Ab763 simplified design

[Lynxtrap]

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.

I may be missing something, but it seems like the schematic doesn't quite match your description..? I see the channels joining at the reverb mixer with no mixing resistors and no use of the reverb circuit?

[martin manning]

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

No problem, that mod to mix the channels is well proven, been around for decades. You have some work left to do in the power supply area, i.e. finding a transformer that will provide the right voltage, and choosing a compatible bias circuit. If your PT doesn't have a bias tap, you can get the voltage from the un-rectified AC as shown. That will not work with a FWB, you need to capacitively couple it in that case.

I see the channels joining at the reverb mixer with no mixing resistors and no use of the reverb circuit?

That's exactly right. Disconnecting the normal channel coupling cap from the 220k and connecting it to he vibe channel output is the Torres mod. Then, since the reverb is not wanted here, I eliminated it, and combined the grid resistances into the 150k.

[Mikante]

I m not following you. About this Torres mod, i don t see how the two channels won t be interacting with each other without mixing, i m not that expert so this is more of a question.
About the PT, i was going to use the standard deluxe reverb, it is also suitable for the tweed deluxe, at least on the paper, so i guess it can be used in this case too.
Here it is, Hammond 290BEX

67277946-A8B3-4AB5-AC7D-F63F15031130.png

About the bias, why should i change anything?

[Lynxtrap]

That's exactly right. Disconnecting the normal channel coupling cap from the 220k and connecting it to he vibe channel output is the Torres mod. Then, since the reverb is not wanted here, I eliminated it, and combined the grid resistances into the 150k.

OK, I misunderstood. I thought it was about utilizing the reverb circuit in some way, but It's a "reverb on both channels" mod.
It's the same as Mikante's own version on the previous page but without mixing resistors. The way I see that, the signal will see AC ground through the other channel's coupling cap with the the caps forming some kind of capacitive divider..?
I would use mixing resistors and maybe lower the 3.3M to keep down the total serial resistance if necessary/wanted.

[martin manning]

There are limits to the copy-paste approach.

About the PT, i was going to use the standard deluxe reverb, it is also suitable for the tweed deluxe, at least on the paper, so i guess it can be used in this case too... About the bias, why should i change anything?

You have eliminated some current draw by dropping the reverb and the trem, and the 20-50V drop of the vacuum rectifier, so voltages will be high across the board with the standard DR PT. I was assuming you would be using some alternative, which may not have a bias tap on the secondary.

The way I see that, the signal will see AC ground through the other channel's coupling cap with the the caps forming some kind of capacitive divider..?
I would use mixing resistors and maybe lower the 3.3M to keep down the total serial resistance if necessary/wanted.

Looking into the output of each stage is not AC ground, it's the output impedance, about 39kΩ. The 220k mix resistors are not needed for the signals to mix. The 3M3/150k voltage division is needed to avoid overdriving the following stage. Adding the 10p makes it look like a fixed volume pot with a bright cap. If the goal is to preserve the DR vibe channel sound with the reverb turned off, I'd recommend leaving that network as-is, at least to start.

[Mikante]

There are limits to the copy-paste approach.

About the PT, i was going to use the standard deluxe reverb, it is also suitable for the tweed deluxe, at least on the paper, so i guess it can be used in this case too... About the bias, why should i change anything?

You have eliminated some current draw by dropping the reverb and the trem, and the 20-50V drop of the vacuum rectifier, so voltages will be high across the board with the standard DR PT. I was assuming you would be using some alternative, which may not have a bias tap on the secondary.

The way I see that, the signal will see AC ground through the other channel's coupling cap with the the caps forming some kind of capacitive divider..?
I would use mixing resistors and maybe lower the 3.3M to keep down the total serial resistance if necessary/wanted.

Looking into the output of each stage is not AC ground, it's the output impedance, about 39kΩ. The 220k mix resistors are not needed for the signals to mix. The 3M3/150k voltage division is needed to avoid overdriving the following stage. Adding the 10p makes it look like a fixed volume pot with a bright cap. If the goal is to preserve the DR vibe channel sound with the reverb turned off, I'd recommend leaving that network as-is, at least to start.

I know but the same transformer is rated for the earlier deluxe design too which only has two channels without reverb and no third gain stage. I assume it can be adjusted. My first thought was to add a further preamp tube so that both channels could have a dedicated tube for the third gain stage and also solve the problem of the phase.
Sure it won t compensate the drop completely.

[martin manning]

I know but the same transformer is rated for the earlier deluxe design too which only has two channels without reverb and no third gain stage...

The earlier Deluxe also uses a 5Y3 rectifier, which will drop about 50 VDC.

[Mikante]

I understand, but the deluxe reverb is also tube rectified. The 5e3 for example, only two preamp tubes, no reverb and no tremolo. Same transformer and larger dropping resistors, won t this solve the problem for me too?
I really don t like tube rectifier.

[martin manning]

You can adjust the preamp voltages easily with the dropping resistors, but IMO it would be best to keep the power tube anodes and screens close to the original, at around 400V.

[Mikante]

Ok, how can i do that?
This is something i m still struggling about, why is this transformer too much, what are the specifications that i should look for?
This Iron is 660on the secondary and 138mA of limit. Beside the 50v output for the bias.

[Lynxtrap]

Ok, how can i do that?
This is something i m still struggling about, why is this transformer too much, what are the specifications that i should look for?
This Iron is 660on the secondary and 138mA of limit. Beside the 50v output for the bias.

Rule of thumb is that the rectified DC voltage will be about 1.4 times the secondary AC voltage of the transformer. In this case, that is half of 660V, so: 330 x 1.4 = 462VDC.
That is about the usual B+ for Deluxe Reverbs, but it does exceed maximum plate voltage specs for 6V6's. You have the option of using a transformer with less secondary voltage or to drop the voltage if you want to run the output tubes within specs.

Your amp will also draw less current than a DR as you have no reverb or tremolo.

[Mikante]

The princeton transformer looks within specs but it has no 50v out for the bias.

CB9E3B13-A098-4B4B-9340-4C109981AC1F.png

[martin manning]

The princeton transformer looks within specs but it has no 50v out for the bias.

That's better, but still a bit high. I'd go with Hammond 372BX. It has a multi-tap primary you can configure to suit your local line voltage.

Rule of thumb is that the rectified DC voltage will be about 1.4 times the secondary AC voltage of the transformer. In this case, that is half of 660V, so: 330 x 1.4 = 462VDC.
That is about the usual B+ for Deluxe Reverbs, but it does exceed maximum plate voltage specs for 6V6's. You have the option of using a transformer with less secondary voltage or to drop the voltage if you want to run the output tubes within specs.

You are right on the approximate DC voltage with solid state rectifier, but the correct DR B+ is ~400V. Note the marked-up schematic below shows 420, and Fender shows the reissue running at 397.

[Mikante]

I see the 372bx has a 50v out, damn it is way more expensive too
For me to better understand, the princeton transformer has 2 secondary and one is 550vac and you said that this one was sill a bit high, i guess you were referring to the 630vac out right? Then why is the 372bx a better choice? Mainly for the 50v out?
I d like to understand because the difference between 600 and 550 is not that much, in my head at least.

[Lynxtrap]

You are right on the approximate DC voltage with solid state rectifier, but the correct DR B+ is ~400V. Note the marked-up schematic below shows 420, and Fender shows the reissue running at 397.

Yes, I wasn't thinking and forgot to take the tube rectifier into account. 460V would not be normal for a DR.
They do tend to read higher than the schematics, but not that high under normal circumstances. I would however be surprised to see a reading below 415V unless biased very hot (I'm not familiar with the reissues).