Can you please check this custom Hoffman AB763 schematic?

[Wil]

Hi everyone,
Leo_Gnardo is going to kill me but... after four years I'm back at it.
The infamous "Wil's desastrous Deluxe Reverb Project" is back!
Still eager to learn, not in a hurry this time and determined to do it right, something I'm unable to do without your tremendous help.

So here I am, back at it again, with a first batch of questions.
I'd like, in advance, to thanks every person that will take some of his/her time to help me complete this amp project.

Here we go:

You will find below the original Hoffman Ab763 with 6G16 tremolo unit with a couple of modifications.
- Custom 68 Deluxe tone stack on normal channel
- Reverb and tremolo on both channel (don't take the normal channel jumper into account, it's a cut wire)
- Reverb noise reduction
- NFB Resistor switch
- Master volume
- Solid state rectifier with switchable sag resistor.




Here are a couple of questions:
- Do you see any mistakes in this customized schematic?
- What is the purpose of isolated inputs and how does it affect my grounding scheme?
- What grounding scheme do you recommend for this kind of build?
- Is the orientation of the PT important (180° rotation, still fitting in the original chassis hole)
- What is the best filter caps values for 235VAC mains (sounds like a silly question since they're after the PT...)? What grounding scheme should the filter caps adopt?
- Where is the best/worst place to put Carbon Comp resistors? I'd like to put some in the most crucial area because I kind of believe they have some kind of mojo when used right.


Thank you very very much!
Have a good day,
RW - from Belgium

[nosaj]

Where is the best/worst place to put Carbon Comp resistors? I'd like to put some in the most crucial area because I kind of believe they have some kind of mojo when used right.

As far as the above quote. Where ever someone suggests that you put them. Do your self a huge favor and try carbon film and even metal film in those locations. Then you can find out if your belief is true or not.
The best mojo comes from the hands and soul of the one with the tools be it guitar, bass, drums. Give any famous guitarist a crappy setup and they will still have that one thing you want but can't buy.

nosaj
Good luck on your build just don't let perfection get in the way of getting something done. You can have great sound with semi inferior parts.

[the fatch]

Assuming that your power transformer is specified for this amp and is designed to operate from European 235V AC, after the PT everything in the rest of the amp is the same as the US version, the only difference being the frequency of the AC, 50Hz rather than 60Hz. The filter caps specified to smooth the 120Hz ripple of the US version should easily cope with the 100Hz ripple due to the 50Hz mains. (American amps for export to Europe are usually identical to the US model except for the power transformer).
The worst place for CC resistors is the grid stoppers and grid leak of the input stage, where the noise that they generate will be amplified all the way through the amp. The resistance of CC resistors changes by a tiny amount as the voltage across them increases, but this becomes significant only at high voltages which suggests that the only place where any additional distortion might be noticeable would be plate resistors late in the amp. However, the effect is so small that it’s likely that any improvement in tone would be in the mind of the listener. The Valve Wizard pulls no punches: “Subjectivists sometimes claim that CC resistors sound ‘warmer’ or more ‘natural’ than the metal films. This is because they add more noise.”
As for grounding, I suggest that you read the chapter in the Valve Wizard’s book, which he has generously made available at http://valvewizard.co.uk/Grounding.pdf

[g1]

Also, R.G. Keen's article on carbon comp mojo here: http://www.geofex.com/Article_Folder...carboncomp.htm

[Wil]

So, one question is already answered:
- Should I use isolated input jacks? YES

From the article on Valve Wizard, as I understood, here's everything I shouldn't do:

Vintage amps often inadvertently created
ground loops by using non-insulating jack sockets, bending over the ground tab on a
potentiometer and soldering it to the case, soldering a ground bus to the backs of the
control pots, or using the chassis as a more-or-less ‘random ground’.

Surprisingly, the buss bar connected to the back of the pots is quite a common design, isn't it?

Would you recommend this diagram?


Regarding the CC resistors. I quite don't get what's written on Geofex. It's written a recommended location is for plate resistors but the opposite is also written as well. Did I miss something?

Can I also ask you to check my master volume mod, to see if everything looks alright to you, please?

Thank you very much.
Have a nice day,
RW

[nosaj]

So, one question is already answered:
- Should I use isolated input jacks? YES

From the article on Valve Wizard, as I understood, here's everything I shouldn't do:

Vintage amps often inadvertently created
ground loops by using non-insulating jack sockets, bending over the ground tab on a
potentiometer and soldering it to the case, soldering a ground bus to the backs of the
control pots, or using the chassis as a more-or-less ‘random ground’.

Surprisingly, the buss bar connected to the back of the pots is quite a common design, isn't it?

Would you recommend this diagram?
[ATTACH=CONFIG]49088[/ATTACH]

Regarding the CC resistors. I quite don't get what's written on Geofex. It's written a recommended location is for plate resistors but the opposite is also written as well. Did I miss something?

Can I also ask you to check my master volume mod, to see if everything looks alright to you, please?

Thank you very much.
Have a nice day,
RW

Just this part at the end

Use CC's sparingly - only where your personal ears tell you that they make a difference.

I'm always amused at people who advertise putting carbon comp resistors in their 9V powered effects to give them some kind of magical vintage sound. Urban legend is tough to kill, though - and magic mojo always makes for dynamite advertising copy.

So now you know what's happening, and something of the numbers involved. The effect is real, though slight.

R.G.

nosaj

[loudthud]

Should I use isolated input jacks? YES

From the article on Valve Wizard, as I understood, here's everything I shouldn't do:

Vintage amps often inadvertently created
ground loops by using non-insulating jack sockets, bending over the ground tab on a
potentiometer and soldering it to the case, soldering a ground bus to the backs of the
control pots, or using the chassis as a more-or-less ‘random ground’.

Surprisingly, the buss bar connected to the back of the pots is quite a common design, isn't it?

Would you recommend this diagram?
[ATTACH=CONFIG]49088[/ATTACH]

That grounding scheme is not the best, but it will work OK. What makes it work is the split in the grounds on the filter cap board (A and B) and the resistors in the B+ line isolate the hum and buzz upstream. What would be better is to isolate all the grounds of all the filter caps and route a ground wire to the stage where the DC from that cap is used.

Isolating the jacks only makes a minor difference. I think the Hoffman layout intends the buss bar to not be soldered to the back of the pots. If you are going to solder a buss bar to the back of the pots, you might as well ground the input jacks to the chassis unless you want to isolate all the pots from ground.

[Justin Thomas]

. I think the Hoffman layout intends the buss bar to not be soldered to the back of the pots. If you are going to solder a buss bar to the back of the pots, you might as well ground the input jacks to the chassis unless you want to isolate all the pots from ground.

Correct. Note in upper left corner: only to be connects to a PT bolt.

Justin

[Wil]

Thank you for the answer.
Regarding CC comp, I think I'll expriment with one channel carbon comp and the other carbon films.

- Do you think relocating the input jacks at the back of the amp is a smart idea?
The signal path to the preamp tube would be much shorter and that would leave me with two hole in the front panel to implement some extra potentiometers (like a presence pot).

Have you heard avec the "Rivera" Studio Deluxe Reverb?
It has a "fat" mod, a 6 position rotary switch which appear to modify cap value is some specific area.
It has a master volume, a presence knob. That could interest me as well!

and another video of the man behind the amp explaining what's inside:

https://www.youtube.com/watch?v=6E91tfguNtI&t=639s

[Mike Sulzer]

Also, R.G. Keen's article on carbon comp mojo here: http://www.geofex.com/Article_Folder...carboncomp.htm

I do not think that there is any point in generating a small amount of second harmonic with a resistor when you have triodes that do it better. You can affect the sound by changing the bias on tube more than by putting all CC resistors.

[Wil]

I think I'll make a separate thread regarding the Rivera 68' Deluxe so people looking for it on Google could find all relevant information on it in one single place.

[g1]

I do not think that there is any point in generating a small amount of second harmonic with a resistor when you have triodes that do it better. You can affect the sound by changing the bias on tube more than by putting all CC resistors.

Absolutely. But I don't think anyone was advocating CC resistors. I got the impression that R.G.'s article was intended to shoot down the CC mojo idea with real observable reasons. Part of being balanced and fair meant that he did have to acknowledge the minuscule effect in some positions. In post #3, the fatch mentioned the same general idea. Personally I think CC's are horrible. But some are going to insist on them, and for those people it would be nice if they at least know where they might do that tiny bit.

[Randall]

Don't believe the hype when it comes to carbon comp resistors. I had an expensive tweed twin build in my shop recently in which the builder included vintage used coupling caps and modern carbon comp resistors. I only measured 4 or 5 of the resistors, and 2 of them in the bias circuit were drifting with heat. And 5 of the 6 Astron caps were leaky, one of them really bad. So put in carbon films and new Mallory 150s and the amp left with a lot more mojo than it came in with. It worked, the bias was stable, the tubes weren't red plating, and the pots were not scratchy.