3M3 and 10pF pass filter in Fender Reverb amps

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ElectronAvalanche
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3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

Hi there,

so after using my trusted Vibroverb Clone successfully in band rehearsals I took it home to fix some residual hum. Turns out a simple shielding of the Reverb can and a shield attached to the underside of the top of the cabinet got rid of almost all the hum. Very good!

Playing at home without a drummer going beserk next to you revealed some hiss that really annoys me.

The 3M3//10pf combo is the likely culprit. Since I did not have a 10pF at hand when finishing the amp back then I used a 2M2 with a 15pF to get (at least in theory) to the 4.82kHz cutoff. As I understand it, the 3M3//10pF as per schematic sets the treble content of the amp.

The installed 2M2 measured a lowly 1.89M, the cap (ceramic 2.5mm) measures 15pF dead on outside the circuit. So the freq would be more like 5.61 khZ. This could be the reason of the hiss IMHO.

Pulling any tube before the Reverb mixer did not reduce the hiss, pulling the Reverb mixer tube got rid of that. Grounding the signal before the 2M2/15pF reduced the hiss a bit, but not much so. Grounding after the 2M2/15pF got rid of the hiss.

So I will order some 10pF Silver Micas and install a 3M3 (this time picking one that is dead on 3M3). Why the 2M2 has drifted so much (found another one from the same batch that also read 1.80M) is a riddle to me (carbon film btw.).

Would you guys opt for a CC 3M3 or stick with a 3M3 Carbon Film I have on hand?

BTW: in the pic you can see a 3M3 with a 15pF I had in the amp way before, the amp was extremely bright due to this. So small changes do have a serious impact.

Thanks for any input,

Dote
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Colossal
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Colossal »

ElectronAvalanche wrote:Would you guys opt for a CC 3M3 or stick with a 3M3 Carbon Film I have on hand?
If you are limited to on-hand choice, I would opt for the carbon film over the CC for precision/accuracy or use a 2W or higher 1-2% metal film and there won't be any drift...ever.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by JazzGuitarGimp »

I believe the hiss is a result of the resistor's noise coefficient (self-noise). This higher the resistance, the more self-noise it generates. Of the available resistor types, metal film has the least self-noise. Also, it seems to be accepted fact that the higher the resistor's wattage rating, the lower the self-noise. You might want to try a 3M3, 2-Watt metal film resistor.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by roberto »

ElectronAvalanche wrote:I used a 2M2 with a 15pF to get (at least in theory) to the 4.82kHz cutoff. As I understand it, the 3M3//10pF as per schematic sets the treble content of the amp.
Yes, it does it, but it's not the same to use a 3M3//10p, 2M2//15p or 220k//150p. While the frequency of the filter is the same, the voltage divider changes alot: more and more signal passes through, and the cap bypass loose its efficacy (there's less difference between the unbypassed and the bypassed signal.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by jjman »

That resistor does not carry DC so I don't think it should generate hiss like a plate resistor does. Is the 220k in good shape. Tube swap?
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Roe »

the reverb circuit is noisy. check grounding and lead dress on v4 carefully
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Darkbluemurder
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Darkbluemurder »

roberto wrote:
ElectronAvalanche wrote:I used a 2M2 with a 15pF to get (at least in theory) to the 4.82kHz cutoff. As I understand it, the 3M3//10pF as per schematic sets the treble content of the amp.
Yes, it does it, but it's not the same to use a 3M3//10p, 2M2//15p or 220k//150p. While the frequency of the filter is the same, the voltage divider changes alot: more and more signal passes through, and the cap bypass loose its efficacy (there's less difference between the unbypassed and the bypassed signal.
Correct, and you also alter the balance between the dry and the reverberated signal (the lower the resistor, the less reverb). Good if you always thought that the amp had too much reverb.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

Thanks to all of you for chiming in.

It escaped my mind that simply scaling down the resistance to 2M2 and using a corresponding cap would not result in the same sound due to the rest of the circuit. Thanks Roberto for pointing that out. Also Darkbluemurder is right, the Reverb could have a bit more intensity with a set setting of the Reverb knob. So I will not raise the return signal as much for the same Reverb intensity. This too will reduce the noise floor.

I have checked the mixer tube btw, but this did not have an influence. And the 220k was dead on as well. So this points back to the 2M2/15pF.

Thanks again to all for their tips and help.

I will report back once the ordered new resistors are here and installed!

Electron!
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

I finally got around to install a new 3M3/10pF combo.

This did help some, but not too much. The CC 3M3 did hiss a bit less than the CF I got hold of.

So this got me scratching my head :roll:

Grounding the 3M3/10pF 500pF junction reduces the hiss a tiny bit, grounding between 3M3/10pF and the mixer tube grid gets rid of all the hiss and some hum.

I have got the two channels both feeding the Reverb by means of two 220k mixer resistors after the corresponding second stage anode caps.

I redid all the grounds to Fender specs and this helped with some ground loop around the Volume pots that had hum on 0 and slightly less hum on 3-5 and then hum again.

The hiss really annoys me and I am at a loss where to look at.

Checked the tubs and stuff, it seems to be the 3M3/10pF.

I have not yet looked at the PS caps, since these were rather fresh Atoms.

Any ideas more than welcome.

Oh, maybe I need to test a Fender RI amp at a music store and see if the amount of hiss I get is just normal? I seem not to remember this amount of hiss with my SF Princeton Reverb that I foolishly sold many years ago.

Thanks,

Electron
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by JazzGuitarGimp »

If it were my build, I would use three 10M, 1%, 1W Metal FIlm resistors in parallel (Mouser p/n CMF7010M000FKBF). This should give you the best hiss reduction without completely changing the circuit topology.

For hum reduction, I would use a shielded cable from the output end of the 3M3 / 10pF combo to its destination input. Ground the shield at one end only, and ground it at the preamp ground lug.

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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Firestorm »

ElectronAvalanche wrote: ... grounding between 3M3/10pF and the mixer tube grid gets rid of all the hiss and some hum.
...and all of the signal. No wonder it's quieter.

The composition or configuration of that 3M3 should make zero difference noise-wise; it doesn't see any DC current so the only noise it contributes is thermal which is a function of resistance, not composition or wattage. Aiken covers it.
I'd look at your first and second stages and try to limit the noise there: CF or even MF will likely be quieter. Tube choice matters, too. One counter-intuitive thing to try is swapping the dropping resistor in the power supply (4K7 on the Fender schematic). Seems like the filter cap should swallow it, but if the dropping resistor is noisy, it somehow makes it into the signal.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

Some odd things that IMHO just point to the 3M3/10pF:

1. Pulling both V1 and/or V2 does not reduce the hiss.
2. grounding 3M3/10pF at point (A) does not completely reduce the hiss, grounding at point B gets rid of all the noise
3. all anode resistors in the amp are Dale R65N Metalfilm, so these should not be the source of the hiss

I would think if the noise/hiss comes from the first stages, grounding the 3M3/10pF at point A should also get rid of all the hiss noise.

See attached schematic for pointers which I modded to also show how I changed the circuit to have Reverb and Trem on both channels.

My layout does differ from the Fender Layout btw, this might have an influence on the hiss. My 3M3/10pF combo is close to the 0.022uF decoupling cap of the Vibrato Channel. But pulling the tube of the vibrato channel does not influence the hiss, so I guess the hiss is not induced by the 0.022.

I will look at the PS dropping resistors. I have a cement and a CF there.

Thanks again for your help, this Fender amp is driving me nuts (as opposed to the Dumble clones that are not noisy at all).

Electron



Firestorm wrote:
ElectronAvalanche wrote: ... grounding between 3M3/10pF and the mixer tube grid gets rid of all the hiss and some hum.
...and all of the signal. No wonder it's quieter.

The composition or configuration of that 3M3 should make zero difference noise-wise; it doesn't see any DC current so the only noise it contributes is thermal which is a function of resistance, not composition or wattage. Aiken covers it.
I'd look at your first and second stages and try to limit the noise there: CF or even MF will likely be quieter. Tube choice matters, too. One counter-intuitive thing to try is swapping the dropping resistor in the power supply (4K7 on the Fender schematic). Seems like the filter cap should swallow it, but if the dropping resistor is noisy, it somehow makes it into the signal.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Milkmansound »

I have done a lot of experimenting with this and here is what I came up with:

i use carbom metal film for the 3M3 and the 470k at that junction (everything else is CC) I use a vintage mica domino for the 10p

I run the lead to the tube diagonally across the top of everything to make it as short as possible (which sucks because it blows the aesthetic a little)

I take metal tape and cover the underside of the top of the cabinet with it ensuring that there is a good bond where the chassis and tape meet

that should drop the noise down quite a bit. Getting the grounding right on the reverb circuit is also essential - but with the reverb off, and the amp at idle you should have minimal hiss without adding grid stoppers and shielding or anything like that.

I used to use Carbon Comp for that spot, but I got tired of putting in 10 different resistors before I found one quiet enough. Carbon metal film sounds pretty good and so far every single one has been silent, saving me time and frustration.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by Firestorm »

ElectronAvalanche wrote:Some odd things that IMHO just point to the 3M3/10pF:

1. Pulling both V1 and/or V2 does not reduce the hiss.
2. grounding 3M3/10pF at point (A) does not completely reduce the hiss, grounding at point B gets rid of all the noise
3. all anode resistors in the amp are Dale R65N Metalfilm, so these should not be the source of the hiss

I would think if the noise/hiss comes from the first stages, grounding the 3M3/10pF at point A should also get rid of all the hiss noise.

See attached schematic for pointers which I modded to also show how I changed the circuit to have Reverb and Trem on both channels.

My layout does differ from the Fender Layout btw, this might have an influence on the hiss. My 3M3/10pF combo is close to the 0.022uF decoupling cap of the Vibrato Channel. But pulling the tube of the vibrato channel does not influence the hiss, so I guess the hiss is not induced by the 0.022.

I will look at the PS dropping resistors. I have a cement and a CF there.

Thanks again for your help, this Fender amp is driving me nuts (as opposed to the Dumble clones that are not noisy at all).

Electron



Firestorm wrote:
ElectronAvalanche wrote: ... grounding between 3M3/10pF and the mixer tube grid gets rid of all the hiss and some hum.
...and all of the signal. No wonder it's quieter.

The composition or configuration of that 3M3 should make zero difference noise-wise; it doesn't see any DC current so the only noise it contributes is thermal which is a function of resistance, not composition or wattage. Aiken covers it.
I'd look at your first and second stages and try to limit the noise there: CF or even MF will likely be quieter. Tube choice matters, too. One counter-intuitive thing to try is swapping the dropping resistor in the power supply (4K7 on the Fender schematic). Seems like the filter cap should swallow it, but if the dropping resistor is noisy, it somehow makes it into the signal.
I see what you're saying. If it is the 3M3, there is -- theoretically -- nothing you can do about it. The noise is all resistance dependant.

When you ground Point B, you also take the reverb pot, the 470K and the 220K out of the picture. Maybe something there?

On your schematic with the added channel mix resistors: shouldn't the bottom 220K go on the other side of Point A?

Good luck.
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Re: 3M3 and 10pF pass filter in Fender Reverb amps

Post by ElectronAvalanche »

Milkmansound wrote:I have done a lot of experimenting with this and here is what I came up with:

i use carbom metal film for the 3M3 and the 470k at that junction (everything else is CC) I use a vintage mica domino for the 10p

I run the lead to the tube diagonally across the top of everything to make it as short as possible (which sucks because it blows the aesthetic a little)

I take metal tape and cover the underside of the top of the cabinet with it ensuring that there is a good bond where the chassis and tape meet

that should drop the noise down quite a bit. Getting the grounding right on the reverb circuit is also essential - but with the reverb off, and the amp at idle you should have minimal hiss without adding grid stoppers and shielding or anything like that.

I used to use Carbon Comp for that spot, but I got tired of putting in 10 different resistors before I found one quiet enough. Carbon metal film sounds pretty good and so far every single one has been silent, saving me time and frustration.
Hi Milkmansound,

thanks for your reply. I put a shield on the underside of the cabinet top which helps alot for keeping out hum and most important cell phone noises.

With the amp at idle I do have more hiss than I want. As said, grounding at point B removes all the hiss, so it is coming from a point before B. With the reverb pan output grounded, there is no change in hiss but any hum picked up by the reverb pan is removed, so I figure the grounding of the reverb section is ok.

I have a shielded cable running from B to the grid of the mixer tube. Why Fender did not put a grid resistor there escapes me. I think I have tried that at one point, but I remember that it sounded different even with a 10k resistor there, so I did not follow that route.

@Firestorm:
Thanks for your reply as well and for pointing out the mistake with the 220k from the normal channel in the schemo. Indeed this one should go to the 500pF input. This is how it is in my amp, I will try to redo the schemo.

So to sum it up:

* The hiss is generated in or before the 3M3/10pF.

* I might want to replace the caps in the power supply and maybe the 4k7 of the ps string and see if that will help. Which is a bit contradictory to the fact that with V1 and V2 pulled I still get the hiss, and with the circuti grounded at B the hiss is gone. The mixer stage also is connected to the same PS cap as V1 and V2 so I guess that the stage should hiss then with B being grounded if it is the PS cap.

* Turning up the Master Volume on my Dumble clones and turning down the Preamp volume leads to a comparable hiss, so it might just be normal given that Fender amps do have the input to the PI fully open, thus all the noises are amplified all the time.

*I need to try out a real Fender amp, maybe an old SF / BF one to see if the amount of hiss is just normal behaviour. My old SF Princeton Reverb must have had some hiss, but I sold it many moons ago and my memory might not serve me right.

Thanks again for all the help. I highly appreciate any more ideas!

Electron


Attached the corrected schematic (Thanks Firestorm!)

EDIT: come to think about it:

Both channels have a 220k after the decoupling cap going to the 500pF. From thisjunction the signal also goes to the input of the 3M3/10pF, so as opposed to the Fender schematic, there is now a 220k in front of the 3M3/10pF.

Maybe this is the reason for the increased hiss?

I have seen this type of arrangement for having Reverb on both channels on a Fender, but clearly this deviates from the original circuit (I also attached this schemo that I found on the web).

Maybe someone with a bit more knowledge about what a 220k in front of the 3M3/10pF concerning freq response can enlighten me, what this does?

Electron
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