Low pass filter after the Body control?

[chance0]

This question might be best answered by SteveD, but if anyone else knows...

Is there low-pass filtering happening in the VM preamp after the "Body" control? The amp is somewhat pedal friendly, but I can't help but hear some attenuation in the high end when the amp is set to low dynamic range and the amp is run fairly clean. If there is attenuation going on, is there a way to circumvent the filtering? Clip a cap, maybe?

I hear that the preamp is like a jumpered Plexi but without the bright cap. Maybe that's all, but it seems like something else is filtering high frequencies as well.

[DeanM]

well the treble will come from the detail side. thats where nearly all the harmonic distortion comes from. The grid stopper resistor R2 will roll off highs. it works in conjunction with the tubes capacitance and the roll off will be subtle. you could lower it to maybe 47k and see how that sounds. but be careful with this as it is used to prevent oscilation and radio interference and also this may effect gain of the stage sligthly and that might require the same mod for the body side to keep em balanced so a better option mite be as follows. there's a treble bleed cap C5 thats 330pF. you could try upping that. try 470 or if thats not enough maybe 680pF.

after the body there is filtering but it effects the lower midrange. thats the mid boost switch section.

[chance0]

Thanks, Dean.

(Assuming that I have my parts correct) I was thinking that maybe C8 and R11 would be the low-pass, but then I realized that they are tied to V2, which shouldn't affect the low range. The 68k R2 appears to be the same resistor found on old Bassmans (which should be the same as JTMs). So maybe I'm just hearing things.

[DeanM]

yeah yer right. C8 and R11 will roll off some highs but that is in high range cos with the extra gain it would be nescessary.

yeah R2 is a fairly common value. id say try what i said about C5. its prob yer best bet if ya really wanna try somethin. but yeah thats true it could be yer ears foolin ya! or even just the guitar and pickups either??

[Cr4nK]

after the body there is filtering but it effects the lower midrange. thats the mid boost switch section.

You mentioned that there is a lower midrange filtering after the body. By changing the value would if increase/decrease the mid boost frequencies, and were is it located?

[DeanM]

yeah R8 and C6 make up a low pass filter and when mid boost is pressed in it switches C6 out for C7. if you increase C7 it will decrease the the midboost frequencies. if you decrease C7 you will increase the midboost frequencies. as it is, it boosts frequencies around 400Hz.

[SteveD]

It doesn't actually boost frequencies at 400Hz by itself. That little network simply selects between two low pass roll off break frequencies. The lower frequency, which is around 70Hz, has no agenda on the amp's natural mid dip of centre frequency 400Hz. The higher one, at around 2KHz, will completely mask the mid dip and flatten the response nullifying the mid dip, which results in the sound of the amp effectively being 'mid boosted'.

[DeanM]

oh sorry steve i shouldve worded that better! i knew that there is a mid dip at 400 hz and that the midboost removes/flattens/compensates for that dip. when i said boost at 400Hz i guess that gives the opposite/wrong impression.

the mid dip...what causes that? is it the cross over point between Body and Detail frequency responses??

[SteveD]

The mid dip is common to pretty much all Marshalls and is the result of the natural response of the whole preamp signal path and in particular the tone stack.

[chance0]

Dean (or Steve, if you're the type that like's to divulge your expertise):

Do you know what c80, R58, and R91 do?

C80 is a 1 nF cap, R91 is a 15k resistor, and R58 is a 30k resistor.

They're coming off the cathode of V4 after a 10k voltage divider. Does this look like a low-pass bunched together or am I misled? Whatever they are, they appear to be part of the negative feedback loop, but I can't find an analogue to them in any other amp that I've looked at.

[DeanM]

yes they are to do with the negative feedback and are part of the negative feedback loop from the speakers. and it looks like they have something to do with the presence control aswel.

thats as much as i can say to be honest. i dont know how they work or function!!!

[SteveD]

R58 and the resistance of the Presence pot set the broadband negative feedback signal ratio from the output transformer secondary back to the phase inverter cathode. C80 in parallel with R58 ensures high frequency stability in high gain mode when everything is dimed. R91 in series with C80 limits its effect so as not to have an excessive agenda on the amp's top end.

[chance0]

Steve:

By stability, do you mean that it helps tame oscillations?

Would I be able to go more broadband and obtain flatter response in the low DR by short circuiting the capacitor and smaller resistor (R91)? I realize this is at my own risk. Since I think the stock sounds of the VM in high range are incredible, maybe I'll install a switch in the extra speaker jack hole as a hard bypass. Doesn't seem too hard to do.

I'd like to open up the low range to let more chime and air into the signal without outboard EQ.

Chance

[SteveD]

By stability, do you mean that it helps tame oscillations?

Yes


You certainly wouldn't want to short circuit C80 and R91 as that would send the whole output signal back to the PI and render the amp unusable.

If anything you would want to lift one leg of C80 or R91 in order to OPEN circuit them.

I would rather you left it alone however as you don't seem technically proficient enough to carry out modifications.

[chance0]

Oh dear.

You're right, Steve. Major oversight on my part.

I guess you wouldn't believe me (or would find it scary ) if I told you that I research and develop radiotelemetry/RF-power systems and advanced conductor materials all the live long day. (And yet RC circuits still puzzle me.)

Crazy world, I guess...