'76 Princeton Reverb reverb tone/level mod?

[mike_mccue]

I have a super clean Fender Princeton Reverb with a factory JBL k110 speaker. The only thing that is not factory original are two well matched 1980's era JAN 6V6GTs that I put in last spring. The amp is pre pull boost.

The amp sounds real nice in general but the reverb circuit is a very muddy and seemingly overdriven. The sweet spot is on "2" and it's a light reverb effect. If I crank it for a surf style deep reverb it just sounds "not so good" and it certainly sounds muddier than any other Fender reverb amp that I own.

When I just drive the amp with out reverb it sounds great all the way to top volume.

I'm starting to try to learn about ideas for modifying the reverb circuit to see if there is a deeper reverb sound available that isn't so muddy.

Maybe I need to tweak the frequency response or, perhaps just tame a gain or driver stage?

Does anyone have some ideas about this particular model of Fender amp? It's my first Princeton Reverb, but I have a Deluxe Reverb and a Super Reverb and I have had some Twins. This amp doesn't have the same bright reverb sound.

best regards,
mike

[Tubenit]

dwell pot

You can add a reverb dwell pot on the back of the chassis which may help.

However, before you do that try a 680k dwell resistor or using alligator clips, wire and a 1M pot to experiment with a dwell pot.

With respect, Tubenit

[mike_mccue]

Excellent, thanks. I would like to "collect" a few ideas and then go focus some study on the options.

Others suggestions are welcome! :-)

best regards,
mike

[TheElectricMoron]

Every, and I mean EVERY, Fender on-board reverb sounds better with a 12AU7 in the driver slot. You will be able to use almost the whole range of the control and it will still surf like a mutha. And of course it is a completely reversable and solder free mod. What more could you ask? But you'll love it as a reverb driver.

The 12AU7 is also a totally GREAT tube for the Fender long-tail PI's too, but not for the Princeton.

Common as dirt and about as cheap the AU is way cool tube for lots of nasty places. Reverb drivers and PI's are not the only use.

JMO of course

Charlie

[slidincharlie (Carlo P)]

Every, and I mean EVERY, Fender on-board reverb sounds better with a 12AU7 in the driver slot.

Once I put a 12AU7 in the reverb driver slot of my '79 Vibrolux Reverb. It's the version with a 680 ohm cathode resistor instead of the BF and earlier SF setup with a 2k2 bypassed with a 25 uF cap. My aim was a mellower reverb more usable through the whole pot range (sort of Mike's same issue).
After a few gigs the 680 ohm cathode resistor burnt. A tech told me that it was due to the 12AU7 that draws more current than the stock 12AT7.
Does it make sense? Would a 1W 680 ohm resistor help?

[mike_mccue]

Is that a result of the the lower plate resistance of the 12Au7 design?

just curious.

best regards,
mike

[TheElectricMoron]

I do not find a schemo of the VibroluxReverb with that cathode arrangement. The schemo I found is dated 2000 ad has the 2k2 bypassed cathode resistor. I think I'd just stuff that in there instead of beefing up the 680ohm that was there. Have you seen a schemo with that 680ohm configuration? Pretty impressive that the tube would take that much heat - and it will take more load than a 12AT7. With the insane volts that Fender stuffs onto those drivers I can't imagine a 680ohm lasting for long there - particularly those silly 1/2 watt toys they use. A 2k2 resistor and 25mf or less bypass cap will make you happy I think.

Charlie

[TheElectricMoron]

Mike - no, just lower gain. And it will take the heat better than a 12AT7 too.

Mostly I like it because it really makes the reverb controlable and it's way cheap.

Charlie

[mike_mccue]

Thank you to both.

best,
mike

[slidincharlie (Carlo P)]

Hi Charlie.
The 2000 fender schematic that you've seen must be the modern Custom Vibrolux.
I attach here the original schematic of my '79 Vibrolux Reverb (pull-boost).
You can see the 680 ohm cathode resistor on the reverb driver (and also an old pencil note where I wonder where the 2k2+25uF arrangement noted in older schematics is...).
Right now I don't remember if I've already switched to the older 2k2+25uF setup in my Vibrolux. I recall that I did it at some stage a while back.
When I had smoke and a burnt resistor it was with the stock 680 ohm cathode resisitor and a 12au7 in the reverb driver slot.