Recto Mods

[afu]

My fav thing to do to Rectos (and other high gain amps) is to change the first two cathode bypass caps the .47u. It brings out the mids and highs a bit more and gets rid of the flubby low end. Another way to add grunt to it is to add a .01u cap in parallel with R242 (470k). A basic treble peaking circuit, but it lets more of the mids through.

That's cool. Vintage amps had high values, but they used less gain and the preamp wasn't configured to cascade like the modern ones. That lower bass response from the ancient ones never foresaw the lengths we now go to to create distortion.

I would be happy with a few tone stack mods.

I've been looking at the Clean mode of Ch 1 for the 3 Ch. The tone stack differs from the Lonestar by only the 100 k slope resistor. The Rectos use 150 k. It won't give a person the Lonestar clean, because the whole concept for bias and filtering is different, but, still. I'm prepping for 2 articles about Clean mode and the second will talk about changing the filtering. Since conversion to a Reborn Clean would require an overhaul, I think it's more going to be mostly about small improvements in a couple of places.

 

[afu]

Another thing I found and then tested: the last (3rd) clean stage rolls off highs beginning as low as about 400 Hz. To get a "better" sound, the gain needs to be increased past Noon and the volume dropped to about 11:00. Since the presence is after stage 3 and before the volume, slightly higher than suggested presence settings open up the top end when the amp is played that way. The overall sound is more compressed and doesn't have large differences between strums and the sustaining note. As long as the volume is low enough, the P.I. won't clip, but the sound is more immediate than having the volume higher and gain lower. To make it easier, I cut mids a little and increased treble. My settings at the end were:

P: 10:45
V: 11
G: 1
B: 12
M: 11
T: 1:15

On mine, the gain didn't begin to make it break up until about 2:00. My Send is a little lower than Noon and that might make a difference.

Basically, the filter on the gain pot will introduce more frequencies as it is increased. Making more signal pass and passing it at a lower frequency will help boost it past the low pass filter in the last preamp stage. Reducing the volume just keeps it clean, of course. The EQ helped control clipping and Presence set the treble response. There was slight overdrive, but it added character, instead of being a "distortion" sound.

 

[afu]

https://warpedmusician.wordpress.co...1-introduction-input-stage-and-clean-channel/

Here's the full text to the clean channel analysis. This will eventually lead into mod ideas. As it is, it will (hopefully) help with tone shaping.

 

[afu]

I had a spine injury on Friday and some trouble from the RA. I may be awol for awhile, in case anyone was to respond to anything I've written. I can't be on long. Thanks.

 

[afu]

One quick idea. Put a dpdt switch in to take stage 3 of the dirty channels out of the signal path. A simple RC filter or two would probably be needed to tune it, but the result would be a Vintage-style preamp ala Marshall, Hiwatt, Fender, Orange, etc. Don't know if it's worth it, but some brave soul could try it. It may not sound very good with the cold biased power amp.


or you could flip it to Raw and forget about the above.

 

[Jorn218]

Reading these over Afu I like the anode mod and may look a little closer into that one. I may have to adjust the values you came up with seeing that my amp is already modded somewhat. I would like to try to be comparing apples to apples if at all possible.

I wish you the best and hope you have a rapid recovery. Remember that all recovery is 90% mental.

 

[screamingdaisy]

One quick idea. Put a dpdt switch in to take stage 3 of the dirty channels out of the signal path. A simple RC filter or two would probably be needed to tune it, but the result would be a Vintage-style preamp ala Marshall, Hiwatt, Fender, Orange, etc. Don't know if it's worth it, but some brave soul could try it. It may not sound very good with the cold biased power amp.


or you could flip it to Raw and forget about the above.

Out of curiosity, what is the difference between raw and vintage? I always assumed they cut a gain stage or two. Is there more to it? Is the cold clipping stage still present?

 

[afu]

The cold clipping stage is still present in Raw. The voicing switches on the schematic show a series of relays for each mode. I explain and contrast them here. Raw is a hybrid of Modern and Vintage with reduced signal flow to the Gain pot. Though it's great for Classic Rock and Blues, it isn't really like the amps originally used for those styles.

 

[Third Age Amps]

I've got to say, I love your site! Enough of the technical analysis for those of us that really get into it, and (much like Merlin's site and books) understandable to the tinkerer. I'm going to try your Loop mod on my Rackto this weekend.

 

[afu]

I've got to say, I love your site! Enough of the technical analysis for those of us that really get into it, and (much like Merlin's site and books) understandable to the tinkerer. I'm going to try your Loop mod on my Rackto this weekend.

Thanks. Merlin is an inspiration. His preamp book is about the most comprehensive book I have for design and he's a nice guy.

 

[afu]

I have been putting a lot of thought into the cold clipping stage (stage 3). Aside from the anode (plate) resistor change for stage 1, stage 3 would be the other culprit for fizz. At idle, the bias sets it almost at cut off. When anything more than (perhaps) a few hundred millivots comes in, it's going to be distorted and short on the positive side and within the headroom of the tube up to 4 volts (no overdrive) or 5 volts (overdriven, about to clip) on the negative side.

The issue is this: if a passive, high output humbucker is plucked and produces a .5 volt p-p signal and the Gain pot is halfway, the signal on the grid of stage would be about 15 p-p. So, the negative going cycle on the anode is clipping and has a nice distortion going on. The positive side can only get around a .25 volt peak (but will increase by a small degree, because tubes allow leakage). While tubes distort softly under most conditions, trying to force something 30 times larger than the available room is going to create a hard clip and square the output to some degree.

Further, when the Gain pot is turned up to the point that the fizz becomes very prominent, the low frequency response becomes blurred. That is also due to this stage. The filtering is cutting high frequencies off above 3 kHz and the cathode is not bypassed. The whole signal is receiving a very small, even amplification with the afore mentioned heavy distortion, which will make the low end lose focus, because the tube is affecting frequencies all the way down to 6 Hz. The amp has no grid resistor following this stage, so the resistance of the stage creates a divider with the 330 k load and passes more than 75% of the signal to stage 4.

While a cold clipping stage is going to produce harmonics which are mostly even-order (if there's no hard clip), the amplifying section of the next stage is going to amplify the crap out of the complex, harmonically rich signal, create more harmonics based of of the multiple harmonics coming in, and pass it to the cathode follower, while providing no filter for the treble harmonics being produced. Then it goes to the EQ, the PI, then the push-pull power section, which is going to produce odd order harmonics from anything it gets and will itself be fizzy, from the cold bias, if overdriven.

Now, let me say that this is exactly what the amp is meant to do. I would also argue that this stage is the heart of the amp and is a large part of its popularity. A person doesn't really need to turn the Gain pot way up to get a great distortion sound and is advised by Mesa to not do it. That's kind of point. Non-Metal players can use the tube rectifier to soften the sound, or switch to Spongy for a darker sound.

However, if a person wants to maintain the heart, but reduce the fizz and flub at higher Gain settings, a resistor change would be a remedy. I haven't figured it out yet, but if 10 k worked for a JCM 800, a value at or above 10 k is probably going to be a good compromise. Too low will change the amp too much. On paper, a value of 12 k to 18 k seems to provide a little more positive headroom and remain low enough to still work the way this stage is intended. I think the key is to get it just under the point of hard clipping. If anyone has done a mod to this section, please respond.

(Aside) As I mentioned before, installing a switch to remove the cold clipping stage would turn the preamp into something akin to a goosed Plexi. I still don't feel it's worth the bother in most respects, but if the bias mod has been performed, it could get a person close to the 6L6 equipped Marshalls, but in a modified form (or just flip the switch and install EL34s if the mod isn't done). Considering all the work involved and what it might do to the resale value, this might just be a mod for the person who plans to be buried with their amp. Otherwise a new design could incorporate this type of switching.

 

[afu]

One more thing, the three dimensional distortion that the Recto produces is due in large part to the asymmetrical clipping of this stage. When not turned up too high, half of the cycle is relatively clear and the other half is being punished for its sins.

 

[afu]

http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

This is a great article about impedance mismatches and what they do to the tone. Talk about an easily reversible mod.......

Note: Mesa only advises mismatches where the cab is a higher rating than the output and no more than 2x the output impedance. Perhaps Roadster/Road King owners and anyone else with blankets over the tone, or excessive, unwanted, dark tones could try this.

 

[afu]

I still don't feel well, but I'm getting an itch. I think I'm going to soon try some experiments with the things I've been looking at. I'm going to use parallel resistors to reduce V1a's anode to 150 k and to reduce V2b's cold cathode to 15 k. Depending on those outcomes, I may also swap out V2b's anode to 150 k. I may see what different cap values do to the bypass on V1a after everything else. If it ends up being a poor set of mods, I can change back pretty easily, overall.

I'm really, really considering a high voltage switch to throw the parallel resistor over V2b so I can go back and forth. I like the fuzz pedal-ish clipping the stage does, but being able to give it breathing room to clean up would be nice when my inner Jimmy/Jimi come out.

On V2b, the resistor combination of a 150 k anode and a 15 k cathode should maintain the overall character of the cold clipping while giving it 1 V room on the positive side of the plate and increasing the gain from around 5.5 db to 15.8 dB. The negative side will cut off or be overdriven more easily (by close to 1 volt less room). It also reduces the hpf on the anode to 1 kHz vs 1.6 kHz to further cut that sizzle out (still gets audible harmonics up over 10 kHz).

If V1a is modded down to 150 k on the anode and its cathode cap is reduced, the signal will have a small reduction in dB and bass content. V2a is unchanged. V2b, will still clip asymmetrically, but less harsh and a little louder. V3a is unchanged. It's already used to reintroduce the lows and highs and now it would get slammed a little harder. This will increase the clipping of this stage, which is desirable, instead of reamping harmonics from early on. The cathode follower will end up compressing a little more than it already does, but this compression should be softer and less abrupt than the compression produced by the stock V2b stage trying to murder the signal.

My predictions are: clearer distortion at high Gain settings; a more articulate distortion until very late in the Gain taper; a softer, tighter compression when the Gain is cranked; more dynamics with input volume and picking variances; a Raw mode that is not so dull and which cleans up more easily with guitar volume; and a cleaner Clean channel.

I'll let you know if this works or not.

 

[screamingdaisy]

Now, let me say that this is exactly what the amp is meant to do. I would also argue that this stage is the heart of the amp and is a large part of its popularity. A person doesn't really need to turn the Gain pot way up to get a great distortion sound and is advised by Mesa to not do it. That's kind of point. Non-Metal players can use the tube rectifier to soften the sound, or switch to Spongy for a darker sound.

One of the things I value about the Recto is how I can get significant distortion without heavy compression, and how varying the pick attack can control the dynamic response of the amp. I feel as though I can control how much the amp sags and how quickly the amp responds by varying my right hand picking and/or muting technique.

One more thing, the three dimensional distortion that the Recto produces is due in large part to the asymmetrical clipping of this stage. When not turned up too high, half of the cycle is relatively clear and the other half is being punished for its sins.

Interesting.

 

[afu]

I don't actually a have a fizz problem with my amp. I have Ruby tubes biasing it a little warmer in the power section (28mA draw), I use the tube rec, and I don't usually turn the gain past 1:00; Noon is my normal gain. On Modern, I used to play fast, old Metallica with no issues and "Harvester of Sorrow" or "Sad But True" sound crushing with the tube rec. AIC songs growl like a mother and Tool sounds appropriately dark and sinewy.

I personally have few complaints about the amp, now. When I put the Mesa tubes back in to play around, the fizz can be harsh, particularly on the silicon setting, but I prefer Normal anyway, and I plan on using these Ruby tubes until they fail.

 

[afu]

I keep going over the circuit by calculating 500 mV signal through the stages, filters, and dividers up to V2b. It's pretty apparent to me now that V2b doesn't need a change. The content up to that point needs to be affected. If the cold clipping is decreased, the signal is going to become so hot that it will be a mess and overload the grid of V3a to greater extent than it already would.

Reducing the amount of lows and highs with the least amount of change would be more sensible and also retain the character of the amp. I still think making V1a's anode 150 k would be a benefit. The cathode cap could be reduced to .47 - .33 uf to eliminate some bass. It still would have enough gain to be punchy, the harmonics are reduced, and now the bass reduction is allowing the meat of the guitar frequencies to come through.

The tricky part of the whole thing is what to do after that. At this point, rather than reducing anymore gain, the objective would be to filter out treble and/or upper mids. I think the Gain pots are the place to do it. That bright cap could be removed or reconfigured. If kept, a series RC filter of 220 k and 1 n would roll off signal at 723 Hz. 150 k and 1 n puts it around 1kHz and 100 k and 1n is 1.6 kHz. In any case, the highs are limited at the pot and the mids will come through better. This RC filter is like the one on the Output pot.

Part of the issue, as I can see it, is that treble passes pretty freely all the way until V2b. V1a and V2a are cathode bypassed and the divider and gain pots are bypassed after V1a. It isn't until the grid of V2b that highs get dumped. The treble cut on V2b is about 1.6 kHz. Any harmonics generated by V2a get a shave at that point. Limiting the content going into V2a would go a long way toward removing fuzzy upper harmonics and would retain the signature sound if one doesn't get too crazy with treble reduction.

I would probably put a slightly lower filter on Ch 3 than Ch 2. Since 3 is set up for Modern, the lack of negative feedback in that mode is going to allow more upper frequencies to be reproduced by the power amp. There is a cap on the Volume pot to shunt highs to ground, but there is ample high frequency in Modern Ch 3.

 

[screamingdaisy]

Reducing the amount of lows and highs with the least amount of change would be more sensible and also retain the character of the amp.

OD-808... Or variations thereof.

Rectos also love the EMG 81. Very clear without much bottom.

How loud are you playing? Keep in mind that much of the "fizz" is attenuated by the speakers once you start to hit stage volumes. Mods that make the amp sound better when solo'd at low volumes may negatively impact the performance in a full mix at gig volumes.

At lower volumes I usually roll my tone knob off slightly to reduce the top end, which takes off some of the fizz with it.

 

[afu]

OD-808... Or variations thereof.

Rectos also love the EMG 81. Very clear without much bottom.

How loud are you playing? Keep in mind that much of the "fizz" is attenuated by the speakers once you start to hit stage volumes. Mods that make the amp sound better when solo'd at low volumes may negatively impact the performance in a full mix at gig volumes.

At lower volumes I usually roll my tone knob off slightly to reduce the top end, which takes off some of the fizz with it.

The mod ideas are for others. I've read all kinds of things about what people want from the amp. I actually find the Recto to be a fine amp as it is, after the series loop mod. I play really loud. When I have to play quiet, I use the Solo as a volume to darken the top end.

I don't like OD as a tone shaping device. It feels too... solid state, I guess. That isn't a bad thing, but it isn't what I want most of the time. When I want that feel, I grab a fuzz. When I use EMGs I really don't feel like I need an OD.

Today, I reduced my guitar volume pot with a parallel resistor and it smoothed the highs out. I even put the Recto on silicon and tried to make it fizzy and it didn't really do it like before and the Treble had more play. My total resistance is now about 240 k with a Duncan JB. I also have a "treble bleed" and spin-a-split going on. Even my single coil sounds are smooth: http://wp.me/a4h97k-bU

I think a guitar wiring mod is cheaper and easier than an amp mod (and also doesn't risk reducing value of an expensive amp).

 

[afu]

I figured out the Presence controls for the modes, how they differ, and how to explain it without a lot of nerd talk. I placed a link in the topic for the Presence Mod, since it applies directly to it, but it may also be of use to someone struggling with matching sounds. There's a really good reason the suggestion in the manual to match the Presence between channels doesn't work correctly in practice.