Need some help with Princeton Reverb Build

After the Trem started working, I played with it some and it's just too danged fast. So, started looking for info on how to slow it down. Found some info here on this site.....damn, for a Telecaster site - you guys do a lot of amp work.

I've ordered parts to slow down the Trem, put in a bias pot, and install the NFB switch.. Everything is here and just waiting on me to start taking the amp apart.

Now to get off my butt and go do it......

While I'm sitting here typing.....is there any way to dial back the reverb intensity a little? A lot? Anything past 3 is just stupid....it would be nice to have a little play in it so that it's not either off or on. Seems like every Fender amp I've seen is like that....

 

Ok, I've torn into it have done the following....

1. added a bias pot as per Uncle Doug
2. Changed to Tremolo caps for a slower Trem
3. Installed an NFB switch as per Robinette's site.

Here's what it looks like - please tell me if you see anything amiss....

Mounting bracket


Trem caps


NFB Switch.....put heat shrink around the resistors...hope that's not a problem.


NFB installed.....switch end


NFB.....coax to board and ground


Bias Pot wired up.....


Ok, so all of that is done....

I haven't fired it up yet as I'm still lost on what voltages at what points in the circuit I should be looking for to bias this amp.

I've watched videos - Uncle Doug's video is great right up to the time he starts to actually test the bias and the camera stays on the multimeter and never does he show exactly where he's putting the probes.

So, I just got through doing more research and ended back up here reading an old thread about how to bias a PR.....based on most the comments there, I probably shouldn't have even put in the bias control because I don't have the knowledge to keep from killing myself.

Thus, I'm at a total loss as to what to do. Can anyone just tell me very simply - what pin or point on the amp am I measuring and what should it be? Or, do I just set the pot in the middle, play it and make adjustments based on what I hear? Trying the tremolo to see that it's working fine, but the amp doesn't sound too cold?

Sorry for the long post.....just trying to figure this out.

 

Thanks, I just read that a little while ago....there was also instructions on how to read the output transformer voltage at the center tap and pin 3 in mA.....I guess that's what Uncle Doug was doing in his video. Because he was manipulating the bias pot and showing the mA go up and down. Although, that's a two-hand operation and little more risky.

I did go ahead and fire it up and measure the bias voltage at the Trem pot. My Mojotone charts showed that should be -31 volts. With the pot, I could go from about -28 to -41. So, at least the pot is working properly.

I adjusted while playing and really didn't hear a huge difference. Now, when I cranked on the Tremolo, I could really hear the difference in the intensity as I turned the pot. If I understand correctly, the Tremolo goes away as the amp runs hotter - so I went to that point and backed it off to where the Trem was engaged, but not overwhelming (man, you can get it to really pump big time if you turn it back too much). This ended up back around the -31 so I just set it there and have left it at this point. I know that I wasn't actually setting the proper bias for those specific tubes, but everything was working properly.

Deciding whether to dive back in try to actually check the true bias. Maybe I've done enough.

The NFB switch is interesting. Definitely changes the dirt as you move it around. I did the 3 way with Light, Normal, and Heavy. The Light seemed to have the best clean headroom.

The Tremolo is definitely a little slower by changing out the caps. I thinks it's workable now. I'm really looking for a slow Trem to just use with chords. I think this will do for that application.

 

This is where I get confused....seems like minus is more.....

So, if I'm at -31v at the trem pot.....would going to -35 be raising the bias voltage meaning running cooler?

 

Ha! Wish I'd have taken a picture of what I started with. I keep pretty much any kind of brackets and small metal parts .... have been for years. I dug around and found this bracket that was shaped kinda' like a U but with turned out legs....like an Omega. And it had a swiveling plate riveted to the top. I liked the size and the foot angle, so I drilled out the rivet, chunked the plate, the started straightening out the U shape in my vise to get it flat. The hole on the foot was already perfect. Drilled out the hole for the pot, then used a hack saw to lop off the rest of it. Then to the grinder to clean it up a bit.... kinda' looks like what Uncle Doug used on his video.

Actually, I was pretty proud of that little bracket - even showed it to my wife.....boy was she impressed....(insert eye roll here).

 

I guess the quarantine stuff isn't that big a deal for me.....I'm retired. The only thing that's really changed for me is that I'm missing playing gigs and going out to eat!

I've missed two country gigs that I specifically built this amp for.....along with my normal Rock gigs....

 

Ok, I'm gonna' play with this some tomorrow and see what kind of changes I see....and I'll fire it up in the dark and see how it looks.

Thanks for the help.

 

Well, I have this nagging feeling that this pandemic is going to kill me.... no, not the virus.....but, I think after being cooped up this long that my wife is gunning for me....

I was outside the other day for a while....when I came back in, she said that she missed me. I thought was really sweet until I went back out and saw the bullet hole in the fence where I was sitting.....hmmmm....what did she really mean?

 

The tube I was waiting for showed up today, so I installed it and put it all back together. Just spent the last couple of hours playing it.

So, here's the final rundown on the project....

1. The new bias pot really didn't do anything for me because I really never could figure out exactly where to set it. So, I turned it hotter until the trem started going away, then pulled it back some. Turned off the lights and left it on for a while - nothing glowing, so that part's done.

2. The Negative Feedback switch wasn't really doing much for me until I got it all buttoned back up and got to play with it some. It let me stay pretty danged clean all the way up past 7 on one of the settings, I labeled 'em with C-clean, D-dirty, N-normal.

3. Swapped out the capacitors in the Tremolo circuit as per some info I found on another thread in the forum. That definitely helped to slow down the Trem to a manageable level. I can actually see using it some on slower ballads with ringing chords.

4. I've always felt like Fender reverbs were just too danged aggressive. Either on or off without much room to dial in what you want....and anything past 3 got stupid. So, once again I found a post somewhere (I don't even remember where now) about swapping to a 12AU7 instead of the 12AX7 in the reverb driver. That was worth the $10 and the wait. The reverb is much tamer now. I was running it around 4 and it was nice with room to back off to 3 without it disappearing.

So, this project, along with the short-scale neck on my Tele has opened up a whole new sonic world for me. I think some chicken-pickin' is in my future. I've played the Tele more in the last month than I have in the last 10 years.

As I reached over the back of the amp to turn it off - a though jumped into my head.....if I ever decide to open that thing back up again, I think I just might move the On/Off switch and the NFB switch to the front panel. Might help break up the blank spot on the front panel where there is normally the Princeton logo. And it would damn sure be easier to turn the thing on and off.

I want to say many thanks to all of you who have jumped in and helped me on this project. I started out as someone who knew nothing about building an amp, but wanted to .... and now I still know nothing about building an amp, but I did....

You guys have been great.

Thanks,
Bruce Weldy

 

A lot of the guys have hit on all of these points, but you put it all together in a step by step. I've printed this out and it will go with all of my Princeton documentation. If I open it up again, I may give it another go. Right now, though - it sounds pretty danged good.

Thanks!