fender Deluxe reverb - Would like opinion on couple of mods

[LoveChet]

I have a 1979 silverface deluxe reverb with some mods I don't understand. It could be possible that what I see as a mod is in fact a stock feature of my year model.

1. There is an added 2 meg resistor from channel 2 preamp stage 1 plate to ground so it is across the entire tone stack.
2. channel 2 preamp stage 2 has a 1.5 k cathode resistor as opposed to what I see as 800 ohms on AB763 diagram.
3. The V4 mix HT appears to use the B+325V as opposed to the B+4 280v as shown on AB763 disgram.

The following are mods that I do understand and assumidly they might be related/necessary for the mods above but probably not.
1. Six position fat switch with various cap values in addition to 25 pf from channel 2 preamp stage 1 plate as it feeds the treble pot.
2. Master volume
3. Added 12AX7 wet and dry stages for FX loop between mix and PI.
4. Wiggle tremolo against 6V6 output tubes instead of preamp tremolo.

Thanks

[g1]

It is not an AB763, so the first 3 items are design changes, not mods.
Refer to the correct schematic for the 79 silverface.
Others here may have comments about the purpose of those design changes.

[Chuck H]

1. There is an added 2 meg resistor from channel 2 preamp stage 1 plate to ground so it is across the entire tone stack.

Well,.. A two meg resistor isn't going to tax the DC voltage significantly and the impedance at the plate pin itself should be low enough that the resistor isn't doing anything at all. Try lifting it and see if you even notice anything. If it's connected as stated then I doubt you will.

2. channel 2 preamp stage 2 has a 1.5 k cathode resistor as opposed to what I see as 800 ohms on AB763 diagram.

It's possible that someone separated the shared cathodes of V1B and V2B. Look to see if these stages each have their own cathode circuit. It's a "popular" modification because it's "believed" that making these cathode circuits independent improves clarity. I'm not at all sure how that should be. Both stages are Class A and the cathode is fully bypassed. Even if both channels are used at the same time there shouldn't be any interaction at all. That said... In my own designs I separate them

3. The V4 mix HT appears to use the B+325V as opposed to the B+4 280v as shown on AB763 diagram.

This might not be a bad thing. You'll notice that by providing DC to that tube from the 280V node that it puts like phase series stages on the same node. This is actually common, but considered less than ideal for higher gain amplifiers. Which guitar amps (even vintage ones) are. The upside to this mod is your amp is less likely to suffer instability at the first sign of filter cap degredation. I have no problem with this change.

1. Six position fat switch with various cap values in addition to 25 pf from channel 2 preamp stage 1 plate as it feeds the treble pot.

Wait... What?!? Why the hell is that there? Just to add another 25p to the cap value? Then again, I once added an extra 3n3 cap to a 22n cap for the midrange in a tone stage just because it "sounded right" to cut a little deeper. I think 25pf added to the treble cap would have about the same audible impact so I'll chalk it up to tweakers discretion. That or the treble cap may have tested on the low end of it's spec and the mod is there to achieve something closer to the stock value? I guess either way this is alright by me.

2. Master volume

I never liked pre PI master volumes in vintage Fender topography amps. YMMV and it's up to the player to decide what THEY like for what THEY do.

3. Added 12AX7 wet and dry stages for FX loop between mix and PI.

This is what it is. If you want an effects loop you need to add amplifiers and put it right where yours is. Not much more to say except that the amp already has reverb and tremolo and most other effects you might want can be effectively implemented with a pedal. In this light the added series stage only add noise floor. Again, YMMV.

4. Wiggle tremolo against 6V6 output tubes instead of preamp tremolo.

This is a touchy one for me. The DR runs a relatively high voltage on it's 6V6's. IMHE it's hard to get really good bias wiggle trem when you have to run the power tubes at higher voltage/lower current to keep them in spec. Maybe that's just my experience though. The best bias vary trems seem to run the power tubes on the hotter end of bias current at idle. This is impossible when you start with 420V on the plates of your 6V6's. I think it might be the "mojo" factor that follows bias vary trem over the photocell type that inspired this change. But if the trem sounds good then it's a non issue I suppose. Whether it sounds good is up to you.

You're relatively new here. Everyone that's been here a while knows better than to ask "me" for an opinion

[LoveChet]

Thanks for all the feedback. As far as your comments about what a particular mod is supposed to accomplish (or not accomplish) I really can't disagree since I have nothing to compare to. My only benchmark is that if it has good tone then I like it. I have nowhere near the expertise or experience that you on the forum have even tho I used to be an electronic tech.

I could go into a lengthly speel about my love hate relationship with this Deluxe however I should save that for a different forum. However in summary I now realize that a Deluxe can sound great but it needs to be in good working order. It took me lots of years to realize that. It needed all new tubes, 6V6 bias set properly (20 not 50), and some tube socket replacements. It also needed to be slanted backwards (duh). In addition to that I think the mods I have finally put the tone over the top. The FX loop in particular allows me to use a Stryman reverb (Plate Reverb setting) since I never liked the spring reverb. Also a 31 band Eq in the FX loop allows me to boost the 500 and 630 bands to fill in what is missing when trying to gets Chets tone in 'I Love How you Love Me'. The 6 position fat switch on the preamp treble although I am not sure exactly how its wired, it sounds much fuller at one end of those settings. Incidently, when I activate the fx loop (by pulling the return volume control), it comes alive with a much fuller sound, hard to expain.
Paul Yandel asked Rivera to mod a Deluxe then I am assuming I didnt go wrong by what Rivera did for me.

Yes, my amp has the channel 1 and 2 cathode circuits separated. Oddly the wiring looks stock.

I'm not finding any schematic that has the preamp 2 meg added, the 1.5 k instead 800 on ch2 stage 2 cathode res, or the higher Mix HT so I am 'assuming' that Rivera did all that when he did all my mods I asked for. Reason being it appears he did other mods that I didnt explicitly ask for such as wiggle tremolo. I suppose he could have separated the cathode circuits as well.

Incidently I havent traced out how he did the Master volume yet.

Thanks again

Gary - Westerm Mass