Fender Princeton Reverb Amps

**Bezoeker** · 01-04-2016, 10:55 AM

In my opinion, the classic mod is the way to go for a Princeton Reverb and an archtop. A 25K pot in the second speaker output hole and wired into the bottom of the tone stack (replacing the 6.8K resistor) gives you a midrange control.

This works great in any Fender tone stack with no midrange control. I suppose you could move the knob to the front if you do not use the tremolo, but the second speaker jack hole is the old-time cosmic fart way to do it.

Arguably a 50K pot would let you really start to lift the whole tone stack away, and you can always just keep it turned down if you do not like it.

EDIT: I honestly thought I would not type "jack hole" today. Shows you how wrong you can be sometimes.

**christianm77** · 01-06-2016, 10:41 AM

And this the bit where I say, Princeton Reverbs are great with single coils or humbuckers, but if you are planning to do gigs with an actual PRRI '58 or something swap out the speaker for a more efficient one for extra headroom. Boutique amps may have better speakers, but it's worth looking into because some cats like break up.

An Eminence Li'l Buddy is a good choice. The speaker swap will take you a few minutes. Very easy.

You could even install a 12" in one which is a little more involved.

**Longways to Go** · 01-08-2016, 12:15 PM

Originally Posted by Bezoeker

In my opinion, the classic mod is the way to go for a Princeton Reverb and an archtop. A 25K pot in the second speaker output hole and wired into the bottom of the tone stack (replacing the 6.8K resistor) gives you a midrange control.

This works great in any Fender tone stack with no midrange control. I suppose you could move the knob to the front if you do not use the tremolo, but the second speaker jack hole is the old-time cosmic fart way to do it.

Arguably a 50K pot would let you really start to lift the whole tone stack away, and you can always just keep it turned down if you do not like it.

EDIT: I honestly thought I would not type "jack hole" today. Shows you how wrong you can be sometimes.

I'm interested in this mod. Any downside, as in, does the amp still sound the same?
be looking to boost mids as opposed to cutting them . . .

**wengr** · 01-08-2016, 01:41 PM

Originally Posted by Longways to Go

I'm interested in this mod. Any downside, as in, does the amp still sound the same?
be looking to boost mids as opposed to cutting them . . .

I can't speak about the mod, but the VS20 that I just purchased comes with a mid control, and after only one day I can certainly say that for my application I would not want to be without the mid control.

**Greentone** · 01-08-2016, 01:54 PM

The mod is a good one. OTOH, the fixed resistor is essentially the same thing as having a middle control turned half-way up. This works quite well with the Fender tone stack. I slightly prefer the bass/treble stacks to the bass/middle/treble stacks for this reason. I would generally leave the middle in the half-way position anyway. Certainly, I wouldn't cut it back--there is already enough of a mid-frequency dip in the Fender frequency response as is. There can be some value in boosting mids and cutting the treble and bass settings--but this can easily be achieved on amps with the two knob arrangement by just cutting back on both bass and treble.

To approximate a baxandall "flat" setting with a Princeton Reverb just set the bass = 0.5 or 1 and the treble = 0.5 or 1. That's a setting that essentially takes the Fender "smile" out of the frequency response and gives you an almost Ampeg-like tone. It sounds much like Grant Green at that point. On his Super Reverb he would cut bass and treble and knock the middle up a bit. Same thing.

I find that with Fender amps the fewer tone knobs the easier it is to get a great sound (Marshall, too). In fact, I would go so far as to suggest that with almost all amps the more knobs and sliders you have it becomes a real chore to dial in a great tone. The early Fender circuits with "tone" knobs have great frequency response and it is difficult to garf it up with one knob. (The tweed Deluxe, for example, sounds more mid-rangey than any blackface amp. This is good. My only complaint is that the inter-stage capacitors are all 0.1mfd values--they pass bass frequencies almost down to DC straight to the output capacitor. The Deluxe is a bass frequency monster. There is no control to tame all that bass energy. This is why many tweed amp people desire a tone knob that shelves low frequencies, rather than high frequencies on this amp.)

**KirkP** · 01-08-2016, 03:40 PM

A 25K or 50K pot in place of the 6800ohm resister will both eliminate the mid cut and greatly increase the maximum possible gain in the second stage of the preamp. That could be useful if you want to be able to overdrive the amp. It's probably not so useful if you like to play clean, but the pot can do no harm and gives you another parameter to experiment with.
Dial the pot back to 6800ohms and you are back to stock.

**Teddyfm** · 01-08-2016, 04:12 PM

Love my PRRI. On my fourth speaker now. Stock Jensen, Lil Buddy, Jensen 10PQ and finally a Ted Weber 10A125. Shedloads of different tubes aswell. All good fun, but I think the Weber is the one for me. Perfect match for me.

I got warned about going down the rabbit hole, tube and speaker wise, but its oh so cozy down there. Plus its cheaper than a guitar.

Ted

**Greentone** · 01-08-2016, 04:17 PM

I am a big fan of the Weber 10a125 speaker. I use them in the amps I build that use 10" speakers. (My 5f6 has four of them. I have a 5f2 with one.) The Weber sounds great.

**Herbie** · 01-08-2016, 05:10 PM

Originally Posted by Longways to Go

I'm interested in this mod. Any downside, as in, does the amp still sound the same?
be looking to boost mids as opposed to cutting them . . .

I have had this mod about half a year now in my SF PR (-79). No regrets.

On contrary, in a way I got my Princeton "back"! Now it is very flexible little amp with plenty of classic jazz and other sounds. In a way I got a totally new amp after doing the mod.

When You turn the mid pot up the overall volume rises a bit. But when You turn it down it is Your original Princeton again.

**Bezoeker** · 01-08-2016, 06:42 PM

Originally Posted by Longways to Go

I'm interested in this mod. Any downside, as in, does the amp still sound the same?
be looking to boost mids as opposed to cutting them . . .

Hi Longways,

As already mentioned, this mod has no sonic downside. Your amp will sound the same when the pot is set to 6,800 ohms.

For a modest mid boost, I would use a 25K pot. But a 50K will also work just fine, give you even more mids, and still be just like the amp is now when set to 6,800 ohms.

If you are doing this yourself, let me know and I can open mine to show where the 6,800 resistor is and how I installed the pot on the back.

The pot location I used is the way we did it way back when. You could certainly install it some other way. But even though it is odd to have it on the back it is also a funny nod to "period correct".

Chris

**Bezoeker** · 01-08-2016, 06:45 PM

As Herbie mentions,...

Past about 25K the mid boost continues but you are also now boosting the entire range of the amp. So yes still more mids past 25 K, but it does get loud.

Really none of this is a "boost" really. It is just gradually lifting the entire tone stack from ground and reducing the overall attenuation in the stack.

Hope this helps.

**Greentone** · 01-08-2016, 07:05 PM

Remember, the typical midrange pot on the Fender tone stack is 10K, so the 25K pot is already pretty radical. A 50k pot is extreme. I personally would go with a 10K pot.

**Bezoeker** · 01-08-2016, 07:21 PM

Great point Greentone.

EDIT: I had remembered it as 25K standard, but I am surely wrong.

I think you will also see 10K on the early Mesa amps too.

In my opinion the Blackface sound of the PRRI 65 sounds great with a bit more mid as "normal".

So I am happy with the 25 K.

Thanks for talking us back in off the 50 K pot ledge.

**Bezoeker** · 01-08-2016, 10:20 PM

It was a few years ago that I put the midrange control in my PRRI 65, so I went and checked the details,...

I used a 25 K linear pot. A sort of crap-o-potamus Alpha pot, but it works just fine.

About 3 1/2 on the knob is 6.8 K, the stock resistor value. I am usually around 6 on the knob, but a full 10 is not absurd at all, and even sounds quite good if you like fatness sometimes.

The 6.8K resistor unfortunately sits on the sub-board with all the control pots.

This board is a little involved to remove for a hobbyist.

I removed the resistor entirely, and ran a shielded lead from the "hot" end of the resistor location to the center wiper of the pot, and grounded the "zero" end of the pot.

The pot and knob are mounted in the second speaker jack hole in back. It is very quiet, and very likely a twisted pair would have worked just fine, but shielded is easy enough to do.

I suppose 50K would be a bit overkill. But my thinnest sounding guitar is a Tele with an SD Quarter Pounder in the neck position, so fairly fat for a Tele. Possibly a very thin sounding guitar might possibly want more than the 25 K I have in there.

But indeed at "10" (so the full 25 K resistance) the tone stack is pretty fat. And as Greentone notes, this is fatter than a stock Fender tone stack with a midrange control using a 10 K pot.

That is my story and I am sticking to it.

**Longways to Go** · 01-08-2016, 11:34 PM

Great info guys! (as usual

)

I'm not doing this mod myself -- beyond my skill level. But I did talk to the local amp guru & he agreed it's a good mod & he's done many of them -- and it won't cost much. So I'm heading his way soon.

It's not that I consider my PRRI lacking but it would be nice to have more flexibility. As Greentone points out, bass & treble set around 1 or less is perfect for jazz, but it makes me wonder -- what if you could go just a bit beyond that! I guess it bothers me that the best tone is at the limit of the controls . . .

FYI, I'm running a NOS 5751 in V1 and a Celestion Gold speaker . . . why not add a mid knob!?!

talk about the rabbit hole . . .

**KirkP** · 01-09-2016, 01:02 AM

If you want to go deeper down the rabbit hole, download the Tone Stack Calculator and you can see how frequency response responds to each component value.
http://www.duncanamps.com/tsc/index.html
Of course seeing isn't hearing, but it at least gives you an idea if you're taking things the wrong direction.

**Herbie** · 01-09-2016, 01:45 AM

Originally Posted by Longways to Go

Great info guys! (as usual

)

I'm not doing this mod myself -- beyond my skill level. But I did talk to the local amp guru & he agreed it's a good mod & he's done many of them -- and it won't cost much. So I'm heading his way soon.

It's not that I consider my PRRI lacking but it would be nice to have more flexibility. As Greentone points out, bass & treble set around 1 or less is perfect for jazz, but it makes me wonder -- what if you could go just a bit beyond that! I guess it bothers me that the best tone is at the limit of the controls . . .

FYI, I'm running a NOS 5751 in V1 and a Celestion Gold speaker . . . why not add a mid knob!?!

talk about the rabbit hole . . .

Hmm...are You thinking a mod that would get the Treble and Bass UNDER zero!? Brilliant! Never thought things that way!

BTW – isn't the Cel Gold a bit trebly spkr for PR? I know that Cel Blue is!

**Elamberth** · 01-09-2016, 11:58 AM

Originally Posted by wengr

I can't speak about the mod, but the VS20 that I just purchased comes with a mid control, and after only one day I can certainly say that for my application I would not want to be without the mid control.

How are you liking the amp? Would you say it is loud enough (clean) to play with a drummer?

I have been looking at the VS112 from Sound Island, but am pretty set on a Ravine at this point.

**KirkP** · 01-09-2016, 01:26 PM

Originally Posted by Elamberth

How are you liking the amp? Would you say it is loud enough (clean) to play with a drummer?

I have been looking at the VS112 from Sound Island, but am pretty set on a Ravine at this point.

I've used the Sound Island VS112-JZ20 for about 5 years in many settings including big band. That's the 20 watt 6L6 version if the amp. It does fine with drummers.

**Elamberth** · 01-09-2016, 01:38 PM

Thanks! How are you liking the amp?

Do you use it for strictly jazz, or other styles as well?

**KirkP** · 01-09-2016, 03:31 PM

Originally Posted by Elamberth

Thanks! How are you liking the amp?

Do you use it for strictly jazz, or other styles as well?

I like the Vs112-JZ20 a lot. It's very well made, including inside the chassis. One preamp tube had to be replaced and I did one bias adjustment on the power tubes. For a 5-year old tube amp that's low maintenance. The point to point wiring will make it easier to repair or mod than a Fender reissue, when the time comes for that.
I mostly use it for jazz, but the big band calls for rock or blues solos now and then. The VS112 is more midrangy and darker than a traditional Fender tone stack, so for rock or blues I usually cut the bass and mids and boost the treble much more than I'd do on a Fender. Mine has no bright switch, so it doesn't have the ice-pick brightness of a Fender.
I love having a dwell knob for the reverb. The presence control is useful for adding highs, but much more subtle than a bright switch.
I sometimes miss the tremolo. All the components for tremolo are in the chassis, but the dwell and presence knob bumped them off the control panel! It's not a big loss though, since there are cheap tremolo pedals.
I use a Jenson Tornado 12" neo speaker, but it sounded great with the stock speaker as well. Neo saves weight.

**Greentone** · 01-09-2016, 03:50 PM

Speakers can do a lot. As a teenager I played through a Teisco Professional. Basically, this was Teisco's home market professional market amp--not imported into the USA. It was a 50-watt circuit based around a Fender Pro Reverb Amp design, but employing the more commonly available in the Asian market British-designed tubes (e.g., EL-34 power tube duet, GZ-34 rectifier, etc.) The speakers in the amp were twin 12" heavy-duty Pioneer drivers.

Given the tone stack (Fender/Marshall design) the amp should have sounded like a Fender combo, albeit louder given the final tubes, transformers, and voltages. Conceivably, the amp could have had a Marshall-like grind/crunch instead of the singing Fender tone. (Actually, the amp sounded sort of like a 50-watt Hi-Watt.)

Due to the frequency response of the speakers, however, the combo amp sounded pretty flat, with a strong midrange--more Ampeg V-series meets Hi-Watt, than Fender/Marshall. However, when I ran the amp through a Fender cabinet or a Marshall cabinet it sounded VERY different. Suddenly, there was the mid-range frequency dip--the speakers in the combo amp were counteracting it, to a quite pronounced extent.

For a few years I put Oxfords in the Teisco and enjoyed a Fenderish combo amp, until I realized that I could just buy a Fender combo and restore the Teisco--and have something that sounded big, unusual, and pretty gnarly--with a deep reverb and a great, bias-oscillating tremelo. Those old Pioneer drivers still work as well as ever, by the way. The amp needs some freshening though.

The point is, you can modify the circuitry of the amp to get your sound, or you can install a different driver to alter the tone. I guess everybody knows this already, but I thought I'd put in a reminder.

**roegtr** · 01-09-2016, 04:33 PM

Just as an addition to the fact... I just replaced the original Fender labeled speaker in my recently accuired Princeton Reverb II with a Celestion Creamback 65w (g12m-65 I think..). It definitely transformed the sound, and to the better. A speaker change has a big impact! Next I'm thinking of adding a bias adjustment pot and to install 6L6 power tubes for tighter lows

**KirkP** · 01-09-2016, 05:22 PM

Originally Posted by roegtr

Next I'm thinking of adding a bias adjustment pot and to install 6L6 power tubes for tighter lows

Make sure your power transformer can handle the filament current of the 6L6. 6L6 tubes might change the optimal speaker impedance. If there's a mismatch you'll lose some of the headroom you were expecting. And of course the bias current must be changed.

**Elamberth** · 01-09-2016, 06:17 PM

Originally Posted by KIRKP

I like the Vs112-JZ20 a lot. It's very well made, including inside the chassis. One preamp tube had to be replaced and I did one bias adjustment on the power tubes. For a 5-year old tube amp that's low maintenance. The point to point wiring will make it easier to repair or mod than a Fender reissue, when the time comes for that.
I mostly use it for jazz, but the big band calls for rock or blues solos now and then. The VS112 is more midrangy and darker than a traditional Fender tone stack, so for rock or blues I usually cut the bass and mids and boost the treble much more than I'd do on a Fender. Mine has no bright switch, so it doesn't have the ice-pick brightness of a Fender.
I love having a dwell knob for the reverb. The presence control is useful for adding highs, but much more subtle than a bright switch.
I sometimes miss the tremolo. All the components for tremolo are in the chassis, but the dwell and presence knob bumped them off the control panel! It's not a big loss though, since there are cheap tremolo pedals.
I use a Jenson Tornado 12" neo speaker, but it sounded great with the stock speaker as well. Neo saves weight.

Thanks! It certainly is one amp I amp I have been looking at closely. I have had to go lightweight recently, sold my heavy amps, and am looking for a new primary live/ jazz recording amp. I'm using a DV Mark Little Jazz, but want an upgrade, and will keep the DV as my backup/ secondary.

I am doing mostly jazz these days, solo and with a trio, but play some blues & the occasional rock song . I can get what I need from a Fulltone OCD for overdrive.