65 Princeton Reverb to Deluxe Reverb Conversion

[TenderTendon]

I have this 65 Princeton Reverb reissue that I have never really been happy with. I decided to convert it to a Deluxe Reverb, using the Hoffman 1 channel AB763 turret board. Before I make the commitment to add another tube socket, I'd like to see if I can get good results from using a single tube reverb and Sluckey's Trem-o-nator. I never used the reverb above 3, so I think I'll get enough reverb with this. I've edited the layout for you guys to critique and for me to have something to follow. I'm sure there are mistakes and things to be optimized. I would appreciate it if you guys could have a look and offer suggestions to give this the best opportunity of working the first time. Sorry for the crude drawing. MS Paint sucks... Thanks for looking!

[tdvt]

I will be interested to see what the more knowledgeable have to say.

You might find this recent thread relevant regarding the Trem-O-nator LFO not likely not being a good neighbor with anything else in the other half of the tube. https://el34world.com/Forum/index.php?topic=30069.0
Thinking that might affect your tube count.

To your comments regarding reverb, I recently completed a 1-channel Hoffman AB673 (2-tube reverb) & added a dwell control to the circuit which I find makes a huge difference in the adjustability of the reverb. The dwell is essentially the reverb "send" & the regular reverb control is the return, if that makes sense. You just change one resistor to a pot to add it (the 1M, R31 on the Hoffman drawing).


Many add the dwell control to the back of the amp, an alternative would be to use a concentric dual pot in the reverb pot position.

You also have the mid control shown in the layout, which I also used & find very useful, but the Princeton is just treble & bass.  A concentric dual pot for that might be a consideration for that as well

[TenderTendon]

Yeah, I read that thread. I'm hoping that the PI driver isn't as sensitive as the reverb recovery and doesn't cause issues. The mid control is not being used. I just didn't take the time to remove it from the layout. I would like to see how good the single tube reverb can be, before adding another pot or additional components.

[tdvt]

Yeah, I read that thread. I'm hoping that the PI driver isn't as sensitive as the reverb recovery and doesn't cause issues.

I would like to see how good the single tube reverb can be, before adding another pot or additional components.

I wonder if that stage would work in there, myself.

I very much like to idea of the 1-tube reverb too, currently working on a small add-on module to install the 1-tube circuit in a 5F2A I built several years ago.

[tubenit]

This is a DRAFT idea that is not proven.  Having said that, I am reasonably confident the one tube reverb would work fine as this is how I typically do the insertion point and have consistently been happy with it.  It is NOT surf reverb intense so if that's what you're looking for, I would advise using a typical Fender reverb instead.

Not sure about the vactrol tremolo.  I have done one before on an amp and was pleased with the results. It got a pretty deep tremolo to it.

Hopefully others and particularly Sluckey will offer a critique. I offer this schematic simply as a discussion point.

Note I changed several cathode cap values and the post LTPI coupling cap values to what I feel is an improvement over the Fender values.

With respect, Tubenit.

[tubenit]

Another possible DRAFT idea to consider trying to keep the tube count the same as on a PR.  I have NOT built this one either, so if you have interest in this as for feedback from the guys on the forum.  I'm posting the idea with the aim to generate some discussion about this.

Check for errors!

with respect, Tubenit

[TenderTendon]

Thanks for those. I am looking at the first draft you posted and comparing it to my layout and Fender layout. I'm sure I'll have more questions, especially considering the insertion point of the reverb how it mixes in. I doubt I'd have the space on the turret board for the second layout you posted. The turret board is already populated and wired according to the Hoffman layout and removing the components from the second half of the tremolo tube allows just enough room for the vactrol. I'm going to start temporarily wiring this board into the chassis.

[sluckey]

I've been looking at your modified layout and have a couple suggestion concerning the TON circuit...

1. Look closely at the VTL5C1. The actual bug will not easily fit the way you have drawn it. The pins are laid out a bit differently.

2. Remove that 47K that sits to the left of the VTL5C1. Use a 50K-RA INT pot and connect it as shown in my attached drawing.

I also have modified Trem-O-Nator layouts for Hoffman 2-channel AB763 and Princeton Reverb if these would be of interest.

EDIT... I just realized that some current production VTL5C1s (such as Xvive) have the LED terminals reversed from the old Vactrol units. Probably best to install according to the markings on your VTL5C1. You only get one shot. If you install it backwards you will need to replace it.

[TenderTendon]

Thank you so much for that! I'll get to work on it first thing in the morning. I have to ask... What program did you use to edit the layout? I'm sure it's much easier than MS Paint...

[sluckey]

I did those drawings several years ago soon after I did the original TON circuit. Hoffman was kind enough to give me his original Visio drawing and I used Visio to modify his drawing. Then I made a pdf copy to share on the net. As you have found, there's no easy way to modify the pdf copy.

[tubeswell]

This schematic is reverse engineered from an actual 22W amp for 6V6s with 3 preamp tubes (including a 1-tube reverb). The amp sounds fantastic. You could easily add a 4th tube for tremolo.


https://el34world.com/Forum/index.php?action=dlattach;topic=25970.0;attach=83666;image

[TenderTendon]

This schematic is reverse engineered from an actual 22W amp for 6V6s with 3 preamp tubes (including a 1-tube reverb).

Thanks for posting that. One thing that makes me wonder (and I've seen this quite a bit lately), is why the traditional Fender 3.2M/10pF blender is being replaced with only 150K-250K. Does this improve something? Doesn't this overpower more of the reverb return? It appears to me that the biggest problem here is asking too much from a single 12AX7 triode. Would a 12AU7 triode do a better job of driving the Fender transformer/tank? If so, would a 12DW7 would be the good option to get closer to that Fender splash? I'm not into the surf sound and I doubt that there's many out there that still are, but if there comes a time that I sell this amp, I don't want it to appear that it is "broken".

[tubenit]

With the one tube reverb design I use, the reverb pot is 1M with a 330k  on the other side of a 150k -220k resistor.  This is adequate in "separating the sound to send and from return. 


The Fender reverb pot is 100k so the resistor to separate send and return needs to be larger. Fender uses 3.3M.


I would suggest simply trying a 12AX7, 12AT7, 5751, or 12DW7  which is what I use most frequently these days.


With respect, Tubenit

[TenderTendon]

With the one tube reverb design I use, the reverb pot is 1M with a 330k  on the other side of a 150k -220k resistor.  This is adequate in "separating the sound to send and from return.
The Fender reverb pot is 100k so the resistor to separate send and return needs to be larger. Fender uses 3.3M.

That makes sense, but doesn't that 150K cause the dry signal to distort the output much earlier on the volume knob?

[TenderTendon]

I also have modified Trem-O-Nator layouts for Hoffman 2-channel AB763 and Princeton Reverb if these would be of interest.

I have played amps that have had the tremolo placed both before and after the reverb. I actually prefer the sound when the reverb return is not affected by the tremolo. In this case, where would be the best place for tremolo injection? Volume pot wiper?

[tubeswell]

Thanks for posting that. One thing that makes me wonder (and I've seen this quite a bit lately), is why the traditional Fender 3.2M/10pF blender is being replaced with only 150K-250K. Does this improve something? Doesn't this overpower more of the reverb return? It appears to me that the biggest problem here is asking too much from a single 12AX7 triode. Would a 12AU7 triode do a better job of driving the Fender transformer/tank? If so, would a 12DW7 would be the good option to get closer to that Fender splash? I'm not into the surf sound and I doubt that there's many out there that still are, but if there comes a time that I sell this amp, I don't want it to appear that it is "broken".

The reverb on the Morgan SW22R sounds great, and plenty of it. Nothing ‘broken’ about it.

One thing I didn’t check tho’ was the turns ratio of the reverb transformer. It looked the same physical size as a typical Fender BF RT (18k-25k load) so quite possibly was ‘the same’ number of turns (but it could have been higher to suit a single 12AX7 triode. All the xformers were labelled as custom made for Morgan). Not a biggie, because you can use a 12DW7 for the reverb with the ‘AU’ side driving the RT. Or go transformerless.

[sluckey]

In this case, where would be the best place for tremolo injection? Volume pot wiper?

See Reply #7.

[TenderTendon]

See Reply #7.

I don't see anything there regarding placement of the tremolo before the reverb.

The reverb on the Morgan SW22R sounds great, and plenty of it. Nothing ‘broken’ about it.

By broken. I didn't mean sounding bad. I meant not sounding like someone familiar with Fender reverb would expect.

[sluckey]

See Reply #7.

I don't see anything there regarding placement of the tremolo before the reverb.

The schematic and layout I posted in reply #7 clearly show injecting the tremolo at the .001µF cap leading into the PI. Use a 50K-RA pot and remove the onboard 47K resistor.

[TenderTendon]

Yes, I see that. My question was in regard to injecting the tremolo before the reverb, so only the dry signal is modulated.

[sluckey]

I have played amps that have had the tremolo placed both before and after the reverb. I actually prefer the sound when the reverb return is not affected by the tremolo. In this case, where would be the best place for tremolo injection? Volume pot wiper?

Yes, I see that. My question was in regard to injecting the tremolo before the reverb, so only the dry signal is modulated.

Sorry. I misinterpreted. I know the trem works well as I showed so that's what I would do. But if you want the trem injected at the volume pot give it a try and as your note says "experiment with placement". Just be aware that the tremolo effect will be going through the entire reverb circuit. I have no idea how that will sound. If you like it, great. If not, well it's easy to move.   

[TenderTendon]

But if you want the trem injected at the volume pot give it a try and as your note says "experiment with placement". Just be aware that the tremolo effect will be going through the entire reverb circuit. I have no idea how that will sound. If you like it, great. If not, well it's easy to move.   

No problem... Yeah, I have a couple Valco amps that wiggle the bias of the first 12AX7, so the reverb return doesn't get modulated. I've always liked that sound better. Hard to explain, but it sounds more like the reverb was applied in the studio. I'll give it a try both ways and decide. It's nice to know that there is a tried & tested schematic to fall back on should experiments fail. Thanks again.

[TenderTendon]

I was powering up sections and checking for wiring mistakes and luckily found none. The only thing that didn't behave as expected was the LFO. It produces a nice clean sine wave with a range of 2.46hz - 6.8hz. I would like to get this down to at least 2hz, maybe a little slower if possible.  Voltage swing was 177v@2.46hz, 260v@5hz and 150v@6.8hz. Is there any way to even this out, or is it just the way the oscillator works? The LFO is currently running without the vactrol wired in, as I'm waiting for a couple new ones to arrive. To simulate the vactrol I wired in an LED (red 1.8v 20mA) and the LED would pulse a couple of times and the LFO would stall. Voltage at the LED was 1.55v and very dimly lit when stalled. The only way to re-start the LFO is to remove the LED. What should I be looking for to fix this?

[sluckey]

Be sure the LED cathode (flat side) connects to ground. Have you had it accidentally connected backwards? If so, get a fresh one because it's likely shot.

Once you have the LED reliably blinking, recheck the voltage swing and frequency range. It's normal for the amplitude to drop some at the low and high end of the frequency range. 2Hz will be hard to get, so wait until the LED is blinking correctly to tackle that.

[TenderTendon]

The LED was connected correctly and still works. It glows brightly, passing 20mA with 1.78v. I removed one of the Vactrols from my SLO-100 board and tried it. Same thing. LFO stalls until removed.

[sluckey]

The LED was connected correctly and still works. It glows brightly, passing 20mA with 1.78v.

Does the LED blink at the rate of the LFO frequency?

[TenderTendon]

That information was just testing the LED with a power supply, to confirm its condition. When the LED is connected to the cathode/10K, while it it oscillating, it will blink for a couple seconds, with every blink getting dimmer. The oscillator then stalls and the LED remains barely on. At this point, there is 1.55v on the cathode and it stays like that until the led (or Vactrol) is removed. Oscillation then resumes.

[sluckey]

Try a different 12AX7. Roll two or three if necessary.

Can you post a hi-rez pic that shows that area of the board and the tube socket. Pic should be clear enough to read component values and show interconnecting wires and socket pins.

[TenderTendon]

I can't work on this again until tomorrow. I tried a couple of tubes. Both are brand new that came in this PR reissue. Both are GT branded. I tried lifting the 10k and biasing with the LED alone. The oscillator came to life and worked perfect. I then tried adding a 22uF bypass cap. This worked perfect too. The LED pulses slightly brighter when the bypass cap is installed. Another interesting (and welcome) result of these changes is that the oscillating range is now 2.07hz to 7.5hz.

[sluckey]

Experiment with the 10K value. It's purpose is to keep some current flowing through the tube when the LED is abruptly switched off. This makes for a fast start when you switch the LED on. I considered even going as high as 47K but the 10K has worked well for me and quite a few others. Maybe I didn't experiment enough.

You're the first to report this issue. LMK what value resistor you end up with. You really don't need the bypass cap.

[TenderTendon]

I ended up trying 15K without a cap and that worked. Once I had it working rock solid, I experimented with the speed range. I ended up with .018uF caps and 1.5meg resistors. This gave me a range of 1.7hz - 5.5hz. Exactly what I was hoping for. Ironically, after changing the caps and resistors, 10K worked fine. Actually, I tried a few resistors, going up to 33K. They all worked fine. As resistance went up, the brightness of the LED went up. Not much, but noticeable. I'll pick a final value when the new Vactrols get here. Probably end up with 10K or 15K. Thanks for your patience with this. Not fun troubleshooting over the internet.

[HotBluePlates]

... injecting the tremolo before the reverb, so only the dry signal is modulated.

Experiment to verify the advice, but...

When people set up pedalboards they normally place Tremolo after Reverb.  The reason is applying reverb to a trem'd sound washes out the tremolo and makes it indistinct.  But trem after a reverb yields a strong trem effect (unless you turn down the Intensity).

[TenderTendon]

Experiment to verify the advice, but...
When people set up pedalboards they normally place Tremolo after Reverb.  The reason is applying reverb to a trem'd sound washes out the tremolo and makes it indistinct.  But trem after a reverb yields a strong trem effect (unless you turn down the Intensity).

I've been experimenting for over 45 years. I don't care for a strong tremolo effect. In fact the only time I have EVER used a strong tremolo effect was in the late 70's, when I was in a band that played "Down by the seaside". Never used it since. When I am playing clean stuff, i prefer a very slow, subtle tremolo followed with reverb. I don't think the reverb washes out the tremolo at all. It just gives it some nice "air". I usually gravitate towards one of my Gretch or Supro amps over a Fender for that. They sound amazing when you upgrade them with a bigger tank. For dirty stuff, I don't use tremolo at all. Just a bit of delay or reverb.

[sluckey]

My Ampeg Gemini II had the tremolo applied prior to the reverb. The tremolo was inserted at the input to the James tonestack. The output of the tonestack was sent to the PI circuit, but it was also sent to the reverb circuit. So, the signal traveling through the reverb circuit had tremolo on it. Maybe the tremolo effect was washed out a bit coming out of the reverb recovery, but no matter. The reverb signal mixed with the dry signal (which had a strong tremolo effect) at the PI input. Net results was a strong trem effect and a strong reverb effect. Sounded very good.

[HotBluePlates]

... When I am playing clean stuff, i prefer a very slow, subtle tremolo followed with reverb. ...

Since you know your preference, go with that!

My Ampeg Gemini II had the tremolo applied prior to the reverb. ...

Blinded by my taste for strong trem, and limited experience with Ampeg amps!

I have an early R12R Reverberocket where the trem is applied at the output tube grids, and previously owned an ET1 Echo Twin where reverb was applied after the first amp's output (with trem), and before the 2nd amp's input (with a second trem circuit).

[TenderTendon]

Well, the tremolo portion of the amp is working fine now and the 12DW7 finally arrived, so it's on to the reverb next. I have to say that I'm quite surprised at the potential of a 12AU7 triode. Plugging an 8 ohm speaker into the tank driver output resulted in some respectable output. More than I expected to hear. I guess that's a good thing. One thing I noticed is when testing with a signal generator, the reverb driver output was strong down to 40hz or so. Is it advisable to decrease the cathode bypass cap, to take some low frequency load off the tube, or is that where the Fender reverb sound comes from? By hitting the springs as hard as they can with everything they got? Also, what is the correct way to go about determining the optimum bias the reverb driver? Plate is at 400v and I have it biased a little over 2W now. Thanks.

[sluckey]

Fender used a 500pF coupling cap to the driver grid to purposely reduce bass frequencies entering the driver. Using a 1µF to 5µF cathode bypass cap should reduce the bass even more.

I've never even thought about biasing the driver. I just go with the stock 2.2K resistor value.

[TenderTendon]

2.2K puts me slightly over max plate dissipation. I'm using 3K now. Pin 1 = 400v. Pin 2 = 0v (-16.2 to cathode). Pin 3 = 16.2v

[sluckey]

2.2K puts me slightly over max plate dissipation. I'm using 3K now. Pin 1 = 400v. Pin 2 = 0v (-16.2 to cathode). Pin 3 = 16.2v

I guess you can't use the stock value since you're not using the stock circuit. Better ask tubenit about his resistor value and tube voltages. Or just don't worry about it.   

[dude]

Is this schematic proven reliable, just confused a bit about the Vactrol being wired correctly.

 https://el34world.com/Forum/index.php?action=dlattach;topic=30097.0;attach=104736

[sluckey]

Explain your confusion.

[dude]

Explain your confusion.

In your reply #7, you mentioned an EDIT, maybe l’m reading something wrong, seems you implied a possible change..?

[sluckey]

In your reply #7, you mentioned an EDIT, maybe l’m reading something wrong, seems you implied a possible change..?

No change mentioned. I was just pointing out that some other VTL5C1 "may" have the LED reversed. Look at the pic and notice the LED polarity markings. Easy fix. Just turn the Other upside down. Now the + lead will be on the same side as my Vactrol.

The schematic has been proven by lots of folks.

[TenderTendon]

OK, I finally got the turret board mounted and everything permanently wired up. Upon the first power up, there was complete silence coming out of the speaker. I immediately got nervous and started checking voltages. Nothing alarming there. Turned volume, treble and bass all the way up. Still dead quiet. I started checking for grounded grids and got pops and hum there. Okay... Plugged a cord in the input and touched the tip. Plenty of loud hum. I was nervous for nothing. This amp is quiet! No hum, no hiss, no nothing. A few things I have to comment on...

1) The one tube reverb using a 12DW7 is every bit as good as the original. There is more reverb than I will ever want. I would never use more than 3-4. Turning up to 10 is definitely the surf thing, with the reverb louder than the dry signal. Now that I have seen what this 12DW7 is capable of, I will never use 2 tube reverb again. It's that good. I am pushing the 12DW7 hard, but I don't think it minds. Also, the reverb return is completely hum free, even on 10. This probably has more to do with the layout than anything else though.

2) Using the one tube reverb and Sluckey's Trem O Nator allowed me to eliminate one tube completely and easily convert the Princeton to the Deluxe. In order to do this though, I had put the tremolo oscillator and the final gain stage in the same envelope. No one knew if the oscillator would pass noise over to the gain stage or not, but I am very relieved to say that it does not. Not one bit! The oscillator is doing it's thing with 300v PP right next to the signal and nothing's crossing over.

3) I have to say that this is the best sounding Fender I have ever heard. I think a large part of that was changing all of the cathode bypass caps (including the reverb driver) to 5uF. It is more "Marshall like" than I expected it would be. With low output humbuckers, it starts to break up at 4-5 on the volume and there's no flub unless you turn the bass up past 7-8. My son works midnights and was sleeping, so there was very brief high volume testing. With the little testing I did do, I am extremely impressed. Clean tones are very similar to the Princeton, but when pushed into distortion, there is no comparison. The Princeton circuit sounded like crap compared to this (sorry Princeton lovers).

I have not tested the tremolo yet, as I'm waiting for a 50K pot to arrive. I know it will work, but I don't know how well yet. I also have to get the C & D power nodes lowered a bit so tomorrow I'll swap some dropping resistors and do some more testing. Maybe I'll take some pics to share my work...

[tubenit]

The one tube reverb using a 12DW7 is every bit as good as the original. There is more reverb than I will ever want. I would never use more than 3-4.

Yep, I have my one tube reverb amps set at around 3 and it's all the reverb I want. 

I also prefer smaller coupling caps than the 22uf Fender normally uses. I use mostly 5uf but also occasionally 2.2uf in later gain stages and occasionally a 10uf in V1a.  It does make a difference in tone and always a smoother tone, IMO with less grind. Just a personal preference thing not necessarily better.


Thanks for sharing your success!! Congrats!  Do you have a final schematic you could share please?

With respect, Tubenit

[tdvt]

I am sure many will appreciate the update with your results & impressions, lots of threads (all over the web) leave you hanging.

Especially good to know about the Trem-O-Nator working well with the second triode in the tube in use elsewhere.

I am interested in the bypass cap values you liked as well. Can you post a rough schematic of where you ended up?

I ended up tweaking the power rail on my PR (vintage) to separate the reverb recovery from the driver, putting the unused B+ node into service. I am not sure where I landed for resistor values. I probably have pics & notes somewhere.

[TenderTendon]

I'll post an updated schematic and layout when the tremolo pot arrives. I still have to experiment with the tremolo injection point.

[sluckey]

Is this schematic proven reliable, just confused a bit about the Vactrol being wired correctly.

 https://el34world.com/Forum/index.php?action=dlattach;topic=30097.0;attach=104736

Hey Dude. Here's purple tele's build with the TON. You can see that he followed my layout exactly. And here's what it sounds like...

2CInc.

[dude]

Sweet, thanks for sharing all your hard work with us.