From mural–(at)–m-us.campus.mci.net Tue Jan 7 10:28:45 CST 1997
Article: 33482 of alt.guitar.amps
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From: “mural–(at)–alimusic”
Newsgroups: alt.guitar.amps
Subject: ADD GAIN TO SF PRINCETON
Date: Mon, 06 Jan 1997 20:07:41 +0000
Organization: Kali Music on the Internet
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CC: Scott A Warren
On Sun, 5 Jan 1997, Kali Music wrote:
> Ffgtr wrote:
> >
> > About a week or two ago, there was a bit of traffic concerning Hoffman
> > Amps. cheers
>
> I was in Sarasota last week and stopped by to meet Doug and try his
> amps.
(snip)
> He also helped me with ideas on adding a gain stage to a ’72 Princeton
> that was gutless. Now this cutie has so much gain I have to keep her on
> a leash! Thanks, Doug, if you’re *listening*.
> Murali
>>
Could you detail that mod? A friend has the same amp, and he is dying
for
a little more gain. Thanks.
Scott Warren
OK! Please note that you lose the trem with this mod. Gecherself a
schemo of the AA1164 Princeton Reverb and a schemo of the AA964
Princeton. You will see that the PR has a gain stage using the second
half of the 7025. This is the stage that is missing in the AA964
Princeton. You *insert* this stage between the volume control and the
second half of the 7025 shown on the AA964 schemo. You have the option
of using either the half of the tremolo/phase inverter 12AX7, or, as I
did, using the other half of the 7025 (1st preamp tube). It’s just a
question of where you want the tone recovery amp stage to be in terms of
how the leads are physically located in the chassis.
*******I don’t want to be a killjoy, but I feel that if you can’t read a
schematic enough to figure this mod out, you should probably have a tech
do it for you.*****
The voltages are lethal and you might just screw up your amp and have a
very expensive repair bill. A competent, reputable tech should be able
to perform this mod for about $50.
FIRST: I tore down the entire trem circuit and opened up some space on
the eyelt board. Then I proceeded to assemble the parts necessary and
built the new circuit in a convenient location on the eyelet board.
NOTE that you will have to disconnect the negative feedback circuit, or
switch the wire going to the 2K7 ohm resistor to the black output trans.
wire (ground) or your amp will OSCILLATE!
NOW: I went a step or two further and routed ALL the leads under the
circuit board, the way I believe Leo intended. This is not too bad
(time wise) on something as simple as a Princeton, but it will increase
the cost of having a tech do it.
THEN: I used the Intensity Control to vary the bias by running a 1 Meg
resistor to ground from what was the brown lead on the Intensity pot.
This enables you to vary the negative bias resulting in about 15ma
difference from 1 – 10 on the pot. I don’t recall offhand, but I might
have tweaked the bias feed resistor a bit to get the tubes to 25ma at
idle with the Intensity Control at 1 (off). So you are now able to get
your tubes anywhere from 25 to approx. 40ma. You WILL hear the
difference. BUT: don’t use Sovtek 6V6s! They won’t last long at
40ma………
SO, now you’ve got this absofuckinglutely SCREAMING Princeton and your
wife or girlfriend or neighbor loves the tone but has a REAL PROBLEM
with the volume. Take the Speed Control and turn it into a Master
Volume by interupting the circuit, going into the Speed Control (lug
that has a Blue wire) from the output of the last gain stage, and going
>from the wiper of the Speed Control (center lug) to the Phase Inverter,
denoted by the letter “X” in the box on the AA964 schemo.
AND you have tamed the beast.
There are a few tweaks you could try, like a 2M2 ohm resistor in place
of the 3M3, or different value caps to bypass this resistor (or no cap
at all). Or you could put in a push pull pot that lets you bypass the
tone stack completely. WOW: talk about gain. Or different value caps in
the new gain stage to voice the amp just right.
These are really fun amps to work with but please be careful.
OH, uh, one more tip: RUN, don’t walk, to your nearest tube outlet and
pick up some NOS 6EY6 tubes. Use ’em in place of your Sovtek 6V6s (or
any other 6V6s that you’re not happy with). You can probably find them
for about $5 each (YES! NOS American made tubes that sing for only
$5!!!!). They spec out very, very close to 6V6s, though I’ve been
warned by Tremolux you may need to reset your bias slightly lower (more
negative volts). I have not found that to be the case in my TV front
Deluxe, though. I plugged ’em in, turned it on, measured the bias and
they came up exactly the same as the NOS 6V6s I had been using. Sound
awesome, dude. The only question I have is how long they’ll sound
good. But see if you can find Mullard or Tung Sols or something good
like this and they should last as long as any good 6V6.
I would appreciate hearing how they like it, from anyone who tries this
mod. I tip my hat to Doug Hoffman for the basic idea, and to Tremolux
for the 6EY6 tube tip. (Always remember to thank your Gurus, dude!)
Good luck and enjoy the
sustainnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn……………………………
m.
