Ab763 reverb circuit in separate mini chassis

[Toxophilite]

I appreciate the insight and advice but unfortunately there's not really any room
and as there are two different sources of B+
I would require 2 different filtering stages, requiring even more room

[Willabe]

Why bring in the B+ twice? You only need 1 B+ coming in to the mini chassis.

Radial caps are great for this, much smaller then axial caps and less then $2 a piece.

Bring in the B+, goes to a small B+ dropping R, say ~470 ohms, then to the 1st filter cap, another B+ dropping R and to the 2nd B+ filter cap.

You might have even been able to get away with only 1 filter cap for both tubes in the mini chassis.

Read this on grounding;

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


                Brad            

[Willabe]

Or............ you might have been able to bring in the B+ to feed the verb driver stage and return the ground from the verb driver tube and it's OT (you'd have to isolate that jack from the chassis) back to that B+ filter cap node, that would also give you a B+ wire and it's ground wire that you would run as a twisted pair, would be quieter. 

Then add a B+ dropping R and 1 filter cap for the verb recovery and the verb mixer tube stages and return their grounds to that filter cap.


                Brad   

[Toxophilite]

...what about my tank idea...??? waaahhhh


I'm not having a big problem with noise anymore, it was primarily a grounding isssue as stated earlier


I have 2 tubes in my mini chassis
Following the fender setup (cause that's what I'm doing and I'm not a huge re-engineerer(,my new word)
THe 12ax7 gets it's power from the same B+ nodes as the preamp wanting around 170vdc
And the 12AT7 via the transformer wants power from point B (the same as the power tubes) and it's looking for 410 vdc I think .
The original schematic only has the same amount of filtering caps as I have in mine


Anywho I really appreciate the suggestions but this doesn't seem to be a problem at all

[eleventeen]

I think you have it right. The driver tube gets monster B+ and the receiver gets small B+.

[Willabe]

I'm not having a big problem with noise anymore, it was primarily a grounding isssue as stated earlier

Yes and I'm trying to point out a few reasons why. Things to consider when you design something, for next time.


                    Brad   

[Toxophilite]

Thanks very much
I appreciate that and will keep it in mind for my next build

[Toxophilite]

Oh and thanks for the link too
that's a good read!

[Toxophilite]

Okay I was reading the article
Haven't absorbed it all
But I think I understood I should be grounding the two different tubes to different points because they have different power supplies??
And maybe some further smoothing caps would be good
I think ideally my phono plugs and my footswitch jack should be isolated and the grounds free from the chassis at that point. Seems to me that would help too
i wonder if my mixing and last preamp stage after the driver should bne grounded to the same area as my entire preamp section(by the input jack) ?


Am I getting totally confused?
At this point I don't want to open it back up and disconnect things so i can work on it again
But I'd probably have another go at it in a week or so

[sluckey]

But I think I understood I should be grounding the two different tubes to different points because they have different power supplies??

yes

I think ideally my phono plugs and my footswitch jack should be isolated and the grounds free from the chassis at that point.

Fender didn't bother.

i wonder if my mixing and last preamp stage after the driver should bne grounded to the same area as my entire preamp section(by the input jack) ?

Yes. That tube is powered by node D which also powers the input preamp tubes. Your node D filter cap should be grounded by the input jack also.

Fender used a slightly different approach with very good results as long as that brass plate has a good electrical bond to the chassis. There are two ground wires coming out of the cap doghouse. One solders to the chassis near the solder connection for the PT center tap (power ground). The other solders to the brass plate behind the pots near the VIB channel input jacks. Take a look at this pic of a '64 DR and also the schematic/layout for a DR or SR. Compare the layout and the schematic. Or better yet, pull the chassis on your SR and study that.

[Willabe]

Doug has a very good grounding system that's been proven to be very quite many times here. It's in his library of information.

I posted the link to the Wizards grounding because he goes into depth about it and has some good drawings with it. (And yes it will take a while to digest it.)

Aikien Amps has some good info on how he looks at grounding, so does R. G. Keen and Kevin O'Connor in his TUT books.

What they all seem to have in common for the most part is getting away from using the chassis for random grounding. Or said another way the chassis is not a ground wire. (Well ground is ground right? They all go to the same place, to ground, right? Wrong.    )  The more ground connections you have to the chassis the more chances you have for those ground connections to cross each other and modulate each other, and if they do modulate each other, they will cause hum. Chassis grounds from a circuit with more current that cross a chassis ground with low current will have a larger chance of modulating the small current circuit ground. Using a wire for ground instead of the chassis, you control where the ground current flows. (A 'wired' ground instead of a 'random' ground.) So the more random chassis grounds you eliminate the less chances you have for hum caused by them.   

You have the amp circuit and you have the power supply (PSU) that feeds it, so you try and return all the grounds of a circuit, like the 1st preamp tube, to the filter cap node ground that feeds it. And you try and locate that B+ node filter cap as close as possible, within reason, to that circuit. That way you keep that circuits 'loop' small. (Doug often, but not always, likes to put his filter caps close to the circuit they supply and not in a Fender type dog house.)

See the high lighted drawing attached. 1 return loop in red and the other in purple from the same filter cap node.


                 Brad     

[Toxophilite]

Cool
All sorts of good information I'll use when I open the amp up again in a bit (going to play it a bit first so I don't spend all my time up to my elbows in it)


On this build because of space and ease  I used a JJ multi-part can cap which only has one common ground for the whole shebang!


THanks!

[Willabe]

On this build because of space and ease  I used a JJ multi-part can cap which only has one common ground for the whole shebang!

Doug, Sluckey and many others like multi cap cans and have built many quite amps/tube Fx units with them. But they do have a grounding schem/plan, they don't just ground anything, anywhere.

Valve Wizard, K. O'Connor and others don't because of the single ground on the cap can.

I've been using the small (modern) radial e-caps, hard to beat them for small size, low price, hi temp and hi ripple. So far I like them very much.


                   Brad         

[Toxophilite]

Hi
I've used axial caps many times, very handy and they definitely do the trick
I agree, I have no attachment to particular components as long as they're the right values etc


Actually prior to installing the JJ can cap on this amp I had a hollowed out old can cap filled with radial caps!
IN this instance I was introduced to the idea of modern production can caps and as it made the build so clean I gave it try and it has worked fine so far I think it's a 40 20 20 20


On this amp there is a definite ground scheme, it's not ground here and there willy nilly
The main chassis prior to reverb unit addition has (for the tremelo , PI ,PA and filter caps) two nodes in the PS right beside each other and both attached by short wires to a ground strap on the rectifier base (Hammond AO-43 chassis)
the filters have one node (they only have one anyway)and everything else goes to the other node


The WHOLE preamp section is grounded at the input jack which is tied to a chassis ground strap right next to it on the input end of the chassis


Prior to the addition of the reverb the amp was dead quiet at idle and still is when the reverb is off


With the addition of the reverb unit, and only when the reverb is on, I have a slight hum.
When I open it up again I will rearrange a few things and separate the grounds from the driver and the recovery stages at the very least

[Willabe]

I've used axial caps many times, very handy and they definitely do the trick

So you know how small a radial cap of the same value/voltage as a axial cap can be.


            Brad   

[Toxophilite]

 
Oooops!
That was a $#&%#* mistake
I meant RADIAL! not axial
I've used RADIAL caps many times
I totally concur, they are compact, do the job just the same, and are a zillion times cheaper!
Like I said, on these builds I splurged and bought the exciting and new (to me) can cap as it was such a tidy solution
I believe I have radial caps on my 6G15 build and on the Gulbransen amp conversion I used radial caps as well


My next build will be a vibrolux in an old SS Harmony chassis and I will use radial caps there as well

[sluckey]

So you know how small a radial cap of the same value/voltage as a axial cap can be.

That's not a fair comparison!   

[Willabe]

      Ok, so I used a rubber ruler.


          Brad   

[Willabe]

Oooops!
That was a $#&%#* mistake
I meant RADIAL! not axial
I've used RADIAL caps many times

I missed that, but I knew what you meant.


              Brad   

[shooter]

That's not a fair comparison

   

I'm missing this one, size doesn't matter?

[Toxophilite]

Next date I'm on, I'm going to say it's not the size that matters it's the rated capacitance and voltage

[sluckey]

I'm missing this one, size doesn't matter?

Everything looks tiny laying next to a Sprague Atom.

[shooter]

Sprague Atom

ahhh, thx

it's the rated capacitance and voltage

don't forget to mention frequency and duration

[Toxophilite]

HI
I opened up my amp today and tried grounding the recovery and extra gainstage (the entire 12ax7) to the same point as the input, by way of an experiment, Which is where the entire preamp is grounded and I noticed no appreciable difference in the slight amount of hum/buzz present when the reverb is really turned up. However the reverb circuit ground point is at the reverb input phone jack anyway and that is quite close to the input ground chassis attachment


I was assuming the transformer should be grounded to the driver stage as the transformer is fed the higher B+ 410 from point B of the power supply
I haven't however added extra filtering stages though that might be my next step
It's not crazy loud hum/buzz and only really present when the reverb is turned up a lot but it would a good learning experience to track it down.
Right now the entire driver stage is grounded to the chassis at the reverb input jack




Next I will try running all the 12At7 ground back to the PS section appropriate
and running the 12ax7 ground to the input jack
however in this instance the ft switch jack and the phono plugs seem to be ground loop potential


and see if that combination helps
If not then I might even isolate the phone jacks and the footswitch jack as well as the above 2 changes


probably throw some extra filtering on the B+ going to the transformer too