Ab763 simplified design

[martin manning]

Not sure how much that extra triode contributes. It's only there to boost the extremely low signal from the reverb tank. It's also half of a 12AX7, leaving the other half unused. If it were me, for the same number of tubes, I'd eliminate the normal channel and keep the reverb.

[Mikante]

I really want the two channels, as for the reverb, I hardly use it.
I don t know if it was for the brighter sound, but the vibrato channel sounded better and I was able to get some crunch or overdrive at reasonable volume.

[Roe]

Not sure how much that extra triode contributes. It's only there to boost the extremely low signal from the reverb tank. It's also half of a 12AX7, leaving the other half unused. If it were me, for the same number of tubes, I'd eliminate the normal channel and keep the reverb.

this is easily seen if you compare the different channels of typical black- and silverface amps. clearly, the extra triode offers extra compression, a little gain, and treble boost (due to the 10pf cap over the 3m3 resistor) as well as more noise and farty bass. the clean sound and breakup is just sweeter and more interesting.

without the triode, you can get closer with at least one high plate gainstage (220k anode resistor) as well as a treble bypass cap (e.g. 150pf over 220k)

[martin manning]

Adjusting the passive elements was my thinking if that half-tube were eliminated. It's a clean center-biased stage, so I don't think there is much more than the EQ happening there.

[pdf64]

…
without the triode, you can get closer with at least one high plate gainstage (220k anode resistor) as well as a treble bypass cap (e.g. 150pf over 220k)

Bear in mind whatever is boosted in one channel is cut in the other.

[Mikante]

Could this work, no trim pots and only one 3.3m resistor to adjust the gain.
No trim pots messing with the “load” will the large 3.3m resistor be a problem too?
In the original design there is a 220k grid resistor for the v4 and it is also placed after that large resistor and bright cap,
Maybe i am confused about this problem with the load.

3C97F474-7672-4859-A1E4-7FAEC64BAA78.jpeg

Could this work?

[Roe]

…
without the triode, you can get closer with at least one high plate gainstage (220k anode resistor) as well as a treble bypass cap (e.g. 150pf over 220k)

Bear in mind whatever is boosted in one channel is cut in the other.

yes, if you do this in the mixer circuit like marshall did, it creates one dark and one bright channel. Doing it elsewhere is perhaps better, unless the other channel is too bright. a bright cap on the volume control seems like a good idea (as does 220k w. 3k3 cathode on the first gainstage)

this case is a little different since the two channels have different eq sections. The first channel with a tone control will be more gainy, or have less loss, than the other one. In this case, uneven mixing resistors work, e.g. 270k and 180k or 330k and 150k.

[Lynxtrap]

Could this work, no trim pots and only one 3.3m resistor to adjust the gain.
No trim pots messing with the “load” will the large 3.3m resistor be a problem too?
In the original design there is a 220k grid resistor for the v4 and it is also placed after that large resistor and bright cap,
Maybe i am confused about this problem with the load.
3C97F474-7672-4859-A1E4-7FAEC64BAA78.jpeg
Could this work?

Take it for what it is, a rough estimate or a shot from the hip, food for thought or alternate take.
But I think I would start out with something like this and tweak it from there. Your own version would work too, my thought was mostly getting a little bit more rock'n roll out of the Tweed channel.

I would also lower the value of the 25uf cathode bypass on the third gain stage to something like 4.7uF or less.

3C97F474-7672-4859-A1E4-7FAEC64BAA78.jpeg

[Mikante]

Hello,
No mixing resistor for the vibrato channel? The 2.2m will serve the purpose instead? Is this going to give me even more gain from the channel?
2700k cathode resistor on v1b what will actually do?
250k pot for the volume, like the early 5f1 design.
You have lowered the grid leak value to 220 for the v4, the ab763 schematic shows exactly the same value for that.
Trying differents cathode bypass, i don t know if i m ever going to make my mind once a start swapping values there

[Lynxtrap]

Hello,
No mixing resistor for the vibrato channel? The 2.2m will serve the purpose instead? Is this going to give me even more gain from the channel?
2700k cathode resistor on v1b what will actually do?
250k pot for the volume, like the early 5f1 design.
You have lowered the grid leak value to 220 for the v4, the ab763 schematic shows exactly the same value for that.
Trying differents cathode bypass, i don t know if i m ever going to make my mind once a start swapping values there

As I said, that is just what I personally might have gone with as a starting point. I'm definitely not saying "do it like this".
The thought was mainly to not have the Tweed channel going through the "reverb mixer" 3.3M. I also think that 3.3M is overkill in Fender amps and especially without reverb.

The alternate values in the Tweed channel were mainly to lower the gain.

And yes, in this case the 2.2M resistor serves as a mixing resistor, so no need to have another resistor in series with it.

[Mikante]

Hello,
No mixing resistor for the vibrato channel? The 2.2m will serve the purpose instead? Is this going to give me even more gain from the channel?
2700k cathode resistor on v1b what will actually do?
250k pot for the volume, like the early 5f1 design.
You have lowered the grid leak value to 220 for the v4, the ab763 schematic shows exactly the same value for that.
Trying differents cathode bypass, i don t know if i m ever going to make my mind once a start swapping values there

As I said, that is just what I personally might have gone with as a starting point. I'm definitely not saying "do it like this".
The thought was mainly to not have the Tweed channel going through the "reverb mixer" 3.3M. I also think that 3.3M is overkill in Fender amps and especially without reverb.

The alternate values in the Tweed channel were mainly to lower the gain.

And yes, in this case the 2.2M resistor serves as a mixing resistor, so no need to have another resistor in series with it.

I got that and i never had the feeling that you were saying “do it like this”.
So the larger value on the cathode will lower the gain on the tweed channel as leaving the cathode bypass out.
The 2.2m resistor will be for the vibrato channel only, it was my intention too but i wasn't t sure.
Here the revised schematic, i will start from here.

11CB8947-3EDC-4C2E-AAEE-305BEB97A5DD.jpeg

[Lynxtrap]

Your previous solution with both channels going through the 3.3M is just one more component, so it's easy enough to try both.

[martin manning]

A solution to this can be derived using the well-known "Torres" mod to run both channels through the reverb, then eliminate the reverb. This works fine, and simulation results with the 6G3 tone control look interesting.

[martin manning]

PS: The extra triode could be put to use as a bias-modulating tremolo, as in the 6G3.

[Mikante]

A solution to this can be derived using the well-known "Torres" mod to run both channels through the reverb, then eliminate the reverb. This works fine, and simulation results with the 6G3 tone control look interesting.

Will the two channels interact with eachother without mixing resistors?
I don t understand the change on the bias circuit, could you explain it?