AB763 DR Master Volume question

[Lectroid]

I now have my AB763 DR clone, (schematic below,) working to where I'm pretty happy with the tone.  This was my most ambitious project yet, and again I want to thank to all the people who nursed me through my previous problems and questions.

I decided to add Mid and MV to the AB763.  The transformers are out of a Hammond AO-43 chassis running into my 12" shop speaker.  The MV I stole from sluckey's Tweed Deluxe Reverb but while it works well, I'm not happy with the amp's responsiveness.  I was hoping for the amp's Volume control to gradually add OD, and let the MV control the output volume. 

Instead, both the guitar's volume knob and the amp's volume knob act to increase gain.  Of course that makes sense but they seem interactive, and not very intuitive.  I'm having trouble getting a smooth transition clean-to-dirt, not as much control as I'd like.

If I take it out of the circuit, I get a more Fender-y response with gradually increasing distortion as the Volume knob is raised.  It sounds like a classic 60s DR or SR.  I think it could easily run 6L6s.

Is this what I should expect from this circuit?  Am I using it wrong?  Anyone else have any advice as to how they use it?  I'd like to leave it in, but right now I'm leaning toward taking it out.

Thanks     

[sluckey]

The AB763 circuit is a clean circuit. You'll have to turn it way up to get any dirt.

Did you read why my DR even has a MV pot?

[pdf64]

I found that a type 2 / LarMar master volume works quite well with that type of amp design.

[Joel]

When I built my single channel deluxe reverb I initially used a 1M pot for the MV.  And I didn't have the 220k resistor following it either. 

I found the 1M MV pot resulted in the amp having too much drive for me.  I replaced it with a 100k and the amp is cleaner as I wanted.  If you want more drive, maybe try a 1M MV?

[Lectroid]

The AB763 circuit is a clean circuit. You'll have to turn it way up to get any dirt.

With the MV out of the circuit, I get that normal "Fender-y" behavior--it's strong and clean and loud, and would be a good pedal platform.  Am I chasing a phantom here with my desire to have a MV, with the Volume knob solely as a preamp gain control?  Maybe I should leave off the MV and quit while I'm ahead?

Did you read why my DR even has a MV pot?

Yes.  In summary, it sounds like your original circuit had too much gain and that the 100K pot fixed it.  Or am I missing something?  I think my circuit is identical to your TDR, including the MV pot.  I don't see why I get a different result. (?)  And why does mine sound better with the 100K pot out of the circuit?

Should I replace the 100K MV pot with a 50K resistor?

What I'd mosts like to know is: Why did leaving out the 50K resistance and its loading create the problem? Is it an issue of impedance-matching?  I really need to understand this better than I do--so hopefully I can figure out the next one myself.

Thanks!

[pdf64]

… Why did leaving out the 50K resistance and its loading create the problem? …

The heavy load of the 50k trem pot loads the 3rd stage, reducing its gain. Without that heavy load, the gain increases.

[Lectroid]

The heavy load of the 50k trem pot loads the 3rd stage, reducing its gain. Without that heavy load, the gain increases.

From sluckey's TDR doc: The tremolo pot puts a 50K resistance on the signal path just prior to the mixing resistors that feed the PI
This is where I'm not clear.  Looking at the Fender AB763 schematic, the Tremolo circuit with that 50K pot ties into just before the PI, and after the 3rd stage.  Is that right, or do you mean that the the 50K pot loads down the output of the 3rd stage? 

In that case, since the 50K pot was gone from sluckey's TDR design, the 3rd stage loading was less.  Does that mean the signal going into the PI was stronger?  Is that why he experienced increased gain?

[pdf64]

… This is where I'm not clear.  Looking at the Fender AB763 schematic, the Tremolo circuit with that 50K pot ties into just before the PI, and after the 3rd stage.  Is that right,

Yes

or do you mean that the the 50K pot loads down the output of the 3rd stage? 

It’s not an ‘or’ scenario, both are correct.
To reiterate, as stock, the gain of the 3rd stage is reduced by the heavy 50k load on its output.

In that case, since the 50K pot was gone from sluckey's TDR design, the 3rd stage loading was less.  Does that mean the signal going into the PI was stronger?

Yes

Is that why he experienced increased gain?

Yes.
Hence him adding the type 4 master volume there, to get gain under more control.

[Lectroid]

@pdf64,

Thank you for explaining that so clearly.  It clears up a longtime gray area for me and explains why the amp seems too 'hot' to me.  I'll experiment with some resistors there to tray and tame the extra gain.

Thanks again!