Single Channel AB763 no verb/vib

[tubenit]

A forum member wanted a clean sounding AB763 without reverb or vibrato. Numerous forum guys chipped in with their thoughts on this. This is the result. 12/'08

Tubenit

[LooseChange]

Do you think you need the two 220k resitors between the preamp and the PI.  Actually, can't you remove one of the coupling caps there too (Just use the .001)??  Just a thought.

[sluckey]

Yes, you could lose the .047uF, 220K and 270K. Just connect the .001uF directly between V1-6 and V2-2. Doing so removes the voltage divider (220K/270K) that the normal channel must go thru to get to the PI, so you'll hit the PI with a hotter signal (100% v 55%).

The reason those components were included was to more accurately copy the circuit of the AB763 as seen by the normal channel. (The vib channel tube impedance was ignored)

[fdesalvo]

Would the hotter signal cause a loss of headroom? 

[darcynash]

Could you do this same thing but drop two 6l6s?

[tubenit]

Sure you can drop a pair of 6L6's and then re-bias it.

Tubenit

[lpresnall]

Anyone have a BOM for this build?  I know it's been a long time....

[sluckey]

Anyone have a BOM for this build?  I know it's been a long time....

I don't recall seeing one. But, making your own BOM is easy when working from a schematic and a layout. Plus, the familiarity you gain from doing so is very worthwhile. If you have any questions about particular parts values/ratings we'll all jump in and lend a hand.

[lpresnall]

Ok, I'm just lazy!  But it's raining today so I'll grab coffee and do it!  I'm sure I'll be back here once the soldering iron is hot with a load of dumb questions!

[lpresnall]

Speaking of schematic...is there one on the forum for this layout?  It'd answer a question or two for me...I noticed the "lite" version of this circuit with the two 6L6's has a rectifier (solid state or tube, I don't recall) and this one appears to have diodes?  If so, are they the usual 4007's?

[sluckey]

I don't recall ever seeing a custom tailored schematic for that exact layout, but just use a Fender original schematic and ignore the vib channel, reverb circuit, and tremolo circuit. IOW, look at only the normal preamp.

What questions do you have concerning this layout? Which 'lite' version are you referring to?

1N4007 diodes will work just fine.

[lpresnall]

The "lite" was a version with only 2 power tubes that I saw somewhere on here.  I'm beginning to get lost in everything I've looked at!  I think I'll go with the 4 tubes though since it's only a little more work and I need to perfect my heater wiring anyway!  I suppose the wattage I get out of this depends on the trans/tube combination that I use?

[lpresnall]

Ok, I'm going on this amp and just have a few questions:

[lpresnall]

Ok, oops!  So now the questions:

I see the caps (22uF) but am wondering if they go on a cap board on top of the chassis, or on the main board, or a smaller board inside the chassis? 

Or, maybe just assorted around the chassis alongside the main board where there's room?

Is there any major underboard wiring that I'm missing?  With the exception of the bias to the 220K's it all seems above board.

The ground(s) are also unclear to me.  Should I take them all to a lug on the PT?

Thanks for putting up with me.

[tubenit]

Are you going to make a Hoffman style layout board?  Or a typical Fender style eyelet board?  Or use a tag board?  Or use terminal strips?

The type of "layout board" that you use will somewhat inform you what type of approach to take for the filter caps.

Take a look at a Hoffman style board.  The filter caps on this might go from the board to the buss wire across the back of the pots.

 http://www.el34world.com/boardmaker/oldboardkitpage.htm

Other ways of doing the filter caps would be to mount them on a board or terminal strips  and run the wires across. to the layout board.

Look at how they do this on Dumble type amps:  http://ampgarage.com/forum/viewtopic.php?t=12873

You have lots of options on how you want to approach this including a cap can or the old Fender "doghouse" type filter cap container.

It's your choice.  I don't see a "right way" of doing it.

Regarding the grounding scheme.  Hoffman covers this quite well in the Library of Information.  Which has tons of great info and I recommend that you look over it thoroughly.

http://www.el34world.com/charts/grounds.htm

With respect, Tubenit

[lpresnall]

Hey tube,

Thanks for the reply!  Yes, the board's a Hoffman style.  Since that was the layout I had I decided to keep "like things like".  The board seems crowded (unless I'm doing something wrong).  My little bit of previous experience is two ax84 amps and an 18 watt TMB.  On this build even the small caps go on the board looking like Godzilla stomping out a city.  I suppose the current doesn't care, just looks odd.  Thanks for the links - every time I look over this forum I find something great that I'd missed before!

[sluckey]

Here's how Doug usually did filter caps on the Hoffman boards. The positive end connects to the board and the negative end connects to a ground buss that's attached to the back of the pots.

[lpresnall]

I did see this pic, but couldn't fill it up and give it to the nurse so to speak regarding where the wires went!  Ah, the back o' the pots...that makes sense.  I'll get thru this thing (especially with the help on this forum) but I may be a little slow...I may've dived into deeper water with this circuit than is wise for me.  I'll be in touch, I'm sure, and thanks again for the quick replies!

[sluckey]

The ground buss is soldered directly to the back of the pots. The caps actually connect to the ground buss, not the pots.

[SILVERGUN]

lpresnall,
Just my 2 cents...consider doing the reverb....
I just built a similar amp, and it sounds great with the reverb on (and you only need a touch of it), but without reverb, this will be a very dry sounding amp

Still a great classic sound, but the reverb just adds that little touch of magic, that is worth the extra couple parts it will take to do it....it will be much easier to include it into your new build than try to add it later.

Good luck and have fun!

[lpresnall]

Quick question...what's the function of the 250p at the left side of the board and can I substitute something else?  Somehow I missed it ordering parts...

[SILVERGUN]

Quick question...what's the function of the 250p at the left side of the board and can I substitute something else?  Somehow I missed it ordering parts...

It's your treble cap for the tone stack....yes you can substitute, anywhere from 150pf up to 500pf (without too much concern about changing the overall treble control)....this stuff is subjective.....how much do you care about it sounding "exact" to the original
If you "just want to get it built",,,you could go as high as .001uf before the AB763 police show up at your house 
Go back and "get it right later"

If you want a correct value, I can put one in the US mail today for you....PM me your address, and I won't even charge you for the stamp

[lpresnall]

Well, you are awesome!  PM on the way and thanks so much!  Let me know how I can reciprocate!

[lpresnall]

Getting into the flow...but I'm confused on the bias...what exactly am I looking at?  I see the 10KL which I'm assuming is the pot, and I have the diode in place to the right of it beside the 470k...but how does this all fit together?  I see something I don't recognize on the photo of the hoffman ab763 build and assume that's the pot/bias...any help for me here?  Thanks!

[sluckey]

Getting into the flow...but I'm confused on the bias...what exactly am I looking at?

Use your "Hoffman Style AB763 Lite" layout to make all connections, particularly the bias connections. There are some slight differences between the Lite version and the full version since the Lite does not have tremolo.

I see the 10KL which I'm assuming is the pot, and I have the diode in place to the right of it beside the 470k...but how does this all fit together?  I see something I don't recognize on the photo of the hoffman ab763 build and assume that's the pot/bias...any help for me here?

Are you talking about the white thing? If so, yes, that's the bias pot. It all fits together IAW the Lite layout.

BTW, the resistor next to the diode is 470Ω, not 470KΩ. It's called the range resistor because you may need to use a slightly different value to get the bias voltage within the range you need based on your particular PT and the output tubes you use.

[SILVERGUN]

Getting into the flow...but I'm confused on the bias...what exactly am I looking at?  I see the 10KL which I'm assuming is the pot, and I have the diode in place to the right of it beside the 470k...but how does this all fit together?  I see something I don't recognize on the photo of the hoffman ab763 build and assume that's the pot/bias...any help for me here?  Thanks!

You are creating a negative DC supply to feed the grids of the power tubes. You are correct that the 10KL is the adjustment pot for that circuit. It will give you a range of bias voltage that you will use to properly bias your output tubes.....it is best to start the amp with the adjustment pot set so that it is allowing maximum -DC volts to reach the grids,,,,which will result in a lower bias reading, because the higher the negative voltage is there, the less current will flow through your output tubes,,,,and at initial startup, it's just safer to start out that way

You should see the capacitor for that circuit has the positive end going to ground, which should look a little odd,,,,but is standard issue for a negative supply

BTW, the resistor next to the diode is 470Ω, not 470KΩ. It's called the range resistor because you may need to use a slightly different value to get the bias voltage within the range you need based on your particular PT and the output tubes you use.

Sluckey does an amazing job of catching stuff like this all day,,,everyday....be very grateful that he is here 

Here's a good article about the whole circuit from the Hoffman library:
http://www.el34world.com/charts/Biascircuits.htm

[lpresnall]

Uh-oh!  Now I'll have to check that I didn't put 470K's in all those spots!  So where is the pot itself mounted?  In the Hoffman build there's something I haven't seen before on the board, but it's too small to be an actual pot. 

[sluckey]

The bias pot is the white thing on the board. Doug sells them.

[lpresnall]

Ok, I will throw the 10K pot I bought in a drawer and get the right thing!  Thanks!

[SILVERGUN]

Just for the record....
The bias voltage is present when you turn the power to the amp on and before you hit the standby switch to the "play" position,,,,,so make sure that your negative supply is working before you flip the standby swtch and apply B+ to the amp

Do this at both power tube sockets on pin #5, and make sure your bias voltage is making it there before going any further

[lpresnall]

Okay, I'll do it, and thanks for the replies!

[lpresnall]

Ok, heater wiring is done...board is in and I got the bias pot in the mail today (thanks Doug!!).  I have a new set of questions...I've never wired 12A jacks before and my plan is a single input jack and two outputs with no impedance selector.  How should I proceed with this?  The common hookup page on the forum has a picture of how the resistors go in, but not any actual wiring (sorry for my being somewhat of a moron...maybe if I built the same circuit twice I'd know something).

[Willabe]

You should ask your new questions here,

http://www.el34world.com/Forum/index.php?board=13.0



                  Brad