Problem with a AB763 build

[rutledj]

I built the  2 channel AB763 circuit using the schematic found on this site with the power tube modulated tremolo ckt.  It sounds great but I can't actually get it to go into overdrive at all until you get up to around 9-10 on the volume (either channel) and it is only a really mild od at that volume.  My voltages seem in order (listed below) but not sure what else to check.  I thought this ckt usually started breaking up around 4-5.


AC to Rectifier 700v


Rectifier output 394v
After choke: 391
After first dropping resistor: 309
After second dropping resistor: 258


Both of my dropping resistors are 10k


v1-1 169.8
v1-6 165
v2-1 182
v2-6 180
v4-1 173
V4-6 174
v5-1 varies with oscillator
v5-6 391
v6-1 175
v6-6 173
v7-3 387
v8-3 384
v7,v8 -4 390
Tubes biased to 24ma


Thank,
Rut

[sluckey]

It sounds great but I can't actually get it to go into overdrive at all until you get up to around 9-10 on the volume

Sounds like a typical AB763 amp. I've never considered the AB763 to be an "overdrive" amp.

[rutledj]

Guess I was mistaken. I thought most Deluxe Reverbs (which I think this basically is) would start breaking up around 4-5 on the volumn knob.

[sluckey]

Did you use a 47Ω resistor in the NFB circuit?

[rutledj]

Yes

[Willabe]

I thought most Deluxe Reverbs (which I think this basically is) would start breaking up around 4-5 on the volumn knob.

So you have 6V6's in it?

One mans ceiling is another mans floor.       Also depends on the where the amps output tubes are biased, the speaker(s) the guitar PUP's and how hard you pick.

I had a BF SR I gigged with for years, mostly with a plan Jane Fender Strat (63 reissue, stock PUP's, .010 set, medium heavy pick) and I had to turn it up to 6(~+) on the amp volume control with the PUP's full up before I got anything I would start to call breakup. (And I can pick pretty hard when I want, had to spend time practicing picking softly. I spent a lot of hours on an acoustic with .012 set before I played an electric.)   

   

[Ed_Chambley]

On some my Fender AB763 amps that are original I simply disconnect the NFB wire tape and tuck it away.  This will change a Super Reverb to begin breakup sooner.  There are many things you can do, but the easiest is to boost your signal.  I have a Deluxe Build with 2 6v6 with a 100K pot on the NFB so I can make it clean or not.  The first tube has a 220k plate resistor and a 4.7uf/1K5 bypass cap.  I added a 25k mid pot.  The tremolo 1meg pot switches and adds gain when not using tremolo which is a roach trem.  Increase the mids and remove the NFB or most of it and plug it into a Greenback loaded cab and it gets very marshally. OTOH, if I add NFB about half turn (approx 50 ohms), increase mids and decrease treble and bass and use a Tele, the amp has a great 80's Country Tone.  Think Pete Anderson.

I have done some other things, but I also have a 65 original Blackface. It will begin breakup with the original speaker 5-6 on the volume.  I currently have a JBL 120 in it and it will not breakup, but I can hit it with a boost and the tubes wake up.  I do not like the tone of a distorted JBL.

What I have found is changing the AB763 to get breakup the first thing you lose is the chime.

There are things you can do to get early breakup, but then you no longer have a AB763.

I guess there is always give and take.  It is my favorite circuit and IMO the AB763 Super Reverb is the best all around amp ever made if you play places large enough.  I love the clean shimmer and I can use pedals for any tone I want as I have never found a pedal that doesn't like this circuit.

[Willabe]

I simply disconnect the NFB wire tape and tuck it away.  This will change a Super Reverb to begin breakup sooner. The tremolo 1meg pot switches and adds gain when not using tremolo which is a roach trem.

I had a friend, who past away, and we did the same on his BF DR, lifted the feed back and put in the switched trem pot, added a lot of ump/breakup and some volume. It was very noticeable. We then put it in a 2x12" cab and that added some/a bit more volume, he liked it.     

I currently have a JBL 120 in it and it will not breakup,

Rut, there you go, speakers can make a big difference.