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previous: MIKE B I would like to mod my sf princeton... -- 7/13/2000 2:03 AM

Re: princeton mods

7/13/2000 12:20 PM
BIG Dave	Re: princeton mods This was discussed a while back. John Stokes came up with a great mod. I kept one of his postings for my files. Here it is: "The PR is rated at 12 Watts. I was wondering why that was, since it is so very similar in output tube compliment and voltages to the 20 Watt rated Deluxe Reverb. Moreover, a friend has a Super Champ (another 2x6V6, single 10") amp that is damn loud. Hmmm...... I found out what the deal was. When driven hard, I noticed severe asymmetry in the output waveform. One side of the output was clipping much sooner than the other. Why?? Further probing revealed that one of the 6V6s was not even being driven fully into saturation!! I then started looking at the PI, which in that amp is a split-load type. It turns out that the PI was running out of dynamic headroom before it could drive both sides of the output to full tilt. I then decided to try to remedy that by increasing the B+ voltage (and hence the headroom) available to the PI. This was easily accomplished by moving the B+ feed to the PI up one notch on the power supply voltage divider. Problem solved. The increased voltage allowed the PI to fully drive both 6V6s. The output was not much more symmetrical under drive, and the measured output power jumped to 18 Watts. All this by essentially just moving a couple wires from one place to another. Another thing I noticed is that the coupling cap into the PI was a .02. The input impedance of a split-load PI is VERY high, since the 1 meg grid return resistor is returned to the top of one of bottom side load resistor, and due to the degeneration, the effective input impedance is increased considerably. Anyway, this provides for way too much low end response, leading to the often mentioned "Fender fartyness". Drop that cap down to .001 and the farts get a dose of gas-x. The stock speaker is a rinky-dink piece of poop. (Mine came with an Oxford 10J4.) The C/P10 Q should work well in an amp that's had it's output boosted. These tweaks make the amp actually gig-able. Mine is not harsh in the least. I'm using the JAN Philips 6V6s, and they're fairly smooth under overdrive conditions. As always, YMMV. John Stokes"

7/13/2000 12:20 PM

BIG Dave

Re: princeton mods

This was discussed a while back. John Stokes came up with a great mod. I kept one of his postings for my files. Here it is:

"The PR is rated at 12 Watts. I was wondering why that was, since it is so very similar in output tube compliment and voltages to the 20 Watt rated Deluxe Reverb. Moreover, a friend has a Super Champ (another 2x6V6, single 10") amp that is damn loud. Hmmm...... I found out what the deal was. When driven hard, I noticed severe asymmetry in the output waveform. One side of the output was clipping much sooner than the other. Why?? Further probing revealed that one of the 6V6s was not even being driven fully into saturation!! I then started looking at the PI, which in that amp is a split-load type. It turns out that the PI was running out of dynamic headroom before it could drive both sides of the output to full tilt. I then decided to try to remedy that by increasing the B+ voltage (and hence the headroom) available to the PI. This was easily accomplished by moving the B+ feed to the PI up one notch on the power supply voltage divider. Problem solved. The increased voltage allowed the PI to fully drive both 6V6s. The output was not much more symmetrical under drive, and the measured output power jumped to 18 Watts. All this by essentially just moving a couple wires from one place to another.

Another thing I noticed is that the coupling cap into the PI was a .02. The input impedance of a split-load PI is VERY high, since the 1 meg grid return resistor is returned to the top of one of bottom side load resistor, and due to the degeneration, the effective input impedance is increased considerably. Anyway, this provides for way too much low end response, leading to the often mentioned "Fender fartyness". Drop that cap down to .001 and the farts get a dose of gas-x.

The stock speaker is a rinky-dink piece of poop. (Mine came with an Oxford 10J4.) The C/P10 Q should work well in an amp that's had it's output boosted.

These tweaks make the amp actually gig-able. Mine is not harsh in the least. I'm using the JAN Philips 6V6s, and they're fairly smooth under overdrive conditions. As always, YMMV.

John Stokes"

Replies:

Richie You described these perfect.. I get... -- 7/14/2000 12:43 AM