I have a few question about FPR, I own a
1976, has a Oxford 10j4 speaker (found that out on a site about Amps it's original I'm the only owner) and naturally its a silver face. Whats the difference in a black face compared to a silver face sound wise? Not looking to upgrade just curious.

very little IMHO with that amp.

there may be a few cap and resister changes but ultimately they are very close.

both good sounding amps

found this on the web a step by step

Deluxe Reverb Blackface mods

I have provided a schematic of an AB868 as a representative example of a Silverface DR schematic. This is the model that did not have the "Pull Boost" function. You can look at the AA763 schematic for a complete Blackface circuit of the early pre-CBS models. There was an AB763 version as well that had minor differences (see http://acruhl.freeshell.org... for a good comparison). I will suggest a series of "baby steps" to make incremental improvements to your Silverface DR rather than trying to do a total restoration to the AB763 design. You can stop at any point when you feel the tone is to your liking. Keep in mind that there may be minor discrepancies between your particular model and the schematics provided, but the principles still apply. Now break out those soldering irons!

1. Remove those dumb 1000 ~ 1200pF disc caps that go from pin 5 (Grid) of the 6V6's to ground. Initially, just snip one lead on each cap just in case the amp oscillates afterwards and you want to restore them. Test the amp to see if it oscillates. Most likely, it will be fine. If the amp does oscillate, you can either resolder the snipped leads of the originals caps, or preferably replace them with a much lower value like 470pF and see if the oscillation goes away. If the amp is ok, snip out the caps altogether. Removing (or lowering the value of) these caps will improve the high-frequency response of the amp.

2. Change the .01uF cap that goes to pin 2 (Grid) of the 12AT7 Phase Inverter (PI) tube to .001uF like in the AB763 schematic. I actually used a 1500pF Silver Mica Here. Test the amp and see what you think of the sound now.

3. There seems to be a consensus amongst the experts out there that the Utah and Oxford speakers that came in the DR's generally suck. I would try a new Jensen or Weber speaker next. I finally replaced my Utah with a new Jensen P12N cause I got a good deal at my local music store. It sounds better, but it did not knock my socks off as I had expected from all the hype. Maybe a lower wattage Alnico would have been better. Definitely tighter sounding though.

4. The next easy mods are the 330K resistors to the Grids of the 12AT7. Change them to 1M's per the diagram below. Also change the 680 Ohm resistor that goes to the Cathodes of the 12AT7 to a 470 Ohm. At this point, you should have the equivalent output section to an AB763. My amp already had the 470 Ohm resistor.

5. You can compare the diagrams below for a complete change (not including the rectifier tube difference). I didn't see any reason to remove the 1500 Ohm series resistors to the grids of the 6V6's, or to change the 82K plate resistor on the 12AT7, so I wouldn't bother changing them. In other words, I like the AB763 circuit.

6. The next thing I plan to do is replace all the Ceramic disc caps with either Silver Micas or Orange Drops. My particular DR already had good quality Blue Spragues for coupling caps. They used ceramic discs for the three .1's and the .01 in the schematic below.

What you want to end up with is the AB763 Phase Inverter circuit and replace any crappy caps with good ones. I also plan to add precision 1 Ohm resistors from the Cathodes of the 6V6's to ground and bring them out to test jacks so I can adjust the Bias anytime without having to disassemble the amp.

I have also thought about converting from Fixed Bias to Cathode Bias to get more tube compression, but I think I'll play with this setup for a while and decide. Let me know what your experiences have been. Have FUN!!!

Alpep thanks for your quick response I was just curious about the sound. I had a friend put a 12in speaker in his back in 76 and it sounded a lot better, don't know what type of speaker. But I've toyed with the thought of replacing the speaker in mine with a 12in jensen. My friend told me at the time that the 12in just fit.

You will have to replace or cut on the baffle to install a 12. And at some point, the SFPR goes to a glued in baffle, which makes this mod a b*tch. It will definitely open up the volume though. I have one in my '65 BFPR.

//and I have tried many different speakers in mine - a vintage Jensen C12Q sounded the best so far, but I am trying to find a vintage C12n or C10n to try.

Thanks Spike for that information maybe my friends Princeton wasn't a reverb. do you think that would have made it easier to install, he didn't sound to me that he had that much of a problem with the installation, also he had it for the blues harp (Chicago type blues). Could be harp players don't need reverb.

Also this is only my second post, I like this GDP.

I have a celestian blue bulldog copy in my 61 princeton and it is one of the best speakers i have ever heard.

everyone and I mean everyone comments on that amp and I attribute a good portion of the sound to the speaker

so even though they are expensive if you have the baffle cut for a 12 go with that speaker.

Alpep Thanks again

Alpep - be advised that most of the Deluxe Reverb mods in your post above won't actually apply to either a Princeton or a Princeton Reverb, as they use different phase inverter circuits. Also, indiscriminate replacement of all the "bad" ceramic caps with "good" silver mica caps is not necessarily beneficial - many lovers of vintage Fender tone prefer the ceramic caps as they do affect the character of the highs and high-mids. (I can recommend Orange Drops as replacements for leaky poly coupling caps, however.) None of that makes as much difference as the speaker, though, so that's where I'd put my time and money first.

Changing speakers is quite simple, obviously - it's just a few nuts. Changing speaker sizes is a whole 'nother matter. If you are currently running a 10" speaker, you have to widen the baffle opening by cutting it to a 12" diameter, or change the baffle board to one cut for a 12" speaker. The absence or presence of a reverb pan isn't going to make this significantly easier or harder. I am pretty sure a 76 will have a glued in rather than screwed in baffle board, so getting the existing baffle out of there will be a pain to do either method.

michael you are correct I just had deluxe on the brain today

Yep, it will be a pain to change the baffle in that cab. You might be better off getting a repro cab with a 12" baffle and keep your original. Or you can experiment with different 10" speakers - there are a lot better 10's out there than a tired old Oxford. You'll pick up more volume with many newer model speakers if that's what you're looking for.

BIG difference in a BF or SF Princeton and a Princeton reverb.

Princeton will not give it up, stays clean all the way to 10.

Princeton reverb may be the best amp ever made.