The New Rivera Fender Concert mod thread

fenderpro

Member
Messages
359
speaker choice is very very subjective, but I can say that I really like the Eminence Lil Buddy, the 10" version of the cannibus rex. the more I play it, the more I like it. YOMV.
 

rosssurf

Member
Messages
136
I can't bear to slog through any more of this. A friend has come up with a 2-10" that's never been modded. It needs output tubes and the bias conversion.

Did you guys (and this sub-group) ever come up with a list of simple changes that really help the stock amp? I'd like to get it improved without the major changes most of us have done. For instance I wouldn't want to split V1 or move the Ch 2 tone stack to post distortion.

Also, I told him that almost any 10" speakers he could come up with, would be an improvement. Actual recommendations would be better. Lol. I've never owned a 10"...just 12 and 15's.
I am very interested in this as well.
 

jay42

Member
Messages
6,971
If I can figure it out again, this is how my bias supply is wired. I think it's less complicated than the SF description.

The lug in the back of the pot is open in this configuration. The 'loose' end of the 33K has been moved off the wiper to one end of the pot. There is a jumper on the two 220K bias feed resistors so they both receive the same bias voltage.
[/IMG]
 
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jay42

Member
Messages
6,971
Bringing this one back around, first the background of this amplifier:

Previous work and condition:
Output tube sockets had been replaced. The 470 ohm and 1K5 ohm screen and grid resistors had been replaced with Metal Oxide parts of appropriate power rating. The bias balance circuit had an extra 33K W metal film resistor in series with the original 33K carbon comp resistor. The NFB resistor is a 27K, unlike mine originally (82K and on a different tap).

Changes:
Bias Balance converted to Bias Level Adjust.
1 ohm MO sense resistors added to pins 8 of output sockets V8 and V9 and tip jacks for both. (Domestic chassis are drilled for dual fuses, but the aluminum rear panel covers those holes. In a similar manner, the combo cabinets are all drilled for castors and tolex is hiding those holes.)
Bad Reverb cables measured 36” and 50”. Replaced with a 6’ cable from Amplified Parts (via Amazon). I need larger wire tab things now...off to HD.
Replaced V1 and V2 plate resistors with 4x 100K RN65 parts for a slightly compressed/smoother tone.
Removed V1 .001uF snubber capacitor per Rivera Rake circuit.
Channel 1 bright pull circuit & attenuation replaced with standard configuration and a 47pF cap per Rivera Rake circuit, instead of the usual 120pF.
Removed the AC Ground switch and stuck in a 50KL NFB pot. I'm thinking that 100KL would have been better, but I didn't have one on hand. Must scrounge.

It sounds better, mostly due to the V1 & V2 tweaks and being able to properly bias it. At this point, the owner needs new output tubes and an electrolytic cap job. I think I'm only into it about $10 on parts. Better speakers would help a lot too. hth out there.
 
Messages
51
I'm a bit late to this party and unfortunately do not have a 10"speaker recommendation. But I will emphasize how important the speaker is. I just picked up Concert 1X12 with an original EVM12L speaker. When I tried it out, I was astonished how great it sounded on both channels because the EVM eliminated the majority of the bass flabbiness. Too bad they don't make an EVM10L anymore.
Another inexpensive fix, because most speakers will not standup to the heavy bass like an EVM, is to put 4.7uf bypas cap off V1, V2A and V4A. Or you could stack them like Tweedlx circuit (though V2 is like his V4 cathode follower): V1-1uf, V2A-2.2uf, V4A-4.7uf. Your 90% on your way to eliminating the flabby base.
Torres and Jackson mods also change the coupling caps to output tubes to .022uf.
Inexpensive fixes that cure a lot of ills.
 

pdf64

Member
Messages
6,931
[FONT=&quot]Could you put Tung-Sol 5881 tubes in concert with whatever bias adjustment to get lower power or will it blow those tubes up.[/FONT]
HI WERTZ2112,
Yes, you can. That's what SMark has in his amp (check out video's and sound on Page 1 of this thread). A re-bias and you're go to go. The 5881 is a version of the 6L6. It is lower wattage (maybe 46 watts or 23 watts per tube). It breaks up earlier. It was used in the Fender Bassman and the JCM 900 (in early 1990's until EL34's became more readily available) among others.
It shouldn't make any difference; 5881 and 6L6GC have the same transfer characteristics and curves and so given the same operating conditions (eg biased to the same static plate current), should perform the same, or as close as any tubes of the same type can be expected to perform.
The amp doesn't 'know' what tube type is in there; it just reacts to their gm, plate impedance etc.
Sorry to dredge it up but I missed this at the time and it's a common misconception.
 
Messages
51
Just to clarify. SMark explained to me in an email that he uses Vintage 1950's Tung Sol 5881's (made in NJ). These specific tubes were made to military spec and could withstand wicked voltage changes. The modern 5881's cannot handle the 495+ v. of this amp and may not be a wise choice.
 

Linster

Member
Messages
17
Anybody come up with any new upgrades lately for the Concerts? I have 2 of them both stock, wondering which way to go. I like TweeDLX idea. I also have 2 other ideas.
 
Messages
51
Hi Linnie,

After completing the Tweedlx and the Jackson/Torres mods, I will offer a couple tweaking ideas.

SMARK, who had the Torres mod done, indicated that he could not get a clean sound just 2 dirty sounds: dirty and dirtier. I found the same to be true with the Tweedlx mod. The reason for this is the imbalance between the volume of the clean and OD channels with the OD channel being overwelmingly lounder. I speculate that is why Fender added a Master Volume control just for the OD channel. And the Tweedlx Mod eliminates that control. Adding the Lar-Mar PPIV Master (upgrade from the Ken Fisher Type 3 MV) volume helped, but in the Tweedlx version (no OD MV) the OD channel channel volume had to be at 2 for proper balance of clean and dirty...not ideal.

Because in both my mods I split V1 using a different side of V1 for the clean and dirty channels, I attempted to tweak or tune the clean channel to give it more headroom i.e. so clean volume could be turned to 6 or 7 (a lot louder) before it begins to distort. In my last attempt, I changed tthe Cathode resistor on channel 1 from the Fender standard 1.5K to 6.2K (and of course used a 4.7uf cathode cap in lieu of the Fender stabdard 25uf to cut the muddiness). Much better, and when it does distort at 7-8-9-10, it sounds good too. But even then there is a imbalance. On the Tweedlx version, I then added a 1meg resistor after the 1st preamp cathode cap on the OD side to try to dial back the OD volume a bit...remember Twedlx has a slope resistor that really kicks up distortion and adds volume...almost plexi-like...pretty cool. The 1 meg resistor didn't do as much as I thought it would. But my suggestion to you is to add a lot of headroom in the clean channel...or dial back the OD volume. Any alternative suggestions you might have to dial back the OD channel volume, I am all ears! Next I'm trying a 8.2K Cathode resistor on the clean channel side for even more headroom.

Second, with the Tweedlx version, I determined that the snubber caps were necessary and had to remain in the amp. The amp squealed when OD volume, slope and gain were dimed. But the cool thing about splitting the first preamp tube is that the snubber caps can remain in the same position, but only affect the OD channel. Splitting V1 frees the clean channel from the caps. Therefore you retain the blackface clean. pretty cool. These snubber caps affect the sound to a much larger degree than I expected. So I did dial them back to 100uf each (from 500uf and 250uf if my memory serves me correctly). Otherwise each channel sounds like it comes from different amps...which could be a good thing if that's what you like!

That's my 2 cents! Be interested in hearing your take on the first issue.
 

Linster

Member
Messages
17
Thanks Ken,
I can see your issues trying to use the PPIMV. If you are use the PPIMV that to control the lead channel you will not get the classic Fender clean because it is killing the signal. I agree the lead can overcome a lot.

I see Tweed did have a MV only for the lead channel. He used the gain as a volume then the Volume has a Lead Master, then the Master Volume on the lead has a PPIMV. Take another look, it been a while for all of us. Tweed uses the MV/PPIMV for the normal channel/total amp volume then uses a Gain/Volume in the beginning, the a Volume/MV at the end of the lead channel to get that volume correct. I think that is correct but he would have to comment for accuracy.

I cant seem to post pictures, or I am too stupid to figure out how. Is there a way?
 
Messages
51
Thanks Linnie,

Tweedlx's master volume is really just a volume control....as he said like a "Marshall Tone stack". So he only has gain and master volume on the OD channel. Even with gain on 5 or 6, the master volume has to be between 2-3 to not overwhelm the clean side which starts to distort with clean volume on 3 with changing things. And the clean at 3 can be really loud if a true PPIV master volume is at 10 (or there is none at all like Tweed's circuity). He suggested a simple MV to one member, but it is not in his diagram. Besides the Lar-Mar PPIV MV is probably a bit better (in concept). Either mod (Tweed's or Torres') holds compromises when trying to control/balance volume.

I think I know where you are going with this though!
 

Linster

Member
Messages
17
The PPIMV is best for a one channel amp or a power amp master. To crank both channels.

I do have some twists Why does the reverb need to be in both channels? Why not just the clean? That opens this amp up to be a BF Fender on the normal channel and something else on the other. You would have to add 2 more LCD's though. The simpler the circuit the better. More stages ad more complexity

Another option is adding a relay and another whole possibilty.

I have 3 concepts going now but not got through all the issues.

I bet you do know what I am going to.
 

jay42

Member
Messages
6,971
I do have some twists Why does the reverb need to be in both channels? Why not just the clean? That opens this amp up to be a BF Fender on the normal channel and something else on the other. You would have to add 2 more LCD's[sic] though. The simpler the circuit the better. More stages ad more complexity

Another option is adding a relay and another whole possibility.
Fair warning for anyone thinking about adding LDRs. I added one to put the midboost on a footswitch. It browned out the -6.2V supply. Without adjusting the resistor on the zener, it doesn't hold up. You end up pondering a better supply off of the heater supply.
 
Messages
51
Thanks Jay42. Thanks good to know.

I have experimented some more to balance the clean and OD channels in terms of equalizing volume. SMark had previously described the clean and dirty channels as OD and more OD.

Here's a fix. I am looking for input on this. I believe it works for the Torres Mod as well as the Tweedlx mod. I am currently applying ths more to a Torres/SMark version with an EVM12L speaker. It assumes you split V1 which turns out to be easy (add a single terminal with leg under the circuit board and add a electrolytic cap and a resistor that goes over or alongside the existing circuitry). Linster previously provided the values.

I changed the cathode cap and resistor on V1A for the clean channel only to 4.7uf (Torres) and 6.8K. (Original Fender spec is 25uf and 1.0K) . Then I changed the plate resistor from 100K to 50K then 25K. On V2B for the clean channel I changed the cathode cap to 1uf and 2.8K (Original specs same as V1A). This was all in an attempt to create headroom for the clean channel so I can turn it up without breakup and more closely match the volume of the OD channel.

With the plate resistor at 50K, breakup didn't start until 6-7 on the volume control. At 25K, the breakup is starts about 8 on the volume. The clean bass control even becomes more useful without causing flatulence on the clean channel bass. Yet it is not brittle. This is strictly for those seeking a clean channel 1. Torres also recommended changing the bright control (pull switch on the clean channel volume) to original Fender specs. I disagree. It was too ice picky. I tried it. Rivera's design makes the bright switch more pleasing and less ice picky. With these changes, the clean channel with the bright switch turned on sounds really good too...with a minor lowering of the treble control. Bottom line is the channels match a better in terms of volume...not perfect, but clearly better. And I got a clean channel 1...hooray! I 'm keeping the plate resistor at 25K for now.

On the other side of the amp, my OD bass is still a bit flabby. I am working on that. Plan to change the cathode cap on V4A to 4.7uf from the original 25uf. Linster said the 4.7uf is as low a frequency as the ear can hear. So maybe the 25uf adds some bass ghosting that needs to go away. I'll let everyone know how that goes.

BTW I added a 50K pot with a switch for the OD mid control (only one I could find was a Peavey from Antique Electronics at $7.00+ ouch...but the switch is probably unnecessary). It's really Tweddlx's idea and in his cathode follower circuitry. But the mid is much more usable and even negates needing a pull switch to add more mids via a cap. Just dial it in where you want it. Thanks Tweedlx. Another superb idea. With the success of that I'm also thinking of eliminating one of the inputs and adding a mid control to clean channel maybe 25K to 50K. I plan to rewire the input so it can be turned from hi to lo via a pull switch on the volume or mid control. I must have too much time on my hands right now....
 

cabkab

Supporting Member
Messages
234
I just got a new to me Concert II serial number f416006. The amp sou ds fantastic with lots of glassy compression. On the clean channel. I'm tracking down some white noise issues and have some new plate load resistors en route that I'll have by Tuesday to put in. The previous owner put in a Vintage 30. The cabinet itself seems to be attributing to most of the weight of the amp. Strange that they would use such a heavy cab.
  • Anyone know what year this is?
  • Haven't had a chance to check the guts against the schematic/layout but I noticed an extra cap and resistor on the top of the board which you can see in the pics below. Any idea what this would be for? Is that only affecting the gain channel?
View media item 15349View media item 15350
 

synaesthesia

Member
Messages
9
Hi

Has anyone had to change out the vactrols in the concert amps? I ask because I noticed them today in the concert when I was servicing the treble pot; I had to change mine in my mesa boogie preamp.
 

MKB

Silver Supporting Member
Messages
8,144
Years ago I had a Rivera Concert head that I modded to the gills and then put back stock to sell. It's been gone for years. Here's a few mods I did that helped:

Place a 330k resistor in series with the grid of the overdrive tube (don't recall it's V number, but it's the one where only 1 triode is wired). Place it at the socket pin. This made the overdrive much smoother and got rid of much of the bass flabbiness.

If you want to play a bit with the overdrive tone, the overdrive section has that unused triode section that you can use for another gain stage or as a cathode follower.

I found my overdrive stage had treble bleed when the master was turned all the way down, and this mixed with the overdrive at low volume levels and made the amp sound harsh. I fixed this by simply wiring a piece of insulated copper FR4 PCB to ground, and moving it around on the amp board until the bleed was shielded and blocked. I then glued it in place. That cured much of the harshness.

The Rivera Concert has to be the poster child of inconsistent tone due to high gain and PTP wiring. The lack of a PCB in this amp is NOT a virtue. Rivera Concerts are like a box of chocolates, you never know what you're gonna get.
 

TweeDLX

Member
Messages
3,756
Greetings! I tore into the old Concert this week. There were some errors in my original mod. Moving the Volume from the last gain stage on the drive channel to directly following the treble pot made a big improvement. Once I fixed those, I realized that while the amp sounded great, it lacked some in the gain department. So I addressed that issue, and came up with this. I removed the cathode follower and added one more gain stage. I also removed the variable slope resistor and went with a 150K slope resistor (ala Mesa Boogie). It took an extra voltage divider to remove the HF oscillation that occurred with the Gain dimed. Loving it so far. The biggest difficulty in modding this amp is finding eyelets to use and routing the signal wires to avoid oscillation. Staying on top of which relay is which also takes some concentration. As a result, my board looks like ass, but it sounds pretty good. The clean channel is pristine Fender clean, and the drive channel can do Santana. I wish the owner could... :)

EDIT: I uploaded a new schematic with errors fixed. The first divider had the 330K resistor going into the Gain pot instead of to ground, I also changed the 500KA Gain pot to a 1MA. I should also mention that I'm using a 12AY7 in V1.

I still don't want to carry it, but I sure like playing it out in the garage.


Mike

 
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phillygtr

Member
Messages
4,459
Just bought an 1983 Concert. I dig it for what it is. Not sure if it has been modded or not since I bought it at GC.

At some point I might consider having Fuchs do the ODS mod, but just wondering if I'm screwing up a potential vintage amp? Right now they go for modest money, but in the future?
 

TweeDLX

Member
Messages
3,756
PHILLYGTR: Kind of your call. For myself, I wanted something that was suitable to the styles of music I play. The original design didn't float my boat, so I fixed it. :) It's been a fascinating journey so far.
 




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