If you could help me out with this topic it would be great. I have heard that a blackface deluxe has a bright cap on channel 2 and no bright cap on channel 1, and that some people decide to disable the bright cap on channel 2 to get that sound but with the reverb. How is a bright switch different than this bright cap, is a bright switch just a switch which will engage/disengage the bright cap? And I am aware that this bright switch is different than a presence control, but how does the presence control work in the circuit? Would there be a bright cap and a presence control or would there be no bright cap with the presence control. My goal is to have a custom build deluxe reverb with a presence control and I am trying to figure out how to lay out the circuit and controls.

Thanks for your help

The bright cap on the Deluxe Reverb is 47pF and is hardwired across the volume pot of the vibrato channel. The Fender amps with the bright switch use a 120pF cap switched in or out across the volume control. The 47pF cap gives a more subtle treble boost than the 120pF cap. Which (if either) of them to choose can depend on a number of factors, but any bright cap across the volume pot has less effect the higher the volume is set; it has no effect at all with the volume on 10.

A Fender's presence control works on the opposite end of the amp, manipulating the negative feedback injected into the phase inverter from the output transformer. They were discontinued by the end of the brownface era and replaced with the volume pot bright cap with the blackface amps. Using both would be pretty redundant functionally, although they can sound somewhat different.

So how did the brown face amps utilize the presence control, i would assume they just had a standard volume pot without a bright cap and then had a presence control? Is this correct?

If I had a deluxe reverb and detatched the bright cap on the volume pot of channel 2, leaving channel 1 alone because it does not have a bright cap, and had a presence control installed would it (the presence control) behave as it does on a brown face amp? In this case the presence control would shape the attack of the amp instead of the bright switch/cap shaping the attack of the amp- this is what I am going for, does it sound about right? Thanks

Yep, the presence control was always located on a separate pot. I don't recall any Fenders having both that and a bright cap on the volume pot.

Adding a presence control to a Deluxe Reverb would require modification of the stock phase inverter circuit. You could just copy it (including the drastically different feedback resistor) off the schematic of the brownface Pro (6G5-A) or Vibrasonic (6G13-A), but note both use a 12AX7 there rather than a 12AT7 (either will function but the12AX7 is better suited to that circuit). It would work like a hybrid of a Deluxe Reverb preamp and a brownface power amp section with 6V6's. It sounds like a good combo actually...

This is very interesting, good info. Thanks!

If you have an A763 (not AB763) circuit BFDR like mine (S/N A 00957), you'll find that removing the RESISTOR on the volume pot disables the tone controls completely. This makes the amp sound like a miniature Marshall, which is not nearly as desirable as you might think.

Thanks michael fried, you have just the perfect info I was looking for. I am actually trying to emulate a blonde bassman which I had heard that the normal channel of a blackface deluxe was very similar to a blonde bassman. In this case it sounds like I can copy the phase inverter circuit from the blonde bassman 6G6-B and add a presence control to my deluxe reverb build and I will be just about there. I am trying to get a low wattage blonde bassman, I have posted the link to another thread I made a while trying to figure out how to materialize my low wattage bassman. http://gretschpages.com/for... Think this would work?

Thanks michael fried, you have just the perfect info I was looking for. I am actually trying to emulate a blonde bassman which I had heard that the normal channel of a blackface deluxe was very similar to a blonde bassman. In this case it sounds like I can copy the phase inverter circuit from the blonde bassman 6G6-B and add a presence control to my deluxe reverb build and I will be just about there. I am trying to get a low wattage blonde bassman, I have posted the link to another thread I made a while trying to figure out how to materialize my low wattage bassman. http://gretschpages.com/for... Think this would work?

Cheers, glad to be of help! You're on the money with your plan - the only thing you'd need to change using the Bassman phase inverter is the 56k feedback resistor - it's for a 4-ohm speaker load vs. the Deluxes' (normally) hm load. That's why I specified the two brownface amps that I did as reference, they use 100k there for their hm load. Please let us know how this works out!

Um, I see that the board's software converted my impedance info into Yikes emoticons! Those both should read "eight ohm load"...

Sorry for the multiple posts but I haven't figured out how/if I can edit these things once they're up...

Looking again at the 6G6-B schematic, I just noticed another difference between the blonde Bassman and the Deluxe's normal preamp channels, one that is likely as significant to their respective sound as the phase inverter is - their second gain stage (fed by the volume control wiper) uses different cathode and plate resistor combinations. The Deluxe uses the common 1.5k cathode/100k plate combination, while the Bassman uses 2.7k and 220k respectively. This will change their breakup and compression characteristics somewhat, especially at higher volume settings. (The Bassman would compress more and distort less into the phase inverter, other things being equal.)

This project has my wheels turning now.... I think I'll convert the normal channel of my Deluxe and see what happens!

The treble control on the blondes is a 350k pot with a 70k tap. The blackfaces use a regular 250k pot.

Big enthousiastic thumbs up for presence control!

In the blonde-inspired amp I just completed (more or less), I could use a 33k NFB resistor without oscillations. That made the presence control a little bit more dramatic.

Here is the link: http://gretschpages.com/for...

Let me know how it turns out Michael, I will be sure to post when i get mine. Could the deluxe 1.5k cathode/100k plate combination in comparision to the Bassman 2.7k and 220k be in response to the speaker combination or power handling? I am really pumped about this amp, I am planning on having a custom build considering there are lots of modifications to be doen anyway- and I am going to do blonde and wheat with the white knobs so it looks like a brown face. And it does not sound like there is a lot to change; just the presnece control pot and cap, changing the second gain stage cathode and plate resistor to 2.7k and 220k- Are these 2 different resistors?, changing the feedback resistor to 100k, and changing the PI tube to a 12AX7. Is this correct? One more question I had; are there different cathode and plate resistor's and (PI) feedback resistor's for both the normal and vibrato channels or are they just for the whole amp and affect each channel?

Michiel, you're correct about the 350k/70k pot, and adding a 82k resistor in series with a grounded .0047uF cap to the "bottom" of the 250k treble pot (the lug connected to the 0.1uF tonestack cap and the "top" of the bass pot) will duplicate that (close enough for 20% tolerance work, anyway). That does revoice the tonestack slightly. I'll A-B with the stock Blackface circuit - it might be worth putting the option on a simple SPST switch! Good catch!

Tennessean, there are actually a couple of other differences in the two phase inverter circuits - the Deluxe AB763 circuit uses 27k and 470 ohm cathode/load resistors as opposed to the Bassman 6G6-B's 6.8k and 820 ohm resistors, respectively. The different preamp cathode/plate resistors just give a different gain response to the preamp - i believe it was probably what their ears were looking for in that preamp circuit as it was in other blonde and brown models at that time as well. Those are two different resistors, and on the Bassman they are separate for each channel.

Also, on the Deluxe Reverb that second preamp stage shares a common 820 ohm cathode resistor, so that needs to be separated out for each channel - one 1.5k resistor for whatever channel would be left "stock" and the 2.7k resistor on the Bassman-modded channel. (You also need to "untie" the two cathode pins of the tube socket if they're connected.) BUT if you're modding both channels, just change the 820 ohm to 1.5k and both 100k plate resistors to 220k. Each channel has its own plate resistor on both amps although they're connected at their supply end.Clear as mud? Not quite as simple, but there are much more complicated mods...

There are differences also in the channel summing resistors (Bassman 2x470k, Deluxe 2x220k) and phase inverter input coupling cap (Bassman 500pF, Deluxe .001uF), but I wouldn't worry about them unless you'd actually use both channels simultaneously with two different instruments (a more common practice back in the day). If by chance you actually would, change both the resistors and the cap to maintain the same bass-end response. Good luck, stay tuned!