Recto Mods

[Shemham]

Thanks from here as well for the effort. Hope you get better soon.

Once I have enough time, I can try some of your ideas on my Rectifier clone. It's a turret board design so component switching is pretty convenient.

 

[afu]

Thanks from here as well for the effort. Hope you get better soon.

Once I have enough time, I can try some of your ideas on my Rectifier clone. It's a turret board design so component switching is pretty convenient.

That's awesome. If you are able to record the differences, please let me embed a media player at the blog. I'll credit you and link to whatever you want to share.

If any results are far from the theory I've posted, let me know so I can edit the offending parts. I've taken a lot of care with the articles, but I always assume there's a possibility for error.

 

[afu]

https://warpedmusician.wordpress.com/2016/03/17/addendum-to-modification-articles/

Just elaborating on established ideas, new ideas for combinations, and some opinion.

I added some information to explain the modifications described, including purpose and effects.

 

[afu]

I'm thinking about adding a Peavey-style Resonance control to my amp. Still letting ideas come and go, but I think it would enable Channel 2 Vintage to be more useful for me. It may not be implemented exactly as a 5150, but I'd like to be able to adjust the bottom end by filtering some of it out.

 

[afu]

I'm not doing a resonance control, as they only add bass by shorting the feedback at low frequencies.

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I've been reading through Richard Kuehnel's excellent book on power amp design with a copy of the 3 Channel Recto schematic. From the PI design, to the coupling caps, to the bias circuit, this power amp was really not meant to be overdriven. These were really, really intentional design choices.

While a person may prefer more distortion from their power amp, I've been calculating that going beyond much further than spec will cause the screen to go over it's maximum dissipation limit and cause a failure. Even if a particular set of tubes' characteristics work with a bias change, the screen may still be driven close to max and fail prematurely.

I calculated that it takes 2.8 ms to make the grid draw current and 12 ms to recover, with a reduction in power of about 30%. By comparison, Marshalls are mostly in the area of 1 ms, 5.7 ms, and 29%, respectively.

There are examples in the book of amps with similar excursion characteristics. The closest are Vox amps, particularly the AC10 and AC15. Aside from Vox, the amps with longer times are mostly Fender, Epiphone, and Gibson, and tend to be super clean push/pull amps or are Class A amps attempting to extend headroom.

So the Dual Rectifier power amp is meant to stay "clean" and retain full power conditions (likely for low freq reproduction). To mod it for distortion, the coupling caps to the grid need to be smaller and the screen resistors need to be smaller, but power reduction, blocking distortion, and Miller capacitance may be issues. Since it's a master volume amp with significant preamp drive, I don't think messing with any of that is necessary.

 

[Shemham]

One mod I can recommend for those who are willing to lift the PCB is to increase the filtering on the first two preamp power supply nodes from 20uf to 30uf. This is one of the main differences between 2ch and 3ch Rectos. Increasing the filtering changes the feel to tad less compressed. To me it made a great difference on my Recto/Slo clone, YMMV.

 

[afu]

One mod I can recommend for those who are willing to lift the PCB is to increase the filtering on the first two preamp power supply nodes from 20uf to 30uf. This is one of the main differences between 2ch and 3ch Rectos. Increasing the filtering changes the feel to tad less compressed. To me it made a great difference on my Recto/Slo clone, YMMV.

That's cool.

 

[afu]

I realized I made a mistake/omission which affects two articles on my blog. I will correct them soon. The V1 2.2M load resistor is in parallel with the voicing circuit and gain pot. With nearly 119k output impedance from V1 this is important. When Raw is selected, the load is 1.159M. For Vintage/Modern the load is nearly 570k. That's purposely bogging down V1, reducing gain and changing the frequency response for the higher gain modes.

A person cannot hear the greater gain of Raw, because the voltage division going on with the voicing circuit and the gain pot allows Vint/Mod to pass more signal, plus the power amp changes are filtering everything by different degrees (or not, with Modern).

Reducing the plate resistor on V1 will make a greater impact than I first realized and the article for the design of channels 2 and 3 will need the information added, since I didn't really go into it.

 

[afu]

I'm ill again and I've not been out of bed as often to be at the PC to help with posts about troubleshooting problems. I did get bored and pieced together a simple clean boost that's tailored to the frequency band of the Recto: https://warpedmusician.wordpress.com/2016/07/04/mcbooster-2-dr-booster/. There's another version which also works really well and is linked in the blog post. Both are basic, textbook, op amp projects originally inspired by the MXR Microamp, but heavily modified from that circuit.

I also discovered the Texas Instruments SPICE program called TINA. I think it's a little easier to use than other versions.

So if any of you are handy with electronics, I recommend you to try the boost and check out TINA.

 

[afu]

I finished tinkering with the Dual Rectifier Boost. The center frequency varies between 320 Hz and 330 Hz depending on the pot's position. Lots o' thump and body, but harmonics still get to do their thing, too. The link in the above post will take you to the schematic. This is an easy build.

 

[afu]

Up in the Presence Pot Mod discussion, I posted frequency plots for Vintage mode's presence pots on both channels. I've completed the plots for Modern, but I'm pretty sick and wish to wait on putting it together until I can think more clearly. The Modern plots are different than I imagined, because I didn't take the input impedance into account. Here's a sample: https://warpedmusician.wordpress.com/modern-25k-preview/. The blue line represents about where the band pass and band stop meet when the control is turned to zero. There shouldn't be any signal below that point on that single line. Each line would have it's own point and it moves up and over as the control is turned.

I've recently tested positive for SLE Lupus and photos show I've had it since at least 1990 and I've never been treated before. I'm a little preoccupied with that, with doctors and lawyers and such, and being a single Dad, while the sun and heat of Arizona are assaulting me. When September rolls around, I should be back to my less incapacitated state, my son will be back in school, and I'll be more active.

 

[afu]

Just put this up: https://warpedmusician.wordpress.com/2016/07/17/contrasts-of-filter-response-for-dual-rec-vintage

It's plots for the presence and the post-tone stack filtering, connected and powered. The inclusion of the signal coming from V5 and the parts between (no fx loop) shows how these things actually filter the sound together. The volume control is at about 11 o'clock and the presence is being swept to 20 points. The 2 Channel Orange and 3 Ch Vintage on Ch2 are almost identical. Any major tone changes would be from other things, like the layout or parts, if anything.

 

[afu]

When a sudden inlet of current hits the preamp, it takes up to 2.4 seconds to charge the power filter capacitors, contributing to sag. V1a is trying to act like a boost by using a 220k resistor, but the charge time takes some edge off of it and softens the impact.

All these sort of things put together still has me thinking the Rectos are meant to emulate a germanium fuzz into a JTM 45, ala Cream (Pushed, Vintage), crossed with parts of the first couple of Led Zeppelin albums (Raw, Blues, Modern), with more gain, and able to do it without (as much) hearing loss. I easily get The Lemon Song with Modern, Heartbreaker or Good Times Bad Times with Raw, and Sunshine of Your Love with Vintage (saggy, baggy, elephant). Going from those to modern tones is just rotation of the Gain control.

 

[screamingdaisy]

Going from those to modern tones is just rotation of the Gain control.

Or, the volume control on your guitar.

Lately, I set my gain on Vintage/Modern around 1:00 and when playing rock I spend the majority of my time with the guitar's volume backed off. There's a certain spot on the knob where you can clearly hear the distortion transitions from kind of a warmer classic distortion into sizzly high gain; I spend the majority of my time in that smoother, classic distortion territory.

As the guitar's volume continues to come down the clipping softens but still retains some of the square wave texture. I've been spending a lot more time in this area too.

Having a treble bleed circuit in the guitar helps immensely. I've always been a big proponent of working the controls on your guitar, but this treble bleed as opened up a lot of new territory for me. I rarely even leave the modern channel anymore.... just been going from clean to high gain on that single channel.

 

[afu]

I've been doing similar things with the guitar controls. I think setting the sound for "7" and using "10" for leads would be about perfect for the person who wants to use it that way. While the other way around could also work, having a volume drop for a lead kinda defeats the purpose. Installing a toggle switch for series/parallel would be another good way to have contrast between rhythm and lead. When I pop back to series, the volume increases noticeably.

One thing I like is using the Jazz neck in parallel, with the tone on zero. Add a little Small Stone and things get really fun. With the JB and the tone on zero, I add an octave down pitch shifter at 70% wet and boost it. It gets that bass-y, fuzzy, Moby Dick sound.

 

[LesPaul70]

All these sort of things put together still has me thinking the Rectos are meant to emulate a germanium fuzz into a JTM 45, ala Cream (Pushed, Vintage), crossed with parts of the first couple of Led Zeppelin albums (Raw, Blues, Modern), with more gain, and able to do it without (as much) hearing loss. I easily get The Lemon Song with Modern, Heartbreaker or Good Times Bad Times with Raw, and Sunshine of Your Love with Vintage (saggy, baggy, elephant). Going from those to modern tones is just rotation of the Gain control.

Oh, thank you for that explanation. I've always loved playing Hendrix songs on my Rev G Dual, without knowing why they sound so good on it...until now. 8)

 

[afu]

Oh, thank you for that explanation. I've always loved playing Hendrix songs on my Rev G Dual, without knowing why they sound so good on it...until now. 8)

I'm just guessing at that, but Hendrix sounds good on my 3 Ch, too.

 

[screamingdaisy]

FWIW - Fuzz Faces stack pretty good into the Recto's lead channels.

Actually, most fuzzes stack well.

 

[afu]

The Output control on the 2001 3ch has a rc treble bleed (boost) going on. This is likely the source of fizz in the amp. I'm not sure if removing it will be good for using the loop, but removal affects presence and above. The circuit lets treble up to over 10k pass to brighten the sound.

When the loop is bypassed, the MVs handle volume for each channel. They have no treble boost. On CH3, there is actually a treble cut. If a person prefers the bypassed sound, but wants the loop and global volumes, this is an easy mod. Removing the resistor and cap from the board is all it requires; No jumpers. I am not able to take apart my amp at the moment, due to poor health. Otherwise, I'd have confirmed it. I'm 99% sure this will work. If a handy person wants to try it, it is totally reversible.