Princeton silverface problem

[DCarter]

I have an early 70's Princeton Reverb that sounds great for about an hour or two, then gets extremely brittle. I replaced all tubes, no change, then took it to a repair shop. I made sure to tell them about my sound quality problem after tubes got good and hot, but he said it checked out ok. Do you guys have any ideas on possible causes? OT, filter caps? I really want this Princeton sound, but if I can't get it fixed, I can't trust it to take out for a performance of more than an hour.

[soundguruman]

The filter caps for B+ and bias supply are the usual culprits. Caps will leak or open more and more as temperature increases...

Technician is NOT guitar player. Amp will test 100% good...and sound like crap.

Technician really has no test for "tone," or sound quality. In fact most techs -- are tone deaf.

I have said it many many times...
"Listening" is the other 50% of repair job. The part of the repair that is almost always ignored.

BUT listening is the most important part, to a guitar player.
Something the tech will never understand.

[DCarter]

Thanks soundguruman. That makes sense to me. The guy I took it to has a good rep in Nashville, but he was not receptive to my mentioning filter caps? However, he didn't overcharge, and did replace the power cable & reverb tank, which was needed. If I can find another guy close, I think you may have solved my problem. I really don't want to have to buy yet another amp, bought this one in 1978.

[Jazz P Bass]

Have you explored the fact that it may be the speaker?

[DCarter]

Jazz P Bass, i've had concerns about it being 40 years old, but it's checked out good a couple of times. I probably will replace it, tho, and box up the original. I'm thinking a new speaker would have to improve the tone. Good advice, thanks.

[Jazz P Bass]

I was only thinking that the speaker may be the issue, because the tech signed off on it as good.

2 hours playing just may be heating up the voice coil to the point that it rubs in the magnet gap.

[DCarter]

I was only thinking that the speaker may be the issue, because the tech signed off on it as good.

2 hours playing just may be heating up the voice coil to the point that it rubs in the magnet gap.

. Ahh, yes, I see what you mean. Definitely something I never would have thought about. This forum is great! Only joined a couple hours ago and already learned so much. What would be your opinion on installing a cooling fan?

[Enzo]

"Checked out" is such a vague term...


Not only should you play the amp through diferent speakers, we'd also like to suspect the guitar. Does someone else's guitar do the same thing through the amp? What kinds of pickups are you using. If they are actives, you might simply have an aging battery.


I remember a guy bringing me an amp once, to tell me the thing played OK the first few minutes, then got thin and distorter, etc etc. What kind pickups you got? "Active" he says. I asked how old is the battery then. And has says "Battery? What battery?"

[DCarter]

Enzo: great idea to a/b the speakers when the Princeton starts sounding funky, I will definitely try. My guitar is a 72 Tele Deluxe hollowbody thinline, Seymour Duncan passive pickups, has always been solid when recording straight in to my board, so I think it's ok. I have priced Celestions and Weber replacements (and reconing) just in case I need to replace the speaker. Talked to another tech today about the filter caps, he seemed to understand what soundguruman was telling me, so he took it in. I will know in a few days whether the problem is in the tubes, board, or caps. Then I will try the speaker test. Think I can install that myself, even tho I am limited in electronic skills. Thanks man!

[g1]

Bad filter caps are not going to take an hour or two to exhibit a problem. And if they are bad, you might have some loss of bottom end, but nothing like what I would describe as "extremely brittle".

[Leo_Gnardo]

I will know in a few days whether the problem is in the tubes, board, or caps. Then I will try the speaker test.

If you do elect to change speakers, might want to consider moving up to a 12. More clear low end with most 12's plus a much wider choice of speakers compared to 10's. Yes you would have to swap out the baffle but if I'm not mistook Mojo sells a Princeton baffle with properly offset 12" hole for @ $65.

Trying to force low end out of a 15 watt open back amp with a 10 in speaker has its limits, and the first thing you hear as you approch max power is flabby tone.

Stock Princetons use a 12AX7 as an output inverter and vibrato tube. With the AX one side of the output waveform collapses just as you reach full power. You can substitute a 12AT7 and get a jot more clean volume (typically 30 to 50%) but the vibrato is diminished or disappears - there are methods for making vibrato work better with an AT but won't get into this unless necessary.

Also - closing up the back of the cab will yield better low frequency response. You can experiment with some thick cardboard, then decide whether to commit to wood. Just leave some space for air circulation around the tubes & maybe fit the power cable inside.

[Enzo]

Before changing speakers, why not simply disconnect the one in there now and connect the amp chassis to some other speaker cab? We don't need it to be the tone you like, all we need to know is whether the tone goes brittle after an hour or two. Once this tone thing is cured, THEN you can shop for a permanent speaker if you like.

[DCarter]

Agreed, Enzo. I have a 1979 Musicman 112 RP (which has nowhere near the warmth of the Princeton, but still a good amp) that I can set side by side with the Princeton, make the connections with cables, then the story will be told!

[DCarter]

Good advice on the mod to a 12, worried about the tube heat issue if I enclose too much. Thanks, man!

[DCarter]

Yeah, I hear what you're sayin'. Makes me wonder a little bit about what other possibilities might be. The sound after two hours is just screwed, hard to describe, which I first thought must be heat-related. But totally unacceptable if I have to represent as a lead guitar player. If I get it tracked down, I will post again. Thanks for the expert help!