1969 Princeton blueprinted with some mods

[KeithStevens]

Hello,
I've been lurking for some time and this is my first post and first real project. I learned a lot from this board, and I really appreciate the group spirit here (not to mention the expertise!). So first off I'd like to thank all of the contributors here.

I've been a big tube amp tone junkie for some time but recently became really motivated to truly understand the inner workings. Not the least of which is I have ended up with a collection to maintain.

I've had this one for close to 20 years. Never really sounded all that great turned up, but it was a good clean practice amp. Then it developed a bad hum and just went into a corner. I ordered the parts for a blueprint and some alternate values based on this board and other sources.

I'm very excited to continue now, because I fixed it and I really like the tone I'm getting from it. It's just so fun to play with now. Attached are samples of my McCarty straight into the amp. Middle position mostly, amp on 7, guitar on 6 and going up or down with the volume lets me control the drive on the amp. The other file is the bridge pickup on a little hotter.

Looking forward to actually build something now. I think I'm really hooked. I'll post some pictures of the interior when I get them off the camera (needs it's own special cable grrrr).

[Structo]

Sounds like you got it working good!

What all did you do to it?

Caps and tubes?

[KeithStevens]

Sounds like you got it working good!

What all did you do to it?

Caps and tubes?

I got a blueprint kit from Area 51, so all the electrolytic and coupling caps, the plate resistors, then from Mojo, balanced the PI section with close spec Dale resistors. The hum was coming from the big capacitor in the bias circuit (which explained why the Tremolo Intensity control turned the hum up and down. I haven't got a replacement for the big cap can yet.

I never liked the tone balance of the controls, so I took the advice from the Aspen Pittman book and used .022's in the tone stack instead of the stock .1 and .047. I used a mix of mojotone caps, spragues and some silver mica's I also got from mojo. I've been trying various tubes to tune the sound. I'd ideally like the tone stack to do something. I guess I need to figure out how to hard wire the bass to 3.5 because I don't think I'd ever need to move it. The treble is at least functional now (not "how much ice pick do you want in your ear.")

I also put in the 470Ohm screen grid resistors. I also tried the Mesa Boogie technique of using a shielded wire from the input straight to the tube. I pretty much decided I would get this thing totally right for my number one guitar and everything else is secondary.

The cool thing has been firing it up after each section to see what affect on the sound each change did as I blueprinted and tried other values. That was quite an education right there. I really enjoyed that.

[Structo]

Which circuit is your amp?

Is it the reverb model?

When Fender changed the Princeton to a 2 x 6V6 amp they really improved that amp.
Some call the Princeton Reverb one of the best recording amps ever.
Along with the Super Reverb of course.

Not sure why the shielded input is called the Mesa technique as it is just a more modern way to clean up an amp and reduce noise.

Not sure if your amp qualifies but some of the worst wiring I have ever seen is in the silverface amps.
They used a lot of wire!
Gerald Weber tells you how to clean up a lot of wire by moving the tone stack component to the pots and eliminate some long runs.

Something you can do to customize the tone stack to your liking is to experiment with the value of the slope resistor.
That is the 100K that comes off the plate of V1.
On your amp it should be the 100K on the lower right corner of the board looking from the rear.

Not sure of your background but Schematic Heaven is a treasure of a site for old amps and some newer ones as well.
http://www.schematicheaven.com/index_HTML.htm

Another cool tool is the Duncan Tone Stack Calculator.
You can plug in different values and then it charts a graph of the frequency response.

http://www.duncanamps.com/tsc/index.html

He also has a power supply designer software, all free ware.

[KeithStevens]

Mine is the AA1164 model Princeton Reverb. I think it was probably the second year of the silverface, but that schematic was untouched for several years and still the same as the blackface version of this amp.

When I first opened it up and got the schematic and googled it, I found a guy that built an exact scratch duplication of this amp, custom cherry wood cabinet, turretboard and all. He was asking 12K for it (subsequently reduced to 5K). I thought, if people are willing to enshrine this amp, then there has to be something there.

The internal wiring was pretty neatly done to my eye anyway. See the photo.

I did find one error that was probably from the factory, the filament wiring was reversed on V2.

I haven't touched the slope resistor yet. The new Aspen Pittman book suggests a 68K there. I did try this other mod, changing the PI feed capacitor coming off pin 6 of V3 from .02 to .001. That did increase the gain of the amp.

Thanks for the tip about the tone stack long runs. I was thinking maybe of trying some different tone control options, so I could just desolder the old runs and try some newer versions right on the controls.

[Blindog]

Mine is the AA1164 model Princeton Reverb. I think it was probably the second year of the silverface, but that schematic was untouched for several years and still the same as the blackface version of this amp.

Yes, it definitley appears to be early silverface. Some tell tale signs are the blue molded caps and the cloth covered wire. The later silverface amps I've been in have had the pvc covered wire throughout the whole amp. The molded caps were eventually replaced with what looks like blue versions of the orange drop caps. The molded ones are preferred by many.

Incedentley, if that pic is before you did all the mods, your input jack to board connection was already shielded. Just sayin'

Welcome and glad you are having fun. There's no turning back now!

Mark

[KeithStevens]

Oh right! I did try the shielded input before I took the pics. I heard the blue ones were good, and I did keep those. I did notice that the values of the ones in the PI had drifted apart quite a bit. I measured the Spragues before they went in and they were spot on.

[Structo]

You lucked out on that amp.
It appears to have blackface style wiring.

The newer ones looked like a spaghetti factory.

I have a theory that after CBS bought Fender they either moved the amp building site and hired all new people to build them.
I makes no sense why the construction techniques would differ so much between the blackface and the silverface line.

Miles of wire, pigtails to cure oscillation rather than find the cause.
Sloppy lead dress, etc.

I read a small article about a guy that worked at the amp plant during the late 60's.
He said he had no background in electronics and he said most of the other people didn't either.
They just followed instructions.
I forget how many amps he built a day but it seems like it was pretty high like 6.
I think he said that someone else installed all the hardware, sockets, transformers.
He just wired everything up.

Your Princeton sounds very nice and it is great that you have resurrected it from oblivion.

[KeithStevens]

Thanks Structo.

I did have a lingering question based on some of the research I did. I have seen 1K5 grid stopper resistors between 1 & 5 on the output tubes suggested.
Is this a worthwhile change? If I don't lose anything by trying it out, I suppose I should just do that.

[Structo]

The grid stoppers or swamp resistors on the grids of the power tubes are mostly to stop any oscillation but also filter some high frequencies on the grids.

1K5 is a common Fender value.

I can't really tell you if the addition of these resistors would be audible at all but it seems that most amps from the late 60's use them.

If you are in to it, you could try them in your amp.

What you could do is solder them in, then play the amp, then turn the amp to standby and short out the resistor by putting a test lead with alligator clips on each side of the resistor, taking it out of circuit.
Then flip the standby switch to play and see if you can hear or feel a difference.
Careful, live amp!

Pin 1 on a 6V6 is not connected to anything (unless it is a metal tube then it is connected to the envelope so it is used for a tie point).
The grid resistor can go from pin 1 to pin 5 which is the grid.
Then the wire from the board is soldered to pin 1.

[Bob-I]

The grid stoppers or swamp resistors on the grids of the power tubes are mostly to stop any oscillation but also filter some high frequencies on the grids.

1K5 is a common Fender value.

I can't really tell you if the addition of these resistors would be audible at all but it seems that most amps from the late 60's use them.

If you are in to it, you could try them in your amp.

They also filter RF interferance. I remember getting CB radio broadcasts through my amp without the grid stoppers.

I can hear the difference with the grid stoppers. I like to up the value some to get rid of the ice pick highs. 3.3k or 4.7k can help. I have 3.3k in my Princeton Reverb.

[Bear]

The stock treble control can be an ear-bleed knob. Wish I had known about tone-stack mods in '93 when I was looking at a couple cheap Princetons. Prices haven't been the same.

The 12 watt power level is just right for practice, recording, and small gigs. Larry Carlton used Princetons (I believe) before Boogies and Dumbles, set so that he could get break-up or clean with his pick attack.

I noticed this amp, certainly a Princeton lift of some sort: http://www.portcityamps.com/twelve.html Some nice tones here: http://www.youtube.com/watch?v=bbtXPxG2TtE Too much non-visible wiring in the gut shot for me to decipher much.

[KeithStevens]

Ahh. Ok, I'm comparing my Bassman circuit that has these to the princeton circuit. The PI feeds pin 1 of the Bassman, but pin 5 of the princeton. The bassman has these standard, but 6L6 vs 6V6.

So I would move the wire from pin 5 to pin 1 before adding the grid resistor?
Do I understand that correctly? Thanks for your patience.

[Structo]

You can solder the resistor from pin 1 to pin 5 and connect the wires from the phase inverter to pin 1.

6V6 and 6L6 have the same pinout.

[KeithStevens]

You can solder the resistor from pin 1 to pin 5 and connect the wires from the phase inverter to pin 1.

6V6 and 6L6 have the same pinout.

Awesome. Thanks Tom. I'm going to try that.