mods to change the tone on the Princeton Reverb II

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...

akw in Germany tells me he knows of someone who felt the mid boost is "very heavy especially on low volume. There is a .0027 uF capacitor in the tone stack next to the pull pot. A .001 cap would shift the frequency and make a milder mid shift." ....and (March 04) see below because Mark confirms this!

After reading this from akw I played with the Duncan Tone Stack Calculator and got the following results for different values of mid boost cap.... (values below are with all rotary tone controls on "5")

(a) mid boost off gives a massive dip (-26dB, about 14 dB down from the rest of the response line) at 500Hz. I imagine this loss of middle in the amp stage is intentional, and cancelled out by the open-backed 12" speaker.

(b) mid boost on with standard cap (0.0027uF =2700pF) flattens the response and gives a slight hump (-8dB) at 40Hz, which might explain the thickness of the sound.(NB April 04 this has been corrected - previously it said 0.027uF which is wrong. Thanks, Armin)

(c) mid boost on with 0.01uF =10000pF doesn't make much difference from standard - the 40Hz hump goes down 2dB.
(d) mid boost on with 0.005uF = 5000pF gives a near-flat line
(e) mid boost on with 0.0025uF = 2500pF gives a curve midway between (a) and (b); small hump at 20Hz (surely this frequency is too low to make a difference) and a dip of -16dB at 120Hz. Perhaps a good place to start experimenting.

I haven't changed any of the tone components on my amp, so the above is theoretical.

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background info - HOW THE TONE STACK WORKS
I am very grateful to "grkeith" on the Fender Duscussion Page - on 9th March 2007 he wrote this as a descrption of the tone stack in the PRII...

"There's a lot going on in that area of the circuit. They were obviously really trying to enhance the tone.

"Like the common BF and SF Fender tone stack, the Treble control acts as a balance control for the highs and the lows. On the schemo I have, there is an error: the 100k should be shown connected to the plate of the tube and the 250pf. The signal is split there; the highs like to go through the 250pf to the 'upper' leg of the treb pot, the lows are resisted there due to that small 250p cap. The lows however more easily go thru the 100k slope resistor than the highs and divide up between .1 bass cap and .047 mid cap, and get drained off to ground according to the position of the bass and mid pot. The highs don't like to go through resistors as easily as lows, so they are attenuated slightly. If the mid and bass pots are set high, then more of the signal gets put on the 'lower' leg of the treb pot. Thus explaining the standard Fender tone stack and why the treb is a balance pot between highs and lows. So now, all the signal is coming out of the wiper of the treb pot, and sent to the vol pot via a network of caps and res, where more is happening that you don't normally see on a Fender tone stack.

"The volume pot is center-tapped. When the volume is at half way, max highs through the 120p and 100p are sent direct to it, less if it's on either side of half way. signal is sent thru the 820k slightly attenuated in the highs again due to the resistor, and goes to the upper leg of the vol pot. The 820k resistor in series with vol pot also loses some volume at the top of the vol pot, and remember some of the highs are already held back in addition cuz of that 820k res.

"Now you have another leg going thru the 500p and the 1m, the one meg is shorted out when the push switch on the treb is closed effectively putting the 500p direct to the top of the vol pot, so when the vol is dimed there is a direct line from the treb pot thru the 500p (the 500p holds back the lows from getting thru) to the top of the vol pot.

"So there should be an apparent tone shift through the range of the vol pot.

"There is a circuit to bypass the 250p treb cap which adds more mids.

"The 'standard' thing Fender did (not on This amp tho') is to put a (sometimes switchable) bright cap on the vol pot, which lets you regain the highs lost through the resistance of the vol pot when it is at low levels. Once you get past 5 it has less and less effect, til at 10 on the vol you hear no diff.

That help?

MORE CLASSIC CHIMEY FENDERY ZING - really easy mod....

Having read on The Gear Page that you can get the Rivera-era Concert closer to the classic 60s Fender sound, I tried it on my PRII (March 2011) and it does have a subtle but good effect in my opinion. This mod is part of the process known as "Blackfacing", in which a silverface amp is modified to sound more like its earlier, blackface equivalent. You can't really blackface a Rivera-era amp -they're too different from anything that came before - but I like this mod. 

Theory - The Rivera-era Fender design team added tiny capacitors to their preamp valves to make absolutely sure the amps wouldn't suffer from oscillation or interference. Called "snubber caps", their side-effect was to remove a little of the top end and make the amp sound slightly sterile compared to the classic Blackface designs of the mid-1960s. This mod takes these caps out of circuit. There are two in the PRII - one for v1a and one for v1b, shown on the schematic as wired from pin 1 to pin 3, and from pin 6 to pin 8; each on the + side of the 25/25 cathode cap. They are both shown on the schematic as 500pF caps, but on the wiring diagram and in real life the v1a cap is 0.0012uF (=1200pF).

Practice - After dismantling, find the socket for v1 (the tube/valve socket nearest the input end of the amp). It has 2 small capacitors actually mounted on the valve socket. For each capacitor, cut one leg close to the solder joint, and bend away/tape up the now-free leg-end so that it can't short against anything. Connect the speaker and a guitar, CAREFULLY switch on the amp and listen for extra noise, radio interference etc. If all is well and you like the small amount of high-end treble you've just gained, that's all you have to do; switch off, disconnect from power, reassemble. Otherwise switch off, disconnect from power, pull v1 out of the socket, re-solder the caps, reassemble.


Dec 2011; Notes from a UK Amp Tech

I've had two PRII owners in the London area recommend Steve Dove of S.A. Ampworks. Here's a note Steve sent to me in 2010 concerning some mod work done to a PRII which had turned up for sale some time after he had modded it; 

"Hello Andrew... this is one amp i will not forget in a hurry. I was asked appr 5 yrs or so ago to give this amp some boogie characteristics. The owner said he was somewhat disappointed with the basic amp, my brief was to make this a portable powerhouse with a great clean sound, and there started the challenge, and that it certainly was. First thing I modded for 6L6 use and this takes the power up to in the region of 35 watts and with some other improvements this sorted the output section,then I turned my attention to the preamp, I felt that the amp just sounded wrong, I was convinced there was a lot to be had from this one, I revoiced the preamp as far as the clean path was concerned, and got that to sound really nice, especially with some volume. The overdrive I found was utterly horrible - I actually think that whoever devised the configuration here,and that applies to all the amps in this series, it was just so poor in conception... my mods gave the amp more gain and took away the inherrent muddiness. I worked on the overdrive aspect over a long period,and with the restrictions re cost I think that a very good end result was the end product. I would like to have fitted another control to open the gain up and make that more versatile, and one or two other features,but the thing with this amp,when it was turned up some, it would sing. A little thin with the vol down, but (customer's name) loved it for use at his performing levels. I hope some of this is of interest, I learned a lot about this type of amp during the time I had it. Regards, Steve."

You will note Steve's not giving away much detail here, which is understandable because this is how he makes a living! Also I haven't dealt with him personally apart from a friendly exchange of emails... however he seems to know what he's talking about and I gladly link to his website.