Debugging a Princeton Reverb High Power build

[axpro]

Hey guys... just typed a long and complete message and i mistyped (stupid laptop) erasing it all.

Heres the just, Noisy amp (not incredibly but more than i would like)

Haven't tried elevating the heaters yet, but did try shielding runs on v1 and to the reverb control.

Any ideas??? Princeton guys?

I also want to make 2 changes.

-Reverb is too big. WHen i turn it up it gets waay noisy. I really can't see myself using it over 3-4 Is there any way to make THAT the top end of the reverb???? RUnning a 12au7 perhaps?

-Trem sounds good but a little too fast. how to slow it down.

Any ideas?

THanks Everyone

[axpro]

Tried the 12au7, reverb is a little smaller now, but still a lot of hum. even with the reverb tube pulled. Which is to say, the reverb control ADDS a lot of hum, even without a tube in the driver position. I would think that would point me towards the recovery stage.

also found info on slowing down the trem, haven't tried it yet. Looks like some work changes are coming, giving me a little more time to work on my projects... not unemployment, just a change in who i get a ride with (giving me an extra hour per day to spend in my workshop. not on company time)

Overall the amp SOUNDS great! exactly the tone i was looking for, spanky clean with a nice thickness... just gotta get rid of the noise and I am there! and then i can devote more time to my JCM800 build lol

[loudthud]

Assuming you have a Standby switch, does the hum go away immediately when you put the amp in Standby mode or does it fade out over a period of a couple of seconds? Did you use a layout that you can post a link to?

[axpro]

Thanks for the Reply LoudThud! I was feeling like "persona non grata"!!!

I am using a Weber kit and following their Layout pretty much, slight differences since my tube sockets aren't exactly the same rotation.

Heres the one from their site
https://taweber.powweb.com/store/6a14hp_layout.jpg

It's not really "realistic" in that some of the boards and parts seem bigger in reality than they draw them. and of course i had to sub in a good Filter cap to replace their Crappy 20 uf cap that exploded on power up!

[axpro]

DOH!


Oh yeah, i forgot to say that when i hit standby, it all goes quiet pretty fast, no lingering buzz or anything.

[loudthud]

Many times "Hum" will have two components. A 60Hz component that comes from the heater supply and a 120Hz component that comes from grounding issues. The 60Hz component is a fairly pure sine wave, the 120Hz component is more of a buzz. Two additional (but rare) causes of Hum are coupling of the magnetic field (60Hz) from the power transformer to the output transformer or speaker and power supply ripple, (120Hz sawtooth wave) usually from a filtering problem.

Power and Standby Switches: Start with both switches off. Turn the Power switch on. If there is any hum before the tubes warm up, it's from the magnetic coupling. You must rotate one or both transformers or move them farther apart and/or away from the speaker. You may hear a little burst of hum when you flip the switch, ignor it. Now flip the Standby on (play mode). Again, you may hear a little burst of hum. Allow a few seconds for the B+ voltages to stabilize. Take notice which of the controls influence the hum. Now flip the standby off. By the time you hear the click of the switch in the off position, any 120Hz buzz will stop and while the B+ bleeds down you can hear any 60Hz hum that is getting into the signal path from the heater supply.

Heater Supply: Hum from the heater supply could be caused by a bad tube. Try substituting known good tubes or swap them to different positions to see if there is any change in the hum level. The wiring of the heater supply is somewhat critical. The wires should be twisted tightly together. Some builders route the heater wiring against the chassis (Marshall style), some in the air above the sockets (Fender style). Both methods work about the same if done well. Signal wires should be as far as possible from the heater wiring and cross at right angles when they must. Distance is your friend. Twisting the wires helps balance the field from each wire because they are out of phase. I noticed that Weber shows a center tap and a "virtual center tap" made from a pair of 100 ohm resistors at the pilot light. Usually the virtual center tap gives lower hum, but try both.

Grounding: The biggest problem is the Quad filter cap because of the common ground connection. The layout shows the yellow striped red wire grounded at the chassis (or possibly a transformer bolt). Move this wire to the ground lug on the filter cap where the extra 20uF cap gets it's ground. This may not be easy, you may have to splice a wire or use a cutoff component lead to form a terminal to solder the wire to. The rest of the grounds are a hopeless mess sorry to say. If moving that one wire isn't good enough, it's going to take quite a bit of work to make a significant improvement.

Elevating the heater center tap will remove some of the 60Hz component to the hum. No sense doing it if it's mostly grounding buzz that is heard.

[axpro]

Ok,

So, on power up (standby off) no buzz.

Turn it to playing position, and the buzz starts. Even with all controls at 0.


The buzz that i hear seems to be from the reverb circuit (Being my first reverb build, it is KILLING me. I guess this is why i usually do Non reverb stuff!) Turning up the volume adds hiss, but not the hum I am hearing. Turning the reverb up increases the hum I am already getting from the verb.

Heater wires are twisted tight, and run along the bottom of the chassis. I will try the virtual center tap, to see if that helps, but I am unsure. TRied some new tubes, and no change. I am a little confused. the hum seems to be the reverb control, but pulling the verb tube doesn't get rid of it. Pulling the first position tube does.

Also, the hum is present with controls all set to zero, but there is still a little reverb bleed when i hit the tank.

I fabricated a ground strap on the cap can, where all the grounds from the amp go. As I said earlier, my lead dress was better, but switching every signal wire to a shielded run is taking its toll on me. hard to rework stuff. I may start re-wiring the first stages.

Thanksa gain Loudthud!

[axpro]

After fighting with it even MORE, I have decided that it is time to take matters into my own hands and change the layout. I have had more luck in the past with PTP wiring on amps, so i have decided to fly a few components (like cathode resitors for preamps) directly from the tube, eliminating some of the wires going to the board. I am hoping that by shortening leads everywhere i can, and cleaning up lead dress i can fix some of the issues.

I have also decided i don't really like weber kits anymore, which is for another thread. They aren't bad per se, just not the greatest when you are trying to get predictable and clean results. (more elsewhere)

[loudthud]

There is one thing I thought of that might work. The idea is to relocate the 20uF 450V cap used to filter point "D". Instead of using section 4 of the quad cap (and it's poor ground), you will use the axial cap (relocated) connected in parallel with section 1. Section 4 will be connected to section 1 to restore the same amount of filtering. The 22K 2W resistor will also have to be relocated. The leads may be too short so you may have to replace that resistor. The location of the 22K could be changed by moving some of the resistors at the end of the board between points "B", "C" and "D".

[axpro]

Cool thud, i'll look at doing that change maybe tomorrow night.

Otherwise, I am HATING this amp.. razzafrazzin.

BUT it does remind me of something, I had some noise issues that wouldn't go away on my 5e3x2 build, no matter what circuit i used... until i replaced the Weber pots.

And the High gain JCM800 build i just finished (with a nice parts kit from Metro and nice higher quality transformers) had NO issues, depsite it being a hot rodded JCM800....

Truth be told, I am not liking the tremolo in the circuit (too fast, not enough effect) The Reverb is too big (WAY too big) and the noise issue is killing me.

I may just decide to build something else out of this thing. Maybe a full tone stack, a different reverb circuit and some better filtering. I could get a new faceplate made up, with Volume, T/M/B Verb and maybe a cut control.

and i could throw this crap circuit out the window.

[Bruce / Mission Amps]

Wow...it is unfortunate you are hating this thing... for the money and the size of it, the mid 60's through early 70's Princeton Reverb amp is probably the single, baddest ass amp Fender ever came up with.
With a few very minor tweaks, a 12" NEO speaker and a little boosting pedal on the front end, they absolutely rock the house and they weigh next to nothing too.

Let me know when you are ready to toss it out.... I'll be there.

[axpro]

I'm just pissin and moanin Bruce.

The whole idea originally, is that i really LIKE blackface clean sounds, and putting my OCD in front of it would make it a nice little amp for jams and the occasional gig. but this noise issue is kicking my ass! Its too much noise to ignore. and running the 12au7 in the verb made it smaller, in reverb... but still had just as much noise as before, so my signal to noise ratio was crap.

I also tried slowing down the trem, and it didn't seem to work at all

Everything is shielded, but it is making way more noise than my JCM800 hotrod build.

I am suspecting low quality pots. I guess i'll do an order snd see if things get better when i start changing out parts.

[tubeswell]

I am suspecting low quality pots. I guess i'll do an order snd see if things get better when i start changing out parts.

Interesting observation. (I am thinking more like "low quality parts" )

I just finished one of these kits (6A14) and had annoying hum, which the vol knob seemed to cancel out at "5", and I tried to rewire the ground every which way, but nothin' seemed to fix it, so I have now put in a new PT and and am rebuilding the amp completely with entirely separate grounding and dumping the can cap altogether etc. So we'll see how that goes.

[axpro]

Hey TUbeswell,

I remember you saying you were having issues with yours too.

I really like the look, and the idea of the amp..... but the parts are shitet o say the least.

Mine does the same sort of thing, volume around 5-6 and the hum goes away.

Ordered good parts and some G10 board and turrets this morning. I am going to re-do the layout on a turret board. with known good parts (probably orange drop caps since i already have a lot)

I understand reverb can be tricky as far as noise problems, but like I keep saying, my JCM800 hotrod had NONE of these problems. then again, it has british made transformers, half decent quality parts (sozo caps, CTS pots) but nothing earth shatteringly good! component wise!

IF its the transformers, that is it,I will be DONE with Weber forever. If i want really inexpensive kit parts, i can get them from Ceriatone, and have better luck with quality.

This puts my combo build on hold. I guess i'll workon the battered shell of my 5e3x2, and build something i might actually like in there. If those transformers aren't shite as well.