Noisy Princeton Reverb Clone

[blewgrass]

Hi gang! I just bought a princeton reverb princeton clone to sharpen my amp techie skills (sigh!! hope this one pans out!)

The amp sounds killer.... just like a vintage princeton reverb... yet there is a bit more 60hz hum than I'm comfortable with, and when you apply reverb, the hum gets worse. I have some fears that this may end up a "long haul" type of troubleshoot, but I have some books (Dan Torres, David Hunter, Gerald Weber) and test equipment to trace the problem before giving up and taking it to a tech.

Where would you guys start here? My first thought is reverb preamp tubes, and power supply wiring. It is not a "famous" clone builder, but Dan Torres and other techs have called it a "good build" whatever that means...

Where should I start?

Steve

[Bob M.]

Grounding...

Grounding, grounding and more grounding! I hope there are shielded wires on the preamp grids. Are you using a can cap for the filter? You could replace with discreet caps and develop a better ground scheme around those.

Bob M.

[blewgrass]

CE Cap can

Thanks Bob,

It has a CE distributing (I think they took over the mallory's) cap can that is 40/20/20/20.

Here are some pictures inside the amp.







Is anything jumping out at you here as a potential buzz problem? The amp is supposedly based on a Fender AA1164 curciut with the addition of a bias pot and the "ground" switch seems to be a standby switch. I noticed that rocking the standby switch while the amp was off and unplugged did not totally drain the filter caps though. I had to use the alligator clip with the resistor trick to drain them.

thanks for any advice and I will take Bob's suggestion of grounding, grounding, grounding.

Steve

[stokes]

Can caps themselves are a common source of hum.You have your preamp filters grounded with the higher current components in the amp.Its a common practice to ground the main and screen of your power tubes near the PT and the preamp filters with the preamp grounds at the other end of the chassis.In your case this may involve more than you care to go through.You would have to either remove the can and put in new individual caps or remove the last 20uf in the can and re-wire a seperate 20uf to replace it,grounded accordingly.And after all that there could be a different cause of the hum.First,are you sure its 60 cycle hum,or are you just assuming it is.If it is from the caps,it would be 120 cycle.

[blewgrass]

Thanks Stokes,

It may very well be 120 cycle. I was assuming... bad practice here I know.
I also noticed that several areas on the tone stack were not grounded to the chassis like the original fender layout calls for. I wonder if the long runs of wire to the grounding bar are creating the noise...

[blewgrass]

much better now

I changed the reverb tank from a short 3 spring accutronics, to a long 2 spring Belton (both which came with the amp) and now the hum has reduced significantly. It is more on the lines of a typical black face amp. There is some hum but not nearly as much. Now when I hit the reverb, 2 is about the most usable amount or you're lost in a sea of echo... this is also true with my deluxe reverb..

At this point, I think I'd be splitting hairs to try and improve the hum. I might still have a tech look at the grounding bar and see if the amp would benefit from some more chassis grounding.

Thanks for the responses here. Feel free to give me any other thoughts that come to mind when you look at the photos above.

Thanks

Steve

[blewgrass]

More Hum

Now the hum has grown louder again. I will take the advice here on the preamp cap grounding. Anything else to check for while I have it apart?

Steve

[tubeswell]

Lead dress. I just built one of these and lead dress is important, esp with the way everything is crammed into that little chassis, otherwise you will pick up unwanted coupling for sure. Look at a Fender BFPR layout ('66BFPR attached) and copy that. Also check to see that your heater filaments are not crossed over. One side should ideally be going to pin 2 on the output tubes and pins 4-5 on the preamp tubes, and the other side should be going to pins 7 and 9 respectively.

[blewgrass]

Less Dress?

Help me out with the newbie lingo a bit. What is less dress? To much wire? To busy with components crowded in together? Thank you for posting the picture of that neat factory layout!

I am trying to take the responses here and reference them with the Torres Book and Gerald Weber volume on tube amps.

Steve

[tubeswell]

Help me out with the newbie lingo a bit. What is less dress?

Lead Dress = shorthand for how you lay out the wires.

Keep signal wires (and even heater wires and other AC wires) close to the chassis and otherwise as short as possible.

Keep AC pairs twisted together to cancel out AC hum.

Keep AC pairs at least 1 inch away from signal wires and where it is necessary to have them crossing, only do so at right angles etc.

Shield sensitive signal wires

Keep ground returns (and especially filter/decoupling cap grounds) in the same vicinity of the circuit as the bits of the circuit that they serve

and so on

[Groover]

The transducer pickup in the reverb pan can pick up hum from the power transformer EMF field, especially if the pan is oriented so that the pickup (output) end is under the power transformer end of the amp. Check this, and try rotating the pan 180 deg. You may need to switch the reverb pan send and return cables around if one is shorter than the other.

[blewgrass]

Thanks for the reply. As I mentioned before, the "reverb" hum drastically reduced when I changed from the small 3 spring accutronics to the large (stock replacement) belton 2 spring tank.

I will also try grounding the tone stack areas to the chassis instead of the way it has been done on the amp. I'll report back results. I may do this tonight.

Steve

EDIT: After looking over Groover's comments, I checked the amp with all the reverb cables and footswitches unplugged. Most of the hum was eliminated. Hmmm. I'm going to try grooves advice and get back to you. Thanks again. If not... maybe the footswitch is bad or noisy.

thanks again for the post!

Steve

[blewgrass]

Another problem that could be a problem is that my amp is in a "head" cabinet, meaning the reverb tank is much closer than it would be in a combo. Is hum a common problem in amps with reverb that are oriented in a head instead of large combo cabinet?

[Groover]

Yes in a head cab it would be much worse. I believe it would follow the inverse square law, meaning that if you double the distance between the pan and the transformer you will pickup 1/4 as much noise. Or halving the distance will make the noise 4 times as strong.

I noticed this when I had the pan wrong-way-round in my Dual Showman Reverb head.

[theunrulychef]

My 72 PR is in a head box & it's very quiet, so I'd think the distance between the tank & chassis should be ok for yours. Though if the tank is oriented the wrong way, as Groover mentioned, it can cause noise. I didn't see anyone mention shielding on the tank (if I missed it, sorry). Do your tanks have the bottom on, often a piece of cardboard, possibly with a layer of aluminum foil making electrical contact with the tank frame? Also, having the tank in a bag seems to help with some noise issues (at least for me).

Good luck getting rid of the noise. A perfect running PR is just about the best amp in the world IMO.