68 princeton "custom" reissue - "r34" mod to reduce extra bass

[ajs64]

Hi all, I have a 68 Princeton Reissue I picked up used awhile back as a small gig amp. I replaced the stock speaker with a Jupiter 10SC and use a 5751 in V1 and am close to getting a sound I like (which is more vintage blackface than the 68 custom series is designed for.) But the bass response is still too heavy for me.

I've read elsewhere about them adding an extra 18k resistor in series with the bass pot, r34. Seems some people just jumper this to have a more traditional Fender tone stack*, and that is an easier mod than replacing the resistor. Others say when you jumper it, that things sound too thin.
So I'm thinking of putting another resistor in parallel, to keep a nonzero value for the resistor...perhaps a 4.7k which would effectively give 3.7k from the resistors in parallel. Any potential downside of doing this? And am I correct that it should also be 1/2w as the other resistors are?

Many thanks in advance.

(*The 68 reissue series uses a 22pf cap instead of a 47pf for "C3" also, so to get the vintage tone stack would require a couple of changes.)

[lonote]

A common mod to tame extra bass is to reduce the value of the V1a bypass capacitor.

A quick look at a schematic I found has your's as a 22uF (listed on the drawing I saw as C18).

You can reduce that down to 1uF through 10uF to change the lower roll-off frequency. I have used 4.7uF in that spot in several builds with good results. See what works for you.





Edit; did not see your attached drawing at first, it is the same as the one I referenced.

[ajs64]

Thank you! I will keep in mind and perhaps give that a try. But first step, I think I want to get the tone stack back to the "classic Fender" design and see where that gets me.

I was probably not so clear in my original post -- these Custom 68 series (Princeton and Deluxe at least) messed with the vintage design of the tone stack to differentiate the sound from the classic 65 reissue series. Some like it better, I definitely don't. (My main amp is a 67 Pro Reverb and if it weren't 53 friggin pounds it'd come to every gig!)

-a

A common mod to tame extra bass is to reduce the value of the V1a bypass capacitor.

A quick look at a schematic I found has your's as a 22uF (listed on the drawing I saw as C18).

You can reduce that down to 1uF through 10uF to change the lower roll-off frequency. I have used 4.7uF in that spot in several builds with good results. See what works for you.





Edit; did not see your attached drawing at first, it is the same as the one I referenced.

[Stevem]

Are you talking about having too much low end when only on your neck pickup?

If so then once at a gig with this amp take 3 seconds to lower the bass side of your neck pickup some.

[solderhead]

Sort of hijacking for a minute...

Steve, that's really good advice. Lately I've been playing a lot through an EL84 amp that has a fair amount of high pass filtering in the front end that effectively removes a lot of bass response. This week I replaced that amp in rotation with a JTM 45 and suddenly there was so much bass that couldn't be eliminated that I was thinking about modding the front end of the JTM45. then I realized that I had tailored the pickup height to sound good with the other amp, and that what I really needed to do was to make some pickup height adjustments for compatibility with the old Marshall.

Moral of the story -- don't always blame the amp.

[ajs64]

Sort of hijacking for a minute...

Steve, that's really good advice. Lately I've been playing a lot through an EL84 amp that has a fair amount of high pass filtering in the front end that effectively removes a lot of bass response. This week I replaced that amp in rotation with a JTM 45 and suddenly there was so much bass that couldn't be eliminated that I was thinking about modding the front end of the JTM45. then I realized that I had tailored the pickup height to sound good with the other amp, and that what I really needed to do was to make some pickup height adjustments for compatibility with the old Marshall.

Moral of the story -- don't always blame the amp.

Thanks guys, but no. This is a well documented issue with this line of amps.

There is an 18k resistor put in series with the bass pot, r34 in the schematic, see attachment to my original post. This is a deviation from the traditional fender tone stack, it beefs up the bass to help distinguish the "68 custom"s from the "65 reissue"s. But it's way way too much bass. Not a pickup issue, I play several guitars including a Tele with a p90 in the neck, a Strat, a Gretsch with coil-split capabilities and a 61 Gibson with real PAFs. They all exhibit the same behavior.

What's odd is that those who have removed/jumpered around this extra resistor say the amp is kinda thin without it. But this is surprising given the only other modification to the Fender stack is a .22 instead of .47 cap. So my idea was to try to put another resistor in parallel to bring the resistance down but *not* bypass it completely. Was hoping others here had tried this and had advice but seems not.

-a

[B Ingram]

Hi all, I have a 68 Princeton Reissue ...

I've read elsewhere about them adding an extra 18k resistor in series with the bass pot, r34. ...

Bottom-Line Up Front: What you could do (that the threads apparently didn't know to suggest) is change the relative-sizes of R34 and R52 (while keeping about 25kΩ total resistance) to increase the mids, which will seem to reduce the bass.

______________________________________

This issue is explained more by the 68 Custom Deluxe Reverb page on Fender's site. They mention "Bassman Tone Stack" on the modified channel.

They're referring to a 5F6-A Bassman, but that amp has a "Middle" control that neither the Deluxe Reverb nor Princeton Reverb have.

- The 5F6-A Bassman has a 25kΩ Middle control, where most blackface amps have a 10kΩ pot (or a 6.8kΩ resistor set to act like a 10kΩ Middle control set most of the way up).

- The 5F6-A also connects its Middle control differently to the tone stack, where a 0.02µF cap connects to the wiper & the pot is not wired like a variable resistor (as was done on the blackface amps).


The "Modified" channel of the Fender Vintage Modified series uses 2 resistors to act as a fixed-setting of the 5F6-A Bassman's Middle control:

- Here, your 68 Custom Princeton Reverb uses R34 (18kΩ) and R52 (6.8kΩ) to mimic a 25kΩ linear taper pot set to a little over 1/4 of the way up.
18kΩ + 6.8kΩ = 24.8kΩ.
6.8kΩ / 24.8kΩ = ~27%


Setting the 5F6-A Bassman's Middle control low has the effect of scooping mids & leaving the bass to seem more-prominent. Try playing with it yourself in the online Tone Stack Calculator.

So now you see where the suggestion at the top comes from. You could just eliminate R34, but if you're gonna walk things back to be like a 65 Princeton Reverb Reissue, you probably should just sell this amp & get a 65 Princeton Reverb Reissue.

[ajs64]

Thanks so much! My hypothesis was to reduce the R34, but not eliminate it, and I wondered how much. I didn't appreciate R52's impact. I did play with the tone stack calculator a little.

As far as selling and getting a 65 RI instead, I probably should, but financially, I think it's significantly cheaper to do the mod. I got the 68 used for a good price, was not really in the market but I really love the "tone per pound" of this amp for small gigs.

Really appreciate the feedback,

-a

Hi all, I have a 68 Princeton Reissue ...

I've read elsewhere about them adding an extra 18k resistor in series with the bass pot, r34. ...

Bottom-Line Up Front: What you could do (that the threads apparently didn't know to suggest) is change the relative-sizes of R34 and R52 (while keeping about 25kΩ total resistance) to increase the mids, which will seem to reduce the bass.

______________________________________

This issue is explained more by the 68 Custom Deluxe Reverb page on Fender's site. They mention "Bassman Tone Stack" on the modified channel.

They're referring to a 5F6-A Bassman, but that amp has a "Middle" control that neither the Deluxe Reverb nor Princeton Reverb have.

- The 5F6-A Bassman has a 25kΩ Middle control, where most blackface amps have a 10kΩ pot (or a 6.8kΩ resistor set to act like a 10kΩ Middle control set most of the way up).

- The 5F6-A also connects its Middle control differently to the tone stack, where a 0.02µF cap connects to the wiper & the pot is not wired like a variable resistor (as was done on the blackface amps).


The "Modified" channel of the Fender Vintage Modified series uses 2 resistors to act as a fixed-setting of the 5F6-A Bassman's Middle control:

- Here, your 68 Custom Princeton Reverb uses R34 (18kΩ) and R52 (6.8kΩ) to mimic a 25kΩ linear taper pot set to a little over 1/4 of the way up.
18kΩ + 6.8kΩ = 24.8kΩ.
6.8kΩ / 24.8kΩ = ~27%


Setting the 5F6-A Bassman's Middle control low has the effect of scooping mids & leaving the bass to seem more-prominent. Try playing with it yourself in the online Tone Stack Calculator.

So now you see where the suggestion at the top comes from. You could just eliminate R34, but if you're gonna walk things back to be like a 65 Princeton Reverb Reissue, you probably should just sell this amp & get a 65 Princeton Reverb Reissue.

[B Ingram]

Thanks so much! My hypothesis was to reduce the R34, but not eliminate it, and I wondered how much. I didn't appreciate R52's impact. I did play with the tone stack calculator a little.

A way to make this happen is to actually install a 25kΩ pot temporarily in place of R34 and R52.

- When you remove both of those resistors, you will have 3 points that correspond to the "Top," "Wiper" and "Bottom" of the pot.

- Turn the pot while playing/listening to decide what setting sounds best.

- Measure the resistance from Wiper-to-Top and use the nearest standard value in R34's position.

- Measure the resistance from Wiper-to-Bottom and use the nearest standard value in R52's position.

[stratomaster]

Just swapping the resistor will be fine. I used a 5.6k in mine (I think). I did a deep dive on this amp over on the telecaster forum. Anything you'd want to know about tweaking it is either there or in my head yet to be written to that thread. I cover the effect of this resistor on the tonestack in depth.

https://www.tdpri.com/threads/princeton ... s.1156402/

[B Ingram]

... I did a deep dive on this amp over on the telecaster forum. Anything you'd want to know about tweaking it is either there or in my head yet to be written to that thread. I cover the effect of this resistor on the tonestack in depth.

https://www.tdpri.com/threads/princeton ... s.1156402/

Note that instead of simulating the stack as a "Super Reverb with a change" it is better to simulate as a "5F6-A Bassman with a 38kΩ source impedance."

Set the "RM" slider to "2.70" to see the curve of the Vintage Modified tone stack.