AB763 PI mods?

[WhopperPlate]

As a basic reference this gives an idea : https://youtu.be/4fW-gTRYBiQ

[Jake1524]

As a basic reference this gives an idea : https://youtu.be/4fW-gTRYBiQ

After even more looking and digging, I think I’m i agree with you. There is a very compressed element to the SSS which is the same reason I do not like my Fuchs ODS amp. Very soft and squishy under the fingers. I like a very direct and snappy feeling when playing.

It seems like raising the preamp voltages altogether could have a positive effect. How would you go about this, and how high can these voltages safely be?

[Jake1524]

Get B+ as high as you reasonably can.
51k/47k plate load resistors with a 12AT7 tube. Maybe add a plate balance pot ala Dumble/Ampeg. Also try a 610ohm bias resistor as opposed to 470 ohm.

Reduce the grid leaks out of the PI to 100k or the silverface value of 68k and reduce the coupling cap size out of the PI a bit.

If you want to get really crazy, you could try an Ultralinear output transformer from a HiFi amp or later Silverface amp.

Okay this makes sense. If I wanted to raise B+ in the whole preamp, would I reduce the plate load resistors at every gain stage?

How high could I actually take the voltages within reason?

[martin manning]

If I wanted to raise B+ in the whole preamp, would I reduce the plate load resistors at every gain stage? How high could I actually take the voltages within reason?

No, reduce the dropping resistor connecting he screen node to the PI node. Easy to try by paralleling another resistor. Another 10k in parallel will raise the whole preamp 40V or so. If you reduce it too much you will lose the decoupling effect, so maybe keep it at 1k or higher.

[WhopperPlate]

As a basic reference this gives an idea : https://youtu.be/4fW-gTRYBiQ

After even more looking and digging, I think I’m i agree with you. There is a very compressed element to the SSS which is the same reason I do not like my Fuchs ODS amp. Very soft and squishy under the fingers. I like a very direct and snappy feeling when playing.

Whereas I agree about the compression, keep in mind , a lot of the compression comes from the local negative feedback in the preamp . Listen to clips of sss5 to hear what a raw Lnfb free preamp can sound like.

It can be less compressed and real snappy , but that extra bandwidth and clarity can feel “compressed” ; one player I heard describe it as “why is it so midrangey?”.

I prefer a sky lineresque tonestack like sss5 , that way the low mids balance out more to my ears. The classic fender stack to me feels imbalanced just adding the cathode driver without the other circuit additions common to an sss. Imo simply adding a cathode driver isn’t a panacea for headroom and tone . Edit : by imbalanced I mean “not enough low mids” imo . Ends up sounding thin and “midrangey “. Not full . Just my experience. Certainly other variables to consider .

[Jake1524]

As a basic reference this gives an idea : https://youtu.be/4fW-gTRYBiQ

After even more looking and digging, I think I’m i agree with you. There is a very compressed element to the SSS which is the same reason I do not like my Fuchs ODS amp. Very soft and squishy under the fingers. I like a very direct and snappy feeling when playing.

Whereas I agree about the compression, keep in mind , a lot of the compression comes from the local negative feedback in the preamp . Listen to clips of sss5 to hear what a raw Lnfb free preamp can sound like.

It can be less compressed and real snappy , but that extra bandwidth and clarity can feel “compressed” ; one player I heard describe it as “why is it so midrangey?”.

I prefer a sky lineresque tonestack like sss5 , that way the low mids balance out more to my ears. The classic fender stack to me feels imbalanced just adding the cathode driver without the other circuit additions common to an sss. Imo simply adding a cathode driver isn’t a panacea for headroom and tone .

This is very interesting. The low mid issue is one of my biggest gripes with fender amps. Going back and forth on this one. Essentially I have 2 bandmasters, one bone stock and one that I use for mods; this way I have a point of reference when actually do the mods.

I am thinking about turning the modded one into a single channel amp. This would leave an extra preamp tube if I wanted to add a cathode follower. Do you have an specific resources/information on the sky liner tonestack?

[Jake1524]

If I wanted to raise B+ in the whole preamp, would I reduce the plate load resistors at every gain stage? How high could I actually take the voltages within reason?

No, reduce the dropping resistor connecting he screen node to the PI node. Easy to try by paralleling another resistor. Another 10k in parallel will raise the whole preamp 40V or so. If you reduce it too much you will lose the decoupling effect, so maybe keep it at 1k or higher.

Ok, thanks for this.

So to clarify:

-Reducing Dropping resistors - Lowers entire preamp voltage
-Reducing Post PI Grid leads - This will attenuate the signal going into the Power section - How would this be different than a PPIMV?
-JD0 also mentioned reducing the plate load resistors at the PI - What is the specific effect of this?


Just wanted to say thanks again for everyone who has taken the time to respond to this. It has been great.

[martin manning]

-Reducing Dropping resistors - Lowers entire preamp voltage

No, reducing a dropping resistor value will increase all of the voltages downstream. Lowering the first one following the choke will raise the preamp and PI supply voltages and give you more headroom across the preamp and PI.

-Reducing Post PI Grid leads - This will attenuate the signal going into the Power section - How would this be different than a PPIMV?

Both will reduce the drive signal into the power tubes, but lowering the grid leaks will load the PI and reduce headroom, shift its AC load line, and reduce bass. PPIMV does none of that.

-JD0 also mentioned reducing the plate load resistors at the PI - What is the specific effect of this?

That reduces the PI gain and lowers the drive signal into the power tubes.

[Jake1524]

If I wanted to raise B+ in the whole preamp, would I reduce the plate load resistors at every gain stage? How high could I actually take the voltages within reason?

No, reduce the dropping resistor connecting he screen node to the PI node. Easy to try by paralleling another resistor. Another 10k in parallel will raise the whole preamp 40V or so. If you reduce it too much you will lose the decoupling effect, so maybe keep it at 1k or higher.

I’m getting around to trying this today! The stock value of that resistor is 1k. So when you said keep it at 1k or higher, I am assuming you are referring to the resistor I am putting in parallel?

[martin manning]

The stock value of that resistor is 1k. So when you said keep it at 1k or higher, I am assuming you are referring to the resistor I am putting in parallel?

Depends on which AB763. Below is the Twin Reverb, with a 1k after the choke. I wouldn't lower that, so you can't do anything for the PI, but you can reduce the 4k7 just after that to raise the preamp voltages. That resistor is dropping 40V, so you can get another 30V or so.

[Jake1524]

The stock value of that resistor is 1k. So when you said keep it at 1k or higher, I am assuming you are referring to the resistor I am putting in parallel?

Depends on which AB763. Below is the Twin Reverb, with a 1k after the choke. I wouldn't lower that, so you can't do anything for the PI, but you can reduce the 4k7 just after that to raise the preamp voltages. That resistor is dropping 40V, so you can get another 30V or so.

Okay, thanks. I had a feeling. How low would you comfortably recommend going with that second resistor?

[martin manning]

I'd say 1k, so you could parallel a 1k3 or 1k5 to get close to that.

[Mark]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

I was wondering if you had a circuit for this sort of thing?

[martin manning]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

I was wondering if you had a circuit for this sort of thing?

See here: https://ampgarage.com/forum/viewtopic.p ... 21#p458121
An eyelet board layout for the bipolar power supply is here: https://ampgarage.com/forum/viewtopic.p ... 08#p365408

[Mark]

Thanks for your help Martin.