sure
No problem
I don't know if it'll help though
Annnd The tank in my amp is screwed into a bag behind the back panel at the bottom!

1st I actually have 4 (including the one in my 6G15 that i made) they're all made by Gibbs and are labelled 1122 - and then a 4 digit number starting with 6

I have one in my standalone 6G15, It works great and sounds like it should
It measures 1.7 ohms input and 176 output output

I have the one that I tried first that had a different sound (a really nice full sound) and was quieter (it has an 'L' on the inside)
It measures input 176 ohms
output 176 ohms!

I have another that got a bit of a hammond oil spill on it but I cleaned it some but haven't tested it. (It has a 'K' written on the inside)

It measures input 1.7ohms.
Output 44 ohms!

Maybe I'll yank the tank (did I just say that??) from the amp and measure it etc
I suspect it's identical to the tank in my standalone  unit as it sounds similar

Hope that helps though but to me it seems hardly definitive, if anything just more confusing!

[Toxophilite]

Ooops I forgot I can measure the tank I'm currently using from the phone cables


interestingly it measures
1.7 ohms input and 44ohms output


So 4 Gibbs 1122 17" tanks all two spring vertical mounting tanks
 2 with the same measurements and the other 2 different


Also my meter adds about .7 ohms..or it measures that if I put the 2 leads together


My super reverbs tank measures about 1.7 ohms input and 199 ohms output and when plugged into my piggyback circuit as Slucky suggested it definitely produce more verb than the 1.7 ohm in 44 ohm out tank that I have in there now(Which still produces a generous amount and way more than the 176 in 176 out tank I was initially testing with)


So I'm guessing the closest to the originally required would be the 1,7 ohms in 176 ohms out tank I fluke-ily put in my 6G15 build


I wonder if one could tweak the recovery stage of the AB763 circuit to accommodate a 44 ohm output?

[Toxophilite]

Or perhaps a better idea swap parts between the tanks
Take the one 170 170 tank (I actually have another on a SS keil unit) and put the output end in one of the 1.7 ohm - 44 ohm units


hmmm

[jjasilli]

Hmm I'm not sure what you mean as there are no filter caps for the reverb tubes


On the AB763 schematic the reverb takes it power from point B for the 12AT7 (same as the power tubes)
and point D for the 12ax7( same as the other preamp tubes)


No specific filter caps
Only electrolytics are on the cathodes of the tubes

It is somewhat possible that this could be an issue.  The tube plate should not be physically distant from it's filter cap.  Fender mostly ignores this with all filter caps on the PS end of the chassis.  OTOH, Marshall's layout is to put the filter cap right near the preamp tube(s) it services.  It might be worth trying this:  add a small value dropping resistor for isolation, and add a filter stage inside the supplemental chassis for the reverb tubes.