Princeton Reverb Tweaks
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Fender
Princeton Reverb Tweaks
From tremolu--(at)--ol.com.no.uce Thu Jul 8 19:44:35 CDT 1999
Article: 188629 of alt.guitar.amps
From: tremolu--(at)--ol.com.no.uce (Tremolux)
Newsgroups: alt.guitar.amps
Subject: Re: SF Princeton Reverb Question
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Date: 08 Jul 1999 17:42:34 GMT
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Xref: geraldo.cc.utexas.edu alt.guitar.amps:188629
>Well hey Tremolux, (my tweed is a Tremolux),
So is mine. A 59.
>I have a 70's Princeton too,
>that's why I felt comfortable saying keep it, it's a nice amp.
Yep. Mine is a 69.
> Is this tweak
>easy to describe?
Fairly.
> Does it compromise the durability?
Not really, but the extra power can make life a bit harder on the speaker. (As
you would expect.)
> Are you talking about
>maybe adopting it to use 6L6's?
>
Hell no. Still uses 6V6s. No change in tone, no change in appearance, just a
little louder. More of what you already have. No big deal.
In most Princetons, the limiting factor in power output is that the phase
inverter starts clipping before the 6V6s. Once the PI clips, there is nothing
left to push the 6V6s any harder. The tweak simply gives the PI a bit more
dynamic headroom so that the 6V6s clip first. You simply move the B+ feed for
the PI up one notch on the power supply voltage divider. Give it more voltage
to work with, it's dynamic headroom increases, and now you can drive the 6V6s
all the way. Leave the B+ feed for the remainder of the preamp stages alone.
Move only the PI. Up one step. That's it. Very simple. You just move a wire.
One other thing I did, which is optional, is that the voltages in the Princeton
are a bit lower than in it's bigger brothers. Yet it uses the identical
circuit to drive the reverb pan. The lower voltages result in a lower drive
level to the pan. IMO, the reverb sounds better if you can bump up the drive a
tad. I did this by reducing the value of the 12AT7 cathode bias resistor from
the nominal 2.2k to 1.8k. Running the tube at slightly higher current gives a
tad more drive and livens up the reverb a tad. If you are happy with the way
your reverb sounds now, just skip this.
You can also help keep it from farting out by reducing the value of the
coupling cap feeding the input of the PI. That split load PI circuit topology
has a VERY high input impedance (several megohms) since the grid leak resistor
is returned to essentially half the load (in the cathode leg), which follows
the signal. That very high input impedance along with the stock input cap
value result in lots of low end crap getting through which you don't need. I
recommend using 1/4 the stock value. cleans up the low end fartyness with no
loss in "real" low end tone.
The stock speaker in these amps is a real limp-wristed turd. These amps
respond amazingly well to a speaker improvement. The WeberVST C10Q seems to be
the best choice, it can handle the (increased) power no sweat, it's WAY
efficient (loudernhell) and had a vastly improved low end compared to the stock
speaker. Yet it still has that classic Jensen style vibe. Try it, you'll like
it.
You do all these tweaks, you'll end up with a nice little Princeton Reverb that
will rattle windows and is usable for small gigs. Yet it still looks stone
stock, and sounds the same as before, only LOUDER. All tweaks are very easily
reversed. No Torres level hacks or butchery.
One last item, and this is to be considered as an optional option (think long
and hard before doing it), is to replace the stock (somewhat wimpy) output
tranny with one for a Deluxe Reverb. The DR unit has more iron, and fits
perfectly (there was a second mounting hole already in the chassis to accept
the increased leg spacing). I just bought a generic replacement DR tranny from
Mojo for mine.
IMO, this is the way Fender should have built the PR, but I'm sure they didn't
because it then becomes too close in tone and volume to a real DR, and they
needed more product differentiation. Oh well....
J.
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