Sluckeys AB763 Deluxe Lite Questions

[fiftynine]

Not just for Steve Luckey, I'm planning a build of the AB763 Deluxe Lite. Thanks for the schematic, Steve.

Regarding the shaded portion where it says 'remove this lot for more gain', I guess this would be left in place to simulate the removed channel. So, wouldn't excluding the components in the shaded area change the voice of the amp, not just the gain? I can see removing the resistors will increase the gain but what is removing that cap actually going to do to the timbre/tone?

[FYL]

The 220K/270K attenuate th signal roughly by half. C6 doesn't play a major role, so tone will be nearly unchanged after you remove it.

[Fresh_Start]

Would there be any value to replacing R13 & R14 with a pot - perhaps 250K - as a pre-PI master volume?

Otherwise, switching the ground connection for R14 might be a useful boost switch.

Cheers,

Chip

[FYL]

You may, with a series resistor in order to keep a nearly constant frequency response.

[tommytornado]

If you go with a 500K you can approximate the voltage divider's gain drop (220k+270k=490K) with the pot @ approx 50%..  or not, with it turned up to 10.  The pot will let you manage how hard the PI is driven.   

And to answer your ??..  Those components in the shaded area are there to simulate the gain lost in a full AB763 circuit due to the trem circuit and reverb/dry mixer...  NOT, to compensate for no normal channel.   

[fiftynine]

Cheers, fellas.

Can anyone tell me what C6 is doing in the full AB763?

[FYL]

I guess that it isolates the vibrato voltage from the anode load.

[sluckey]

Or put another way, it prevents the dc plate voltage of V1B from getting into the following circuit. Specifically in this case, keeps the dc off the Vibrato Intensity pot.

Look at the full AB763 DR. You wont find a 270K that you see in the shaded area of my schematic. But you will see another 220K mixing resistor from the VIB channel. And you should see that the 50K INT pot is in series with that 220K mixing resistor. And the INT pot is also connected to ground. That's a total of 270K to ground. That's the whole purpose of R14 in the Lite circuit.

[fiftynine]

Yes, neglecting any resistance of the other route to earth round the top of the opti-isolator.

So if I didn't use C6 I wouldn't notice and, if I used the a 220k resistor and a 250k pot or just a 500k pot I could simulate the shaded area and anywhere in between. GOOD. This means I can tweak for the sort of gain I want and choose a fixed resistor to suit.

I'm glad I asked. I wonder how it compares to a PostPIMV.

[sluckey]

I did use a pot at that position on my TDR. I used a 100KA simply because I already had one.