The New Rivera Fender Concert mod thread

TweeDLX

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3,756
I sorted out a few more things on this. Mainly where to run leads to the Vactrols to get the correct tube order. The illustration below shows the Vactrols as they appear from the back of the amp looking at the circuit board. I numbered them from left to right. Wound up with more gain than I knew what to do with, so I added an extra divider between V2a and V4a, and an un-bypassed 8K2 resistor on V4a. Now there's enough gain for all but the most hard core metal heads, but you can still get a nice bluesy crunch if you want. Now if it only weighed 40 pounds less...

 

Bork

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13
Hey guys, I'm new and a new Concert II owner in the UK (mine already has an Eminence RW&B speaker installed). I also have a Burman Pro501 and a couple of Ibanez TSA5 5E3 based combos. I've read this thread and the previous thread with great interest and learned a lot. Really appreciate everyone sharing their experiences and experimentation. I have a couple of questions to ask based on the following:

What I'd like to achieve with my Concert II are three things (with planned mods listed underneath):

1) Improve general reliability:

a) Replace the 3.9K bias resistor with a 0-10K pot (also install a 5K, 1/2 watt resistor in parallel with pot so that the sweep would be from 0 to 5K and reduce risk of overheating valves if the pot should ever fail)
b) Replace the inadequately rated 47 ohm 1/2 watt resistor (on bias supply?) near the transformer with 1 watt version​

2) Make the clean channel as Twin-like as possible (love the midrange and compressed spank):

a) Snip V1 snubber cap to allow more high frequencies on the clean channel
b) Switchable sag mod - insert resistor to equal the resistance of G234 rectifier tube (20w?) and find somewhere on the back to install the switch​

3) Make the gain/lead channel more Super Champ/Boogie like:

a) Move the tone stack after the first preamp tube V2A for more gain (use V4B as cathode follower after the tone stack).
b) Replace the stock 25uf cathode bypass cap on V4a with a .68uf to tighten up bass on Ch 2​

May also consider:
Changing the .0022uf cap for the midboost (behind the volume pot on the board) to something that will help centre the mid boost frequency on 320Hz. I have no idea what value cap to go for at this stage, perhaps it'll need to be a separate post.

Two questions:
1) Have I interpreted the mods recommended in this and the previous thread above correctly?

2) Are there any other mods I haven't listed above that might help with progress towards my objective?

All advice gratefully received.
 
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pdf64

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6,948
Sorry if I've misread your post, from it I assume that you may not have much experience with working on tube amps, or grasp of the theory behind them?
This may not be the kind of response you were hoping for, but the best advice may be that without a good level of competence and resource, not to start with the project you propose.
These aren't suitable amps to be cutting your teeth on.
Has the amp been appraised by a competent person (this is an essential with any old item of mains electrical equipment)?
If not, how do you know it hasn't already been subject to mods, bad repairs, or that it's even safe to use?
 
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Bork

Member
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13
Sorry if I've misread your post, from it I assume that you may not have much experience with working on tube amps, or grasp of the theory behind them?
Yep. I don't have much experience with amps and I didn't ask for advice because I was going to do the work myself. I'm asking because I'm planning for someone else to do it. I'm not even in the same country as the amp at the moment and probably won't be for at least another year.

If you saw a flaw in my message above, feel welcome to correct it in a constructive and non-critical manner. I copied stuff I saw while perusing these threads.
 

Bork

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13
The lack of responses here is fairly disappointing. I was hoping for more but at the moment I feel like that new cowboy in town who has just walked into a bar only for the piano to stop playing and the conversation to die.
 

fatztreeboy

Member
Messages
64
hello from san francisco. I am a longtime concertII owner. curently i have an 82 4x10.... this is my 2 cents. try a great speaker and fresh tubes and just play the amp for 6 months. IMHO modding these is a crapshoot. Stock they sound awsome with good speakers. also, for kicks you can jumper the send/recieve loop jacks and use the send/recieve pots on ther back as another cool gain stage to boast your tone. personally i do not thing a single 12 does this amp justice. maybe set it on top a cool single 15 . an 8ohm 15 plugs right in. ( i think it does. it does on my twin). there were a finite number of these built. and half of them have hacked at w/ mods. keep it stock and running 100%.
 

xtian

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2,318
The lack of responses here is fairly disappointing. I was hoping for more but at the moment I feel like that new cowboy in town who has just walked into a bar only for the piano to stop playing and the conversation to die.
There is probably very little traffic here, because this thread has 473 posts! I just wandered in, like a tumbling tumbleweed in your cowboy metaphor.

Anyway, don't go planning mods to an amp you don't have in front of you. You should play and evaluate it, and if something strikes you, like, "gee, wish this thing was saggier, like an old grey elephant," then tackle that issue. Making a bunch of mods at once means you'll never know how they change the amp individually, thus whether they were a good idea at all!
 
Messages
51
The lack of responses here is fairly disappointing. I was hoping for more but at the moment I feel like that new cowboy in town who has just walked into a bar only for the piano to stop playing and the conversation to die.
.

Hi Bork,

I cut my teeth on this amp having never modded an amp before. I did build my own ham radio receiver as a kid, so my only skill was questionable soldering skills.

I now have modded 6 concert amps and have 3 more in the works: a Twin II (same circuit), 1x12 Concert and 4x10 Concert. I get them to a really good point. Then I try another set of mods on a new amp. I live in LA and there appears to be a plethora of these amps usually for about $400 or sometimes less.

Here’s my take:

1. These reliability mods are fine and should be done.

a. The combination of bias and bias balance permit you to use unmatched tubes because you can balance them.

b. 47 ohm replacement. Yes!

2. Standard Fender Circuit

a. The snubber caps on the tubes are interesting. Without them the signal is raw. Yes that is an original Fender circuit. With them the some harmonics are eliminated, but some should be eliminated. When overdriven, the signal is smoother with the caps in. Make your choice by trying each. I presently like the snubber caps in.

b. I tried the switchable sag mod. It doesn’t do much. There is a site that helps you determine what kind/size aluminum wirewound resistor emulates which rectifier tube. Andy Fuch’s offers this feature in his amps too, but the resistors he uses are a much higher value. Maybe that makes a bigger difference. It’s a lot of work for a little result.

3. Make the gain more Super Champ/Boogie like

a. If this were so simple it would be the cat’s meow (or maybe the tiger’s growl) because the Fender Rivera-era Super Champ OD channel is near perfection right out of the box. I have not been able to emulate its sound. It’s still a work in progress. But hint hint leaving the snubber caps in, for example, get you closer to that sound than without them. As part of the continuing search, in one of my latest amps I am inclined to try Tweedlx’s latest version where he deletes the cathode follower and instead creates a new gain stage which I take it is more Boogie-like. He is the all-time MASTER OF THE CONCERT UNIVERSE in terms of knowledge of these amps. I have to agree with him in questioning the value of the cathode follower because it fundamentally does not change the tone of the amp one iota (though it has internal electrical benefits). And its a ton of work to create it. So why do it? I say this from experience because in one of my amps I tried his previous mod. He knows what he is talking about.

b. The flubby bass is best resolved by a good speaker unless you have the 4x10 in which case there is almost no need for changing speakers because the 10” speakers are so clean despite being low quality. My favorite recent choice is the WGS ET65 or ET90 because it tames the ice picky Fendery part of the clean (note: Robbin Ford uses the Celestion version of those speakers with his Fender or Dumble amps)…unless you like that Fender sound then the WGS G12C is a good choose. There are many others. These happen to handle the power well. Vaugh Skow who is assocated with WGS speakers does an extensive comparison of speakers for the Concert on youtube. Check it out and make your own decision.

Changing the cathode bypass caps is a good idea. There is too much bass sound rumbling around this amp (see Rivera below) possibly caused by inclusion of the reverb circuit according to Linster on this thread. At a minimum, change the clean channel to 4.7uf that cut off the frequencies below 50hz which do nothing for the sound your ears hear except adding flub. Maybe consider 2.2uf or even 1uf. The .68uf is a good cathode bypass cap for the lead channel a la Marshall. All good moves.

4. I don’t know what cap creates the mid boost freq. you are looking for. My amps all have had a .0012uf mid cap. I did change it to a .0022 and it sounded better to my ears. The Lee Jackson mods add a 6-way mid cap to provide 6 different cap choices. I like it and am adding back to my amps again. He worked at Fender when these amps were being designed by Rivera and Ed Jahns. So he knows what he is talking about.

Speaking of this, there is a new youtube video out done jointly between Paul Rivera and Sweetwater discussing mods Rivera is now adding to the RI 68 Deluxe reverb for which he charges $2200 or so. He used to perform these mods on Fender amps used by the "Wrecking Crew" etc. They were the reason Fender hired him in the 1980's. But coincidentally, he is essentially describing everything you might want to do to your Concert amp. He includes a 6-way mid cap switch. And he cuts all frequnecies below 50hz to contain the natural flubbiness of standard Fender circuit in the amp. Check it out and listen carefully to what he says. To conclude, those bypass caps changes are a good call.

5. Last, these amps sounds much better cranked, but by today’s standards are too powerful. One of the absolute keys IMO is adding a Post Phase Inverter (PPI) Master Volume. This Master Volume allows you to balance the clean and OD channels at any volume. I love that. The best one is called Lar Mar and was created by a couple of engineers described fully and publically on the Metro Amp website with full description of how to build it. It is quite simple. It is an advanced version of a Ken Fischer (RIP) master volume he used for Marshall amps when he was "the" amp guru. The so called Torres mods include one. It is terrible! Don't use it. The advantage of the Lar Mar is that it allows the amp to see the same 220K ohm resistance in the output circuit when turned all the way down that the amp sees when fully cranked. This means that there is far less “blocking distortion” at low levels on the master volume. So the sound is much closer to the cranked sound. Now you have created an amp that can be scaled for almost any music situation. I put the Master Volume where the Presence was located on the amp because the MV has the effect of nearly defeating the Presence. The presence pot can be replaced with a wire (zero resistance) or any resistor from up to 25K (0-10 on presence knob) because the Presence is a 25K pot. Choose the value that suits you. Eliminating the presence for the Master Volume is totally worth it.

Finally, there was some discussion about removing an 820K resistor in the clean channel tone circuit between the Volume and Treble pots. A Premier Guitar "Ask the Amp Man" article on the web coincidentally explained what this resistor does for the Concert amp. I never understood this before. So like a lot a people, I just followed the party line and wanted the standard Fender circuit and clipped it out. The resistor controls the gain sent to the next stage of the clean channel tone circuit. In other words, without the resistor full gain was sent along and the clean channel becomes much gainier. Maybe you want that. It explains why SMark with Torres Mods has essentially two high gain channels. He explains earlier in the thread that he has no clean channel. If you like that great! I prefer a clean channel and a OD channel. I left the 820K resistor in the circuit and am experimenting with 1 meg resistors and even higher to see if I get more clean headroom. Just food for thought.

OK. That’s a much as my feeble brain can muster at the moment. (I should be putting this much thought into my wife's Christmas present!!) Really good question. And if you’re comfortable with a solder iron, try it yourself, of course, observing the rules to prevent electrocution. It’s a totally fun process.

Best...
 
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Tsquaredz

Member
Messages
30
.

Hi Bork,

I cut my teeth on this amp having never modded an amp before. I did build my own ham radio receiver as a kid, so my only skill was questionable soldering skills.

I now have modded 6 concert amps and have 3 more in the works: a Twin II (same circuit), 1x12 Concert and 4x10 Concert. I get them to a really good point. Then I try another set of mods on a new amp. I live in LA and there appears to be a plethora of these amps usually for about $400 or sometimes less.

Here’s my take:
Without looking at a schematic, many of these mods sound very similar to what I have done to a Princeton Reverb II. Good info.
 

jay42

Member
Messages
6,974
Minor quibble on the Lar-Mar circuit. Mark Abrahamian was the lead guitar player for Starship when he worked on that circuit and sadly passed away. Not an engineer, but possibly one of the nicest and energetic guys I've known on the internet. Search here for Rockstah. Don't know the Larry part.

-------

The most recent one I've worked on...I put JJ 6V6s in and rewired the OT switch to show the tubes 8K. I did other stuff, but that was unique. B+ is around 470V on that one. Mine wants to be 495V, fwiw.
 
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arlum

Member
Messages
20
I own two Fender Concert II's from 1982 and '83. A 1 X 12" combo sitting atop a 1 X 12" extension cab both loaded with Celestion G12H-75 Creamback speakers and a 4 X 10" combo loaded with two Celestion G10 Gold Alnico and two Eminence Ramrod speakers. I admit up front that I don't use the gain channel. I'm strictly a clean channel guy using a hefty pedalboard to fulfill my needs. The clean channel on the Concert II amplifiers are, IMO, as close as you can get to old school late '50s / early '60s Fender single channel amp tone.
 

Zaches

Member
Messages
8
Ok! My first post! I have been obsessing over these concert mod threads for months! I purchased a mint late 82/early83 concert 2x10 off eBay after playing through a luthier friend’s concert 4x10 and loving it. The one I bought is a different amp altogether: stiff, not musical, bity and harsh, etc. It has two Jensen c10q’s and new jj tubes. I replaced the v1 tube with an old rca blackplate and that helped add some depth to the notes, but it’s not nearly enough. I bought a 73 twin reverb with jbl’s at the same time as the concert, and the concert has been mostly collecting dust since then.
I’m hoping to get some good advice on opening up the clean channel to get the sparkly touch sensitivity and musicality of the twin reverb, and enhance those same aspects in channel 2 but with a usable overdrive type gain.
I’m replacing the harsh Jensen’s with jbl e110’s this week, by the way.
 
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Zaches, have you checked the bias??? and...ime....there is no JBL that will be less harsh than a modern Jensen....and I don't like modern Jensens. OMMV..... Some Eminence ALK 1028 Alnicos would work in that amp...as long as you didn't push the amp to its max.
But...imho, there is no reason to do anything else until good general service is done and bias set to some warm setting. In other words, no matter what speaker one uses one cannot make an amp sound good IF it is biased to the cold side of things...it will be thin and lacking richness and harmonic content if the bias is cold, ime.
I do agree that this amp and the TRII are quite loud by the time they yield what I would call good sound. IF I had one, I would have to have a good attenuator to run it through.
I suppose that is why I keep my two super Champs...they are not quite as loud as the bigger amps by the time they yield good sonics.
 

Zaches

Member
Messages
8
Zaches, have you checked the bias??? and...ime....there is no JBL that will be less harsh than a modern Jensen....and I don't like modern Jensens. OMMV..... Some Eminence ALK 1028 Alnicos would work in that amp...as long as you didn't push the amp to its max.
But...imho, there is no reason to do anything else until good general service is done and bias set to some warm setting. In other words, no matter what speaker one uses one cannot make an amp sound good IF it is biased to the cold side of things...it will be thin and lacking richness and harmonic content if the bias is cold, ime.
I do agree that this amp and the TRII are quite loud by the time they yield what I would call good sound. IF I had one, I would have to have a good attenuator to run it through.
I suppose that is why I keep my two super Champs...they are not quite as loud as the bigger amps by the time they yield good sonics.
Thanks for your insight. Ive been plugging the concert amp into my twin reverb’s jbl’s when I have been playing and it’s lovely and round compared to the harsh, thin bite of the Jensen’s. I wish the concert had room for two twelves in the cab. I’m not worried about volume. I live on many acres and we have band practice here regularly.
The bias is a good place to start. The amp is pristine and has zero hum, but I’m sure it could still use servicing and definitely some mods to open it up. I may try removing the snubber caps on my own when I replace the speakers. I wish there was an adjustable bias pot in it. Has any one here installed one?
 

7thbass

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Messages
33
I need to turn mine into a head. Anyone know if there is a pre-existing head cab the Concert can fit into? Or a generic one?
 

jay42

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Messages
6,974
I need to turn mine into a head. Anyone know if there is a pre-existing head cab the Concert can fit into? Or a generic one?
There was a head version, though I don't think all that many were sold. You'll probably have to get a custom one from JDNewell or one of the other vendors. Small warning here...keep the height taller than 10". Mine is 9 3/4" and it's pretty cramped with the tank.

fwiw, I just futzed with mine recently. I converted the BF tone stack to a brownface tone stack from the 6G6-B. I think I like it better.
 

Zaches

Member
Messages
8
Help. Advice please. Volume is cut drastically! After pouring over this forum for months, reading everything I can about amps and mods - I have finally opened up my 82/83 concert 2x10 to remove the 850k resister between the volume pot and treble pot an the clean channel, and replace it with a straight wire. I had already clipped the snubber caps on v1, v2, and v3 last week and figured I’d do the next easy and recommend mod to bring about a more open clean channel. After putting the amp back together my volume is drastically reduced. I plugged into the speakers from another - still super quiet. I then put the 850k resistor back in place. Still low low volume. Am I missing something?!

Edit: I did one other thing that MAY have had some influence. I resoldered a capacitor and resistor connected to a green wire that goes strait to the reverb input back down to the board. It appeared as if it had popped off the board. I removed it back into its “floating” position when I replaced the resistor. Could I have fried something? I’ll try to post a photo.
Maybe one of my power tubes has gone bad coincidentally. I’ll go through the tubes tomorrow...

UPDATE: I believe I’ve isolated the problem to the return side jack of the effects loop.... The effects loop hadn’t been working and when I pulled the chassis I fiddled with the contact leaves on what is a kind of complex looking jack. I don’t know exactly what I did or what I’m doing - just following common sense for now - but I get full volume using a patch cord between the effects loop. Without the patch cord I get a scratchy, low-volume version of the amp.

UPDATE - PROBLEM SOLVED: I got some cleaning solution from r-shack and doused the jack. I then bent the leaves to make good contact in the way that made the most sense to me. It took a few attempts to get the jack bent correctly but I finally got it right. Now back to nodding!
 
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Zaches

Member
Messages
8
[/QUOTE] Using the clean channel only I moved on to V1. I still have the 47K plate installed. I already had changed the 680 cathode previous to a 1500 which mellowed the tone, cut a little gain and warmed up the amp, I liked that change. I went to a 2700 cathode resistor and the amp suffered, got dull and lost the nice clean headroom. I went to 2200 and still I liked the 1500 there the best. I changed out the 47K to a 100K plate with the 1500. Increased the gain but caused the amp to break up to early for me. The 100k helped the low end but almost too much. I had a 82K I went to next and it sounded great with the 1500. I had about 10 of the 82K resistors ranging from 72K to 86K. The first one I installed measured 77K. The 82K that measures 86K acts too much like the100K. I settled for the 82K that measured 72K. I had a 56K that measured close but I still like that 72K range. I still want to split V1 ....
.[/QUOTE]

Reading through your post has inspired me to simply change out the 680 cathode resistor to 1500... it did just what you described: warmed and mellowed the amp. I’m looking for a mini version of my lovely twin reverb in the clean channel, and this simple resistor swap did wonders for the tone. I was afraid I was never going to get away from the piercing harshness that the amp seemed to embody, and now I can really see the light at the end of that tunnel. The jbl’s are starting to shine with the newly warmed tone.

I’ll be moving to the phase inverter section next to change another resistor to the twin reverb’s values, and do some of the cap upgrades that have been described earlier. I’d like to split v1, before I tackle any big changes in the second Channel. Anyone, with any advice on that please let me know.

I’ve also gone through a box of old preamp tubes this past week and settled on a really nice ge 12ax7 in v1 (big warm and 3-d), an articulate and less bass-heavy rca 7025 in v2, new jj’s in V3-v6, and another fender-labeled ge 12at-7 in the phase inverter slot which softened the tone a bit. These preamp tubes helped tremendously with rounding out the character of the amp. I’ve still got jj 5881’s in the power tubes, waiting on tad 6l6gc-str’s to come in. Hoping for more 3-d clarity.
 

Zaches

Member
Messages
8
I hope some one out there is still reading thread.

I’ve been enthusiastically diving into my amp - doing many of the basic mods: snipping of the snubber caps on the preamp tubes, removing resistors going into the clean channel’s volume, and removing the resistor between the volume and treble controls. I also replace the 680 cathode resistor on v1a with a 1.5k, which opened up the sound in a lovely way.

Here is my question!!! When I remove the 47k resistor going into channel 2’s volume pot (and replace with a shielded wire) the amp opens waaay up and the bass gets reallly, reallly boomy. This over-the-top openness effects both channels, not just the second channel. I went back and replaced the 47k resistor with a 27k and the sound was a little tighter but still super boomy in the bass. What’s going on here? Has anyone experienced this?

I like the openness, 3-d sound, and rawness - but the cavernous bass is outta control. I feel pretty committed to the 20-25uf range of cathode bypass caps and will replace the stock aluminum ones with nos Sprague tantalums. I am also thinking about changing the bass caps on the tone stacks down to .022uf.
 




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