I'm re-tubing a WONDERFUL little AA964 Princeton for a friend, and was wondering if any of you had suggestions as to how to get the power section running hotter and breaking up sooner. I've not tinkered with that bias/tremolo circuit before so I don't know what would be best. Give me a bias pot or a cathode resistor....none of this tremolo nonsense.
I welcome your recommendations. Thanks in advance.
Put an RFT in it? I dunno. Good question. What would happen if you increased the resistance of both the plate and cathode resistors on the cathodyne? I really don't know. Just guessing.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
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You can increase the mids with a different mid resistor. You can dump the NFB or put it on a switch. Tonestack lift switch. Increase the grid leaks to drive the 6V6s harder. You can also brown the sound by using a 5V4, 5U4, or 5Y3 to lower B+ and as a result lower plate voltage which gives you earlier breakup.
Make the bias adjustable if you want to control how the output tubes behave (replace the 27K load resistor with a pot). Change the 100K slope resistor in the tone stack to 56K or 33K if you want to boost mids and bottom. Besides that, the best thing you can do to a Princeton is make it into a Princeton Reverb, which I strongly suggest you do NOT do with an original Blackface.
Hello Dartanion. Where have you been? Expect a PM. Question for both you and Firestorm. I recall someplace reading that a cathodyne PI is a unity or less gain circuit. Still, if one makes the plate resistor larger, doesn't that increase the voltage swing across it? What would happen with my off the cuff suggestion?
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
My first inclination is to keep it totally stock for potential resale value. These things just keep going up. Having said that if you must tweak then read on.
The following is something From David Allen I found interesting on another forum and saved for future reference.
From David Allen
I worked on a non reverb Princeton over the weekend where the customer wanted more headroom and gain so he wouldn't have to mike the amp in his church band. I always wanted to know the exact impact in power of certain mods so I checked the output power as I made some changes. I thought some of you might find this interesting.
With a tired looking set of RCAs mismatched about 7ma and a low wall voltage around 116V I got 12 clean watts out of the amp before clipping. I did the Stokes and Paul C mods including new 51K metal film plate and cathode resistors, 2.7M/1M resistor divider for the grid and changed the first 18K power supply resistor to a 10K. I measured 15.5 watts before clipping.
Then I installed a well matched pair of JJ 6V6s and changed the rectifier from the stock 5U4G to a 5AR4 as I felt confident the JJs could handle the higher B+ voltage which will likely not exceed 450V. I biased the tubes around 23ma and measured 18 watts before clipping.
For more gain, I changed the 100K plate resistors for V1 to 220K and got about 30% more gain at the power tube grids with the volume dimed and the treble and bass on 5. Disconnecting the negative feedback increased the gain about 70% but I ended up changing the 2.7K NFB resistor to 6.8K which netted about a 30% gain increase.
The real kicker was installing a PEC 2W 250K audio pot where the vibrato pedal jack was located on the back to replace the 6.8K middle resistor. This particular pot has a very gradual taper (about 6-10%) so you get a middle control up to about midway and then you get lots of gain as you impede the tone stack's path to ground. I put a chickenhead knob on it so you could feel where it was set from in front of the amp.
The amp sounded very good. It was very quiet with more clean headroom plus the ability to overdrive it with the new found gain.
So I think it is reasonable to assume you can get 18W from a Princeton or Princeton Reverb with matched JJ 6V6 power tubes, Paul C and Stokes mods to the phase inverter and 120V input voltage.
I have tried the above mods with the same results.
Interesting. I was thinking more of how to adjust bias settings in the power section, but those are all good suggestions. I think I'll just order some JJ's and be done with it. Most of my curiosity was how one goes about adjusting the bias in this amp when there is no bias pot or cathode resistor.
dehughes wrote:Interesting. I was thinking more of how to adjust bias settings in the power section, but those are all good suggestions. I think I'll just order some JJ's and be done with it. Most of my curiosity was how one goes about adjusting the bias in this amp when there is no bias pot or cathode resistor.
There's a 22K resistor (27K in the non-reverb version) across the bias cap. Change that for a 25K or 50K cermet pot and you can adjust the bias. Pretty unobtrusive mod.
skyboltone wrote:Hello Dartanion. Where have you been? Expect a PM. Question for both you and Firestorm. I recall someplace reading that a cathodyne PI is a unity or less gain circuit. Still, if one makes the plate resistor larger, doesn't that increase the voltage swing across it? What would happen with my off the cuff suggestion?
Dan
The thing with a cathodyne PI is that you don't want the signal coming off the plate to be larger than the signal coming off the cathode or the PI will be off balance. So the plate resistor and the cathode resistor are essentially identical. The "gain" of the signal at the plate is pretty much set by the fact that the cathode resistor needs to be largeish (47K or 56K) which locks the tube down to unity gain (at best).
RB wrote:So I think it is reasonable to assume you can get 18W from a Princeton or Princeton Reverb with matched JJ 6V6 power tubes, Paul C and Stokes mods to the phase inverter and 120V input voltage.
I think we've learned that you don't want 18 Watts from these unless you upgrade the iron. The MacIntyre Prince of Wails mod was the first wave and I believe Stokes has recanted on his mod as it kills either the PT, the OT, or both.
skyboltone wrote:Hello Dartanion. Where have you been? Expect a PM. Question for both you and Firestorm. I recall someplace reading that a cathodyne PI is a unity or less gain circuit. Still, if one makes the plate resistor larger, doesn't that increase the voltage swing across it? What would happen with my off the cuff suggestion?
Dan
Hey Dan,
Yes, the cathodyne or concertina PI is a no gain affair. I haven't really given any thoughts to tweaking one. Need to do some reading to get my head straight on these again. I've been doing nothing but LTPs for a couple of years now. I always used the stock 5E3 PI as it works great and sounds good.
<strike>I'm a little surprised no one has mentioned the 'Stokes' mod - moving the PI up one node in the preamp power divider/filter string. Then again, I don' really know if that will produce the result you describe.</strike>
that class of inverter really is the limiting factor of the amp.....
it has some really good qualities....
its simple, only one gain stage.....
it has pretty good AC balance.....
its stable..... to voltage and variences of the tube .....
its got an acceptable bandwidth and responce for audio
but it also has some significant draw backs.....
no gain......
a limited handling capacity.....
unequal capacitances across the plate and cathode resistors.....
the gain issue is plain
it will color the tone a bit (which is good for guitar)
but your only gonna be able to drive it only so hard before it goes to crap
that nasty distortion thing it does when turn'd up full
the tone is wonderfull and then....... it falls apart
I think "RB's" suggestions to optimize the power supply, tubes and feedback
in a reversable manner would be a good place to start....
then as "jaysg" suggested.... eventually replace tranny's......
but I wouldn't go mod'ing it unless your friend (customer) expressly tell you to.....
