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doclarson 8th April 2019 12:52 AM

Fender Silverface Princeton Reverb Most Quiet @ Volume = 10?
 
Hey All:

I have a 1972 Fender Princeton Reverb that generates no perceptible noise when I dime the volume, but for some odd reason develops a slight hum as I back it off. Noise level of the hum is tolerable at lower volumes, just annoying given how pristine the amp sounds at 10.

Just wondering if anyone has a good guess regarding the most likely suspect, and/or a suggestion on the first components I should check, e.g., volume pot, something else? I’m hoping that this might be a known problem w/ Princetons, but my searches didn’t turn up anything relevant.

Many thanks in advance for your suggestions!

Doc

RBHan 12th April 2019 05:10 AM

You should probably change the power supply caps. Have the bias checked. Check power tube.

enorbet2 12th April 2019 04:29 PM

Hello Doclarsen
It would be helpful to have a sound file or at least a description of the noise but I can still venture a "guesstimate" since, as you likely suspect, it is odd to experience less noise at higher potential signal levels and actually the Princeton Reverb is a bit unique in it's design that can cause or exacerbate such odd problems.

First a bit of background. All of the Princeton (and most post Tweed fenders) amplification stages take output from the preamp tubes' plates and the plate signal is 180 degrees out of phase with the input. This means that unless some phase shift occurs any output signal that bleeds back to the same stage's input cancels BUT the next stage inverts too so it's plate output is IN PHASE with the PREVIOUS stage's input and any bleed can be re-amplified instead of cancelled.

What makes the Princeton Reverb unique is that it has no less than 5 plates connected to the same decoupling stage of the power supply. Think of that as a sort of 'Common" which is only diminished by the filter capo's ability to pass alternating current (signal voltage) to dissipate to the ultimate Common, Ground. The cap's ability to pass AC to Ground is affected by ESR, the Effective Series Resistance of the filter cap, which is higher on lower capacitance caps and the Princeton uses a mere 20 mfd.

All of that may seem a lot to explain that you may solve your problem by replacing that last leg filter cap with a new one, and preferably one of 40-100 mfd value, but I have another idea in mind as well.... well actually two ideas.

1) If the amp were mine I'd alter the PS circuit by inserting a choke in place of the first decoupling resistor using one from say a Tremolux. This will substantially reduce ripple and it's associated noise and other issues in less than pure DC supply voltage. If for some crazy reason you don't like it you can always go back but I'm betting you will love the improvement. In some 50 years of amp repair, design and modification not one single client has ever asked me to reverse that particular mod. It's subtle, but noticeable and noticeably sweeter as well as less noisy since the ripple can cause a "beating" with the signal voltage which muffles clarity and harmonic richness. This isn't to solve your noise issue, it just has some affect on it but is a terrific bang for buck for a better sounding amp since chokes are still pretty cheap.

2) Either replace, or better, upgrade that last filter cap will likely solve your noise problem but if you'd like to venture further into rich tonal focus here's what you do, or have done. In the schematic included below if you look for that point "D" which is the supply at that last 20mfd filter cap note the decoupling resistor of 18K @ 1 Watt. What I would do is get a second 18K @ 1 Watt resistor and a second filter cap and turn 1 last leg into 2 parallel legs - duplicate the last leg. Then connect every other plate to the same leg and the others to the remaining leg. This prevent any Crosstalk, signal getting to a stage that is In Phase. This has several beneficial effects but rather than make this already long response into a book, just think of it as isolation, where each stage does it's job and is not affected by other stages imposing any variables. One hint - while in phase feedback is in phase it is also slightly delayed and this causes 'smear" which reduces boith harmonic richness and clarity. It's hard to translate words into sounds, and vice versa, but it is a profound subtle change that musicians almost always love. Thankfully it is also fairly easily mitigated by any Treble controls or Bright Inputs or switches, since this mod works on most amps. It's just especially effective on the few amps like a Princeton.

I probably should mention that Princeton Reverbs are my 2nd favorite amplifiers right next to Tweed Bassmans. Enjoy yours and use whatever you find helpful in my suggestions..

https://elektrotanya.com/PREVIEWS/63..._sch.pdf_1.png

Jim Williams 12th April 2019 04:55 PM

Sometimes accumulated hum noise sources can cancel each other out if the gain is matched. Most likely that is what is happening here. Replace the power supply and bias caps. Use big ones and all the noise will go away.

dpsbb 15th April 2019 06:39 PM

Always start with trying different known-good tubes in every position before you start going into things like replacing capacitors or other components in the guts of the amp. If you have to buy new tubes and it ends up being something else then that's OK, you should always have a spare set on-hand anyways. Once you know it's not the tubes, then it's time to start looking elsewhere.

John Eppstein 18th April 2019 02:25 AM

Quote:

Originally Posted by dpsbb (Post 13927356)
Always start with trying different known-good tubes in every position before you start going into things like replacing capacitors or other components in the guts of the amp. If you have to buy new tubes and it ends up being something else then that's OK, you should always have a spare set on-hand anyways. Once you know it's not the tubes, then it's time to start looking elsewhere.

This is typical "folk medicine" spread by people who really know little of how and why these problems actually occur.

Usually the only people who benefit are "toob pimps".

Real techs don't automatically shotgun their tubes. Consider this - a typical tube of reasonable quality will cost you around $20 and up these days. A capacitor costs you around $5 , give or take a bit. A resistor costs pennies.

dpsbb 18th April 2019 02:45 AM

dude what is your problem? Seriously. I know what I'm talking about. I'm sorry I disagreed with you in another thread, but now you are following me here to do more attacking of my knowledge and experience?

Disagree with me if you want, but stop the condescending tone and attacks already.

shiee

enorbet2 18th April 2019 04:23 AM

dpsbb - The thing is that tubes, caps, resistors, transformers and such have been around so long and were crucial to WWII that records were kept for many decades of component instance of failure (MTBF). Granted, tubes were better back then but Capacitors are about 5 times as likely to fail as tubes. I know it's sometimes hard to tell when or if John is being condescending but the simple fact is he is just telling it like it is. Replacing tubes first has long been a mantra of tube sellers, not techs... or at least techs that actually know tube circuitry.

If you continue to tout tube replacement as the first thing to do you really are risking any reputation you may have or hope to have.

Unless a tube won't light up at all or the plates glow bright red and light up the amp cab like an orange flashlight it's really rare that a tube has failed. It should be one of the last places you look and certainly not the first. Don't waste your money. Go with the odds and that means caps first..