Adventures in Fractal's Dual Rectifiers

[MirrorProfiles]

Different potentiometer values in schematics 2 channel Dual Rectifier

Hello, I see 2 schematics of the Dual Rectifier on the internet. Basically those are identical however, I see different potentiometer values. Namely: Schematic 1: Gain - 1M Master - 1M Treble - 250K Presence - 25K Mid - 25K Bass - 1M Schematic 2: Gain - 250K Master - 150K Treble - 250K...

boogieforum.com

Trying to find how common 2 channel Rectifiers with 1M Ω MV pots are. So far it feels like 150k pots are more common (or possibly even 250k)

My Rev G pots measure 190k

My friends Rev E measure 228k.

Mesa schematics aren’t reliable at all, and those show 1M Ω, at least on the 2 channel ones I’ve seen. Trying to work out if a particular revision has 1M Ω pots, or if it’s just random, or if it’s some Mesa schematic BS going on.

If anyone has a 2 channel rectifier (Rev G or earlier) I’d be curious to know what the MV pots measure. Still curious if you have a 3 channel variant or single or triple rectifier.

 

[SwirlyMaple]

@James Freeman or @MirrorProfiles

I’m not familiar with the circuit of these amps, but is measuring the potentiometers in-circuit going to give a reliable measurement? Are there any resistances in parallel with them that could be causing an artificially low reading?

 

[James Freeman]

Yes, you have to look at the schematic and make sure nothing is in parallel with the probes or if there are caps that help to block dc path, also if the pot is wired as a variable resistor (lug to wiper).
Not recomended to measure in circuit in general.

 

[MirrorProfiles]

if it helps at all… Global master at the top, channel master below. Red channel is closest to camera, orange channel nestled beneath.

 

[MirrorProfiles]

messaged Mesa directly and they said:

Thanks for reaching out. They are actually two different value pots for each channel and I have listed them below:

Channel 1 Master:
https://store.mesaboogie.com/products/pot-592739-270s-1meg-10-long-shaft.html

Channel 2 Master:
https://store.mesaboogie.com/products/pot-592792-270s-1m-large-shaft-20-a-taper.html

Values did not change between board revisions.
Hope that helps.
Thanks!

so presumably the values I’m getting are to do with other things in the circuit. The part number on my pots checks out with 1M Ω

 

[Orvillain]

There's your answer. Some Rectos had 150K pots, most had 1M.

Hey Cliff, are you 100% sure on this???

 

[FractalAudio]

Hey Cliff, are you 100% sure on this???

No but I have schematics from Mesa themselves that show 150K pots.

 

[0XI0]

Flubby BassTM

 

[FractalAudio]

 

[MirrorProfiles]

Rev G came out in 1994, so this scehmatic would have been at the tail end of the Rev F era (92-94). Interesting that the other schematic floating around is from 5/93 and has the different pot values even though it should be the same PCB. I'll try and dig around more but so far all other schematics (including for Tremoverbs and 3 channels have the 1M ohm listed).

I suppose the only other question is whether that 150k on the 12/93 schematic is a mistake based on measuring the value in circuit? What would a 150k pot read when in circuit? Has Cliff (or anyone else) come across an amp with 150k pots (with different parts numbers to those posted above from Mesa)?

 

[FractalAudio]

Rev G came out in 1994, so this scehmatic would have been at the tail end of the Rev F era (92-94). Interesting that the other schematic floating around is from 5/93 and has the different pot values even though it should be the same PCB. I'll try and dig around more but so far all other schematics (including for Tremoverbs and 3 channels have the 1M ohm listed).

I suppose the only other question is whether that 150k on the 12/93 schematic is a mistake based on measuring the value in circuit? What would a 150k pot read when in circuit? Has Cliff (or anyone else) come across an amp with 150k pots (with different parts numbers to those posted above from Mesa)?

A schematic published in late '93 would likely be for a product that was getting ready for production sometime in '94. Schematics from a manufacturer aren't drawn by measuring a circuit. It's the other way around. The schematic is created and the circuit board and parts list created from that.

 

[SwirlyMaple]

What would a 150k pot read when in circuit?

If the pot was in-spec and truly 150k, then it would read 150kOhm or less in-circuit, but not more.

Other parts of the circuit can act as parallel resistances when connected to the pot, which lowers resistance. But if you measure at the pot’s lugs, those other parts of the circuit can’t raise the resistance measured there.

One thing you could try is measuring the pot, and turning all of the other knobs on the amp except the one you’re measuring to see if it changes the value. If it does, that’s a sure sign that you’re measuring more than just that one pot. But even if it doesn’t change, it’s still not a guarantee that other parts of the fixed circuit aren’t affecting the reading .

The only way to be truly sure is to disconnect the leads from the pot and measure it in isolation, or to trust the mfg code stamped into it. Since both yours and your friend’s measured significantly higher than 150kOhm, and given the digits we can make out from your photos, I think it’s highly likely that yours and your friend’s are both 1M.

 

[SwirlyMaple]

View attachment 15971

View attachment 15972

Could this be one of those intentionally-wrong schematics that mfgs sometimes unleash on the public? I’m wary of anything I find floating on the web.

Searching Mesa’s webstore, I don’t see any 150K pots at all.

 

[MirrorProfiles]

Could this be one of those intentionally-wrong schematics that mfgs sometimes unleash on the public? I’m wary of anything I find floating on the web.

Searching Mesa’s webstore, I don’t see any 150K pots at all.

Yeah, if it was any other company than Mesa I'd trust it more but so many of their schematics have errors.

This 12/93 schematic looks identical value and circuit wise to the 5/93 one, except for these 150k pot values. The 5/93 one would have come out midway through the Revision F era which was released in 92. Because its Mesa, it just seems prone to changing quickly/randomly/going off piste from any printed schematic.

Whats odd is the 12/93 one has hand drawn corrections for some values (680k and 330k), which were printed correctly on the earlier 5/93 schematic. The 5/93 schematic has 1M Gain pots which I don't think were ever factory (there's a corrected version that states "Gain Always A250k).

Since both yours and your friend’s measured significantly higher than 150kOhm, and given the digits we can make out from your photos, I think it’s highly likely that yours and your friend’s are both 1M.

this coupled with with Mesa saying that they have only ever used 1M ohm pots makes me think the 150k on the schematic is a mistake. Maybe there are Rectifiers out there with 150k pots, if so it would be cool to know which amps they went into and what the codes were on those pots.

 

[2112]

Recto, pimped

 

[eggpl@nt]

I toyed with the bright cap suggestion, but honestly, I like Recto 1 Red without any modifications beyond Tone tab. Just a few BMT tweaks and boost out front, and that's what I like. I understand we're maybe chasing a crispier tone here, and I won't sweat anyone's swag, but I like a little less crispiness, I guess.

 

[texhex]

Recto, pimped

Sounds excellent. That Precision Drive is so good with these.

 

[EOengineer]

Ugh I just missed a used Rev G at GC for < $1200. Clicked buy and it was gone.

F.

 

[holoholo]

Sounds excellent. That Precision Drive is so good with these.

Precision drive is a god damn cheat code

 

[MirrorProfiles]

Did we ever establish what Rectifier Recto1 and 2 are?

@FractalAudio does Recto1 look more like

F (small Mesa logo, series loop):

or G (big Mesa logo, parallel loop):

There’s some transitional F/G ones, like Leon’s and mine that are basically a G with some F cosmetics.

Recto2 could be either:

- early 3 channel (I believe the board says Rev C, supposedly sound better than the ones after, until MW)
- 3 channel (with the woofy bass)
- Multiwatt (easiest to spot as the others won’t have power scaling)

The 3 channel Rev C (not to be confused with the 2 channel Rev C) has no text above the global/loop master controls:

anyone from Line 6 fancy joining the party? I think their recto is a 3 channel, would be cool to know what version…. @Digital Igloo @SHwang_Guitars ?

www.rectifierguide.com