Want to build a Princeton Reverb but don't understand the power supply

[Mike_J]

A friend of mine sold his blackface Princeton Reverb years ago and now wants to make a clone to replace it.  He asked me to help him with the build.  I looked at the AA1164 schematic.  It looks as though at least three 12a_7 tubes share an electrolytic capacitor and the node C capacitor doesn't go anywhere that I can tell.

Doug has a good selection of parts for this build which I am going to recommend as the foundation for the build.  His chassis, faceplate, back plate and circuit board among other parts look like must haves. 

The cap can on his part list has me a little confused.  The schematic calls for a 4 X 20uF cap can.  The parts list shows a 40-20-20-20 cap can.  My experience with using a 5Y3 build is you don't want to have more than 32uF on the HT but I have never done a build with a 5U4GB rectifier so I don't know its limits.  Can anyone enlighten me regarding same?  Also, why wouldn't you want to stay with the 20uF value on the HT?
 
I am sure the node C cap must do something but does anyone think it could be replaced by some 8uF caps on each of the first three tubes?  I am hoping someone has a better idea than the original. 

I have heard that at one time electrolytic caps were very expensive.  Wonder if that is why the power supply looks like this or maybe the supply works just fine.  Any suggestions would be appreciated.

Thanks
Mike

[jjasilli]

It looks as though at least three 12a_7 tubes share an electrolytic capacitor and the node C capacitor doesn't go anywhere:  That is correct.


5U4GB:  Tube specs are stated in the tube chart; 40uF is OK:  http://www.mif.pg.gda.pl/cgi-bin/vs4.pl

You can use 20uF if you want.  Also today's caps are smaller and can probably fit inside the chassis.  I.e., you don't need to use a cap can.

why wouldn't you want to stay with the 20uF value on the HT?:  40uF may give better bass response and possibly less hum & noise.


I am sure the node C cap must do something:  It provides a stage of filtering even though it doesn't power anything.


does anyone think it could be replaced by some 8uF caps on each of the first three tubes?:  Yes, but why? 


I am hoping someone has a better idea than the original:    This is subjective.  For someone who loves a Princeton, maybe there is nothing better.  I hotrodded mine; then someone insisted on buying it.  Schematic below.

[Mike_J]

It looks as though at least three 12a_7 tubes share an electrolytic capacitor and the node C capacitor doesn't go anywhere:  That is correct.


5U4GB:  Tube specs are stated in the tube chart; 40uF is OK:  http://www.mif.pg.gda.pl/cgi-bin/vs4.pl

You can use 20uF if you want.  Also today's caps are smaller and can probably fit inside the chassis.  I.e., you don't need to use a cap can.

why wouldn't you want to stay with the 20uF value on the HT?:  40uF may give better bass response and possibly less hum & noise.


I am sure the node C cap must do something:  It provides a stage of filtering even though it doesn't power anything.


does anyone think it could be replaced by some 8uF caps on each of the first three tubes?:  Yes, but why? 


I am hoping someone has a better idea than the original:    This is subjective.  For someone who loves a Princeton, maybe there is nothing better.  I hotrodded mine; then someone insisted on buying it.  Schematic below.

John

Thank you for the reply.  I see you used a 40uF filter on the HT and replaced the 1K resistor going to the screen supply node with a choke.  I did the same thing for a 5e3 build once because it had a pretty loud hum (increased the HT cap to 32uF which is max for a 5Y3).  Made the 5e3 very quiet but it did not sound anything at all like a 5e3 after I made the changes.  Reminds me of a low power 5f6a Bassman more than a 5e3.  Takes pedals real well and has decent headroom for a 12 watt amp so I left it the way it is.  It is not anything like the Deluxe I hear in Neil Young records though.

The friend I am going to help has experience with the sound of the original Princeton Reverb and obviously liked it enough to want to recreate it.  My concern and the reason for the 40uF question stems from my experience with the 5e3 mentioned above.  I am sure he is expecting to hear a certain sound and I hope to help him achieve that with little or no hum or hiss.  Just trying to anticipate potential issues before starting the build.

Could you tell me why you added the IN4005 between the B and C nodes and what it does?

Thanks
Mike

[jjasilli]

Could you tell me why you added the IN4005 between the B and C nodes and what it does?   It "rectifies" the preamp.  When the power tubes call for more el power, like for strong signal or signal spikes, it can cause voltage sag.  They can suck it from the wall and/or the preamp.  The diode prevents them from pulling power from the preamp.


I felt my stock Princeton was a bit weak.  Maybe the PR is stronger with the extra gain stage for reverb recovery. Anyway, I rebuilt this amp for sag, overdrive and saturation.  The 6L6 power tubes are over-spec for the OT which causes saturation & sag.  The choke I think contributes to sag, and to me yields a more pleasing tone. (I never met a choke I didn't like.) The diode keeps sag from immediately reaching the preamp, like from a spike in signal voltage.


The SF Princeton I modded went from "student amp" to a fine example of vintage Fender tone.  However I did not care for its overdrive tone.  I think the Deluxe Reverb, because of its LTPI, is better for that (in a simple amp w/o cascading gain stages to shape OD tone).  But that's subjective and may vary from amp to amp.  One of our members, Psychonoodler is a major Princeton fan, loves their overdrive tone, and builds variations.  You may wish to search some of his old posts.

[stevehoover]

Are you guys talking about the diode in the bias supply by chance?

[HotBluePlates]

Are you guys talking about the diode in the bias supply by chance?

No, they're talking about a diode in Jjasili's schematic attached to his first reply.

If he exports the schematic from Jschem as an image, then it will be readily viewable. Otherwise, you need the program he used to draw the schematic in order to view it.

[drew]

One of our members, Psychonoodler is a major Princeton fan, loves their overdrive tone, and builds variations.  You may wish to search some of his old posts.

Good idea, but note that he spells it "phsyconoodler"

[alerich]

Regarding the unused node in the power supply Google "Stokes Mod". Many PR owners (including myself) do the mod to provide the PI with a larger voltage to work with. Gives the amp more headroom because the PI can swing the voltage wider before it distorts. Many feel that Fender (Leo and his crew) configured the stock circuit to have the unused node to limit the headroom of the amp so that it would not compete as effectively with it's big brother the Deluxe Reverb. You basically disconnect the plate resistor of the PI and the node before it and reconnect them to the unused PS node or wire it that way from the get go if it is a new build.

My 65 PRRI came stock with a GZ34 rectifier. I added another 22uf cap to the first node. The 65 PRRI has discrete axial filter caps on the board. No cap can. The increased filtering is no sweat to a GZ34 and tightens up the bass. Flabby bass is a problem with the PR especially when you push it.

[Mike_J]

Regarding the unused node in the power supply Google "Stokes Mod". Many PR owners (including myself) do the mod to provide the PI with a larger voltage to work with. Gives the amp more headroom because the PI can swing the voltage wider before it distorts. Many feel that Fender (Leo and his crew) configured the stock circuit to have the unused node to limit the headroom of the amp so that it would not compete as effectively with it's big brother the Deluxe Reverb. You basically disconnect the plate resistor of the PI and the node before it and reconnect them to the unused PS node or wire it that way from the get go if it is a new build.

My 65 PRRI came stock with a GZ34 rectifier. I added another 22uf cap to the first node. The 65 PRRI has discrete axial filter caps on the board. No cap can. The increased filtering is no sweat to a GZ34 and tightens up the bass. Flabby bass is a problem with the PR especially when you push it.

Alerich

Thank you for the excellent information.  Did the 65 PRRI have the first three preamp tubes share a single 22uF cap?

Thanks
Mike

[alerich]

Thank you for the excellent information.  Did the 65 PRRI have the first three preamp tubes share a single 22uF cap?

All four preamp tubes (V1 - V4) share the same single node on the power supply rail (the last one furthest from the PT) in the stock circuit whether it is a blackface, silverface or reissue. That's the idea of the mod. It leaves V1 and V2 on that node and moves V3 and V4 to the unused node that is closer to the PT on power supply rail and therefore provides a higher working voltage. This is not just a mod for the 65 PRRI. This is a mod people have been doing to all PR amps for years.

[sluckey]

Did the 65 PRRI have the first three preamp tubes share a single 22uF cap?

Not exactly. V1A, V1B, V3A, V3B, and V4B are all fed from Node Y with a 22µF cap.

V2 is the reverb driver and V4A is the trem oscillator. Both are fed from Node Z (screens) with a 22µF cap.

http://support.fender.com/schematics/guitar_amplifiers/65_Princeton_Reverb_schematic.pdf

[Mike_J]

Did the 65 PRRI have the first three preamp tubes share a single 22uF cap?

Not exactly. V1A, V1B, V3A, V3B, and V4B are all fed from Node Y with a 22µF cap.

V2 is the reverb driver and V4A is the trem oscillator. Both are fed from Node Z (screens) with a 22µF cap.

http://support.fender.com/schematics/guitar_amplifiers/65_Princeton_Reverb_schematic.pdf

Sluckey

Don't you think this design is less than average?  The Deluxe Reverb seems to be suffering from the same malady.  Can't we do better?

Thanks
Mike

[sluckey]

I don't see anything wrong with either design. The AB763 line of amps is the most successful and popular line that Fender ever did. They all feed a bunch of tubes from one B+ node. Nothing wrong with that if you do so wisely.

[Mike_J]

I don't see anything wrong with either design. The AB763 line of amps is the most successful and popular line that Fender ever did. They all feed a bunch of tubes from one B+ node. Nothing wrong with that if you do so wisely.

That's the last word on that.  Your argument is indisputable.

Thanks
Mike

[jjasilli]

My recollection is that the Stokes Mod & the Paul C mod have fallen into disuse.  I tried running my SF Princeton cathodyne PI off the unused C node for more voltage.  OD Tone was horrible, so I reversed the mod. 


Base on research & personal experience, but not scientific testing, my opinion is: a cathodyne PI is fine, but should be used as designed.  A properly designed cathodyne PI should be left alone.  It should not be thrown into imbalance with higher voltages or component swapping. There are definite choices if you want overdrive, but don't overdrive the cathodyne itself which sounds bad (IMHO, others may disagree).  1.  Get OD in the preamp, and let the cathodyne do it's unity-gain thing in peace.   2.  If you want power tube OD, and the cathodyne puts out insufficient voltage signal, then switch to a LTPI.  Now the PI itself can be overdriven w/o ratty tone (no hate mail please). 


In a simple amp w/o cascading gain stages, like a Princeton, the PI is maybe the only place to get overdrive before the power amp.  And overdriving the PI is maybe the only way to generate enough signal to overdrive the power tubes.  If an overdriven cathodyne is not your thing, and you convert to an LTPI, then you've transformed a Princeton into a Deluxe Reverb.