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68 Princeton -- can it be made better?

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12/14/1997 7:56 AM

68 Princeton -- can it be made better?

I just picked up a 68 Princeton Reverb yesterday (my first vintage!), and was wondering if there was anything (like the caps on the tube grids?) that could be done to improve the tonal characteristics of the amp. Also, this thing still has the original 2 prong power cord, which has got to go! Not knowing squat about electronics, I'm VERY hesitant to fry myself before getting a chance to enjoy the amp. Right now, the thing sounds great, but I figured I'd see what you guys might have in your bag of tricks to make it sound even better. I know the pots need to be cleaned, and I know that involves getting close to those deadly caps, so if anyone could point me towards some clear consise FAQ's or something to help me out, I'd appreciate it.

Thanks!

Steve

12/15/1997 2:51 AM

Steve:

    As for those deadly caps, make up a test cable with a 15K resistor in series covered with a few layers of heat shrink tubing. I put a green alligator clip on the longer end (8 to 12 inches) and a red clip on the short end (4 inches). Clip the green clip to chassis ground and then touch the red insulated clip to first one and then the terminal of the standby (of course your amp is unplugged). Once you learn where B+ comes into the circuit board, ground out those points through the resistor next. Some people recommend leaving the test clip on the standby switch as the big filter caps are known to have a memory and can build up a charge even though the amp is off and unplugged.

    I learned about filter caps the hard way when I was a kid. Someone gave me an old tube hifi amp and I thought I'd see if I could turn it into a guitar amp (you just need to change the input jack to 1/4" mono- right? ). I plugged it in to make sure it worked and then unplugged it before reaching in and getting the shock of my life ("but it wasn't plugged in!")

    Torres has some great rebuild mods for Princeton Reverb amps, but since you mentioned its value as a vintage amp (no cigarette burns on the Tolex and the grill cloth wasn't painted up psychedelic?) I'd stay away from such drastic mods. Play it for awhile and decide what you'd really like to change about it- like whether you'd like it to be a little bit brighter or deeper or whatever. There are a lot of simple mods involving replacing some of the caps with different values or different types. Keep the old caps in a bag- maybe label their location with a piece of tape in case you want to put them back.

Steve Ahola

12/15/1997 10:47 AM

I recently bought a '68 Princeton Reverb that sounded good, but a little bit flat out of the store. I bought the Torres "Blackface Mod" which I originally thought would put the amp back to blackface specs. It actually just made some changes that were not original to the blackface but supposed to simulate a blackface sound. It made the amp sound GREAT, very warm and sensitive. It basically changes the cathode bypass circuits, plus some other small things, just a few capacitor and resistor changes.

Torres is known for being very sloppy about shipping the right parts and for telling you what to do to your amp but not why you're doing it (ie, telling you that you're making your amp a blackface when you're actually doing something totally different), but if you know little about electronics, this mod may be for you because Torres takes you through it step by step. Just keep in mind that you're doing a "cathode bypass mod" plus some other things, not a "blackface mod". If you can soder, you can do it. I would suggest ordering the directions for how to do it from Torres, then buying the parts from Mouser Electronics. You can get better caps, and you can also get carbon composition resistors, which are cosmetically exactly the same as the resistors already in the amp. If you do a neat job, you can make the electronics look and sound really nice. I compared my modded PR to a friend's blackface PR and mine was actually a bit better.

Chris Collier

12/15/1997 3:17 PM
steve watson	I can solder, but I'm still waiting for someone to give me an understandable explanation of how to drain the killer voltage. Before that happens, there's NO WAY I'm going inside this thing.

12/15/1997 11:10 PM

The message before mine tells how to do it. Maybe it's not quite clear enough.

O.K. There are 2 ways to do it. One is supposed to be easier on capacitors, the other way is quicker.

First way: (easier on caps)

1. Get a 10k 2 Watt resistor. Connect it to two insulated alligator clips, so you have a device with alligator clips at each end and a resistor in the middle. Tape it all up with electrical tape so there's no exposed wire.

2. Find the filter caps. On the Princeton, they're in the long metal cylinder that hangs down from the chassis just behind the power tubes. Looking down on the inside of the chassis, this unit will look like a circle with four contacts sticking up out of it.

3. Attach one clip of the discharging tool you just made to one of the contacts. Then touch the other clip to any part of the chassis. Repeat this with all four contacts. (One of the contacts is negative, and you don't need to deal with it but I don't know which one it is, so you might as well assume they're all positive, just to be safe.)

The quick way is to take a screwdriver with an insulated handle, touch the end to both contacts of the power switch simultaneously, then, with the end still on the switch contacts, rotate the screwdriver down until it touches the chassis. Keep your hand on the handle. There may be a pop and a spark. (with an amp with a standby switch, you'd do this on the standby switch, not the power switch)

If it makes you feel better, wear rubber soled shoes and use only one hand when you work, that way you know there's no path through your body that the electricity will be even remotely tempted to take.

12/16/1997 7:24 AM
Whit	Chris, With all due respect, method 2 (the screwdriver shorting) can potentially damage the cap. For a high capacitance, it could melt the tip off the screwdriver as well. I sense a little tongue-in-cheekiness perhaps? ... Whit

12/18/1997 3:06 PM
bubba big amp	unplugging the amp and turning on the standby does a good job of drainig the initial high voltage but always discharge them to ground to make absolute sure they're harmless.

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