changing filter caps on a marshall

singlecoil · September 8th, 2003, 05:47 PM

i read somewhere that changing the values on your filter caps can alter the tone. I have two 50uf/50uf 500 V.D.C. filter cans on my jcm800 combo and that changing one of those to a 100uf/100uf 500V.D.C. will give me more bass. I need to have them replaced anyway was wondering if this is true.

Mr. Droopy-Drawers · September 8th, 2003, 09:56 PM

AEBE (All Else Being Equal), you might hear a tighter bass response. But that JCM800 is pretty thin to begin with, so the effect is negated via the design. A good tech can 'un-neuter' that amplifier, and then you should hear a tighter bass response at high volumes (not really more bass, but it won't mush out on you). If you are 100% sure you need new filter caps, try it. You only have time and a few extra $$$ to lose. Keep in mind as you increase the filter capacity, the stress on the diodes at 'turn on' increases, as does the ESR of the filter. A suggestion; get rid of the 'Brite' caps all over the place, and think about using FRED's. That should be cheaper, and more dramatic of an upgrade. Remember, this is all opinion, and I may be full of crap, so proceed with caution

. There's gonna be people here with more theory chops than I have anxious to tell you about the problems of going nuts on the filtering, so let's listen in.............

singlecoil · September 9th, 2003, 10:37 AM

Mr. Droopy-Drawers, thanks for the reply. I don't have a clue what FRED's is or are though LOL. I forgot to mention the amp is a 1983 chanel switcher ( 4210) but i'm sure it has bright caps everywhere also. Wally usually does my tech work, but i'm not sure if he does the marshall pcb thing. Guess i better call him. BTW this thing sounds really good with a tele, i just want to tone it down a little bit.

Mr. Droopy-Drawers · September 9th, 2003, 05:29 PM

It gets technical, even for me (although opening a pickel jar is technical for me). FRED's are the Fast Recovery Epitaxial Diodes. The modern guru says your typical tube amplifier power supply should have the lowest possible impedance to sound the best that it possibly can. In the equation is the ESR of the filter capacitor, and using FRED's. A lot of guys talk about measuring the ESR of the capacitor with an ESR meter, but to my mind using a meter that utilizes a 100kHz test tone doesn't make sense. I still haven't found out if ESR is frequency or voltage dependant (I think it may be, but you know what thought did

). I also like the mush of an old Fender, so FRED's don't thrill me like they might the modern player. I'm sure if I was playing a 7-string guitar, and lots of 64th notes, I'd love to have an amplifier with low ESR capacitors and FRED's. But then I'd also be using a SS amplifier with torroidal transformers, if you know what I mean.
It may be cheap to experiment, so I say try anything twice (just in case you did it wrong the first time). Then decide if it's for you. If you want a tighter bass response with a faster attack, by all means try it.

Dacious · September 9th, 2003, 07:29 PM

If your Marshall has a full-bridge rectifier like all the Plexis-on, diodes arranged in a square or triangle with the AC from the power tranny, an earth and the B+ to the filter caps coming off it, (I'm sure it does) it needs half the filtering of a half-bridge rectifier.

This is just a pair of diodes - one on each lead converging at the B+, with a ct on the tranny going to ground. Older and lower power equipment uses this.

The more filter cap you have, the 'spongier' the response is likely to be, as the caps need to charge/discharge. In general, for quicker and more accurate power supply and hence playing response, the smaller the caps the better - the flip side is, you need sufficient filtering to knock out the ripple or pulsing of the rectified AC - it is not a continuous stream, but a series of power spikes. That's what caps do, or the pulsing would be apparent as a hum or thrum in your amp. The caps charge in the peaks and discharge in the valleys to maintain a more constant voltage downstream.

Cap values is not an arbitrary thing - just doubling the value of the filter caps can have other, unintended consequences. Values are selected in concert with the rectifier type, voltage, current and other considerations like the choke filter. What you're suggesting will not increase bass, but may change bass response, in that it may be 'flabbier' and more distorted. It will put more draw on the power transformer.

The hardest frequencies, those requiring most power for an amp to reproduce, are bass frequencies - one reason why bass guitarists use humungous 200-400 watt amps when we get away with 30 or 50. So straight, hi-fi, undistorted, clean bass response is best served by an efficient power supply with no sag.

So Mr D.D. is right, in that using F.R.E.D.s in an amp will increase power supply efficiency and hence enhance bass response from a hi-fi point of view. But if you are really trying to get bluesy sag like a JTM Bluesbreaker, Super Reverb, or a Tweed Bassman, you might get close, but probably only by losing something else.

You can tweak cap values in the preamp to get more bass response, but in doing so you might alter the overall sound of the amp in negative way. That is how Marshall differentiated the Super Lead from Super Bass heads before master-volume days.

I think Tim Swartz knows more about modding Marshalls, maybe he might chime in.

September 8th, 2003, 05:47 PM	#1 (permalink)
singlecoil Tele-Meister Join Date: Mar 2003 Location: SE New Mexico Posts: 206	changing filter caps on a marshall i read somewhere that changing the values on your filter caps can alter the tone. I have two 50uf/50uf 500 V.D.C. filter cans on my jcm800 combo and that changing one of those to a 100uf/100uf 500V.D.C. will give me more bass. I need to have them replaced anyway was wondering if this is true.

September 8th, 2003, 09:56 PM	#2 (permalink)
Mr. Droopy-Drawers Tele-Meister Join Date: Jul 2003 Location: Canada, eh? Posts: 223	AEBE (All Else Being Equal), you might hear a tighter bass response. But that JCM800 is pretty thin to begin with, so the effect is negated via the design. A good tech can 'un-neuter' that amplifier, and then you should hear a tighter bass response at high volumes (not really more bass, but it won't mush out on you). If you are 100% sure you need new filter caps, try it. You only have time and a few extra $$$ to lose. Keep in mind as you increase the filter capacity, the stress on the diodes at 'turn on' increases, as does the ESR of the filter. A suggestion; get rid of the 'Brite' caps all over the place, and think about using FRED's. That should be cheaper, and more dramatic of an upgrade. Remember, this is all opinion, and I may be full of crap, so proceed with caution . There's gonna be people here with more theory chops than I have anxious to tell you about the problems of going nuts on the filtering, so let's listen in............. __________________ "You can lead a horticulture, but you can't make her think!"

September 9th, 2003, 05:29 PM	#4 (permalink)
Mr. Droopy-Drawers Tele-Meister Join Date: Jul 2003 Location: Canada, eh? Posts: 223	It gets technical, even for me (although opening a pickel jar is technical for me). FRED's are the Fast Recovery Epitaxial Diodes. The modern guru says your typical tube amplifier power supply should have the lowest possible impedance to sound the best that it possibly can. In the equation is the ESR of the filter capacitor, and using FRED's. A lot of guys talk about measuring the ESR of the capacitor with an ESR meter, but to my mind using a meter that utilizes a 100kHz test tone doesn't make sense. I still haven't found out if ESR is frequency or voltage dependant (I think it may be, but you know what thought did ). I also like the mush of an old Fender, so FRED's don't thrill me like they might the modern player. I'm sure if I was playing a 7-string guitar, and lots of 64th notes, I'd love to have an amplifier with low ESR capacitors and FRED's. But then I'd also be using a SS amplifier with torroidal transformers, if you know what I mean. It may be cheap to experiment, so I say try anything twice (just in case you did it wrong the first time). Then decide if it's for you. If you want a tighter bass response with a faster attack, by all means try it. __________________ "You can lead a horticulture, but you can't make her think!"

September 9th, 2003, 07:29 PM	#5 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,612 35 Photos	If your Marshall has a full-bridge rectifier like all the Plexis-on, diodes arranged in a square or triangle with the AC from the power tranny, an earth and the B+ to the filter caps coming off it, (I'm sure it does) it needs half the filtering of a half-bridge rectifier. This is just a pair of diodes - one on each lead converging at the B+, with a ct on the tranny going to ground. Older and lower power equipment uses this. The more filter cap you have, the 'spongier' the response is likely to be, as the caps need to charge/discharge. In general, for quicker and more accurate power supply and hence playing response, the smaller the caps the better - the flip side is, you need sufficient filtering to knock out the ripple or pulsing of the rectified AC - it is not a continuous stream, but a series of power spikes. That's what caps do, or the pulsing would be apparent as a hum or thrum in your amp. The caps charge in the peaks and discharge in the valleys to maintain a more constant voltage downstream. Cap values is not an arbitrary thing - just doubling the value of the filter caps can have other, unintended consequences. Values are selected in concert with the rectifier type, voltage, current and other considerations like the choke filter. What you're suggesting will not increase bass, but may change bass response, in that it may be 'flabbier' and more distorted. It will put more draw on the power transformer. The hardest frequencies, those requiring most power for an amp to reproduce, are bass frequencies - one reason why bass guitarists use humungous 200-400 watt amps when we get away with 30 or 50. So straight, hi-fi, undistorted, clean bass response is best served by an efficient power supply with no sag. So Mr D.D. is right, in that using F.R.E.D.s in an amp will increase power supply efficiency and hence enhance bass response from a hi-fi point of view. But if you are really trying to get bluesy sag like a JTM Bluesbreaker, Super Reverb, or a Tweed Bassman, you might get close, but probably only by losing something else. You can tweak cap values in the preamp to get more bass response, but in doing so you might alter the overall sound of the amp in negative way. That is how Marshall differentiated the Super Lead from Super Bass heads before master-volume days. I think Tim Swartz knows more about modding Marshalls, maybe he might chime in. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

September 9th, 2003, 10:37 AM	#3 (permalink)
singlecoil Tele-Meister Join Date: Mar 2003 Location: SE New Mexico Posts: 206	Mr. Droopy-Drawers, thanks for the reply. I don't have a clue what FRED's is or are though LOL. I forgot to mention the amp is a 1983 chanel switcher ( 4210) but i'm sure it has bright caps everywhere also. Wally usually does my tech work, but i'm not sure if he does the marshall pcb thing. Guess i better call him. BTW this thing sounds really good with a tele, i just want to tone it down a little bit.