Ground resistor

[Leevi]

I'm building a DR ab763 based amp with the Vibrato channel.Since there is not a channel mixing I would like to add a ground resistor on PI to compensate the missing Normal channel and reduce noise. What would be a suitable value of the resistor?
See the attached picture
https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763_schem.pdf

Leevi

[tubeswell]

start with 220k

[glass54]

Hi Leevi
Rob Robinette uses 270k https://robrobinette.com/RR763_BLACKVIBE.htm
I simply used the Hoffman AB763 1CH Deluxe Rev Sch https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf.
Works very well 
I can't see that you would have any real noise issues as the levels are in volts at this stage and potentially "tens of volts"
However it is a huge issue with microphone preamps, ie passive mixing at front end of preamp with relatively high value (carbon) resistor values.
Kind regards
Mirek

[Leevi]

Thanks for the fast replies.
I have already tried 220K which makes the idle noise very low but it of course drops the volume as well.470K increses the idle noise remarkable. I think the optimal value is somewhere between 220K and 470K.
Nice to hear that there is a proposal published by Rob Robinette.

Based on my experience the biggest noise source is around the current ampfilication stage.The amps where you have this third preamp stage generate always noise. I have spentlot of time with Bassman circuits in order to eliminate that.
/Leevi

[sluckey]

If your Vibrato channel will also include the tremolo circuit then you don't need any resistor to simulate the missing normal channel. You also don't need the 220K mixing resistor.

Rob Robinette's BlackVibe is the NORMAL channel. He eliminated the VIBRARO channel so he uses a 270K to simulate the missing 220K mixer and the 50K INT pot of the vibrato channel.

Hoffman's AB763 does not use the standard tremolo circuit so he uses a 47K to simulate the missing 50K INTENSITY pot (I would use a 51K).

[Leevi]

If your Vibrato channel will also include the tremolo circuit then you don't need any resistor to simulate the missing normal channel. You also don't need the 220K mixing resistor.

That's pretty clear but the 220K serial resistor eliminates hiss/noise.
If I bypass it the idle noise level doubles from 1mV to 2.2mV (AC).

The best result (deep quiet) I get when I connect the tremolo after the 220K resistor but
the signal level drops remarkable.

Rob Robinette's BlackVibe is the NORMAL channel. He eliminated the VIBRARO channel so he uses a 270K to simulate the missing 220K mixer and the 50K INT pot of the vibrato channel.

Yes, I noticed that as well. Furthermore there is not the 3. stage at all.

Hoffman's AB763 does not use the standard tremolo circuit so he uses a 47K to simulate the missing 50K INTENSITY pot (I would use a 51K).

Yes, I have used that way in some previous projects.
/Leevi

[sluckey]

The best result (deep quiet) I get when I connect the tremolo after the 220K resistor but
the signal level drops remarkable.

Of course it does, because putting the INT pot after the 220K creates a voltage divider of 220K and 50K.

[Leevi]

The schematic is AB763 without Normal channel.The tremolo is implemented with your "tremor-lator" using LND150 as the oscillator and 50K reverse pot.Otherwise the circuit follows the schematic.
/Leevi

[sluckey]

The schematic is AB763 without Normal channel.The tremolo is implemented with your "tremor-lator" using LND150 as the oscillator and 50K reverse pot.Otherwise the circuit follows the schematic.
/Leevi

OK, in that case the INT pot should be connected as shown in your hand drawn schematic and you don't need the resistor with the question mark.

[Leevi]

Yes, I'm ending up to this solution, but I keep the 220K serial resistor since it eliminates noise i.e the tremolo pot is connected after the 220K resistor.The noise level shows now 0.5mV which means in practise silence.
Thanks/Leevi

[pdf64]

Just to note that by swapping a -5dB potential divider after the trem pot to a -15dB divider before it, you're making quite a change to the gain structure / total max system gain, of the amp.

[Leevi]

Just to note that by swapping a -5dB potential divider after the trem pot to a -15dB divider before it, you're making quite a change to the gain structure / total max system gain, of the amp.

Yes, I'm aware of that, actually I have noticed it in practise.
/Leevi