Recto Mods

[afu]

I know about the Presence, Pre-500, and Bias mods. I'm looking for info on other mods to the preamp for smoothing gain. I have some of my own ideas, but if anyone has experience and media, I'll post links to your results from my blog when I get that far.

My ideas:

Change input stage anode resistor to 150 k. It reduces gain. Less gain = mellow clean.
Change bright cap on Gain pot. Not sure which direction to go here. Want to fix the mush at 2:00 - 2:30. Probably 470 pf to 220 pf.
Decrease cathode resistor on the coupled cathode follower to 68 k. Increases compression, filters out the fuzziness a bit. Very subtle.
Change the treble cut on stage 3 to 2.2 n. That moves the knee down to around 720 Hz or so. It's like a mid boost ala tube screamer.

Has anyone done these or similar mods? Any ideas?

 

[afu]

Another idea is using the space from an fx loop mod to add a rotary switch for selecting cap values on the Output pot or defeating the bright cap altogether.

Pro: No need to drill or otherwise risk defacing the panel.

Con: I have concerns about the wire runs, but twisted pairs and straight lines away from power sources, at 90 degrees to power or signal, would probably be fine. The Output pot is far enough down the line to not be as affected.

 

[vick1000]

I would like to reduce the fizzyness, but I guess that's part of the recto "tone", and where the hi-gain feel comes from.

 

[afu]

I would like to reduce the fizzyness, but I guess that's part of the recto "tone", and where the hi-gain feel comes from.

I really like the Recto tone. I just can't leave things alone sometimes and I like to experiment. If I were a biker, my Harley would be totally custom.

I think my biggest want is to be able to have that massive saturation sustain without the blurring in the low end. I imagine it has something to do with where the gain pot bright cap frequency knee is at.

My other aim is to provide a resource to people that is at least semi-organized and laid out in a straightforward way without a person having to read 200 posts, on 5 boards, with 32 differing opinions. I am thinking about laying out the different ideas and the reasoning behind them in a helpful manner, include competing strategies. Eventually, I would index all the stuff on my blog regarding Rectos so it's easy to access. If anyone has complete ideas and/or media, I'll give them their credit and links/embedding.

People could use some clear information before they dig into a $2000 amp and I need a hobby.

 

[vick1000]

I doubt the people that spent $2k will be doing the digging, more people like myself that found a deal on an old RevG.

 

[Shemham]

I think my biggest want is to be able to have that massive saturation sustain without the blurring in the low end. I imagine it has something to do with where the gain pot bright cap frequency knee is at.

Have you actually tried to jump the 100ohm resistors on the preamp cathodes? Sounds like it could be what you're looking for.

 

[afu]

I think my biggest want is to be able to have that massive saturation sustain without the blurring in the low end. I imagine it has something to do with where the gain pot bright cap frequency knee is at.

Have you actually tried to jump the 100ohm resistors on the preamp cathodes? Sounds like it could be what you're looking for.

Nope. I want to preserve the clean channel. I have been thinking of bypassing the 100 ohm at stage 4, because it won't affect the clean.

I think I might have found a solution for the Dirt channels and it would also improve the clean. V1 is biased to be at about -1.6V with the 220k anode and 1.8k cathode resistors. Dropping the anode R to 150k places the bias at about -2V. It reduces the overall gain a little and makes the signal both harder to send into cutoff and increases 2nd harmonic. It also raises the cathode bypass frequency to around 100 Hz, making the lows a little firmer.

I think the fuzz/fizz is the cutoff/saturation being generated and treble bypassed in stages 1 and 2. Stage one is really strong with a 220k Ra. A hard strum can exceed the headroom provided by that bias point. The positive side of the signal begins to be limited (tube sound) as it swings up past -1V and continues to be limited until cutoff, when it becomes a rounded square wave. The negative side has plenty of room to swing, but introduces distortion (the tube sound) from being non-linear as the current approaches zero.

After this stage, there are two high pass filters. The lower frequencies are being lopped off, but the harmonics of those frequencies may not be. From stage to stage, the harmonics from all frequencies are being reinforced. After stage 2, there aren't anymore treble boost circuits directly in the path for the amp section of the preamp, but there doesn't need to be. The high frequencies pass the easiest and that job is already done. The only stages to not reinforce the harmonics are stage 3, with a voltage gain of <2, and the cathode follower, with a voltage gain of <1. Every other stage has large gain, with cathode bypass caps, and is adding to the harmonics generated at the beginning. Imagine chaining two distortion pedals and distorting the distortion. It's about like that. Changing anode resistance on stage 1 is like turning the first pedal down a little.

There is still going to be a LOT of distortion on Ch's 2 and 3. It'll be clearer at greater values on the Gain pot, I believe. The added benefit is a slightly smaller, less compressed signal also going to Ch. 1. A cleaner clean.

The Dirt cathode follower can also be changed for the compression and high frequency smoothing. I believe 47k to 82k cathode resistors would help with that.

I've been pouring over my books/info from Merlin Blencowe and Richard Kuehnel, comparing load lines, gain, harmonics, and frequencies. I can't say with complete, concrete, confidence that "This is IT!!", because it's theory. Until I, or another person, can test it, I can only say that it looks freaking awesome on paper.

 

[vick1000]

Wouldn't a 12AT7 in V1 do the same basically?

 

[afu]

Not exactly. The gain would fall to the same as changing the resistor, but the bias would be around -1.2 and the response would be flatter. So, less low attenuation and more distortion of larger signals than changing a resistor.

 

[afu]

https://warpedmusician.wordpress.com/mesa-dual-rectifier-index/

I added an Index for my blog posts regarding Dual Rectifiers, links to Mods or content, and the tools I use. If anyone wants to be included, let me know by PM, and provide links or content. I'll consider listing different ideas or solutions from a person and provide credit and links as applicable. People need help and answers and providing a hub would go a long way toward helping the Recto community at large.

 

[afu]

https://warpedmusician.wordpress.com/mesa-dual-rectifier-index/

People need help and answers and providing a hub would go a long way toward helping the Recto community at large.

I wanted to clarify:

I didn't mean any implication about the quality of The Boogie Board. This BB is awesome and provides an incredibly useful and supportive resource. As is common with any board, old threads fall away and have to be searched for. That isn't a big deal, except for the limitations of the search program to find the exact information quickly and person's understanding of how to use the advanced options. Some threads are stickied, but it would be impossible to make every conversation of value stuck to the top.

I mean to make available a hub for my own information that is simple and to include information from here that doesn't shift from incoming topics. It is redundant in a way, but it's a useful redundancy for people who don't have time to read through several threads to find the one piece of information they need. Including input from other points of view is just another way of making the hub a better resource for the community.

 

[afu]

Charts, graphs, and explanation for the input anode mod.

If the admins and moderators are ok with it, I'll post future mods here to keep from causing clutter.

 

[vick1000]

What about another 470k resistor in parallel so I don't have to pull the board?

 

[afu]

Sure. You could put a switch on it, too. I'm going to wait until I have a few more things up to talk about variations or ways to combine different things.

P.S. I swear I wasn't drunk this morning. I have now fixed spelling and grammar.

 

[afu]

I found a small error which actually benefits the mod. The harmonic distortion increases with a lower anode resistance only when the bias is kept at the same grid voltage. Since the cathode resistor is unchanged, the 2nd harmonic changes from roughly 9.7% to less than 1% with a 1 volt signal. A very much cleaner clean and a clearer distortion would be the benefit.

In any case, anything below 1V is super clean. That would be cool with most passive pickups and a person who doesn't have kung-fu grip. It would be unlikely that a person could consistently overdrive stage 1 without a boost or active pickups.

I also did the math for the gain reduction for 100 ohm resistors. It can be hard to wrap my head around something when it's made of rocks and info out there is conflicting. Even people who aren't going to mod will find this interesting. Totally stock, the resistors change the gain from 76.2 V (37.6 dB) to 51.3 V (34 dB). Wow!

It seems to be close to a 3 dB loss with the 150k, too. I imagine the 100k resistors on stages 2 and 4 would be similar. After accounting for the dumps between stages, it would be negligible, but still.

I've been sick enough to be in bed for most of the last two weeks. These maths were the only things I missed, but I should have waited until my head was clear to triple check the all the math throughout. I am satisfied it is 100% correct now.

 

[Jorn218]

I would be happy with a few tone stack mods. What would the Zsrc and R5 be on the DR tone stack to plug into Aiken's Tonestack calculator?

 

[afu]

I would be happy with a few tone stack mods. What would the Zsrc and R5 be on the DR tone stack to plug into Aiken's Tonestack calculator?

Zsrc= 615 ohm, load is 1M according to this:http://www.ampbooks.com/mobile/amplifier-calculators/cathode-follower/calculator/

The Zsrc might actually be slightly higher, because I've seen it quoted up to 680 ohms, but both are close enough to be reasonable for a graph. Even the default 1.3k doesn't change the response a great deal, so tens of ohms is not too much of a problem.

 

[afu]

I've accounted for the 100 ohm resistor manually, since the spreadsheet doesn't allow for it. The AC load line is now calculated and estimated on a graph. This AC load accounts for the 100 ohm limiting. Math for both additions are shown below the blog post.

The 100 ohm resistors actually reduce the low end attenuation/flatten response somewhat for stock and modded. The improved clean channel and the bias change to reduce overdriving the stage still apply to the mod. Other assertion I made have been modified by the results of the math.

 

[afu]

I was looking into a mod for the cathode follower before the tone stack. In older designs, the follower sat in an area where some grid current would be flowing into it at idle, but it had room to move. In my DR, the 435 volts and 220k/1.8k resistors in the amp stage coupled to the follower appear to already be pushing the grid into cutoff. I think the follower is attempting to sit at 2.2 ma, but the max is 1.97 ma. It's acting like a tube diode and chopping nearly one half of the signal off. Crunchy.

I'll look at it again another time, because it seems really odd. The higher plate voltages and the way the amplifying stage is maxed for gain could be doing it. The math is this:

Vk/Rk = Iq
220/100= 2.2 ma

The CF grid steals current from the amp stage. I figured it to be around .12 ma, but the difference here is .23. Weird.

Anyone have thoughts?

 

[Third Age Amps]

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.