1972 Pinceton Reverb problems

[MGW]

I have my uncles 1972 Princeton Reverb. I believe it's the AA1164 version given the age and the reverb driver 2K2/25uf R/C combination. If I don't drive it real hard from the guitar, it sounds pretty good. The minute I hit it with a humbucker it sounds like a blown speaker. This happens with or without the 12AT7 installed. This leads me to believe it is something other than the reverb transformer...but I could be wrong. I have replaced all of the 25uf bypass caps as I have heard tell they could be faulty...went that route cause it was cheap. The reverb xformer is pretty cheap also. The voltage looks pretty good. Maybe a titch low in some spots, but nothing crazy. The schematic specs the plate of the 12AT7 at 395 and it's at 380. Not sure how that impacts the situation. Ideas on where to look next?

[Structo]

You may want to try some grid stoppers on the power tubes.

This will help those symptoms with the humbuckers.

Try 1K5 resistors on the 6V6 grids.

[rock_mumbles]

I have my uncles 1972 Princeton Reverb. I believe it's the AA1164 version given the age and the reverb driver 2K2/25uf R/C combination. If I don't drive it real hard from the guitar, it sounds pretty good. The minute I hit it with a humbucker it sounds like a blown speaker. This happens with or without the 12AT7 installed. This leads me to believe it is something other than the reverb transformer...but I could be wrong. I have replaced all of the 25uf bypass caps as I have heard tell they could be faulty...went that route cause it was cheap. The reverb xformer is pretty cheap also. The voltage looks pretty good. Maybe a titch low in some spots, but nothing crazy. The schematic specs the plate of the 12AT7 at 395 and it's at 380. Not sure how that impacts the situation. Ideas on where to look next?

I think you may be describing what happens to the amp when it's driven too hard (blocking distortion) ...
(personally) I love a Princeton BUT stock they have too much low end for a guitar with hot HB's ... they are a wonderful Strat amp ...

What happens if you back off the guitar volume on the HB equipped guitar?

I'd look at putting a 470k grid stopper on the cathodyne ... that may not completely fix the issue but should at least help some.

There are a couple of common cathodyne mods for Princetons

[MGW]

Good stuff. I'll give all of the above a whirl and report back.

[Leo_Gnardo]

Why why why suspect the reverb transformer? Oh, my car won't start. Blame the windshield.

Would be good to see what the amp's doing on a scope. I've seen healthy Princetons turn out anywhere from 12 to 20 watts, depending on what tubes are installed. Even more with SS rectifier and 6L6 output tubes with 12AT7 driver.

Have you checked the resistors in the power surprise chain? The 1K first in line likes to burn. That would drop your screen grids, reduce available power, also reduce voltages at lower voltage nodes.

Very worn-out output tubes are suspect.

Of course it IS possible the speaker is shot. Try the amp into a known working speaker.

Don't panic. Whatever it is, you'll get it. And a properly working Princeton is a wonderful thing!

[Firestorm]

Check your bias voltage, too. The Princeton is not adjustable so if the resistors drift the bias can wind up way off. Have you tried the low input with the humbucker?

[MGW]

I would love to see it on a scope too, but I have this little problem of not having one.

About the reverb transformer...from what I've read, they seem to be a bit problematic at times...however, that seems to be less of a problem with the AA1164 version. BUT...the problem is worse with the reverb pan in. Also, the problem is worse with the reverb turned on. I tried to see if the pan was having issues so I jumped the reverb in and out togther and that made it a lot worse. Mostly...the reverb driver was because everything I had read pointed in that direction.

Tried a good speaker...same thing. Drove the PR speaker with my Bassman...PR speaker is fine.

I'll check the power supply line and bias as well. I assume if the 1K is shot, then a higher wattage 1k resistor is in order or I'll be asking for a repeat performance.

[Colossal]

Rock Mumbles is onto it. Overdriving the cathodyne in Princetons sounds like crap and is easy with humbuckers. Way too much low end and the blocking distortion that results is awful. Try a large value grid resistor on the entrance to the cathodyne. You've got to keep the grid from drawing current from the coupling cap preceding it. Cathodynes are very linear and great PIs to the point where they collapse taking the tone with it.

Try running a 7 string with a low B into a Princeton if you want to hear what blocking distortion sounds like. Sounds like someone pulled the instrument cable out the guitar and left it on the carpet with the amp dimed: A huge horrible blatting sound that wants to shake the speaker loose from the cabinet.

[Firestorm]

I think they left off the 220K to reduce the attenuation of the 3M3 voltage divider, compensating for no gain in the PI, but yes, that makes it easier to drive the Cathodyne into ugly territory. A good 12AU7 does sound wonderful in Fender reverb circuits.

[Milkmansound]

adding that 220k back in saps the soul from the amp - try changing the cap at the input to a .001 instead. That strips out the low end that you will never hear anyway

I posted my special princeton recipe on this site before - it will not be a princeton anymore if you do what I recommend though. If you want it to be a princeton, then keep it the way it is

[MGW]

I think I may have a voltage issue. The PT is giving 714VAC. My rectified voltage is about 385 VDC. Could the rectifier be part of the problem? Im thinking if the voltage is low from the beginning, then it's gonna get nasty pretty fast

[Firestorm]

Tubes in? That could be underbiased tubes pulling down the supply.

[MGW]

Tubes are in. I've made a couple of changes and it sounds much better...until I plug the reverb tank back in. Then it gets nasty.

[Milkmansound]

Did you try different tanks and reverb leads?

[Leo_Gnardo]

I tried to see if the pan was having issues so I jumped the reverb in and out togther and that made it a lot worse. Mostly...the reverb driver was because everything I had read pointed in that direction.

Hah! That was the basis for the old "Ice Cube" gadget back in the late 70's. Run the reverb drive right into the return, oh with a 100K resistor but who's counting... yeah that's a sure fire nasty overdrive.

I'm thinkin' the reverb drive tube, or possibly its cathode circuit. Sometimes the resistor's roasted or cap shorted or both, from a shorted 12AT7. Often I cut out the cap paralleling the Rk just to dial down the reverb drive level which is insanely high. Another method I use to calm down the reverb, is put a 10K 2 to 5W resistor in series with the reverb drive transformer primary. Think about this - the reverb drive is attached to the same circuit point as the screen grids. If the reverb is raising hob with some sort of noisemaking, that will show up in the screen grids and add distortion, not the kind we like.

And of course, it could be a defective transformer. But it would be a drag to order one up, install it and find all the same problems.