Recommended Fender BF Princeton Reverb Clone Mods

[PeaveyBandit]

Hello,

I'm new to the forum and very glad to read so much helpful information. If this is not the right section to post in please feel free to direct me.

I'm wanting to build a beefed up Princeton Reverb super clone amp that would replace my beloved black face Deluxe Reverb and give me nearly the same performance characteristics in a more portable package. I'm thinking of putting a speaker like a Celestion Gold 12" and making some various mods to improve the bass performance and get it in the ball park of a deluxe reverb.

I've read some like the Paul C mod and it seemed within my grasp. I guess some are more involved than others. I'm good with electronics but I'm not a guru. Just able to assemble and repair amps at a hobby level. I'm seeking some advice or shopping list of the best most stable mods I can implement with my medium level of skills. I'm very good at soldering and wiring but I've never engineered any mods or anything like that.

I'm going to be ordering caps, trannies etc. So now is the time for me to consider more options. Thanks for your help. I've read quite a bit so far but it's a little over whelming to decide what mods or parts are the right choice for my situation.

I'm even considering a slightly taller custom 18.5x18.5" cabinet to give the speaker a tad more room up against the tubes. Any ideas will be greatly appreciated. Or guidance to the proper places to read. I'll continue reasearching in the meantime.

Thanks
PB

[hasserl]

First off I'd suggest going with a Deluxe Reverb output transformer. Allen Amps has a couple that are upgrades for a DR, these work very nice for a Princeton Reverb, and when mated with a good 12" speaker will make the amp sound huge.

Second, I'd up the first stage filter cap to 40uf for better and tighter bass response. Then I'd also crop the input stage cathode bypass cap down to 4.7uf, which will cut some bottom end, but the amp has more than enough and it gets loose. This will help tighten it up some more. The PaulC mod is a good one, as is the Stokes mod. Give the PI a larger voltage and you'll pickup some much needed headroom and it'll put out more power. Along with the Deluxe OT and 12" speaker this is a great upgrade. Makes a little PR into a giant killer. Unsuspecting DR players won't know what hit them.

A couple other things I like to do, drop the Reverb mixing resistor way down in value. I think stock is like 3M3. Drop it down to say around 2M. You can play around with that value to find what suits you. This will tame the reverb a bit, which is over done IMO, and will give the amp a lot more gain. Then you can try increasing the nfb resistor, from 2700 try 5K. SOme guys like to remove the nfb, but I like the effect, just not so much as Fender did stock. Increasing the value of that resistor will cut back on the nfb and give more gain and a more raw sound.

[PeaveyBandit]

Hasserl,

Thanks for the great advice. This was a huge help. I'm going to try and not ask many dumb questions but if you can bear with a newbee for a few more minutes I'd love to make sure I understand this. Can you double check me?

1. "First off I'd suggest going with a Deluxe Reverb output transformer."

Okay I found a T025 on Allen Amps site that says it's a beefy upgrade for DR or PR.

2. "up the first stage filter cap to 40uf for better"

What I found that looks good is a sprague 50/50 and hopefully that's okay

3. "crop the input stage cathode bypass cap down to 4.7uf"

I'm really not sure which cap you are talking about on this one?? What is the original value? Thanks

4. "The PaulC mod is a good one, as is the Stokes mod"

On these mods should I do both? One at a time? Is this a pretty much slam dunk to do both or should I maybe do the stokes and then the PaulC? Sorry to ask this but I've not read enough about people's results to know.

5. "Reverb mixing resistor"

I'm not 100% sure which one this is. I do see a 3.3m in parallel with a 10pf capacitor up in the area the reverb comes back into the chain. I'm guessing that's the one. I'll attach the schematic for refreshing memory.

6. "Then you can try increasing the nfb resistor, from 2700 try 5K"

Okay I'm just guessing that means negative feedback and it's the one across the top of the amp schematic. I'd love to understand that a little more but this sounds like some cool stuff.

[PeaveyBandit]

P.S. Adding on to the last post... I need to ask one more question about another mod I found. Any info appreciated. It states install 470ohm resistors onto the 6V6 grids. It seems like most mods have good explanations but I don't understand this completely. I can explain why.

It looks like on the stock Deluxe Reverb the wire comes in on pin 4 and daisy chains to the other 6V6's pin 4. Then both 6V6's have a 470ohm resistor sitting across pins 4 to 6, and again 4 to 6.

On the princeton I see only a wire daisy chaining from one 6V6's pin 6 to the other 6V6's pin 6. And nothing connected to pin 4's at all.

So my question is how to mod. Just slap some resistors into the Princeton Reverb and not touch the wires coming to pin 6? Or should I add the resistors and also move the wires to pin 4 and duplicate the way it's done on the Deluxe Reverb? Can anyone comment on that?

Here are the rather general and vague instructions for the mod
http://www.schematicheaven.com/mods/hotrodprinceton.htm

Reference the stock layouts for both amps below in the attached gifs.

THANKS!

[hasserl]

See replies below

Hasserl,

Thanks for the great advice. This was a huge help. I'm going to try and not ask many dumb questions but if you can bear with a newbee for a few more minutes I'd love to make sure I understand this. Can you double check me?

1. "First off I'd suggest going with a Deluxe Reverb output transformer."

Okay I found a T025 on Allen Amps site that says it's a beefy upgrade for DR or PR. YEs, that would be a great tranny, but in a stock Princeton Reverb chassis you might find the fit a bit tight. I believe I used the TO22 with great results. It is quite a bit larger than the stock PR tranny, and can be a tight fit with a 12" speaker. But if this is a new build, not a modification of a PR you've got more flexibility. In my PR I was able to install a 12" speaker, with this tranny, but it took a Celestion style speaker basket with the four large openings, and I oriented it so the OT sat inside one of the openings. I actually custom ordered the speaker from Weber, it was a 12F150 speaker built into the Celestion stype basket.

2. "up the first stage filter cap to 40uf for better"

What I found that looks good is a sprague 50/50 and hopefully that's okay

Depending on the rectifier used, this might be a little large. If you're using a GZ34/5AR4 rectifier you'll be OK, but if you use a 5U4GB 40uf should be the max capacitance used. Are you using a multi-section cannister type cap like used on the PR, or individual caps as used on the larger Fenders? You can get a 40/20/20/20 cap from Weber and from Antique Electronics

3. "crop the input stage cathode bypass cap down to 4.7uf"

I'm really not sure which cap you are talking about on this one?? What is the original value? Thanks

The first gain stage has a 25uf cap bypassing the 1500 ohm cathode resistor. Reducing that cap down to 4.7 will roll off some bottom end and reduce the tendency for it to get flubby.

4. "The PaulC mod is a good one, as is the Stokes mod"

On these mods should I do both? One at a time? Is this a pretty much slam dunk to do both or should I maybe do the stokes and then the PaulC? Sorry to ask this but I've not read enough about people's results to know.

Yes, do both of them. The Stokes mod will give a little more headroom, raising the point at which the PI starts to distort. It will also result in a larger voltage swing from the PI, which will push the power tubes harder for a little more output. The PaulC mod will make the distortion of the PI a little smoother. It's not a huge difference, but IMO it helps.

5. "Reverb mixing resistor"

I'm not 100% sure which one this is. I do see a 3.3m in parallel with a 10pf capacitor up in the area the reverb comes back into the chain. I'm guessing that's the one. I'll attach the schematic for refreshing memory.

Yes, that's the one. Reducing the value of that resistor will let the preamp drive the PI a little harder for more breakup. It will reduce the strength of the reverb, but IMO this is not a bad thing. The reverb is a little over the top IMO, so you can stand to lose a little.

6. "Then you can try increasing the nfb resistor, from 2700 try 5K"

Okay I'm just guessing that means negative feedback and it's the one across the top of the amp schematic. I'd love to understand that a little more but this sounds like some cool stuff.

Yep, you got it. The nfb reduces the gain of the amp and smoothes the operation, adding stability. SOme people like to disconnect it entirely to increase the gain, but IMO that is too much. I like some nfb in this amp, it helps keep it refined. It's just a little too much in the stock config. Increase the value of this resistor to reduce the amount of nfb, the amp will respond warmly to this. You can try different values to get what you want. You might try installing a 10k pot as a variable resistor temporarily, adjust the pot while you play the amp. When you get a setting you prefer measure the resistance and install a resistor of that value (or the next largest).

[Bruce / Mission Amps]

P.S. Adding on to the last post... I need to ask one more question about another mod I found. Any info appreciated. It states install 470ohm resistors onto the 6V6 grids. It seems like most mods have good explanations but I don't understand this completely. I can explain why.

It looks like on the stock Deluxe Reverb the wire comes in on pin 4 and daisy chains to the other 6V6's pin 4. Then both 6V6's have a 470ohm resistor sitting across pins 4 to 6, and again 4 to 6.

On the princeton I see only a wire daisy chaining from one 6V6's pin 6 to the other 6V6's pin 6. And nothing connected to pin 4's at all.

So my question is how to mod. Just slap some resistors into the Princeton Reverb and not touch the wires coming to pin 6? Or should I add the resistors and also move the wires to pin 4 and duplicate the way it's done on the Deluxe Reverb? Can anyone comment on that?

Here are the rather general and vague instructions for the mod
http://www.schematicheaven.com/mods/hotrodprinceton.htm

Reference the stock layouts for both amps below in the attached gifs.

THANKS!

You don't need that heavy duty Deluxe Reverb OT from D Allen.
A standard reissue Deluxe Reverb OT is already +25% bigger then the Princeton... but that and the Allen are different primary impedances compared to a stock Princeton OT.
Before you spend countless dollars on transformers and all kinds of Net driven MODS... recap the amp (all the electrolytics) dump that stock 10" speaker, get a WeberVST C10NT and just install that.
Then play the amp for a while. You'll be surprised how much that helps.
The 470 ohm resistors are wired up just like you'd see in a black face amp using 6L6s.
Find a layout for one of those and look it over.
Replace the 1K resistor on the far side of the eyelet board with a 1K@2w.

[hasserl]

These are screen grid resistors, the purpose is to reduce strain on the screen grids of the tubes. They are not required, obviously or they would be there, but they do help for tube life. Though I've seen plenty of Princeton Reverbs with decades old tubes still sound and play fine without these resistors. Still, conventional wisdom is they are a good mod. Pin 6 is unused in the 6V6 tubes, so the tang on the tube socket for that pin makes a handy place to hang a resistor so it's not hanging out in the open. You could run the wire to pin 6 of each 6V6, then run the 470 ohm resistor between pins 4 and 6, just as you described on the Deluxe Reverb.

P.S. Adding on to the last post... I need to ask one more question about another mod I found. Any info appreciated. It states install 470ohm resistors onto the 6V6 grids. It seems like most mods have good explanations but I don't understand this completely. I can explain why.

It looks like on the stock Deluxe Reverb the wire comes in on pin 4 and daisy chains to the other 6V6's pin 4. Then both 6V6's have a 470ohm resistor sitting across pins 4 to 6, and again 4 to 6.

On the princeton I see only a wire daisy chaining from one 6V6's pin 6 to the other 6V6's pin 6. And nothing connected to pin 4's at all.

So my question is how to mod. Just slap some resistors into the Princeton Reverb and not touch the wires coming to pin 6? Or should I add the resistors and also move the wires to pin 4 and duplicate the way it's done on the Deluxe Reverb? Can anyone comment on that?

Here are the rather general and vague instructions for the mod
http://www.schematicheaven.com/mods/hotrodprinceton.htm

Reference the stock layouts for both amps below in the attached gifs.

THANKS!

[hasserl]

Note, that Bruce has a ton of experience with these amps and many others. His opinion is well worth considering.

Bruce, one thing about this is it's not a mod of a Princeton Reverb, but a new build, if I understood him correctly.

Even so, the stock PR is a fine amp and Bruce's comments are well taken. I guess I look at it from a perspective that the PR is just a little too small of an amp with too little breakup, and the breakup it has is just a bit flubby and ratty. My mods are to address those things, but that's just me. There is a lot to be said for stock Princton Reverbs, and certainly a lot of music has been made and recorded on them over the years. So, I got no argument for Bruce, and think his comments are good advice. Even if I would still mod the PR anyway

You don't need that heavy duty Deluxe Reverb OT from D Allen.
A standard reissue Deluxe Reverb OT is already +25% bigger then the Princeton... but that and the Allen are different primary impedances compared to a stock Princeton OT.
Before you spend countless dollars on transformers and all kinds of Net driven MODS... recap the amp (all the electrolytics) dump that stock 10" speaker, get a WeberVST C10NT and just install that.
Then play the amp for a while. You'll be surprised how much that helps.
The 470 ohm resistors are wired up just like you'd see in a black face amp using 6L6s.
Find a layout for one of those and look it over.
Replace the 1K resistor on the far side of the eyelet board with a 1K@2w.

[J Martin]

If this is a new build and you like the sound of your Deluxe Reverb amp, why not build one channel of the Deluxe instead of messing with all these Princeton mods?

[hasserl]

If this is a new build and you like the sound of your Deluxe Reverb amp, why not build one channel of the Deluxe instead of messing with all these Princeton mods?

JMO, but I prefer the sound of a modified PR vs a DR. The PR has a better trem IMO, and once modded the overdriven PR has a superior distortion. That's a good reason IMO to build a modded PR than to build a one channel DR. JMO, of course, YMMV, etc.

[PeaveyBandit]

Yeah I wish I could have them all but I've pulled the trigger on a super accurate replica '64 PR Chassis coming from overseas. It locks me into some aspects of the design but building a single channel Deluxe is something I'd love to do. The hotrod PR is something that would equally please me and might offer the kind of sweetness I like about Princetons only larger. The custom cab I'm ordering is designed for a PR chassis and 12" speaker so it comes 17.5" tall by default. Then I plan on testing all of these mods and using a medium decay reverb pan and find the right amount of reverb. I like the feedback about the vibrato because it's something I don't like about my DR's in the past. The tremelo/vibrato just doesn't convince me at all. I'm not sure I like the vibrato on my BFSR either. So that's encouraging. I'm putting a Celestion gold 12" speaker in there to start with and if I don't like it they are selling as fast as they can go. I may also check into the Mercury BFDR OT as well. I've used their stuff and seemed very nice too.

The help I've gotten here has been amazing. I will come back and post on the forum when I complete the project. It may take me a while but I think it could come out real nice. I'll take some photos.

[casey73]

I recently did a complete redux on my old Princeton Reverb. I picked up a SF PR that had been sawn in half and made into a head at a pawn shop long ago. I fixed it, modded it many times and learned with it. Then it sat on a shelf for many years. Last summer I bought a Marsh amps blackface repro cabinet with a 12" baffle and the blackface faceplate. I'll spare you all the details except to say it is now it is a hybrid between a PR and DR capable of running 6V6, 6L6 or EL34s. After using a Vintage 30 in it for six months I took the plunge and bought the new Jensen Jet Blackbird, 12", 100W alnico speaker. Definately a sound investment and a much smoother tone than the Vintage 30, and more "Fenderish". At the end of the day, I ended up sinking some serious cash into it, but way less $$ than restoring a 68 Mustang . At least that's what I told my wife....

[Bruce / Mission Amps]

JMO, but I prefer the sound of a modified PR vs a DR. The PR has a better trem IMO, and once modded the overdriven PR has a superior distortion. That's a good reason IMO to build a modded PR than to build a one channel DR. JMO, of course, YMMV, etc.

I agree 1000%!
.... a modified PR kills when compared to any other 12-18 watt amp.
That amp (with a good sounding sensitive 10" or 12" speaker) is incredible because it does just about everything any other Fender Reverb amp does but it has a much more fun, bouncy, juicy overdrive tone to it due to the smaller transformers and the cathodyne phase inverter.
Plus, clean is wonderful but since the amp is not as loud as 22w-50 watt amps, you can really crank it up in most smaller club venues and get that great power tube distortion going at those lower and USABLE power/volume ranges.
I've played out a number of times over the last year with guys using my modified Princeton Reverb amps and the whole band just sounds much better at enjoyable volume levels with great guitar tone.
These amps are so much fun with a few mods and tweaks that the new Mission Amp spring-summer project amp is going to be my version of the Princeton Reverb and NOT a Boogie with big ass transformers and 6L6s!!

[PeaveyBandit]

Bruce, you mentioned the smaller transformers. What about my build with a deluxe OT ?

Thanks
PB

[Bruce / Mission Amps]

Sure you can use those... it will be a little more like a Deluxe Reverb by using Deluxe Reverb transformers.
The Princeton uses an output transformer with a true rating closer to 12-15 watts and the Deluxe is closer to 18-22 watts.
Also, the PR uses a power transformer that is also used in the Vibro-Champ... a 3-4 watt amp!!
That means it probably is a 70-90ma PT while the Deluxe PT is around 120ma to 130ma.
You'll still get a great sounding amp if you use the Deluxe iron on a fresh build.