Princeton Reverb II only! If it ain't got the II, this page ain't for you.

Fender Princeton Reverb II

Modifications - Gain, Distortion, Volume
for mods to the tone of the amp see here

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SPAMDEFEATstratopastor@hotmail.com

GETTING 35 watts
MORE CLEAN VOLUME
MORE GAIN SRV-STYLE

FATTER TONE, MORE GAIN - COMMERCIAL FIRM'S MOD KIT DETAILS
USE OF POWER ATTENUATOR
LOOSER, MORE ORGANIC SOUND (NFB MOD)


IN GENERAL I AM SUSPICIOUS OF MODIFICATIONS and I prefer only to list mods which have actually been done. I don't like listing mods of the "I heard from this guy once..." variety. Nor do I  like the attitude which says "Fender's R&D knew nothing, and an amateur with a DMM on his kitchen table is more likely to get something right...."

As with all the other pages on my site, the details are believed correct but I won't take any responsibility for what happens if you attempt them. Valve/tube amps store lethal voltages even when switched off and unplugged from the wall, and if you don't know what you're doing, you can be badly hurt.

If you have tried any modifications, I would be delighted to hear from you - even if you're not satisfied with the result, it's useful information for the rest of us! I'm specially interested if you've converted your PRII to cathode bias, or to tube (valve) rectification, or added a sag resistor.

EASIEST WAY TO GET BETTER CLEAN SOUND; CHANGE VALVES (TUBES)

You can use different preamp valves to reduce the gain in your amp, which might make for a clearer clean sound. If you're already happy with the clean sound, you can stop reading now, but if you're convinced that the sound is just a little overdriven no matter what you adjust, then you could try putting a lower-gain valve in position 1, 3 or 4 - most likely position 1 (nearest the input end of the amp, the right-hand side viewed from the back). If the usual 12AX7 (7025, ECC83) has a gain of 100, then on that scale a 12AT7 is 70, 12AY7 is 40 and 12AU7 is 20. I use an Electro-Harmonix 12AY7 in position 1 - the 12AY7 is the original front-end valve in the 1950s "tweed" Fender amps.

The difference is, you're sending less signal through the preamp stage of the amp. To get the same volume as before you'll find you have to turn the master volume knob up to a higher number than before. And the amp will have less absolute-maximum volume than before. (Note, this will NOT give you "cleaner and louder"... the only way to increase your max.clean volume, as perceived by the audience, is to make serious changes to the power stage, or get a bigger amp.)

Anyway, fitting a lower-gain valve is cheap, non-invasive and takes only 5 minutes to find out if you like it or not. (Even if you don't like it, the 12AY7 is an acceptable emergency spare for v1,v3 or v4.)  I think only Electro-Harmonix make the 12AY7 right now (12 or 13 pounds in the UK, 12 dollars US  (March 2011)), though you can buy unused ones in the older, more expensive brands.


LOUDER: GETTING 35W

Pat Morford (BC, Canada) writes....
"my amp has been modded to use 6L6 type output tubes. this is a HUGE DIFFERENCE, as the entire interplay of the amp circuits change for the better...the output transformer and power supply sweet spot is PERFECT with the simple mod, and speaker change, with the right tubes. This is a very simple bias mod, and I have been running it flat out for hours, the transformer is WELL within heat range, same with the output transformer..  this is a full 35 watt amp now... I will attempt to map the changes to the bias circuit, it's probably ONE RESISTOR only." (Pat is right, you can use the rebiasing method on this website and it will involve only one resistor value, but with 6L6s all the target numbers will be different to 6V6 operation. The static dissipation wattage for 6L6s, and the rough-guide cathode current range, will be on the web somewhere - good luck.)

I emailed Pat (June 03) and, while he didn't have time to write this up, he confirmed that (a) his amp still hadn't melted (b) it's simply a case of fitting a matched pair of 6L6's and rebiasing.

But for longterm operation there's more to it. John Philips says it's vital to add the screen-grid resistors with 6L6s; also that the speaker impedance requirement changes to 4ohms due to the difference in plate load requirement. I have had this confirmed by another tech (July 06). My guess is that, while Fender output transformers can usually stand a factor-of-two mismatch in speaker impedance, it's asking too much to have the mismatch AND pull 35W from a transformer designed for 22W. It's been said on the Weber Amps board that 6L6s develop a higher voltage across the cathode bypass capacitor (the 25V/25uFs) and these should be replaced with 50V. That was referring to a Champ, but the principle probably applies here too.

Pat seems to know what he's talking about, but this mod comes with no guarantees from me or him.  I believe the PRII to be loud enough at 22W. If I wanted a 2 x 6L6 amp of around 35W I'd save up for a Hot Rod Deluxe - also Pignose used to make one the G40V; a used bargain now. I used to list a link to Pat's audio business here, but it seems to be defunct and I can't find another (Mar 2011).

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MORE CLEAN VOLUME (see mod 5 below)
MORE GAIN ON LEAD CHANNEL, SRV-STYLE) (see mods 1-4)


Martin Kuhn lives in Jarna, about 30Km south of Stockholm in Sweden - NOSPAMTHANKSmartin.kuhn@telia.com (delete the capital letters). He builds and design amps, and does repairs and mods as a hobby. There's a great set of photos of an amp-builders' meeting on http://londonpower.com/wks04swe.htm which includes some of his work.

Martin writes, in excellent English....

"Last week (Dec 04) a guy came in with a PR II in excellent condition. He wanted it to have more juice, more of a SRV-sound (everybody wants to sound like SRV, right? ) This amp has lots of juice hidden in it! There is lots of gain that is just thrown away between the stages, so one has plenty of room for tweaking the circuit, and turn it into a high-gain monster if you like.

It is basically the same circuit as the mighty Trainwreck (which basically is a Fender with an extra gain stage), plus the reverb! The mods are presented in the attached schematic, feel free to publish them on your website.
  
All I can say about this mod is that it is kind of an experiment, and a place to start to investigate this circuit. The sound is much more powerful, and you can easily push the amp into poweramp-distortion with a Strat. It gets much more touch-sensitive.

The
(unmodified) lead sound sounded just horrible to my ears, I wonder if anybody uses it at all. After the mod it is quite useful, and can be further tweaked to one's taste. You can easily get lots of sustain, and get the amp to sing and the tone to transform into singing harmonics.

Both the customer and I, after trying out different speakers, felt that the Celestion G12H 30 really made the amp bloom
".

Here is Martin's very helpful schematic with the mods, and his notes at the bottom, shown in red. I have not tried this mod myself. Here are some extra notes which Martin emailed to me.

Mod 1 - the main point here is to replace the original cap with a plastic one for better tone and clarity. Martin used 47pF because he didn't have a 22pF in plastic.
Mod 2 - see the notes in red - those resistor values are in kohms, not ohms. This mod reduces the signal for better balance and clarity.
Mod 3 - reduces down the signal, and cuts bass, for less mush, more definition and improved harmonic balance. Whatever value you choose here, plastic caps will improve the tone.
Mod 4 affects clean AND lead.
Mod 5 was not done - it's just an idea - but increasing the original 47K resistor should give more clean volume; increasing it further will introduce overdrive. Any changes here will mean new values for the other mods to get the levels right.

(New Oct 07; My esteemed correspondent Al Z says he has done Mod 5, but "installed a 50K pot in series with the 47K resistor so now I could dial in the "strength" of the clean sound  to suit the pick-up I am using. Stock seemed too cold for single coil and 90K-the resistor I tried first-was too hot for humbuckers; the sound would distort." The 50K pot is a 1/2-watt mini-pot on the back panel. Thanks, Al. )


schematic with high gain mods


PROFESSIONAL MOD COMPANY'S "FATTER TONE, MORE GAIN" MOD


There's a firm who sell mod kits for various amps. They sell one for the PRII which (I think) makes the bias measurable and adjustable from outside the amp. When you buy this kit, they include a "fatter tone, more gain" kit as an added extra. Sometimes people ask me if I know what's in this extra kit. Well now I know, so here you are. Together, they're a kind-of combination of Armin W's second mod (see above) plus a small change to Martin Kuhn's mod 5 (above). The "fatter tone" refers only to the clean sound, not the 'lead' sound.

I have redrawn this mod on the Fender wiring diagram. Really this is 2 mods which do not depend on each other. For "fatter tone" do the cap change (that's Armin's mod, shown on the left here) and for 'more gain' do the 2 steps on the right (a little like Martin's mod 5).


fatter, more gain mod sketch

I hope this is clear enough - it shows the right-hand end (input end) of the circuit board with the valves/tubes nearest you.


Power Attenuator
I've made myself a variable speaker attenuator ( a very crude imitation of a Weber MASS, or THD Hotplate, or similar). This allows me to use the sound of the power valves running at maximum power, but with the final volume being as quiet as I want. WOO HOO! The folks who write about this are right  - it opens up a new world of tone. Just to be clear, this is an extra box which connects between the amp's speaker output jack and the speaker. It dumps actual power - it's not just a 'volume box' which some folks put in their amp's effects loop, which only works at line level. (In any case the PRII hasn't got an effects loop...)

With a PRII and a speaker attenuator, there are 2 basic options.
(1) With the amp set for 'clean' (so you're not selecting the 'lead' sound). I have the volume (input gain) set at 9 or 10. Then turn the attenuator down to low volume and turn the amp master up to 8, 9 or 10. This gives an on-the-edge blues sound. Add mid-boost for even more ZZ Top feel.

(2) For a more high-gain, squealing harmonics blues-rock sound, select 'lead' . Turn the lead level up to 10, turn the attenuator to low volume, and use the volume control to adjust how much preamp gain/distortion you want. Even if you don't like the 'lead' sound, you'll probably like this because it has the 'buzz' removed, and warmth added, by the 6V6s running at near-max power.


Obviously all this will change in character with a different set of 6V6s and/or with a change to the bias setup.

If you only ever run the amp on '3' the power valves will probably last forever.
A power attenuator allows you to wear out the power valves faster, because you can play at max power all day without hurting your ears; with the master or the lead level on '10' the amp is working to the max whether it sounds loud or not!  This makes a clip-on fan a really good idea. It won't save the power valves but it will stop the valves and transformers heating up the rest of the amp so much.

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LOOSER, MORE ORGANIC SOUND; MORE POWER AMP DISTORTION;
NEGATIVE FEEDBACK LOOP SWITCH

For info about Negative Feedback Loops and what they do, see here. (no point in my trying to re-write this kind of thing!)

July 09 - I have done this mod and only got a subtle change. However another PRII owner says it makes for a big difference. Maybe it's something to do with my power valves being biased cool. 

Referring to the schematic, there's a 100k resistor at the top of the diagram, shown above v5 and v6, forming a loop back from the power transformer to the  inputs of v4. If that loop is broken (just unsolder one end of that resistor) then the amp's power stage becomes less hi-fi, looser, more organic, with power amp distortion (i.e., nice distortion) appearing earlier when the volume is increased. This is a very subtle effect at low volume and only makes a real difference when the volume is increased to rehearsal or gig level.

Advantages  - slightly more maximum volume, earlier onset of power amp distortion, more touch-sensitive, warmer, less clinical sound.

Disadvantages - maximum clean volume is reduced, slight increase in operating noise, slight loss of extreme top and bottom end.

Side-Effect - the presence control won't work. On my amp I can't hear any loss of presence, possibly because I've added a little top end by cutting out the snubber caps.

Risks -  there is a theoretical risk that an amplifier becomes unstable (makes a hooting noise with no instrument connected) when a designed-in negative feedback loop is removed. This wasn't a problem on my amp and I don't think it would be on any PRII.

IN PRACTICE - SIMPLE EXPERIMENT
Identify the 100K resistor and unsolder one end. (Find the speaker socket - there are three wires coming off it. One wire runs to the circuit board, to the relevant 100K resistor). Play the amp, keeping in mind the effect doesn't really kick in at low volume. If you like the sound and the amp isn't too noisy, consider leaving it like that. If not, re-solder the resistor. (Never touch a live, open amplifier while holding a connected guitar.)

WHAT I DID - MAKE IT SWITCHABLE - fully reversible mod
In summary, what I did was to park the 'hum balance' control inside the amp and use the hole to mount a small switch on the back panel. Two wires run from the switch to the circuit board where the resistor is. I'm writing this up after 4 months of using the amp in various settings and I'm happy with the effect and the reliability, and as stated above, the loss of the presence control when switching to modified operation doesn't seem to lose anything tone-wise.

I don't know anyone who has ever needed to adjust the 'hum balance' so I put it inside the chassis and used the hole for the switch. This means there are no changes to the chassis.  Note the hum balance control must be grounded to the chassis, otherwise the amp makes a bad noise.

Parts needed - about 600mm of insulated single-conductor wire, pretty much any kind so long as it's insulated
                      - a switch, sub-miniature, latching, 2-position, single-pole or double-pole, single-throw
                      - I used a 50-60mm strip of the steel  strip they use to help mount car stereos. 2 of the holes had to be drilled out a little.
                        This strip re-mounts the hum balance control and grounds it. You might  think of a better way.

after removing the chassis from the cabinet...

Step 1
solder about 300mm of wire to each of 2 tags on the switch (make sure it's 2 tags which get connected in one of the 2 switch positions)

2
unscrew the hum balance control, slide it into the chassis.

3
screw the hum balance control onto one end of the steel strip.

That's the hum balance control on the left.
view
                          of switch and hum balance

4
Use the switch to mount the strip and the hum balance control to the chassis. In the picture above the switch body is red. The hum balance control is pushed against the inside of the chassis (not sticking through it, there's no hole there). The strip is under tension. The new wires are brown and head away to the right of the photo above.

5
feed the new wires toward the 100k resistor (described above) which has one end lifted. I ran the new wires under all the existing wires - they hold it firmly against the bottom of the chassis (see brown wires in the photo below). It's all about keeping things neat and vibration-proof.

nfb
                          mod wire routing

6
solder one wire to the lifted end of the resistor, and the other wire to the circuit board where the free resistor-end came from (see photo below).

new
                      wires terminate on circuit board

The new wires arrive from the left. In this photo the valve sockets are to the left. I used a green cable-tie (just visible on the left) to tie the new pair of wires with some existing wires. Because I used thick wire, the raised resistor is rigid and I'm happy to leave it like this. 

Here's the switch viewed from the outside (sorry it's out of focus). On mine, 'down' is normal operation and 'up' is modified, i.e. with the  Negative Feedback Loop disconnected.

rear
                      panel nfb switch

For info about Negative Feedback Loops and what they do, see here. (no point in my trying to re-write this kind of thing!)


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