Princeton Reverb Clone Build Hum Problem

[Chuck H]

It COULD be a bad can cap. It may also be the way it was designed, but it's innards are somehow just more prone to interaction (inductance?). If the latter is the case then the new can will hum too. All you can do is try I suppose. Like Enzo says, never think up reasons to NOT test something. Of course I always translated this as 'to not test something it could actually be.' That is, of course you wouldn't look for your wallet in the kitchen if you lost it in the living room But I think a new can is a valid thing to try.

[justsoldern]

Hi again. You called it Chuck! I put in the new cap can and the problem is still there. Although maybe it's my imagination but I think it's diminished just a bit. The B rail is holding voltage a little longer on power down too - like it did with the outboard cap I was playing with earlier, so I think this cap can might be a little healthier. So the conclusion after all this is that this amp is a little haunted and wants to induct a little 60 hz into my B+ for whatever reason. Whether its a physical thing or a ground loop.

So I added in the extra filter stage as described above but with some different values. So on the last bit of B+ run going from the "D" that feeds V3 to the "D" that feeds V1 - I added a 4.7 K resistor which leads to a 10 uf electrolytic cap. The cap is connected to the common node for the 100 ohm V1 plate resistors and grounded with the V1 cathode resistor ground. The extra filter stage only cost me about 5 volts on the V1 plates and the amp is now good to go. It sounds great - it looks good- and I'm finally satisfied!

Thanks to all who followed along and contributed. I hope my little problem helps somebody else out there. These Princeton Reverbs are just fabulous sounding amps - good to have it performing well.

[Bob M.]

Princeton Reverb Hum problem...

Hi All,

Justsoldern's Princeton Reverb hum problem mirrored one I'm having with a 70s SF PR by about 99.9%. I tried many of the same remedies suggested in this thread without nailing the problem. Today, I added an extra discreet filter cap (22uf@350V) for the preamp filter node that directly connects to the 2x 100K plate load resistors for V1 and V3, as was advocated for prior in this thread. The previous filter for this stage was one section of the 4x 22uf@450V can cap capacitors. This can cap is very recently brand new. This discreet filter cap helped somewhat (lessened the hum a little bit) but the problem was still there and unfortunately, this added filter cap solution did not reduce the hum enough in my particular situation to merit calling it the solution.

My next step eliminated the hum completely: I moved the ground for the volume control (left solder lug, 1Meg-A pot) from the brass shield underneath the pots (standard Fender volume control grounding practice) to the circuit board ground point where the cathode components from V1b (resistor, 1.5K and cap, 22uf@50v) are grounded and then in turn connected to the star ground point (power transformer bolt) by its own wire (black wire works best). This step made the amp completely quiet, recording quiet, and basically you cannot tell if it's even on by having your ear to the speaker. I do hear a very little blow-by hiss when I advance the volume control (with no input) but that is pretty normal and the same is true for the reverb control but the previous background hum, regardless of volume (or reverb) control position, is gone.

I'd been scouring the 'net and reading lots of grounding articles as this problem was bugging me and I was determined to get to the bottom of it. I had tried about a dozen or twenty ideas without eliminating the hum until re-grounding the volume control. I did make the reverb circuit a little quieter along the way by using shielded wires from the right and wiper lugs of the reverb pot to their respective connect points on the circuit board (.003uf and 470K). I also moved the V3a grid load resistor (220K) so it connected directly to the tube socket solder lug at pin 2 and connected the other end (via a black wire) to the ground point for the cathode components of V3a (1.5K and 22uf@50v). For good measure, I also added a shielded grid wire from the 470K, 3.3Meg and 10pf junction to V3b, pin 7 and the shield being connected to the cathode ground point for V3b. I'm not sure if these heroic steps were necessary or essential but they are in now and I'm not taking them out, as the amp is sounding great and is noise/hum free. These were steps I implemented prior to discovering the re-grounding of the volume control was the real cure.

I'm telling my experience within this thread because someone else will come along with a PR hum problem and will have all this information in one place and can experiment and see what solution will work for him or her.

There were alot of good ideas and information in this thread and like music, sometimes it's not the first idea, but the idea derived from the first idea that carries the day in the end. So thanks to all who contributed.

Bob M.

[justsoldern]

Hey Bob, Glad you nailed it! It's funny that with so many amps that have been made using that "not so text book wonderful" topology, and nearly all of them get away with it and are at least relatively quiet. And then for some reason you get one or two like yours and mine that simply need something better. Enjoy the amp. I'm loving mine these days

[tubeswell]

Looks tidy, and I understand your frustration with the hum. I personally think grounding is where you can get the most improvement over the stock circuit in these amps, but also going to separate filter caps (like a DR). I don't bother with cap cans any more myself. Having said that, you might be able to do reasonable grounding with a cap can if you can manage a single star ground somewhere, and ditch the grounding to the brass plate. I don't know how fender got their PRs so quiet - maybe it was the steel that the amp chassis' were made from? Have you read Merlin Blencowe's article on grounding?

[Bob M.]

Good comments from both Justsoldern and Tubeswell,
You're right about most builds coming out relatively quiet and then, oops, you get a problem child that needs extra care and grounding attention. I agree with Tubeswell about using discreet caps for filtering as opposed to the can caps. I don't use them myself but when repairing a friend's amp with a somewhat new can cap, I try to work with that limitation. It's hard to justify the expense of new filter caps in this situation unless a faulty (or fault-inducing) can cap can be proven beyond doubt. The can cap amps tend to want to be star grounded because of the can cap grounding. Once you leave Fender brass ground plane with your cathodes or other ground connections, it's probably a good idea to completely abandon it and use other grounding means. Hum tracking many times is tracking down ground loops and they can appear in many forms, in many places. I should've been sharper and noticed right away that I was splitting my ground activities between the (stock) ground plane and a star grounding. Like Justsoldern, I thought I was being led to a localized problem in and around V3a (reverb recovery) by doing many of the exact same preliminaries as he did. This is why I found this thread so close to my hum situation. Anyway, it was found, it took a little longer than I hoped but I learned more valuable lessons. It was great to reach and have that Ah-ha moment. Tubeswell, thanks for the article. (Yes, I've seen, and read, Blencowe's article earlier).

Electronically yours,

Bob M.