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I want to boost the baseline mid frequency level on my 65 PRRI. I don't think that I need to go all out with a mid control, and I'd prefer to keep the circuit as simple as possible. So I want to change the resistor from stock. What is a reasonable resistor value to boost the mids and make my Princeton a bit more Gibson-y? I'm hoping to do the mod myself, so if anyone has additional resources on how to open up the chassis and find the resistor I want to change. Also, is there a preferred make for the new resistor? I've done pickup swaps myself but this will be my first time working on an amp.
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04-15-2017 06:51 PM
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The stock resistor is 6.8K ohms.
10K ohms will give a noticeable mid increase, but still blackface in character.
15K ohms will be getting a bit fatter than we typically think of as blackface.
25K ohms will be a little fatter than 15K but will also start to give a noticeable overall boost.
I suggest 15K and see what you think. If you are not used to soldering then the PC board work could be slightly tricky.
THERE IS NO PREFERRED MAKE OR TECHNOLOGY TO RESISTORS AND CAPS. The value and voltage rating and tolerance are the things that matter.
You can do this, but it will require some care. If you overheat the board you can damage a trace, which is a real thrill to repair - but still possible.
I know of no particularly tutorials but this is all fairly standard stuff.
Note that the tone stack (so the midrange resistor) is NOT on the chassis board. It is on the front board on which the pots are mounted.
Hope this helps.
Chris
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OH and,...
Note that the stock midrange pot in a Twin (for example) is 10K.
I use a 25K pot in my PRRI, and typically have it in the range of 10 to 15K for a bit more fatness than typical.
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That information is extremely helpful! Do you think it's worth it to install a pot? If I want to scoop some mids, I can always raise the bass and treble, right?
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To blather about this a bit more:
Tone stacks are a bit of a mess. Once you go above 10 to 15K on the midrange resistor you are, to a great extent, just lifting the entire stack up off ground. I still suggest a value as mentioned above, but please understand that as you get above 10 to 15K you will have noticeably less treble and bass control. The Treble and Bass pots will still do something, but will have noticeably less effect on the overall sound.
I believe you can do this yourself, and also be happy with the reduced "scoop". I just want to mention that you will lose some dynamic range in the treble and bass controls.
Also, resistor capacity is rated in watts - not voltage as I earlier said. An 1/8 watt resistor (a really little resitor) will be fine in this application. But a larger resistor will still work perfectly fine, it will just require some work to fit onto the board.
Chris
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In my opinion, the best thing is to install a 25K pot in the external speaker jack hole, then use that in place of the 6.8K resistor.
Originally Posted by omphalopsychos
This gives you the normal Fender midrange control from 0 to 10K, and an overall tone stack lift (so louder and with some slight mid emphasis) past around 15 K.
Regarding scoop: If you were to install a fixed 15K, for example, you would not be able to get a deeply scooped sound even with the bass and treble at 11.
This is all in the opinion of my ears. You may hear the results differently.
Chris
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Have you tried turning the treble and bass controls nearly all the way down? You'll need to turning the volume control way up to compensate. This has a similar effect to increasing the value of the resister. Originally Posted by omphalopsychos
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I have. That's how I usually play it, and I still use an EQ to boost some of the mid frequencies a bit further. I'd like to get this sound without an external EQ. Originally Posted by KirkP
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Somewhat clueless SF PR owner here ... "past around 15k" means that the knob control is some where north of "5" on the dial? Originally Posted by ptchristopher3
Are you saying that turning the hypothetical mid control up this far (or higher) gives the amp more overall volume than it has with the stock resistor?
John
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HI John, Originally Posted by John A.
Sort of a long answer needed.
The tone stack (the tone control circuitry) in a Fender tube amp does not boost anything. It selectively cuts the signal. Even at 10 on all knobs there is notable loss in the tone stack.
The design of the Fender tone stack happens to work in a way that the final resistor (or pot in some amps) between the tone stack and ground provides a midrange control UP TO A POINT as you increase the resistance.
At even greater resistance values this resistor (or pot) starts to simply reduce the overall signal losses through the tone stack. So yes it can seem to increase volume.
So back to the PR:
- The resistor between the tone stack an ground is a fixed 6.8K ohms.
- A midrange control would most typically consist of a 10K ohm LINEAR TAPER pot.
- So the PR arguably has a midrange control fixed at 6.8 on a hypothetical knob.
- In my (often flawed and inconvenient) opinion, a PR (BF or SF) can be more useful for jazz if the midrange control is not only added but also extended past 10K ohms.
- In my still sub-par opinion, extending past 10K ohm sort of messes with the overall operation of the tone stack. The dynamic range of the bass and treble knobs start to diminish and the added midrange (past 10K) starts to act a bit more like an overall boost.
- But (and it is a big butt) I very much like a 25K pot added as the midrange control on a PR. Even way up near 25K you are still get an effective midrange control - albeit with some mild reduction in the isolation of all three controls to their assigned frequency ranges. But really, if you play jazz try it,...
Now to maybe just answer your questions:
1. "Past around 15K" would have you way past "10" on a typical Fender midrange dial. "10" on a typical Fender midrange control would be at 10K ohms.
2. A hypothetical midrange control that goes past 10K ohms will give the preamp more total gain. This does not equal more clean volume necessarily at all. Clean volume depends on at what stage in the amp distortion is occurring. But if you were sitting and playing below the breakup level of your amp and turned the hypothetical midrange control past 10K ohms, yes the overall sound would get louder.
I specifically like 25 K because even though the "midrange" increase starts to drop off, you still get some noticeable increase in fat-ness up to around 25K.
Other very useful opinions may differ. On the other hand I have messed around with this quite a bit over around 40 years - so maybe not completely useless on the subject.
Hope this makes something resembling sense.
ChrisLast edited by ptchristopher3; 04-17-2017 at 06:59 PM.
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Thanks, this makes a lot of sense. I'm not a DIY'er when it comes to amps, but this is food for thought the next time I have mine serviced. Truth be told, though, I feel like my PR as is is about as close to the perfect amp as I've ever been able to get. Much trepidation about messing with this, plus amp service in this neck of the woods is super expensive, so best to go in informed. Originally Posted by ptchristopher3
John
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Well do as you see fit of course.
Note that a 25k pot will seem unusual to many amp techs. But at about "3" on the knob it gives you the stock setup on a PR. Then you have quite a bit of midrange fatness available should you want it.
In any case, good luck with this.
Chris
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PTC,
Thanks for the great info/descriptions. I did this with a PRRI once, I don't remember what resistor I ended up with... I always left the 25K pot at about 3:00, so I think I measured that resistance, then replaced it with a resistor. I just cant remember... probably something like 18K...?
Anyway, a similar question:
re: brown Princetons (6G2): I realize there's more differences between a 6G2 and a BFPR than the tone stack, BUT... "more mids" is a hallmark of the browns... couldn't you get alot of mileage towards "brownfacing" a PRRI with a big mid resistor, perhaps a mid-dier speaker, maybe even swapping the PI to a 5Y3?
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FWIW, a Twin Reverb amp has a 10K mid potentiometer. With the mid knob on 10, bass on 1, treble on 1, you have about as flat a response as you are going to get on a Fender blackface amplifier. It's about like having an Ampeg or Polytone with the bass, mid, and treble controls on all 12 o'clock.
I would suggest going with a 10-K potentiometer...maybe a 15K value.
As stated above, be really careful with the delicate traces on the circuit board. You can solder them, but they won't take a lot of heat without lifting.
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A tip: when I did this mod, I clipped the stock mids resistor out, leaving little "legs" on the PCB.... that way, you can just solder to THEM instead of to the traces themselves. Still have to be careful tho. Originally Posted by Greentone
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Ruger9
That's a solid tip.
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Hard to think of what to say past what is already written on this. Originally Posted by ruger9
In my opinion, you get noticeably more fatness past 10K, but also start to get an overall increase in gain.
But why not just put a linear 25K in the Ext. Speaker hole and have something you can adjust?
Or even drop in a 50K, you can always turn it down.
As for general messing with the circuit, I really have not personally tried the things you mention, so I can not make any useful comment.
Chris
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Originally Posted by ruger9
That's a tip I wish I had heeded when I did this mod. I ended up messing up the trace and soldering further down. Worked out fine in the end. I started with a 15K but then ended with 10K. Worthwhile and inexpensive mod.
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I should still have the final resistor I chose (along with the pot and the switch I used) in my "electronics box", I'm going to look today to see where I ended up... because I am seriously considering buying another PRRI, I miss mine too much.
As for "more mids", it's amazing what a speaker swap can do, check out this video of 2 PRRIs: 1 with the stock speaker, and a special edition with the Celestion G10 Gold:
Also, here's one comparing the '65RI and '68Custom, which has more mids:
And, regarding my question, check out this awesome-sounding 6G2 "Brownface" Princeton, altho this one had the stock Oxford speaker replaced with a Weber Legacy 10, which is basically a Greenback (sounds MUCH better than the PRRI as you crank it up!)
(he's also playing a brown Deluxe, he goes back-and-forth)
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Once again, I am BLOWN AWAY by the knowledge here and the willingness to share it. Originally Posted by ptchristopher3
Thanks Chris.
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Funny but after maybe 40+ years of using the term in this context, it occurs to me how moronic it is to use the term blackface for certain Fender amps. I mean, the origin of the term is clearly based on the front panel color of the amp, and the subsequent amps are typically called “silver face”.
But surely I can find a better term.
I will leave the old posts alone, but consider maybe using “black panel” as I have seen used elsewhere.
Being less of a moron always seems like a good idea.
Paradoxically, I think saying, external speaker “jack hole” is fine.
Life’s mysteries.
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I think "blackface" (as well as "silverface" and "brownface") are fine. If Fender ever made white panel amps we'd be calling them "whiteface". Let's not get too P.C. with this, as that was never the intent of those terms.
I have heard people say "-panel" as well. All that matters is that we understand what the person is talking about.
Now, where the REAL "trouble" is run into is, tonally: when Fender puts out a "blackface" Custom Vibrolux Reverb, which is much closer to a "brownface" amp than a "blackface" one in tone.
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Tricky marketing decisions for Fender. They can run afoul of their fans if the appearance of a 2018 amp does not match the generally understood tonal character of the historical use of a given appearance. Originally Posted by ruger9
I have no idea if they even consider this when designing an amp such as you mention. It does seem clear that they made some attempt to shape the “65” and “68” PRRI amps to match the historical appearance.
As for “P.C.” It is not even remotely related to someone else’s politics. I just know I can do better, so I do.
Chris
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Oh, Fender does not care! Let's see.... let's just go with Princetons... Originally Posted by ptchristopher3
65 PRRI black tolex C10R
65 PR brown tolex P10Q
65 PR sunburst Celestion Gold
65 PR tan tolex Celestion Greenback
65 PR knotty pine Celestion Cream 12"
65 PR bordeaux P12Q (12")
68 CPR "silverface" totally different tonestack, speaker.
..."Princetons" all.
Altho, I will note Fender only uses the word "Reissue" with the ORIGINAL, black tolex version.... so perhaps they are "trying" LOL.
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Funny indeed. FMIC are over the place with the “65” , but if there has been a variation on the “68” I have missed it.
I saw the Claret colored PR with the 12” at GC last week. I did not try it. I am more than happy with my modded “65” with the 10” Weber.
Unrelated:
I am from the distant past when actual Princeton Reverbs were modded to various extents. Minimal was the midrange control, maximal was to make a semi-Mesa Boogie out of one. Two 6L6, a “fetron” up front, an extra gain stage, and a master volume - all into an EVM 12L speaker. The whole thing would weigh about 600 pounds.
Good times. Better times now.
Chris
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