Princeton Reverb - strange high pitch squeal

[choosebronze]

First, thank you to Hoffman, and to everyone on this site. My PR build was pretty easy using his great layout and trove of information. And I read a ton of posts on here before and during my build. I'm happy to say it's up and running, and it sounds great, but with two (probably connected) issues.

I had the amp running Sunday, and everything worked. The reverb worked as expected, and was switchable. The tremolo worked as expected, and was switchable. But the RCA jacks I used were a little small for the holes drilled, and a couple times they fell right out of the chassis when plugging/unplugging.

So today I installed some wider shaft RCA jacks, and now I've got problems. I used the gold Chinese ones available everywhere. I have the ground ring up against the chassis. So from outside-to-iside I assembled the jacks as plug, plastic washer, chassis, ground ring, plastic washer, nut.

Now the reverb will not engage. Footswitch does nothing. As for the tremolo I can hear it oscillating, there's a whoosh whoosh behind the sound, but it doesn't actually affect the guitar signal. It just oscillates on its own, under the guitar. The tremolo also is now NOT switchable. The knobs do control the depth and rate of the oscillation.

I built the switch myself, just a couple DPDT's, LEDs, and 9v. I have a lot of experience building pedals, and the footswitch worked Sunday so I'm fairly confident that's not the issue. I built the pedal with a TRS 1/4" jack and then am running a 2 RCA -> 1/4" TRS cable, if that's somehow an issue. I'm thinking all the switch is doing is grounding out the reverb/trem jacks. If it were a 9v pedal I'd just put my thumb on the ground lug and see if that fixes the problem - but I don't love the idea of my hand in a live amp.

I'm really scratching my head. It was all working, so I'm pretty sure I understand how it should be wired. I'd have to imagine the odds of getting 4 defective RCA jacks is pretty slim. They're simple jacks. Any help would be greatly appreciated. I'm so close, it'll feel really good to get this thing done!

[choosebronze]

Update: I figured it couldn't hurt to just rewire all 4 RCA jacks. Easy task. That did something. The tremolo now works as it should, footswitch and all. Maybe I'd accidentally had wires touching, though I didn't see any.

When I hit the reverb footswitch all that happens is a very trebly hum that gets louder as I turn up the Reverb knob. I have no idea if this helps.

[sluckey]

I suspect a wiring error on one or more of the reverb jacks. You don't need the footswitch plugged in for the reverb or tremolo to work so just unplug that while you look for the problem.

As for the hum with the reverb footswitch, sounds like a cable problem. The reverb must have a shielded cable to the footswitch. Look on this page to see the type cable to use with a Princeton Reverb footswitch...

     http://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=WireCable&ORDER_ID=539501739

[choosebronze]

Thanks Sluckey (I learned a lot from your site along the way too btw!). I took the footswitch out of the equation for now, and did some more testing. I rewired all 4 RCA jacks anew, for about the fifth time. Just in case I had something wrong.

If I turn on the amp without the reverb tank it's completely silent. A joy to behold. When I connect the tank, I immediately get a high pitch hum. That hum still gets louder (and very shrill) as I increase the Reverb knob. It's not coming along with reverb. It's just clean signal and hum.

The wiring of the reverb jacks is so simple compared to the entire amp I just built... maybe I'm missing something right in front of my eyes.

[choosebronze]

A couple of the solder joints look cold in the picture but I've made sure they're flowed well and shiny in person.

It won't let me attach a picture for some reason. But here's what I've got wired up: http://jaredmcohen.com/storage/PR_ReverbJacks.png

[sluckey]

Remove the plastic washers from all RCA jacks and see if things improve. Here's a pic of Hoffman's PR (I suspect you have seen it)…

     http://el34world.com/Hoffman/images/Img_8412.jpg

Your pic file was too large. Hoffman has a 1M file attachment limit and your png file is 9.12M size.

[choosebronze]

Well thank you Sluckey. That did the trick. It's all working properly now. I used shielded cables for the footswitch and that's working properly too.

The only tiny issue I see now is at the end of the Depth pot's rotation, high pitch noise is introduced. But if the dial were a clock face I'm talking about maybe 1 minute of rotation at the very end... So I don't even know if it's worth troubleshooting. I can't imagine myself ever cranking the depth all the way up.

Thank you!

[sluckey]

That sounds like a pot issue. Spray some de-oxit or electronics cleaner into the pot to see if that helps.

[choosebronze]

Thanks for all the help Sluckey. This was one of the hardest projects I've even taken on - in any field. I think the cabinet was even harder than the amp. But it's all working perfectly. This amp sounds amazing!

[sluckey]

Really looks good! Do you have a sea foam Strat to go with that?

[choosebronze]

Oh I wish! Believe it or not, I have zero Strats. But after playing this amp I can already tell a Strat is its perfect companion. Guess it's time to go shopping...

[purpletele]

Oh I wish! Believe it or not, I have zero Strats. But after playing this amp I can already tell a Strat is its perfect companion. Guess it's time to go shopping...

How about a Charvel?

https://reverb.com/item/12701427-charvel-san-dimas-pro-2017

the amp looks great

[choosebronze]

I guess you've got do to ALL the testing before you go online to brag... I went to label my chassis today (just doing waterslide decals) and I realized I've had the speaker into what should be labeled Ext Speaker. I tried the main speaker jack and no sound at all. Guess it's time to pull the chassis back out and have a look. Bummer.

And thanks purpletele. It has a lot of imperfections up close, like the corners, as you'd expect. Also, I listened to the people who said spray adhesive would work on tolex. It doesn't. I can already tell I'm doing to have to reapply the tolex soon. But overall I can't complain. Reglueing won't be that bad.

[sluckey]

I realized I've had the speaker into what should be labeled Ext Speaker. I tried the main speaker jack and no sound at all. Guess it's time to pull the chassis back out and have a look. Bummer.

Probably no big deal. If you followed the original schematic, when nothing is plugged into the main speaker jack, the jack puts a short across the OT secondary and you will not get any sound to the EXT jack. IOW, you must have something plugged into the main jack to be able to use the EXT jack. Easy to swap jacks or wiring around to correct. Even easier to just swap labels.

Try this... connect the speaker to what should be the main jack. You get no sound. Now just insert a blank plug into the EXT jack. Bet you now get sound. If this is the case, then swap jacks or labels. THE MAIN JACK MUST BE A SWITCHING JACK (three lugs).

[choosebronze]

Sluckey, I really do appreciate all your help. I'd be totally lost without it. This amp is becoming a two steps forward, one step back project.

Somehow I'd managed to wire both jacks wrong. Completely wrong. The jacks are wired properly now, and they work as you said they should.

I've been lightly noodling on the amp for a couple days, so now that it's all up and running I decided to give it a few big chord strums. Volume on about level 3. Treb and Bass set to about 4. I strum a big open cord and WHOOOOMMMMP. A loud buzz/hum. I don't know how to explain it. It sounds more like a hum, like a refrigerator noise, but electrified and LOUD. Much louder than the amp volume was set to. Loud enough that I thought "I hope that didn't bother the neighbors." The controls had no effect on the noise. I could no longer hear guitar, just hum.  Turned the amp off, let it cool and came back a bit later. Same thing as soon as I hit a big chord. The noise has no audible changes in pitch or anything. It's just a steady loud noise. I can't say I've ever heard anything like it.

Any insight would be appreciated. Took a sound recording. When I listen back to the recording I do hear a bit of shifting in the pitch: https://www.dropbox.com/s/4em9axwvgqv84vg/Whomp.m4a?dl=0

[tubenit]

My guess is that when you rewired the jacks, somehow you got the negative feedback NFB wired incorrectly?   IF that is the case, you can temporarily disconnect the NFB and if the sound/noise stops when you turn the volume up that would be a sign that the NFB is wired incorrectly?

OR ……….. that a power tube has redplated?   IF you have 1 ohm bias resistors on the build, check the current going thru both tubes at idle.  Is the current "matched"?

My 3rd guess is that a solder joint is cold.

With respect, Tubenit

[MFowler]

One needs to remember when ordering reverb pans, doing your home work pays off.  Some indicate grounded input or not and grounded output or not.


Those letters and numbers in their part number have meaning and designation.


Believe me I have to look all that up on antique Electronics Supply (AES.com) website to make sure I get the right pan.


Great looking build.


Mark

[choosebronze]

My guess is that when you rewired the jacks, somehow you got the negative feedback NFB wired incorrectly?

IF you have 1 ohm bias resistors on the build, check the current going thru both tubes at idle.  Is the current "matched"?

It's going to end up being the NFB. Because I had the jacks wired wrong at first, I don't think NFB was ever in the circuit to begin with. So I looked at those components. They're all the correct value and the joints all look fine. I tried another 12ax7 in V3 and no change. But as you guessed, the problem stopped when I snipped the NFB connection to V3. But that's where I get stuck - because my wiring matches the schematic and the layout. I guess I can just reflow all the solder, it's only like a dozen connections.

I don't need to actually connect the "grounded" sleeve lugs back to a ground connection right? Because they're grounded by the chassis? I figured maybe I was looking at the layout wrong, so I drew what I've wired, and compared it back that way. Looks right unless I'm missing something.

On your question about the tubes - when I biased them current across the 1 ohm resistor was very close (I think one was 22 and one was 21.2 mA.). But when I just checked, one was 22 and one was around 16. Don't know what's going on there - but like I said, the hum stopped with the NFB disconnected so I'll go with that as the bigger issue.

[choosebronze]

I rewired the jacks. Then I even tried with just the main jack wired to make sure it was correct. No change.

I checked all the solder joints in the NFB. They all looked good but I reflowed them all anyway. I also noticed a couple other iffy looking joints throughout the amp and reflowed them too. The NFB portion is so tiny it's hard to imagine component failure. What would happen if that 25uf cap fried? It doesn't look bulging... Just brainstorming.

It's all back to where it started. If I disconnect the NFB, it's all good. I can't get it to fail. With the NFB connected it runs for about a minute and then just hums.

[sluckey]

It's all back to where it started. If I disconnect the NFB, it's all good. I can't get it to fail. With the NFB connected it runs for about a minute and then just hums.

Swap the OT primary plate wires.

[choosebronze]

Thanks Sluckey - That would just be moving the OT wire at Pin 3 on V5 over to V6 and vise-versa right? Sorry for the dumb question but I'm still new to this & I figure it's better to verify than to assume.

[sluckey]

yes

[choosebronze]

Sluckey, you are the master. That appears to have fixed it. Now that I said that, demons will come in the night and break something by morning... but it's working right now, with no loud noises. My sincere thanks.

I just spent the last hour reading about feedback loops and don't understand why it would matter which power tube is connected to which side of the OT - but did connecting it the other way around create a positive feedback loop? So when a strong enough guitar signal came through the amp was just thrown into a state of self-generated feedback? If it's just a 50/50 proposition it seems like half of NFB builds would have a similar issue at first.

I guess I've still got to figure out how the power tubes got un-matched, but at least the amp works!

Thanks again for all the help.

[tubeswell]

I just spent the last hour reading about feedback loops and don't understand why it would matter which power tube is connected to which side of the OT - but did connecting it the other way around create a positive feedback loop? So when a strong enough guitar signal came through the amp was just thrown into a state of self-generated feedback?

The primary and secondary windings are deliberately wound with the turns in a specific 'turn-direction'. If the turn direction is reversed on either winding, the signal phase from primary to secondary will be inverted (which will affect the source phase of the NFB signal).


With single-ended OTs the Pr:Sec winding turn direction is straightforward because each winding has a single turn direction. With push-pull OTs, its a bit more complicated because each half of the primary winding is wound with opposing turn directions, and only one half of the primary has the same turn direction as the secondary (And this needs to be so, because the signals from the tubes on each side of primary are in opposing phase - due to the action of the phase inverter - but these opposing signals need to end up with only one phase on the secondary, so the phase on one half of the primary winding needs to get 'flipped', otherwise it would cancel out the phase from the other half of the primary). Either way - there is a phase-relationship between primary and secondary, which needs to be taken into account when installing global NFB.

If it's just a 50/50 proposition it seems like half of NFB builds would have a similar issue at first.

Its quite common to have to switch Primary (or Secondary) winding phase around once you have the OT installed and the amp has been tested. Even though factories color-code their OT winding leads, sometimes someone at the factory gets the colours out-of-whack, and you don't find out until you hook the OT up and turn the amp on.

[choosebronze]

Sorry, I've strayed a bit from the topic title now, but I've been trying to playing the amp as much as possible. If there are any problems I want to find them while everything's fresh in my head. Overall the amp sounds fantastic. I've realized how much mediocre amps cover up bad playing - and how bad of a guitar player I am.

I have noticed one problem. Very intermittently, maybe once every few minutes, I hear a faint pop/crackle. The best way I can explain it would be a similar sound to when there's a speck of dust on a record. It's the kind of thing you might not even notice if you weren't really paying attention. Tried with 3 different guitars and on both amp channels.

I had the same noise once before on a 5F1 build. That turned out to be a bad 12AX7, but I gather that was an unlikely cause. But my hunch now would be a solder joint. Is this where I'd try the chopstick technique? I've never done that. Am I just turning on the amp and poking every solder joint with a chopstick to see if I can recreate the same type of pop?

Or is there something else you guys think I should be looking at altogether? Thanks.

[shooter]

realized how much mediocre amps cover up bad playing

  That's the biggest "complaints" on my builds; "man, I can't get away with nothing playing your amp"

hunting your static/crackle; can also be things like arcing between points, loose socket pin, flacky/sketchy plate R's.
turn all your knobs to 0, leave it cooking for awhile, then start "thumpin" on it, bang the side, drop it an inch or so, shake it.  Repeat turning 1 knob to 10 at a time, your trying to "guess" which stage is causing it.

[choosebronze]

Ok well here's one I didn't expect. I chop-sticked the heck out of the amp. I didn't hear any noise on any component or joint. No noise on any tube pin or potentiometer. Here's where it got weird (to me anyway). With the reverb level turned up (and the tank disconnected) I hear an almost microphonic noise when I chopstick the back of any of the vib/rev jacks. If you've ever touched a piece of metal to a pickup, and you hear that magnetic pop with the pickup grabs the metal. It's that same noise. Except from a bamboo chopstick.... I'm not sure how the bamboo could be conducting anything to the jacks. Is that normal?

[sluckey]

With the reverb level turned up (and the tank disconnected) I hear an almost microphonic noise when I chopstick the back of any of the vib/rev jacks.

That's exactly what you should hear. Don't do that.

[PRR]

The reverb recovery amp has high gain, sometimes the highest path in the amplifier. With the tank disconnected, it won't take much to make a sound. Bamboo is not utterly non-conductive. Especially if it is now summer in your land.

Tie the bamboo to your meter red probe and poke it at a 300V supply. You may read dozens of Volts. Assuming 10Meg meter, 30V means the chopstick is about 100Megs. High, but not infinite.

[choosebronze]

You know, I think I'm pretty smart in my real life. And then I go on forums and every message is a humbling reminder that I still have a lot to learn. I'm going to let the amp run even longer and go chop-sticking again.  I'll throw in some spare tubes just in case that's the problem.

[PRR]

> I think I'm pretty smart in my real life.

Everybody has developed different skills. I got to know way too much about penny-ante audio (where I might have to know the resistivity of a chicken-stick; I have used Popsicle sticks). When I go in a record mastering facility, I'm totally out of my world. And if I came to your real-life work (whatEVER it is) I'd probably be pretty clueless for the first thousand hours.

My car A/C is weak. I know the first thing about freon machines, and that is about it. I'm going to a shop who does sick A/Cs every day, and I will try to stay out of the way.

No need to be over-"humble". Nobody can know everything.

[choosebronze]

Every time I think this build is done, something new comes up. This is the weirdest problem yet. I heard this noise today, and was able to record it. Can't say I've ever heard anything like this - it almost sounds like RF interference or something. This recording is with nothing in the amp's input. Knobs had no effect. Also happened with a guitar connected. It seemed to cut in and out randomly. I've played this amp a lot and never heard anything like this until today.

https://www.dropbox.com/s/c9sepxj547nx0kp/PR_Noise.mp3?dl=0

I was playing in my garage, which only has fluorescent lighting. I also had a window AC unit running. Had 2 mics on the front of the amp. Just mentioning it because I guess my first question is whether this is more likely something inside the amp chassis, or interference coming from somewhere else. I've always read about lead dress causing hum and other strange noises, so I don't know if I just need to re-route some internal wiring, or what. Any help is greatly appreciated.