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Princeton Phase Inverter

12/26/1998 9:33 AM
Princeton Phase Inverter
I've read in G. Weber's book and followed some post on this board about raising the voltage to the phase inverter on a Princeton (mine is a 1979 SF) by moving to a higher voltage tap. These were in articles that also changed the driver from a split load to a long tail. My question is, would this work on a stock split load driver? Also, voltage for the pre amp tubes continues on from the driver. Is it correct that I separate these and only boost the voltage on the phase inverter? I am also contemplating installing a 12" speaker but the baffle board is fitted into slots in the sides of the cabinet. Any suggestions for what to do in case of a screw up? Also, has anyone done this with this particular amp and is the baffle board glued to the bottom of the cabinet? I'm just wondering how difficult it would be to remove if I had to replace it.  
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12/26/1998 12:13 PM
Re: Princeton Phase Inverter/Mods
You don't say if yours is a Princeton reverb or not.  
Measure the voltage of the anode on the  
triode section of the no gain PI tube.  
Some of these are higher then stock now.  
You don't want the supplied voltage to be much over 350vdc.  
Do you have a schematic for the amp?  
The long tail conversion is a bit tough and you'll loose the vibrato tube or you get to install a new tube and wiring to do it.  
Here are a few easier ideas:  
You can move the PI triode section's B+ tap to between the two last dropping resistors or replace the second dropping resistor to a lower value.  
Better... move the driver triode section and the PI triode section to between the last two dropping resistors for more headroom and a bit more gain.  
That is about 50v to 75v above the stock configuration.  
Now only the first preamp tube is on the lower voltage tap.  
Personally, I like changing the second dropping resistor to a lower value (about 7K to 10K to bring all the preamp voltages up about 30v to 50v.  
You don't have to change B+ the wiring around.  
While you have the amp apart, install a 5 watt 1K resitor in series with the B+ lead to the common point of the two screens on the 6V6s.  
Move the negativre feedback point from the bottom of the 1K5/25uF combination on the cathode of the driver section and put it right on the cathode pin itself. Shortout the 47 ohm resistor.  
Now change the feedback resistor from the 2K7 now to about 6K8 (use your old mid range resistor) to up to 15K.  
Oops... back up to the bass control pot and replace that 6K8 mid range set resistor with a 10K to 12K 1/2 watter.  
And for an additional good sound, replace the .1uF bass cap with the .047 mid range cap and replace the .047uF mid range cap with a .033uF/630v cap.  
You can try a .022uF in both spots later if you want... good sounding too.  
Replace the 250pF ceramic treble cap with a 270pF silver mica to finish it off.  
These are my basic Hot Rod Princeton Mods.  
The amp will sound a bit different but to my ears they sound absoultly great for a 15 to 18 watt amp.  
You might want to try a few of these at a time to see what you like.  
Mission Amps
12/26/1998 2:28 PM
Why thank you Bruce....
I have a Princeton clone that I just built. Over the next couple of weeks I'm changing the circuit over from the perf board(yuck!) I built it on to a turret board. I might try some of the mods you mentioned.  
Got two birds with one stone on that one....  
12/27/1998 9:35 AM
Re: Princeton Phase Inverter/Mods
Thanks for the reply. It is a Princeton Reverb and yes I do have the schematic. Right now it is cathode biased with a GZ34 and I'm going to install a Deluxe output tranny. It has no feedback at this point either, although I may try some of your mods. with regard to that. BTW what type of speaker are you using with these mods? Are you keeping the stock 10" or putting in a 12"? Sounds like changing the dropping resistor is an easier way to go for changing the voltages. What is the effect of changing the bass and midrange cap? How does it affect the tonal response?  
12/29/1998 8:26 PM
Steve Watson

Just a note on the 12" speaker, TG. I have a 71 PR with baffles for both 10 and 12 inch speakers, and when I used the 12, the bottom end got extremely flabbly and farty. Swapping out the stock OT for a Mojo replacement Deluxe Reverb size OT helped quite a bit. Might be something for you to consider.  
12/29/1998 8:58 AM
Re: Princeton Phase Inverter
Bruce covered the electrical mods with no stone unturned. To address your speaker conversion concerns:  
A while back, I modified a SF Princeton (using the McIntyre Prince-O-Wails mods), including a change to a 12" speaker.  
My princeton's baffle was removeable, so it was easy to make a new one for the 12" driver, and save the stock one for when the Smithsonian calls me and asks to display my rare SF Princeton. I understand your concern with a fixed baffle. About a month or so ago, we were discussing installing a 10" driver in a SF Champ. John Martin, who posts here occasionally, suggested mounting the speaker through the existing hole from the front. John always has some excellent ideas. Although I sometimes build speaker systems for the stereo, always mounting the drivers from the front for better sound dispersion, this method hadn't occurred to me (Duh!). Maybe this method will work for a 10 to 12 conversion, also. (Your princeton's grill frame is removeable, and the cloth stands about 3/4" away from the baffle.) If there isn't quite enough clearance to allow the rear of the 12" driver frame to sit down against the board for mounting, the front edge of the hole can be chamfered a little. Just a suggestion, in case you didn't want to cut a larger permanent hole to mount a 12 from the rear.  
If this front mounting is feasible, you get an added bonus for alleviating possible clearance problems between the speaker magnet and existing chassis-mounted components & tubes.
12/29/1998 5:04 PM

Thanks Doc. Since you did the "Prince" conversion, could you explain how to calculate the correct bias with the circuit that is used in it?  

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