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AB763 circuit Master Volume Mod Reverb/Delay Processors (HW)
Old 19th January 2019
  #1
Gear Head
 

AB763 circuit Master Volume Mod

Hi,

I'd like to add a master volume to my 64 Deluxe Reverb. I found this site a few months ago:

AB763 Mods by Robinette, Master Volume Mod

I sent the author of the site an email asking for help, but I've yet to hear back from him. So I come to you experts now.

I'm confident with a soldering iron, I'm sure I can tackle the project. I have some questions tho, I want to make sure I do this right before I go destroying a great amp.

1. I'm familiar with schematic symbols and signal flow, but I still struggle relating a specific area of a schematic to it's real-world physical location. In other words, if y'all could describe (or even have a picture on hand) where exactly the two wires reside in the amp for this mod, that would be super helpful.

2. The most convenient place for the pot is the space where the old auxiliary 2-prong outlet resides, I'm sure y'all know that. If I were to rip out the power outlet, what do I do with the leads that terminate at the outlet? Is it dangerous to just put a threaded cap on them (individually of course, not joined)?

3. Related to the previous question, it seems the outlet space is located directly above a transformer. The site suggests coax wire to elminate possibility of interference. I would want to use coax anyways to be safe, but I'm a bit confused where it says to only ground it at signal input. Does that mean only connect it to ground where the leads join the factory circuit? Leave the bare wire disconnected at the pot?

I appreciate any answers at all, thanks ahead for your time
Old 19th January 2019
  #2
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Quote:
Originally Posted by BrassElephant View Post
Hi,

I'd like to add a master volume to my 64 Deluxe Reverb. I found this site a few months ago:

AB763 Mods by Robinette, Master Volume Mod

I sent the author of the site an email asking for help, but I've yet to hear back from him. So I come to you experts now.

I'm confident with a soldering iron, I'm sure I can tackle the project. I have some questions tho, I want to make sure I do this right before I go destroying a great amp.

1. I'm familiar with schematic symbols and signal flow, but I still struggle relating a specific area of a schematic to it's real-world physical location. In other words, if y'all could describe (or even have a picture on hand) where exactly the two wires reside in the amp for this mod, that would be super helpful.

2. The most convenient place for the pot is the space where the old auxiliary 2-prong outlet resides, I'm sure y'all know that. If I were to rip out the power outlet, what do I do with the leads that terminate at the outlet? Is it dangerous to just put a threaded cap on them (individually of course, not joined)?

3. Related to the previous question, it seems the outlet space is located directly above a transformer. The site suggests coax wire to elminate possibility of interference. I would want to use coax anyways to be safe, but I'm a bit confused where it says to only ground it at signal input. Does that mean only connect it to ground where the leads join the factory circuit? Leave the bare wire disconnected at the pot?

I appreciate any answers at all, thanks ahead for your time
I can tell by your questions that you have no business working on this amp.

Pack the amp up and take it to a tech and describe what you want to do....
Old 19th January 2019
  #3
Gear Head
 

Quote:
Originally Posted by UKMK00 View Post
I can tell by your questions that you have no business working on this amp.

Pack the amp up and take it to a tech and describe what you want to do....
Its two wires and a pot, it's stupid simple, I just wanna make sure I'm putting the wires in the right place, and choosing a good spot for the pot to reside.

But thanks for your encouragement!
Old 20th January 2019
  #4
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Quote:
Originally Posted by BrassElephant View Post
Its two wires and a pot, it's stupid simple, I just wanna make sure I'm putting the wires in the right place, and choosing a good spot for the pot to reside.

But thanks for your encouragement!

And another thing...Mr Robinette was nice enough to include a wiring diagram which shows exactly how to wire this mod. It could not be more precise and direct.


Do yourself a favor and take the amp to a tech.

High voltage is no place for hubris...
Old 21st January 2019
  #5
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Radardoug's Avatar
 

It might look stupid simple, but it probably isn't.
Old 21st January 2019
  #6
It is very simple. You lift the cap before the 12AT7 driver tube and wire in a 250k volume pot between it and the junction of the two 220k mix resistors.
Old 10th February 2019
  #7
Here for the gear
 

64 deluxe

are you nuts it don't need a MV don't drill holes in it go buy a 5 watt amp anything but a MV in your vintage amp crazy fool
Old 11th February 2019
  #8
You don't need to drill a hole or permanently modify the amp. Some use input jack #2 to fit it. Is it a tool or an investment?
Old 16th February 2019
  #9
Gear Head
 

Quote:
Originally Posted by Jim Williams View Post
You don't need to drill a hole or permanently modify the amp. Some use input jack #2 to fit it. Is it a tool or an investment?
This is the most helpful post here so far...

Yes, its just a tool.

Quote:
Originally Posted by Bobtec View Post
are you nuts it don't need a MV don't drill holes in it go buy a 5 watt amp anything but a MV in your vintage amp crazy fool
I understand its a piece of history, an icon, blah blah blah

To me, its just (an awesome sounding) piece of electronic equipment and I'd like to mod it to fit my needs. Its MY amp, not yours.

I came here to ask how to do something...
not to ask if I should do it...

Jim, thank you for your level-headed input.

I'm no tube-amp-expert-technician, but I figured this mod was simple enough to handle, and to get my foot in the door towards becoming confident with this kinda stuff.

I (errantly) came here for some encouragement and some specific guidance because I'm a very visual person, and not good (yet) at matching schematics to real world physical locations

I came here to learn, but everyone (not you Jim) just decided they know what's best for me and my gear haha. Classic slutz I guess.
Old 16th February 2019
  #10
Gear Nut
 

Hate to tell you this, but that guy's master volume mod/add ins are incorrect.

That's probably why you don't hear from him.
Old 4 weeks ago
  #11
Lives for gear
The Master Volume is actually implemented the same way it is done in the Matchless DC-30 amps, and it does work through cancellation of the two signals that are opposite/inverted to each other.
Old 4 weeks ago
  #12
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Quote:
Originally Posted by audiospecific View Post
Hate to tell you this, but that guy's master volume mod/add ins are incorrect.

That's probably why you don't hear from him.
Why is it incorrect? Looks like it will work.
Old 3 weeks ago
  #13
Gear Nut
 

Quote:
Originally Posted by UKMK00 View Post
Why is it incorrect? Looks like it will work.
This is the only master volume mod I see that is correct.

The change to the attachment would be going with two 1 meg resistors in series (2M) instead of a 1.8M wiper stabilizer.
Attached Thumbnails
AB763 circuit Master Volume Mod-type-2orlar-mar_master_volume_schematic.jpg  
Old 3 weeks ago
  #14
That circuit is dependent on matching pot elements. Standard dual pots with a linear taper are 20% tolerance. It's worse with log taper pots. The phase inverter is dependent on matched inputs, a matching a 20% dual pot will never have. Special order 10% pots are available from Bourns with a large minimum order.

Even if you get a pot to match at one setting, move it and that match will go away. It's like trying to push down a Ping-Pong ball in water with your thumb.

A stepped switch with 1% resistors will work. So will a single element pot before the phase inverter like all the other companies do.
Old 3 weeks ago
  #15
Gear Nut
 

Problem is that its not a master volume control. when it gets adjusted, yes the signal goes down, but the bias current would increase.

This technically is the right way, however, just have to add 180K-200K resistors in series before the input of this so the previous stage maintains a decent load.
the caps would be twice the value the two coupling caps are now. the control is a simple 500K pot. two 500K resistors can be added from wiper to the outside terminals of the pot to reduce wiper noise.

Either that, or make it more like a VCA and have a low and high set point for the bias and the operator adjusts between those set-points.
Attached Thumbnails
AB763 circuit Master Volume Mod-screenshot-2019-03-02-03-57-03.png  
Old 2 weeks ago
  #16
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Quote:
Originally Posted by BrassElephant View Post
1. I'm familiar with schematic symbols and signal flow, but I still struggle relating a specific area of a schematic to it's real-world physical location. In other words, if y'all could describe (or even have a picture on hand) where exactly the two wires reside in the amp for this mod, that would be super helpful.
Jim is correct -- what is most commonly called a "master volume" control on a guitar amp is installed in between the preamp and power-amp sections, and this is how Fender did it in the CBS era. On the AB763 chassis, this point is in between the .001uF ceramic-disc capacitor and the pin 2 grid of the 12AT7 driver tube. To install, disconnect the end of the cap that's closer to the back of the chassis, and wire the now-disconnected cap's leg to the input of a potentiometer, and then the pot's wiper connects to the point on the tagboard where the cap was connected originally. Connect the pot's ground to the tagboard at the same grounding point as the 47 ohm feedback resistor - this is directly across the tagboard towards the front of the amp.

I think you'll need a 1 Meg audio-taper potentiometer to avoid loading the channel-summing resistors excessively, and you can disconnect the 1 Meg resistor that goes between the 12AT7's pin 2 grid and ground.

Quote:
Originally Posted by BrassElephant View Post
2. The most convenient place for the pot is the space where the old auxiliary 2-prong outlet resides, I'm sure y'all know that. If I were to rip out the power outlet, what do I do with the leads that terminate at the outlet? Is it dangerous to just put a threaded cap on them (individually of course, not joined)?
I don't like this location for three reasons:

1. It's a bit clumsy for wiring . . . the AC socket is actually used as the connection point for the power cord, so at the least you'll have to remove the cord's strain relief, strip back more of the jacket to make the leads longer, reinstall the strain relief, and connect it directly to the switch and/or fuse holder. You'll also need to fabricate a small metal plate to mount in place of the socket, in which you can mount your volume pot.

2. This is some of the "hummiest" real-estate inside the amp, as it's next to the power cable, power transformer, rectifier tube, and filament wiring . . . and using shielded wiring only helps with electrostatic interference, not electromagnetic, so it's of limited effectiveness here.

3. Of all the wiring inside the amp, a fault in the primary-side power wiring (between the power cord and the power transformer) is the most likely to be able to cause a shock hazard, so only dig into this if you're completely confident about safe wiring practices.

I'd suggest a location made by removing something you're not using, that's already the right size for the volume pot. Jim's suggestion of an input 2 is a good one . . . the "extra speaker" jack is also a good candidate, and perhaps a bit easier to get to.

Quote:
Originally Posted by BrassElephant View Post
3. Related to the previous question, it seems the outlet space is located directly above a transformer. The site suggests coax wire to elminate possibility of interference. I would want to use coax anyways to be safe, but I'm a bit confused where it says to only ground it at signal input. Does that mean only connect it to ground where the leads join the factory circuit? Leave the bare wire disconnected at the pot?
While it won't help with the magnetic interference from the power transformer, using shielded wiring still has some merit. I'd use two pieces of shielded coax (i.e. cut apart a cheap siamese stereo RCA cable), and connect both shields together at both ends. You then have a three-conductor cable that's suitable for the three terminals of your volume pot . . . just make sure the shields are used for the ground connection.

----------------------------------------

Having said all that, I have to say that I don't like master-volume mods to Fenders. The first problem is that they don't have enough gain in the preamp to get satisfactory overdrive and distortion at this point in the amp - when CBS added master volume controls, they used the reverb drive tube to add some switchable extra gain, but even that wasn't enough compared to i.e. a Marshall or a decent gain pedal. The second issue is that given the choice of operating point of the preamp tubes and the values in the tone stack, distortion caused by overdriving a Fender preamp is inherently kinda flat and muddy sounding. I think there's a reason why the market value of the Silverface Master-Volume Fenders is consistently much lower than the equivalent earlier models.

If the main point of your mod is to be able to have the crunch/distortion character of your existing amp, but be able to get it at a lower volume level, then one of the loadbox/power-attenuator products on the market will do a far better job. They aren't cheap, because they need to be made in such a way so as to dissipate some of the amp's output power. But since the real character of the AB763's distortion comes from overdriving its power-amplifier output stage . . . you'll never get that same tone from merely overdriving the preamp.

If on the other hand you're trying to get an overdrive/distortion that's a completely different character from what you already have . . . then you simply need to find the right distortion pedal . . . there are so many on the market these days that there's bound to be one that you like.
Old 2 weeks ago
  #17
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Quote:
Originally Posted by audiospecific View Post
Problem is that its not a master volume control. when it gets adjusted, yes the signal goes down, but the bias current would increase.
Methinks you need to re-read the circuit diagram.

Not that I'm a fan of the Trainwreck-style "master-volume" design; it has many issues. Being within the NFB loop of the power amp, it will affect output impedance as it's varied, and the additional wiring to the pot will negate or complicate the proper functioning of the grid-stop resistors. But . . . the DC grid bias on the output tubes will remain unaffected, whether it's in a cathode-bias circuit or a fixed-bias circuit.
Old 2 weeks ago
  #18
Gear Nut
 

Quote:
Originally Posted by kirkus View Post
Methinks you need to re-read the circuit diagram.

Not that I'm a fan of the Trainwreck-style "master-volume" design; it has many issues. Being within the NFB loop of the power amp, it will affect output impedance as it's varied, and the additional wiring to the pot will negate or complicate the proper functioning of the grid-stop resistors. But . . . the DC grid bias on the output tubes will remain unaffected, whether it's in a cathode-bias circuit or a fixed-bias circuit.
no error. The bias feed resistor that was in the circuit was replaced by a potentiometer its function has always been to limit current flow from the bias supply.

Now if the bias was self bias circuit, the resistor from the bias rail (commonly the dc ground) to the grid sets up the bias potential.

I know what they are trying to accomplish. Which is the same mod I used to do for people over 20 years ago. But they went about it wrong. Because I varied the bias voltage, and its not just a simple cookie cutter modification either.
Old 2 weeks ago
  #19
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Take a look at the diagram below (as shown in the OP's link) . . . both sides of the potentiometer are at the same DC potential. Of course this is shown in a cathode-bias circuit, so both grids are at 0VDC with respect to ground, but if the grid-leak resistors are fed by a negative voltage in a fixed-bias amp . . . both sides of the pot are still at the same DC potential. Therefore there's no virtually no DC current that flows through the pot, and by consequent it cannot affect the DC bias.

The thing that makes this really NOT a "master volume" mod is the fact that it changes the open-loop LF bandwidth of the circuit as it's adjusted; that is, rolling off the bass as the volume is turned down. Presumably the designer felt that this was more of a feature than a flaw, as the Trainwreck amps I've worked on always struck me as being a "twiddled and voiced" type of product. I have no problem with that in a boutique amp . . . but if you install this control on a Fender, it takes the instrument very much away from being a Fender, and being something else . . . and in my experience, that's not what most people who play Fenders are looking for. I think it makes even less sense now (as a Fender mod) than when I first saw the Trainwrecks in the 1990s, as there are so many boutique amps on the market, and resources for building your own . . . that you don't have to use a Blackface Deluxe as a starting point.

I'm also suspicious of the motivation for reducing the AC grid impedance on the power tubes - this is frequently justified by a discussion of "blocking distortion", which in and of itself does not exist. I've read the armchair circuit analysis that supposedly causes the phenomonon several times, and the reality is that if one measures and observes bias behavior of an AB763 as it transitions in and out of heavy clipping . . . the "blocking distortion" postulation is actually the opposite of what's going on.
Attached Thumbnails
AB763 circuit Master Volume Mod-5e3_type-3_mv_mod_schematic.jpg  
Old 2 weeks ago
  #20
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Radardoug's Avatar
 

Thats just a really stupid circuit. It would compromise the action of the grid stoppers. Much better to replace the 220Ks to ground with a conventional dual pot.
Old 2 weeks ago
  #21
With a 20% tolerance?
Old 2 weeks ago
  #22
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Quote:
Originally Posted by Radardoug View Post
Thats just a really stupid circuit. It would compromise the action of the grid stoppers. Much better to replace the 220Ks to ground with a conventional dual pot.
Agreed; stupid. In my opinion, it's much better to give some thought to whether having variable signal attenuation at this point really has a chance of improving the particular amp's tone/versatility/musicality . . .
Old 2 weeks ago
  #23
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Quote:
Originally Posted by Jim Williams View Post
With a 20% tolerance?
Of course a single pot before the phase splitter would be even better.
Old 1 week ago
  #24
Gear Nut
 

For the cathode bias output, the more logical approach would be to rework the circuit so the output tubes operate with a remote cutoff circuit. Like this:
Attached Thumbnails
AB763 circuit Master Volume Mod-f8pxhe5i66csdn7.large.jpg  
Old 1 week ago
  #25
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Quote:
Originally Posted by audiospecific View Post
For the cathode bias output, the more logical approach would be to rework the circuit so the output tubes operate with a remote cutoff circuit. Like this:
Adjusting the screen impedance and/or bias does not in any way resemble the circuit behavior or subjective effects of a master volume control.
Old 1 week ago
  #26
Gear Nut
 

Quote:
Originally Posted by kirkus View Post
Adjusting the screen impedance and/or bias does not in any way resemble the circuit behavior or subjective effects of a master volume control.
Well, about 30 years ago I wanted to call it the "Bias Envelope" adjustment. The purpose of it was to vary the conduction of the tube instead of signal level so that a more of a compressed signal at a lower volume can be accomplished. I did this, then this other dude in Cali copied me a long time ago.

The guitar players nicknamed it "the master volume mod".

Adding a potentiometer to G2 should work, but since I never did it to that to a 6v6 cathode bias, so I never worked out the formula for that control. I'm guessing its (((Rp1||Rp2)+Zout) X amplification factor )X 10) but maybe it needs to be X 100 so either its 50K or 500K. Just be aware it has to handle the current and working voltage, so you would be looking for a 5W @ 500V control to go with that 6V6 circuit.

If you have one of my Marshalls I modded a long time ago and opened it up, you will see that I'm varying the grid bias. I chose this route because its a lower voltage on an external control.

Lifting G2 out of circuit should be the same effect as putting it in standby. As some amps use this as a "mute" or "standby" when G2 is switched out of circuit.
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