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Post subject: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Sat Jul 28, 2018 6:36 pm
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This is going to be long and detailed. I think it’s warranted considering the nature of this line. So buckle up.

* A few prefaces to this post:

Those who know this amp know the elephant in the room. The extremely prominent bass tone, and it’s a big fat elephant right from the factory. The midrange is also prominent. Listen to the many YouTube comparisons to the 65 Princeton and you’ll notice the dull blanket sounding difference.

I’m not an amp tech, I know nothing about amp schematics or why the amp is what it is.

What I do know is regarding NOS preamp tubes and understanding how they react to a tube layout. This is my first 6V6 amplifier, so I won’t be addressing the power section. I don’t have any others to compare.

(Initial reaction)
What the hell is going on here? Why does this amp sound like it’s covered by a blanket? I like mids, that character of the amp is why I bought it along with a low price tag. The amp lacks vibrancy and color. Some guitars sound terrible, some pedals sound terrible. I spent 2-days fighting the breakup tone which was frustrating.

(The plan)
To come up with a tube plan, you need to understand the amp and the different characteristics of NOS tubes. If you don’t know NOS tubes, you’re shooting in the dark. Someone might recommend you to get some, but you could turn this big fat elephant into a mammoth if you don’t understand the differences of NOS tubes.

***The amp lacked vibrancy, heavy in bass, murky breakup. Remember this***

NOS tubes can be put basically into 3 different categories.

•American made: GE, RCA, Sylvania, Tung Sol, Raytheon, etc. These tubes are bold, big lush bass tones, darker bone crushing breakup.

* European made: Amperex, RFT, Valvo, Siemens, etc. These tubes have sizzling crystal highs, flat eq mids and bass. Hot searing highs in breakup.

* Mullard: Mullards are kind of their own class. Brimar are in this category as well. Singing midrange, mellow and smooth highs and bass. Smooth mellow breakup

There are exceptions. Telefunken and EI have different offerings with characteristics that range from US sounding and Eu sounding. But those categories are fairly solid for a plan.

Note: buying NOS tubes can be tricky. There was a lot of tube relabeling done in those days. The label can say Mullard but the tube could be an RCA. Ask questions to any seller on eBay. If you don’t know physical tube characteristics, you could buy something unintended.


Knowing this amp has more than enough bass, and lacks vibrancy, that took American made off my plan of attack. The bass tones and mids in the amp need to be tamed. Putting in a GE or Sylvania exacerbates the problem.

The amp has ample mids. No need to embellish it further.

That left me with a plan of going with the Euro types. The plan was to suppress the elephant by heightening the characteristics of the high spectrum to counteract the lows and mids.

This detail is extremely important for this amplifier. It’s a different animal from other Fender amps. Normally, I use US made tubes in Fenders to beef them up a little. This amp is like an anti-Fender by nature, so the opposite tube direction is in order.

I’m going through this in detail so people understand that there is planning behind my tube advice. I wouldn’t just spout out a list and say “Just do this and you’re good” or “Just get some NOS tubes and you’re set.” without any reasoning behind it.

(The swap)
Here was my final tube selections when all was done, and I’ll give my reasoning for each V-section:

V-1 Amperex ecc83 or *EI Silver plate (Yugoslavia)
V-2 GE 12at7wa
V-3 GE Jan 12ax7wa
V-4 Amperex ecc83

V-1:
Amperex ecc83
I tried Mullard, RFT, and Amperex in this slot. The Mullard couldn’t bring it to a vibrant enough nature in breakup and it lacked articulation. The RFT was very cool. I could have easily stuck with it and been happy. The breakup came to life, articulation surfaced, but there was still a bit of bass drag. Not enough to be bothersome though. The Amperex took it over the top. Clear articulate highs in breakup, tamed the bass to where I could use the bass knob up to 4-5. Has more of a Fender sound. Crystal clean under Vol 4-.

*edit: Since my initial tube swap, I tried a few more in V1. The EI silver plate (Nos Yugoslavia) was incredible in this amp. I’m adding it to my list as an alternative. It’s every bit as good as the Amperex in this amp.

V-2
GE 12at7wa/6201
I still like thick lush bassy reverb. Eu tubes made the trail very prominent. That didn’t surprise me. The GE 12at7wa/6201 is my favorite reverb tube. Very quiet, bold attack, lush thick trails. It tames stabbing highs.

V-3
GE jan 12ax7wa
I wanted to keep those big bass tones in the reverb, GE is my favorite for the trails and don’t see a need to change that. I would have been ok with most US made tubes in this slot. I tend to favor GE long plates for reverb. RCA can be slightly grainy, so that wouldn’t be an option for me here. Sylvania, Tung Sol would be good choices as well.

V-4
Amperex ecc83
Although half is used as trem oscillating, the other half is used as the PI. This tube keeps the idea of taming the elephant bass. It works with V-1 to bring out the vibrancy and color.

(The result)
It became very apparent to me that a big part of the problem with this amp lies with the stock GT preamp tubes. It reaffirmed to me why I hate them so much. The murkiness of the breakup is all but gone now. There are people who dismiss this idea, or take it for granted, or ignore it. But the modern ecc83s tubes have a way of sucking the life out of amplifiers. I’ll never understand why people will go through mod after mod and continue to keep those GT/JJ “special” preamp tubes in their rig. Maybe they are good for other amps, but not this one.. At all.. Ever.

For 2-days I was frustrated by the breakup of this amp, now after swapping 4 preamp tubes, it’s one of the best sounding amps I own. I have a whole different perspective on this amp now.

I took my frustrations out on Fender regarding the breakup when I should have been taking them out on GT and JJ for those ridiculous tubes. Still not sure why Fender chose this huge bass aspect, so I’ll halfway apologize. Most people won’t think to do what I did and won’t understand how to pull the blanket off the fat elephant.

This amp is now crystal clean 4 and below. Colorful and vibrant. I can use a neck pickup above 5, I have more control over the bass, I can use a fuzz pedal. The breakup isn’t a muddy mess. Big problems solved.

I could decide to swap the speaker later, but right now, I’m liking this amp a lot. I’m not in a big hurry to do that.

The power tubes appear to be fine to me. I never had a problem with them, and still don’t. If it ain’t broke, don’t fix it. For me concerning this amp anyway.

Anyway, to wrap it up. Don’t be too quick in being frustrated with a 68 Custom Princeton. I understand why you might first hand. I was initially frustrated too. But I hope this post helps you understand that there are ways to manipulate that preamp. It’s sensitive to tube changes, and for that I give it a big thumbs up. I think the amp is better than perceived, but I honestly can’t blame anyone for that perception in its stock form.

Now I love it. I’m going to play it a lot. Enough said. Peace.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Tue Jul 31, 2018 3:14 pm
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Thanks for the preamp tube-rolling info shred. I appreciate your write up and your perspective. The way I use the amp seems to be a little different. I own a Fender 68 Princeton Reverb.

I’m not really into NOS tubes. Just my opinion, I think those are really old “used” tubes, being sold as new (New Old Stock). Perhaps someone pulled a bunch of tubes out of an old piece if electronic equip. Question: Is something that old really new? I think some might have a high end roll off and reduced cathode emission being used. I do own some old RCA, GE, etc, preamp tubes. The RCA 12AX7’s, 7025’s do seem quieter (roll-off?) but I don’t use them. Don’t let me stop you, because I’m not an expert.

I enjoy my 68 Princeton Reverb as is and think it sounds great. I use the 68 solo as a standalone guitar amp, therefore the added warmth and Bass (and I still dime the bass control and use the neck pup on my strat) give it a fuller dynamic sound for my guitar hobby. To me that’s an advantage, the extra low end, it fills up a room, as a solo guitar. Also I suspect it’s the kind of guitar music I play, jazzy space-out songs, surf tunes, Ventures, TV/movie themes. I use modulation effects like reverb, delay, chorus, tremolo, and vibrato, with No overdrive. So I’m not pushing the 68 Princeton into any breakup, rather searching for a cleaner Fender shimmer sound, using my Strat’s vibrato bar a lot. I probably don’t turn it up past 4 on the volume. Sounds like you found the 68 PRRI usable with the tube swaps, so that’s good.


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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Tue Jul 31, 2018 11:30 pm
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jackhammer wrote:
I own a Fender 68 Princeton Reverb.

FMIC's contemporary 68 *Custom* Princeton Reverb sounds nothing like your original
AA1164 model and any sonic comparison between the two is totally pointless.

Arjay

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Wed Aug 01, 2018 3:07 pm
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Jackhammer, regarding NOS, you can still get old tubes that were never used. Most, most likely have been used at one time. And there are many that absolutely were used. Generally if they test as high as a newly bought modern tube, they’re listed as NOS. I use the term in general to distinguish old from modern. Tung Sol for instance is one that you would definitely want to reference.

Tubes that are old don’t age in a sense that the inner workings become brittle over the course of decades to the point that they become duds and not worth buying. A NOS tube made 60 years ago and stored properly will perform the same as the day it was made. (Allegedly, I’m not a scientist, but I have gotten long tubes lives out of most so far)

Years ago I ran across an in-depth article concerning modern and old tube manufacturers and learned that the general consensus among tube makers is that due to the difference of supply and demand from those old days where tubes were used in everything from radios, TVs, Amps, military, etc., tube makers wouldn’t turn a profit if they manufacture them as intricately as the old factories did.

The old ones offer a more broad spectrum of different sounds. There isn’t a tube made nowadays that sounds like an Amperex or EI silver. The new Mullards really don’t sound like the old Mullards. Psvane is the only brand that claims to make them like NOS, but look at the price of them and you’ll see why the other companies don’t.

The newer tubes can be fine in many amps. They’re probably just fine in the newer 65 Princeton. But this amp needs serious help in clipping mode. The low end is so prominent it drowns out high end articulation.

I can’t imagine the fight of buying a new speaker and still dealing with that problem. It’s my personal assessment that the preamp tubes should be addressed before thinking about a new speaker. In most cases, I wouldn’t say so. For this amp, I absolutely think so.

My own general rule, through trial and error over many years, is that I never put a bassy tube in a valve chain ahead of a high strung one (like an Amperex). You’d never see me put a GE long plate in V1 and an Amperex in V2 of a stack chain. The GE will drown out the high end goodness of the Amperex. Not so if you flip them around.

So here, with the Custom 68 Princeton, That problem is there right out of the box. The amp itself is an overload of bass. Since I’m not an Amp Tech, this is the only way I know how to manipulate that problem (subjective problem I suppose). I took my knowledge of NOS tubes and found a way to push the highs up to or past the bass and give it high eq articulation while clipping.

The power tubes that came with the amp are good tubes. The stock preamp tubes are a detriment to this amp in my opinion of course.

I’ve really been enjoying this amp since the swap. Very happy with this purchase.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Thu Aug 02, 2018 10:26 am
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Interesting topic. I’ve never owned or played through a Princeton so have no personal experience of that amp to draw on but I do own a limited edition DRRI (same spec as the regular DRRI except the speaker) and I have tried a variety of different tubes in it. It came with Groove Tube JJs (ECC83S) in the pre-amp and Groove Tube JJs (6V6S) in the power section. Within 6 months one pre and one power tube went microphonic so I decided to try some different tubes.

My experience was quite different to the OP’s. My view is that tubes can influence the tone of an amp but it is only tinkering at the edges and two supposedly completely different tubes can be made to sound, as near as makes no difference, the same simply by adjusting tone controls on the amp, guitar and even pedals. I would go as far as to say that my choice of pick can have more influence on my tone than my choice of tubes. I am no electronics expert but I believe that the tone of an amp is the result of the entire circuit and will be essentially the same no matter what tubes are in it. Provided they have been tested and are of good quality.

To me, it’s a bit like choosing free range or battery eggs to put in a cake mix. Will it make the cake taste any different? I don’t think so!

The tubes I tried included two 12AX7/ECC83 Mullards from 1957 and I have to say my excitement changed to disappointment because I thought they were nothing special. I also have some NOS Beijing Factory ECC83/12AX7WA (6N4-J Military) and whilst they do appear to be slightly louder than some other tubes, tonally I don't think they are exceptional.

It is my view that modern tubes are only as good as the testing that is done before we get our hands on them. It is for that reason that I only buy tubes that have been rigorously tested. There will, of course, be some that fail soon after purchase but that is only to be expected. Very few things in life are perfect and tubes are no different!

I now have TAD tubes in both the pre and power amp sections of my DRRI………and it sounds amazing!

I do, however, have NOS 12AT7 tubes in the driver positions because they supposedly have greater longevity. The reverb driver is a JAN Philips 12AT7WC and the phase inverter is a Mullard CV4024. I guess only time will tell if I made the right choice.

This is my experience of different tubes and I expect there will be many on this forum who will disagree and have their own view on those little glass bottles of tonal magic.


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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Sat Aug 04, 2018 1:17 am
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Hi 65blackface,
I have a few responses to your post. The first would be to address your plan with what you were after. I can’t tell by the tubes you bought.

As I said in the OP, simply buying NOS based on average recommendations is something I run across a lot. If you don’t know enough about them, you can be disappointed.

The first thing I want to mention is that I did say Mullards weren’t a good choice for my amp. The reason being, Mullards are geared toward midrange. People generally use them for distortion in amps like Mesas and Marshalls. Some people like to take the edge off the top without adding bass. I don’t find them to be beneficial in Fenders either. They’re really nice in my H&K Triamp however.

I was surprised to read that you bought NOS Beijing tubes. I wasn’t aware of such tubes. Perhaps because the period of buying NOS moves forward in time I guess.

I looked at a couple listings for them on eBay. Those are triple mica tubes. They’re built to be quiet. I find those kinds of tubes to be dull and lacking flavor. You can tell they’re more modern than what would typically be called NOS by the smooth discs. It would be like buying JJ specials from the 80s. So essentially, you swapped your JJ for an older tube exactly like the JJ you took out.

Having a Mullard CV4024 in your PI will give you a slight bit more headroom and clean up the clipping a bit. But sonically, again, I didn’t find Mullards to be beneficial in this amp or any fender amp I ever tried one in. Yet strangely, I keep trying one just in case.

You bought very good 12at7s. Any one of those would be great for your reverb. But you kinda swung and missed with the others. Not saying that to be rude. I swung and missed a few times in the beginning too. It took me years of experimenting and studying the effects of NOS types in different amps, and this one was a fun challenge for me.

They do absolutely have an effect on the preamp. It’s not like the egg comparison you made. Your amp sound begins in the preamp. I can make my amp cleaner, dirtier, brighter, darker, dull, vibrant, just by swapping preamp tubes. It’s not as prominent as a speaker swap as far as drastic change, but it’s a good foundation for your power tubes and speakers to feed off. I didn’t really like my stock speaker initially, now I think it’s fine.

I envy amp techs. They can manipulate the internals. I don’t have that knowledge. A tube swap isn’t going to turn my 68 custom into a 65. But it is noticeably different than when the stock tubes were in it. Most notably, the Amperex cleaned up the muddy mess of the clipping above 4.

If your amp is already on the bright side, you would be better off with NOS GE, Tung Sol, or Sylvania. (GE being my favorite of the 3) They give a nice musicality to the bass and mids. This is the only fender amp I favored euro types over US.

Anyway, thanks for your input. It’s not for everyone. I just wanted to share my experience with my 68 custom. Hope I helped clear the air with what you have.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Sat Aug 04, 2018 7:17 pm
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Let me point out one more thing 65Blackface.

The reason I call my stock GT JJs, is because GT doesn’t produce tubes, these are GT labeled JJs. They only screen, test, and relabel them with their brand. TAD is the same. They don’t manufacture tubes. There’s a good chance you once again swapped JJs with another set of JJs that now say TAD on them.

It’s unfortunate when these relabelers don’t disclose what the tubes really are. They could be anything from Russian made, China made, JJs, Sovtek, Tung Sol, who knows? Mesa discloses what their tubes really are. I think it’s worth noting that they’re at least up front about it.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Sat Aug 04, 2018 8:49 pm
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Its true that all 12ax7s amplify evenly across the audio spectrum all the way up to RF frequencies. Its also true that each has its own "voice", due to differences in mechanical construction and coatings.
The length of the plate affects the voice the most by adding mechanical vibrations to the electronic signal. The long black tube is the plate. There are two in a 12ax7. Their length and thickness affects these subtle microphonic resonances. The electrical charges physically attract the plate to the cathode. As signal rises and falls, the attraction changes and stresses both mechanically. At 2 khz you can plainly hear a tube without need for a speaker.
The main quality of a plate box, though, is the carbon coating on the plate. The electrons that are boiled off the hot cathode have a tendency to bounce off the plate instead of being absorbed. They bounce with a pop and the result is the sizzly hiss in the speaker. The quality of the carbon coatings improves electron absorption by 60 to 90%. Obviously a tube that gets 30% more electrons is going to sound better and be quieter.
Where I notice the difference the most is when a difficult chord change sounds muddy, good tubes will play through and sound good.
The other intangible is the headache factor. The edginess of many amps that make you want to hang it up after an hour of playing. A glassy, brittle preamp tube leaves you with a headache. A good tube changes it to "S***, I'm late for work cause I got lost in the music".
They are subtle differences, but my guys put the good tubes in whatever amp they're playing tonight.

There are many NOS tubes that aren't special, but even my non-musician ear can tell the difference between a Telefunken, an Amperex Bugle Boy, and an EI smooth plate 12ax7.
Not in the wild, but my comparison presentation for guitarists was to start with their tube in V1, then put in a clean Telefunken, then an edgy Amperex Bugle Boy, then a raucous EI smooth plate. The musicians could tell the difference right away and my untrained ears could too.
Acoustic guitarists liked the $300 Telefunken. Blues pickers liked the $80 Bugle Boy, and hard rockers chose the $40 EI tube.
A beginner probably wouldn't be able to tell the difference and frankly, they should work on their chops before plunking down money for these tubes, but when your tone hits a wall or playing better doesn't make you sound better, think about better tubes before you get a different amp.


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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Sun Aug 05, 2018 7:19 am
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shred6 wrote:
Let me point out one more thing 65Blackface.

The reason I call my stock GT JJs, is because GT doesn’t produce tubes, these are GT labeled JJs. They only screen, test, and relabel them with their brand. TAD is the same. They don’t manufacture tubes. There’s a good chance you once again swapped JJs with another set of JJs that now say TAD on them.

It’s unfortunate when these relabelers don’t disclose what the tubes really are. They could be anything from Russian made, China made, JJs, Sovtek, Tung Sol, who knows? Mesa discloses what their tubes really are. I think it’s worth noting that they’re at least up front about it.


You may rest assured that I did not replace GT JJs with TAD JJs. It is easy to find out what tubes GT and TAD are relabelling to avoid making such a silly mistake. Sometimes you can tell just by looking.

The TAD version of the JJ ECC83S is the ECC83-Cz (RT003) and 7025/E83CC (RT030). The latter being the highgrade version. I am using the 7025WA (RT080) and 7025S (RT011). The RT080 is basically the same as the Sovtek 12AX7WC with a few tweaks specific to TAD. As such it is not available in another brand. The RT011 is a Chinese made tube constructed the same as the old Mullards but it does not claim to sound the same! The same tube is also available from The Tube Store as their Preferred 7025 tube and other relebellers as well.

The power section of my DRRI has TAD 6V6GT-STR and these tubes are made in China, but designed and specified by TAD. They are not available under any other brand. I may be wrong but it does not appear that TAD relabels JJ 6V6 tubes.

In regard to the NOS Beijing Factory tubes, more information can be found here https://www.watfordvalves.com/product_d ... sp?id=5309

It is a few years old now but Watford Valves produced an excellent testing report on pre-amp tubes that can be found here https://www.watfordvalves.com/cgi-bin/d ... _12AX7.pdf


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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Mon Aug 06, 2018 8:50 am
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If a tube is relabelled, all bets are off on comparisons.
Even jj come in different grades with identical appearing construction. The lighter color of the carbon coatings on the gold label accounts for the lower noise rating. Apparently good coatings are expensive as they are twice the price.
The same jj construction can be found in the Marshall VLVE10067 ECC83. JJ built these with no coating. But look closer. The bright silvery surface doesn't look like nickel. Older versions of this tube featured a silver anode and the jj looks more like silver plate than bare nickel. Plates have to have a nickel base. Silver conducts too well to spot weld connections to it.
So I wouldn't be surprised if TAD, Fender, Ruby, and Mesa also spec their own coatings. They can effectively manipulate sound and cost, so why not?
So how do you tell if you have good coatings on a tube? Put them in V1 with nothing plugged in. Turn up the volume and compare the hiss and sizzle of the electrons bouncing off the coatings. Less hiss = more music.
Tap on the side to compare microphonics. Every tube will be slightly microphonic. But they will ring, not rattle.
The longer the plate box, the more prone it is to microphonics, producing squealing feedback. The jj ecc 803 is too sensitive to be used in a combo in the v1 position. It sounds fine in a lower gain application.
At the other end is the short plate jj that isn't a box at all. Its plate is overlapping metal plates that aren't touching. This removes the resonance a welded box suffers from, making them resistant to mechanical vibrations.
So look closely at the construction and listen closely to the noise of each tube. With NOS, there are a lot of turds out there and only a few gems.


Last edited by TimsAudio on Mon Aug 06, 2018 10:44 am, edited 1 time in total.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Mon Aug 06, 2018 9:30 am
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shred6 wrote:
...It’s unfortunate when these relabelers don’t disclose what the tubes really are. They could be anything from Russian made, China made, JJs, Sovtek, Tung Sol, who knows? Mesa discloses what their tubes really are. I think it’s worth noting that they’re at least up front about it.


GT discloses where their tubes are made, namely by the suffix letter. "C" is Chinese, "R" is Russian, "S" is Slovakian (JJ). There are other suffixes for tubes made exclusively for GT, such as "GE", which is a remake of the old GE style, but now made in China.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Mon Aug 06, 2018 9:34 am
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jackhammer wrote:
...Sounds like you found the 68 PRRI usable with the tube swaps, so that’s good.


There is no such amp as a 68 PR "reissue". There is a 68 Custom PR, which is not like a real 68, nor is it like the 65 reissue.

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Post subject: Re: 68 Custom Princeton Reverb Tube Swap (detailed)
Posted: Wed Sep 05, 2018 11:35 pm
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I have a couple updates on the amp.

I encountered a friend who has the same amp. He told me I should do a jumper mod on the amp. I hesitated, because generally, I don’t touch the innards. In my early days I was shocked by the innards by not understanding that it could still shock me unplugged.

Once he explained to me where to put the jumper (you can Google search it), I gave it a shot.

The Jumper mod is the greatest thing you can do to this amp. By far. (For us non-techs ok). I highly recommend it.

The amp has a lot of bass even at 1. After you do the jumper mod, what used to be at 1, is now at 4 on the knob. There is still plenty of bass on hand for the amp. I have more versatility when cranked now.

—————————-

The other thing I tried was another speaker.

I bought an Eminence Copperhead for my Pro Jr. but decided to try it in the 68 cpr first.

The Copperhead sounds exactly like the stock Celestion Ten 30. The only difference is a volume spike with the Eminence. Something good to know. You can get the same sound if you want the amp to be louder.

I rather like the Celestion and the lower efficiency. I consider that to be an asset for a mic’d Stage gig. While some consider the Celestion a cheap POS based on the price, I think it’s pretty cool to be able to get a Copperhead sound at a budget price. I’m keeping it in there, and wouldn’t have a problem using one in another amp down the road. It’s a good speaker in my opinion.

The Copperhead took a bit to break in in the Pro Jr, but it sounds great now and takes pedals perfectly. Exactly what I was after.

I’ve got both amps where I want them, been playing a ton lately, life is good.

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