Princeton Reverb

Production years:

1964 -1967 “blackface” circuits AA1164 (pre-CBS), AB1270 (CBS) and AA764
(CBS)

1968-1978 “silverface” circuit AB1270 without Push-pull.

1978-1981 “silverface” circuit with Push-pull volume boost.

The Princeton Reverb is the smallest Fender amp with tremolo and reverb.
With a 10 inch speaker and 6V6 tubes it delivers a
true ”American” scooped Fender clean tone with punchy and
responsive lows together with chimey highs. When cranked it tends to sound a bit
browner than the bigger two-channel amps.

Compared to many bigger Fender amps the Princeton Reverb (PR) was just
slightly changed during the CBS/silverface periods. The rectifier tube was
mainly the only thing where 5U4GB was replaced by GZ34. In build
quality all silverfaces are not as robust as the blackface.
Many people consider the silverface tone and sonic qualities are
just as good as the blackface models. Being almost a blackface amp with a
wrong faceplate the silverface models became popular value-for-the-money
amps. Collectors hunt the blackface models and players hunt
the silverface for its quality and price. First in 1978 when the
puch/pull volume boost was introduced, there was a significant difference
from the blackface circuit.

Tube layout and tube replacements

AA1164 Tube layout (Seen from behind, V1 is to the right side)
V1
12ax7 = Preamp
V2 12at7 = Reverb send
V3 12ax7 = 1/2 Reverb recovery and
1/2 Reverb mix
V4 12ax7 = 1/2 Vibrato and 1/2 Phase inverter
V5 6V6 =
Power tube #1
V6 6V6 = Power tube #2
V7 5U4GB = Rectifier tube

Popular mods and upgrades

So, why would you want to enhance or mod a PR? Due to its small size many
players want to increase the power, volume and clean headroom. A
few also wants to change the distortion characteristics. We’ve seen no
other amp that has been analyzed so thoroughly in terms of mods and tricks
to make it bigger. Does this mean you should implement the mods too?
Not necessarily. Just because many people does so, it doesn’t mean it’s
right for you.

In general we don’t recommend the mods that increase the power and clean
headroom unless you are playing your PR at gigs on medium-large stages
and you need the sparkling clean fender sound to cut through. If you’re
micing the amp the audience should have no problem hearing a
stock PR (we know: it’s really hard to trust the sound
engineers). Your job is to make sure your band and yourself can
hear the amp. If you are using your PR at home or in recording
situations we think you’ll appreciate the early breakup of the
original 6V6 power tube circuitry, the ineffective phase
inverter, a smaller output transformer and a low-medium
inefficient 10″ speaker. You are lucky to have a tube
amp that breaks up nicely and shines at you every day at practise volumes.
It’s what everybody really wants. So why increase the threashold of tube
heaven?

You’ll need schematics to implement some of these mods. http://www.ampwares.com/fender.asp. We usually start with
explaining a mod from a functional perspective where we relate
to components in the logical schematics diagram. Finally we point out
location of components in the physical layout diagram.

Speaker

The easiest and most effectful mod of them all. The speaker is the most
important component in your amp which physically produces
the air waves that your ears detect as a tone. If you
have an original Oxford speaker in your PR you’ll experience
a significant change (to the better, in many’s opinion) by
swapping to almost any other speaker. The
Oxfords are by most guitarists considered as flat and weak
sounding. You’ll have to push the amp and tubes hard to get any volume
and punch. Because of this they have gotten a bad rumor,not rightfully
deserved in our opionion. The Oxfords may actually be desirable
for those who seek lots of breakup ad moderate volumes. They’re not
loud, nor do they have lots of sparkle and punch, but they
do have some vintage vibes and a nice musical voice.
You may like such inefficient speakers, particurlary in
multiple speaker applications 2-4 x10″ where the total areal of speakers
will provide enough volume for you anyway. In a Princeton Reverb, and any
gigging 1x10″ or 1x12″ context, we personally prefer a speaker with
higher efficiency than the Oxfords. We have listed a few 10″
recommendations for PR below:

“Vintage” type:

Warehouse Veteran 10″. Light, snappy, warm, firm, they have
the vintage appeal of upper mids and higs. Excellent replacements of the
original CTS Alnico 10″ that came originally in blackface Super Reverb, one of
the absolute best speakers we’ve heard in SR. Lots of touch sensitivity and
warmness. The speaker is designed so that the cone breaks up when
pushed. Don’t be afraid of this before you’ve heard it. Fantastic,
mellow and compressed.

Jensen P10R. Beautiful Alnico compression. True vintage Fender sound.
Distinct, clear and neutral.

Jensen P10Q. Much like the P10R, little more punch and darker
sounding.

Weber 10A125. Much like Jensen P10Q, with more bite and crunch. Firm low
end and nicely balanced with mids and sparkling highs.

Eminence Legend 1028K. Very popular modern replacements for the
original CTS Alnicos. Bluesy and snappy.

More powerful speakers:

Warehouse G10C. More efficient and louder than the Veteran, more punch and
power. Somewhat darker too.

Weber 10A150. Loud and darker color than the 10A125. Lot’s of punch and
massive low mids requiring an EQ adjustment on your amp. Loud and heavy. Can
practically turn your 10″ Princeton into a loud Deluxe Reverb.

Jensen NEO 100. Powerful, dark, warm, efficient, massive low end, light
weight. We think it combines the Alnico compression and ceramic punch
well.

Jensen C10r and C10q. The efficiency of C10q makes the amp loud, and it
offers lots of punch as well as chimey highs. Good replacements for the
ceramic 10″s in many blackface amps.

Weber CVC10. Weber’s equivalent to C10r and C10q.

Weber 10F125. Powerful ceramic speaker, bright and powerful with lots of
headroom.

Check this video to hear the Weber 10A150 in a 4x10″
Fender Super Reverb (video on
Youtube).

This video demos the Jensen NEO 100 speaker in a 2x10″ Deluxe
Reverb (video on
Youtube). (Amp is biased with 6L6 tubes and has tremolo disconnect mod and
V1 tube pulled out.)

12″ speaker upgrade

You can either replace the original 10″ baffle with a 12″ baffle,
or you can drill new holes in the existing baffle. Don’t mind if the baffle hole
is tailored for a 10″. If you have seen or played a few
vintage amps from the 50′s and 60′s, you’ll know that the speaker hole
doesn’t need to be a perect circle that matches the speaker perfectly.

A 12″ speaker will push more air and give you a bigger sound with
more low ends, particularly more lower mids. A 12″ can normally handle
more power  than a 10″ at both lower and higher frequencies. The
12″ speaker is slightly harder to drive  and if you’re
really into a soft touch and finger tips you’ll probably appreciate
the dynamics of a 10″ more than with a 12″. An
interesting effect with 12″ speakers is the “3D” sound effect.
While 10″ speakers can be front directional (depending on cabinet also)
there is a better spread with 12″ speakers which contributes to a
better sound on stage. People will hear you better wherever they are
positioned relatively to the amp. A 15″ speaker has even more spread and it will
cut through skin and bone. We have never heard anyone who upgraded to a
12″ go back to 10″. Maybe that will tell you something.

If you have a large speaker chassis be careful so that it will fit under
the outpout tranformer. If you cut a new baffle your self (out of a
particle board plate), you might want to align the speaker vertically as low as
possible. Try to dry fit the speaker with the amp chassis and cabinet loose
before you drill the screw holes.

Warehouse G12C. Our best recommendadtion if you want a new “replica” of
the vintage Jensen C12n that came in i.e. blackface Pro Reverb and Twin
Reverb. A full tone speaker with lots of touch sensitivity. It
is warmer and deeper than newer Jensen C12n, not that hard and cold.
Highly recommended in both Princeton Reverb and Deluxe Reverb.

Warehous Veteran 30. Slightly darker and bolder than the G12C. Lovely and
complex tone. We have this speaker in one of our silverface princetons.

Jensen C12N. It is said: “If an alien came to earth and wanted to hear an
American guitar sound, I’d play him my Twin with a set of Jensen C12N’”. This
speaker is used in newer Deluxe Reverb reissues (the 100W model) and it has
lots of punch and sparkling highs. If you can track down a vintage model
from the 60′s, you’re a lucky man. The new Jensen speakers are more efficient
and brighter.

Jensen P12Q. Beautiful Alnico compression. True vintage Fender sound.

Weber Michigan 12″. Huge power.

Weber California 12″. Huge power. Be careful that it will
fit under your OT.

Weber 12A125. Very much like Jensen P12Q, we like it even better.

Weber 12A150. Darker than the 12A125. Lot’s of punch. Loud and
heavy.

12AY7 or 12AT7 as preamp tube

If you want a cleaner and spankier tone you can replace the V1/V2 12AX7
preamp tube with 12AT7 or 12AY7. These tubes have lower voltage gain and
different frequency responses, particurlarly when distorting. 12AX7 tube has a
voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45. One could
describe these tubes to have less harsh and buzzy distortion. This mod does
not alter the tone significantly when amp is played clean’ish. You’ll have to
increase the volume setting to achieve a similar volume as before. If you hit
sweet spot at volume=4 with 12AX7  you may go as high as 6-7 before
reaching it with 12AT7 or 12AY7.

Diode rectifier

This mod is for those who wants more power, clean headroom and attack.
It is very practical to bring with you those small Sovtek plastic
adapters that easily can replace the rectifier tube (either the 5U4 or
GZ34). You’ll increase the power of the amp with a few watts. The
diode rectifier will feed your amp’s filter caps with a higher
and ”faster” DC voltage compared to a tube rectifier. The fact
that it delivers a slightly higher DC voltage to your power tubes (via the
filter caps) means an increase of the tube voltage gain factor.
When you hit a hard chord the preamp and power tubes will require
energy and current is flowing from the DC filter caps which are
charged up by the rectifier. If the recitifer can deliver more energy
more quickly, you’ll have more power and attack.

Check also out Ted Weber’s “Copper Cap Rectifiers” that emulates the sag of a
tube rectifier.

Blackfacing

It takes very little effort to blackface the silverface PR models
without the push-pull volume boost. If you have a blackface PR
you should inspect the circuit and not just the tube
chart to verify which model you have, either the AA1164, AA764 or
AB1270. Sharp readers will see that the silverface AB1270 circuit
is identical to the blackface AB1270. Otherwise, there are minor
differences within all blackface and silverface circuits.

To our knowledge, the
following changes were introduced in AB1270 and AA764
(compared to the pre-CBS AA1164):

The 5U4 rectifier tube was changed to a GZ34 in AA764. GZ34 has noticably
less sag than a 5U4. So if you want the sag (compression effect) of
a pre-CBS blackface PR, go down to 5U4.

The electrolytic cap providing dc voltage to tremolo circuit changed from
25μF/50V to 50μF/70V  in AB1270  and 50μF/70V in
AA764.

The reverb recovery circuit in AB1270 got a 2000pF highpass cap
leading higher frequencies to ground. This makes the reverb slightly
darker with less presence, if noticable at all. See the green circled
cap to the left.

Logical schematics

Layout

Click the images for full size versions.

Increase/remove/switchable NFB resistor

This is an interesting mod, particularly in the PR. It is
applicable in all Fender amps with a negative feedback (NBF) loop. The
purpose of the NBF loop is to clean up and cancel out high frequencies and upper
harmonics (distortion). It makes the volume pot more sensitive and
aggressive and you’ll find the amp’s sweet spot at a lower volume knob
setting. If you remove the feedback loop entirely you’ll achieve
maximum effect of this mod. You do not get more or less clean headroom
in the amp. You get a rougher and rawer tone with more of
the higher frequencies, aka hizz. The reasons we like this mod in a
PR are:

The PR’s is not as sharp and trebly as bigger Fender amps. It
doesn’t have the common bright switch. In other words, having a
switchable mod that enables more treble and distortion could be useful and
interesting.

We use the PR for distortion anyway. We do not play the PR for its
clean sound but for smokin’ blues. So why not make it growl
even more? If we want clean sound we pick a Super or a Twin.

You may choose to implement the mod in several ways:

Simply disconnect the feedback loop (a wire). The 2,7KOhm resistor
can easily be removed with a soldering iron. See green circled resistor in
figure below.

Increase the feedback loop resistance value. This reduces the effect
of the feedback loop, making the amp break up more.

One of the two alternatives above in combination with making the NBF
switchable. You can remove existing wires to and from the
ground switch if you plan not to use it. Then wire the ground switch in series
with the 2,7KOhm NBF reistor.

Logical schematics

Layout

Bigger output transformer (Deluxe Reverb)

Simply replace the output transformer (OT) with a Deluxe Reverb OT.
This reduces the output transformer sag and limitation that
the Princeton is known for. With a bigger transformer you will experience
more attack and a bigger sound due to more efficient power
transformation from your tubes. You should consider this mod if you
are increasing the headroom in your amp (Paul C mod, Stokes mod,
Diode rectifier, etc ). In this case the original OT is not able to deliver
this power to the speaker without absorbing some energy and introducing sag
and too much compression.

Output transformer with multiple impedances (built-in
attenuator)

This mod requires the knowledge of how to replace a transformer in your
amp. This mod will give you different speaker
outputs impedances converting the the external speaker
jack to an independent output.

The original OT is sized for an 8 Ohm speaker
impedance. It will make sure the power tubes “see” the correct
impedance and can operate at optimal conditions in terms
of clean headroom potential and frequency response. If
you had an OT designed for a 2 ohm speaker load the tubes
would operate at their best with a 2 ohm speaker load. If
you use a 8 ohm speaker with a 2 ohm OT, the tubes are
pushed harder since they see a bigger load than expected. Both
volume and clean headroom is reduced significantly, and the tone is not
significantly changed. This effect is similar to an attenuator, which is
very nice when you want to achieve the amp’s sweet spot at lower volumes.

Go to Mercury Magnetics web site. Order the Deluxe Reverb multi-tap OT
with 2, 4 and 8 ohm taps. Yes, we mean the Deluxe Reverb OT.

Remove the original OT carefully in your Princeton Reverb and
store it away. Install the Mercury Magnetics OT.

Wire the OT’s primary circuit just as before. Clip the cables nicely and
twist them as Leo Fender’s gently old ladies did.

The OT’s secondary circuit will be wired differently. Wire the 8
ohm tap to the main speaker jack. Then wire the 2 (or 4 ohm) tap to the ext
speaker jack. You must first remove all existing wires to the external speaker
jack so that it is totally separated from the main speaker jack.

Now you have two speaker outputs; 8 and 2 ohms (or 4 ohm). You may use
them independently and the ext speaker jack does not require the main speaker
jack plugged to work.

We like this mod a lot. Especially since we’re using the original jack
outputs. Choosing the 2 ohm output with a 8 ohm speaker will attenuate the
volume a lot. Having an amp with several different output impedances
is practical for those who are using extension cabinets.

Middle pot

The PR did unfortunatley come without a mid pot. Many players are happy
with the default 6.8K mid resistor in the EQ circuitry, but some want what
the others have; a 10K potmeter mounted in series with the 6.8 Kohm
resistor. When the potmeter is set at 0 you’ll have an original tone, and when
turned up the mids increases and your amp growls with a British
Vox’ish tone (especially together with a Celestion speaker). For a
wider tone range you can replace the 6.8 KOhm resistor with a 4 KOhm,
and use the 10 K pot to go beyond the original 6.8 KOhm and up to 14 KOhm. If
you do not use the reverb or tremolo pedal it is very practical to install the
middle pot here. It requires a simple solder and removal job to
replace the phono plugs with a mid pot. Or you could install the mid pot at the
left most side of back of the amp where the additional power supply is
located.

Logical schematics

Layout

Weak tremolo

Some Princeton Reverbs have weak tremolo. This can sometimes be solved
by changing the vibrato tube V4 12AX7 or even the power tubes. If this does not
help one can change a resistor value to increase the strength of the tremolo.
See figures below which resistor this is. Replace the 1
MOhm resister attached to pin 1 of the V4 tube with a
lower value. One can use 500K or even lower, 270K.

Logical schematics

Layout

The Stokes mod -- increases voltage to the phase
inverter

This mod, and the Paul C mod, is so thoroughly discussed
elsewhere that we will only make a few comments on it. It will give your
amp more headroom meaning you can play cleaner at a louder volume. Otherwise
the tone of the amp stays much the same. You should also replace
the output transformer with a larger one to cope. Here is one reference http://www.blueguitar.org/new/articles/blue_gtr/amps/john_paul_rick_PRmods.pdf.

The Paul C mod -- convert PI to fixed bias

This mod will give you smoother distorition characteristiscs and a
fuller sound. If you think your amp and speaker are sharp and edgy this mod
could be worth a try. It will not increase the power and clean headroom. The
mod requires some rewiring with a soldering iron and a new
resistor. For description and schematics take a look here http://www.blueguitar.org/new/articles/blue_gtr/amps/john_paul_rick_PRmods.pdf.

Bass and tone characteristics.

The following mods are for those who want to change the tone
in terms of more or less bass. To tighten up bass means removing the farty,
low bass frequencies which are not handled well in a low power amp like the PR.
These mods should be considered together with the particular speaker and effect
pedals you are using to suit your music style. We do not say these
mods should be implemented by everyone, we just inform you that there are
techniques to shape your tone to suit your musical expression.

Reduce input-stage cathode from 25 to 4.7 to “tighten up” bass.

Increase 1st filter cap to 40uF (increase low end).

Reduce coupling cap that feeds PI, also a means to “tighten up” the
bass.

Questions and comments are welcome at the bottom of this page.

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10 Responses to “Princeton Reverb”

Morten W says:

September
7, 2010 at 1:10 am

Wow, this site it great! Thanks for sharing your vast knowledge man!

Reply

Steve M says:

September
17, 2010 at 6:11 pm

I found your youtube videos first, then the site and am truly impressed
with you playing and knowledge. I am a bass player convert to guitar who is
now interested in getting my first tube amp. I am a little concerned about
home volume of a mere 15 watt, but am mostly interested in a PR. I would very
much like to hear your opinion about the PR 56 Vintage Reissue. Do you feel it
is worth the money or should I try to find an earlier model? Your views ont
the PR Recording Amp would also be appreciated as it is nice with the built in
attenuator.

Reply

Jens Mosbergvik says:

September
18, 2010 at 3:20 pm

Hi Steve. Thanks for those kind words. I’m honored.
The reissue of the
popular Princeton Reverb is very very good tonewise. It’s tone and noise
levels are better, IMHO, than other new PCB board amps like Super Reverb RI,
Twin Reverb RI, Vibrolux custom and Deluxe Reverb RI. In A/B tests I doubt
anyone will hear a difference between a blackface PR and the PRRI if the tubes
and speakers were identical. The tubes and speakers are what make individual
amps sound different.

The only drawback with the PRRI is that you cannot service A PCB board amp
yourself and try out different mods to the circuit (this might not be any
relevant to you)

BR
Jens

Reply

Steve says:

September
21, 2010 at 2:47 pm

Hi Jens,
Thanks for you opinion. Did you have a view on the PR recording
amp? I understand it also has an effects loop so was wondering you felt 1) the
attenuator was worth the extra money for home/studio use… and 2) does the amp
take effects well? I hear so often about some amps that do. like the mesa
5:50, and others that don’t take pedals well.

thanks,
Steve

Reply

Jens
Mosbergvik says:

September
21, 2010 at 8:52 pm

Unfortunately I have no experience with the PR recording amp.

There’s a lot of amps, speakers, guitars and players that sound great
with some pedals, and crap with others. I think all players must decide for
themselves. You know what they say? Opinions are like assholes. Everyone’s
got one. By
that I mean that you shouldn’t really trust anybody else’s ears. I have
heard the same thing with the Fender Super Reverb; it doesn’t handle pedals
well. That’s just nonsense. Some players have not been able to dial in a
good tone with his specific guitar and pedal. Thousands of other are getting
great tones out of the same amp.

Peace

Jens

Reply

Steve says:

September
23, 2010 at 8:39 am

Hi Jens, I could not agree with you more about opinions… But I do know
the sounds I like and the styles of play I like to hear, which your
playing is that one of those styles, although I don’t have such high
aspirations as a singer/player. It is a little hard venturing out into
tube tone, but for bell clean sounds, plate reverb, … it seems the only
way to go. Of course the player and what feel they intonate with their
fingers is crucial to overall sound and I doubt that I will ever be any
exceptional as that is not my goal.

I guess when it really comes down to it, and I am playing at home, I am
afraid a 15watt will be too loud, if I have to push the valves for the
right sound… that is the one advantage to solidstate as I see it. I want
to play but not annoy everyone else in the house. As I walked the dog this
morning, I thought again about the new Fender G-Dec (I have the second
version, which I don’t think is so great). The new one is versital they
say, plus very customizable with the software provided, so maybe as a new
player, I should keep my sound goals on the cheap side financially. It
sucks when one knows what great sound is, but can’t feel I can justify it
to myself to spend 8-15,000Dkr for it, when maybe I will be nearly
satisfied with a small 2500Dkr.

When I look at setups of players I like, that split their signals to a
Fender & Vox amp, it makes me think that is what I’d really like to
do, but wonder if it is too stupid just by 15watters like the Princeton
& AC15C1.

God, life was much more simple as a bassplayer. Great basses and 1
great amp, and your done. The sound pallete is so much bigger with a
guitar / amp and having versital sound is important when one has to pay so
much to make it happen.

I am just blabbering on… too early in the morning and not enough
coffee…. well, you can see I am puzzeled about what to do. Wish you had a
store in Copenhagen and I’d come in, that would probably help me make a
decision.

take it easy,
Steve

p.s. not to mention I toy with the idea of a mesa 5:50, mostly for the
5watt switch, but again justifying the expense at the level of player I
am…. it is a little hard to do.

Reply

SLL says:

September
30, 2010 at 8:06 pm

I think that you meant to say increase the NFB resistor value in
alternative 2. Increasing the value reduces the NFB applied to the circuit.
Removing the resistor increases the value to infinity.

Reply

Jens
Mosbergvik says:

September
30, 2010 at 8:16 pm

You’re definitly right. Thanks very much for letting me know!

Reply

Jerry says:

October
25, 2010 at 5:57 am

I have a ’69 Princeton and want to add the “middle pot” mod to it. But I am
not clear on how you are wiring the pot into the circuit. I lifted the ground
side the 68k resistor off the back of the bass pot, soldered a wire to it and
attached it to the #2 connector of a 10k pot. I then wired the #1 terminal on
the pot to ground. Don’t I need to wire an output signal someplace using the
#3 terminal of the pot? I am bit confused by your diagrams. Can you explain in
more detail?

Reply

jimi says:

October
25, 2010 at 8:56 am

that should work. Ground the #1 connection to pot chassis and ground the
chassis with a wire to ground. Better that way.

Reply

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