Dual rectifier two channel, why is it more scooped than newer tremoverb, roadster, RoadKing?

[FedWar]

Hi guys,
so I've always wondered why the first dual rectifier series (up to Rev G) was more mid-scooped than all of the later rectifiers, including the tremoverb, which is based on a rev G.
I've started to download and compare all of the available schematics, and started looking specifically in the tone stack area.
All of the later rectifier shows similar tone stack RC values in the filters, but there is a difference:

Up to Rev F and I think also up to rev G the schematics show a 1MOhm resistor between the red and the orange tone stack, labeled R215
So my understanding is that the signal would go through the active stack but some signal will flow to mass via the other tone stack as well.

Rev C schematic

Rev F schematic

This resistor or a circuit equivalent is not present in the Tremoverb

Nor in the Road King

Nor in the 3 channel rectifier

What do you guys think?
I am onto something or did I totally misread the schematics?

thanks to those who take some time to reply!

 

[electric mayhem]

Is this what you are referring to on a rev F? My experiences with recto solos have always been brief and usually rev G triples or roadsters but this rev F fell into my lap recently, my 1st dual surprisingly.

In looking for your R215 I see Rev F was still using 400V coupling caps on the PI. Were these re-spec'd to 630V like the OG, Mark II's and III's? This F also has Mark III tranny's which is different from the rev G's.

 

[FedWar]

Is this what you are referring to on a rev F? My experiences with recto solos have always been brief and usually rev G triples or roadsters but this rev F fell into my lap recently, my 1st dual surprisingly.

In looking for your R215 I see Rev F was still using 400V coupling caps on the PI. Were these re-spec'd to 630V like the OG, Mark II's and III's? This F also has Mark III tranny's which is different from the rev G's.

Hi,
with rev F and rev G I mean the first dual rectifier solo series, check www.rectifierguide.com


Rev F has Mark III transfomers true and it might have different PI coupling values, and it is less muddy than the rev G, IMHO due to a different bass frequency handling in the first RC network after the first preamp stage.

But I don't think that would be the reason for this sculpted mid scoop sound that was so typical of the first two channel dual/triple rectifier series.

Let me turn the question around.

What do you think it is the purpose of that R215 connecting the two tone stack, considering that only one tone stack is connecting the signal path to the master at a time due to channel switching LDRs?

 

[electric mayhem]

Let me take this in reverse starting with an electrical not sonic explanation as I see it. Using my F as the example the 1st Vdrop of the tone stack is a 47k (according to the schematic). Not much signal is going to take another option of 1M. Downstream of the 47K is plenty of resistance but I wouldn't know the impedances LDR8 and 9 are looking into.

That said the downstream fixed impedance looks about the same for orange and red so if the pots were at the same values I would expect similar impedance on each channel with a fat-in-value 1M sitting in between them. While not much signal would pass to the non-selected channel there is still a path to ground through a voltage divider so there is very small (I would expect) bleed thru.

Why implemented? I wouldn't know but smoothing comes to mind when channel switching. It just doesn't seem substantial enough to make a difference to my unlearned head. If all the pots were providing minimal impedance in the RC network of the non-selected channel and relative higher impedance in the active channel you theoretically could work the voltage divider against the active channel and into the ghost channel. Just a guess.

I was looking at the rectifier guide yesterday trying to cement some of the differences in my head. I've only had 2 channel solo's and each time had to sell them right away for other shiny things so I don't have the comparison except all the recordings made. Mike just made a rev C for @GJgo and I am really interested in how that feels to play. Sonic wise I'd really like to try an E but talk about unicorn.

As for the coupling caps, all the old Marks used 400V and Mesa respec'd to 630V after numerous failures. I have to imagine for the same reasons any recto with 400V coupling caps needs updating. With all the recto gurus on here someone has to know for sure but it certainly won't hurt to put new high voltage R&R caps in there.

The pic I posted was my guess at which R was R215 on the board. It sits between LDR8 and LDR9 physically.

 

[FedWar]

Why implemented? I wouldn't know but smoothing comes to mind when channel switching. It just doesn't seem substantial enough to make a difference to my unlearned head. If all the pots were providing minimal impedance in the RC network of the non-selected channel and relative higher impedance in the active channel you theoretically could work the voltage divider against the active channel and into the ghost channel. Just a guess.

Thanks for this, I was keep thinking about the reason why they put it and I could not find any

Then if you think the effect is just for smoothing, I don't understand why the newer rectos have a much mid heavy tone, while schematics shows that tone stack values are the same.
For example on my roadking II, which schematics for channel 3 and 4 are pretty much similar to dual rev F orange and red channels, I need to put mids almost to 0 to have the same mid scooping of the old rev F.

 

[electric mayhem]

Again just a guess, so a guess on a guess makes a second derivative guess in diminishing value:

They removed the R so the factory didn't see the value (pun intended) of it. Like it or not gone stops it from acting as a voltage divider with no path to ground. No Bleed thru. No worry about non-active pot values effect.

You are calling out mids, the last in a tone stack and on some amps not even there as a variable pot. It is the tail of the tone stack.

Sorry for the financial example but it reminds me of how bond yields fluctuate. Normally a closer in 5 year note rate will be much more behaved than a far out 30 year bond yield that gets whipped around by events like the end of a whip or tail.

So let me turn it back around once more since you likely have more amp experience- wouldn't the tail end be more sensitive to changes upstream in the tone stack including the existence or lack of a ghost circuit? There is a slight difference on the 1st part treble cap from orange to red on the Rev F schematic. Not huge but it changes everything downstream.

 

[FedWar]

So let me turn it back around once more since you likely have more amp experience- wouldn't the tail end be more sensitive to changes upstream in the tone stack including the existence or lack of a ghost circuit? There is a slight difference on the 1st part treble cap on the Rev F schematic. Not huge but it changes everything downstream.

ok, understood, to my limited theoretical knowledge (I'm not a qualified tech mind me so I might be wrong ) usually a RC network would affect with some kind of shelved shape or at least small bump and then shelve.
first dual rect up to rev G have a "V" shaped scooped around 1000 hZ (judging by ear, did not verify with oscilloscope which I dont have )

roadking II has fairly comparable schematics, check attached schems
Do you find any RC network that can explain this difference in sound between the two?
thanks again for going with me down the rabbit hole

 

[bjosephs]

The 1M resistor is there to keep the DC voltage stable on those tone stack caps. The signal is imposed on a DC of a hundreds volts and without a pull up/down resistor the unselected tones stack will start to drain down through the cap leakage. The switching pop could be big and a bit unpredictable depending on how long between switches.

An LDR, I would have to thought, would have a low enough off resistance to keep them charged but a relay based switching system will surely need it. Mesa amps absolutely have parts in them that didn’t get on the schematics every time.

I can’t imagine that the drain on the signal through that 1M would be audible. The source impedance of the follower is very low and the load of the tone stack great compared to 1M.

 

[FedWar]

@bjosephs thanks for your input!
By comparing roadking II schems vs rev F, as far as the ch3 and ch4 are concerned, I cannot find any reason why the RKII has mids so much prominent.
Must be something not shown in the diagrams as you said.

 

[bjosephs]

Could be pot tapers. They are marked as linear in most of these schematics but I know they switched Mark series mids from L to audio taper so that the 5 setting was actually lower than it appears. They could be using something custom.

It can also be something outside of the tone stack. If lows and highs are emphasized it can make the amp seem scooped so maybe continue your investigation into the presence circuits and low end emphasis.

 

[FedWar]

Could be pot tapers. They are marked as linear in most of these schematics but I know they switched Mark series mids from L to audio taper so that the 5 setting was actually lower than it appears. They could be using something custom.

It can also be something outside of the tone stack. If lows and highs are emphasized it can make the amp seem scooped so maybe continue your investigation into the presence circuits and low end emphasis.

I will.
FYI red channel has no feedback on all rectos (it's the trademark) thus this reduces the variables quite a bit hopefully