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Unread 10-17-2009, 05:03 AM   #1 (permalink)
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Filter Caps Explained

Since the subject of filter caps came up, I thought I'd share an article I wrote for the MetroAmp Wiki page so here it is.

Filter Caps Explained
By Jon Wilder of Wilder Amplification

Filter capacitors (aka filter caps) are to electrons like buckets are to water.

Just as buckets store water, filter caps store electrons.

The larger the bucket -

* The greater the volume of water it takes to fill it.

* The longer it takes for a pump of a given flow rate to drain it.

The larger a filter cap's value -

* The more stored electrons it takes to fully charge the cap.

* The longer it takes for a circuit of a given amount of resistance to discharge it.

Like all other capacitors, filter cap values are measured in microfarads (1uf, or 1 microfarad is equal to 0.000001 Farad) and unlike other caps, they are only measured in microfarads. This is because any value in the nano or picofarad range is simply too small a value to store enough electrons for the cap to perform efficiently as a filter.

The larger the value of the cap -

* The more electrons it can store

* The more electrons it will take to charge it up to the supply voltage.

* The longer it will take to discharge

* The more current it can supply over a given time constant before fully discharging

Notes on filter capacitor configuration

Capacitors work backwards from resistors in that -

* Cap values are additive when hooked up in parallel (C1 + C2 + C3...etc etc = Total Capacitance)

* Cap values are halved when two equal values are placed in series (2 100uF caps in series gives a total 50uF capacitance)

* "Dual" filter caps of equal value placed in parallel with themselves, then in series with each other, gives the value of one of the dual caps. Example, connecting a 50uF + 50uF cap in parallel with itself, then placing it in series with another 50uF + 50uF cap connected in the same fashion (parallel to itself) will give 50uF total capacitance

* 2 or more filter caps in parallel will have the same voltage rating as the cap with the lowest voltage rating

* Voltage handling is additive with filter caps in series (i.e. 2 filter caps in series that have a 500WVDC rating will increase voltage handling to 1000WVDC)

* "Bleeder resistors" are typically two equal value resistors arranged in a "voltage divider" configuration (see Series Circuits) across each cap for voltage balancing (i.e. to make sure that each cap sees the same voltage and one is not going over voltage). They also serve as a quick means to "bleed" the charge from the filter caps upon power down, hence the name "bleeder" resistors.

What Is Happening

In a filter circuit, filter caps act as "temporary batteries". When the DC pulse is rising in voltage, the cap is charging. The cap reaches full charge when the pulse hits its peak.

Imagine your house is hooked up to the power company with a backup battery. As long as the power from the power company remains on, it powers the house while also keeping the backup batteries charged. Once you experience a power outage, the household electrical now runs on the battery backup. While the house is running on backup battery power, the batteries are discharging at a rate dependent upon how much electrons they have stored versus the amount of electron current being drawn by electrical devices in the house (i.e. supply & demand). As long as the household current draw does not exceed the amount of electrons the batteries have stored, the batteries will continue to supply power to the house until the power from the power company is restored. Once power company power is restored, the house will now run on power company power again and the power from the power company will recharge the backup batteries until you experience another power outage. This is exactly what is happening in the filter circuit. The only difference is that due to the pulsed DC from the rectifier, on a tube amp that runs on 60Hz line frequency, a full wave rectified power supply experiences about 120 power outages per second (120Hz = ripple frequency of a full wave rectifier).



The illustration above shows the pulsed DC output of the rectifier circuit. You can see that there is a gap between the pulse peaks. These gaps represent the "power outages" that frequently happen so many times a second in a tube amplfiier. When the pulsed DC output of the rectifier circuit is at its peak, it charges the filter caps up. Once the pulse starts to fall from its peak and start its way back down to 0 volts, the circuit will draw current from these "temporary batteries" known as filter caps. This picks the voltage in the power supply back up to the peak value of the pulsed DC. However, as the circuit draws current from the filter caps, the filter caps discharge, which causes the supply voltage to drop at a rate determined by the filter cap value and the amount of current being drawn from it (again, supply and demand). The circuit continues to draw current from the filter caps until the DC pulse rises to a value above the remaining charge in the filter cap. The pulse then recharges the cap while the circuit now draws current from the main supply source, until the pulse reaches peak value and falls again, thus the cycle starts all over again.

Below is a diagram that illustrates what pulsed DC looks like once it's been filtered by 1 stage of filtering.



The dashed lines in the illustration represent the pulsed DC itself that has been filtered out by the filter circuit. As you can see, when the voltage from the rectifier wants to fall, the filter caps act like a buffer of sorts and take over as the primary power supply while the pulsed DC output is falling, then rising back up. The fall and rise of the rectified DC is still there...however, the amplifier circuitry never sees it because the capacitors keep the voltage up by supplying current to the amplifier during the fall and rise times, hence the "filtering" effect. As the circuit pulls electrons from the filter caps, the filter caps slowly discharge and drop voltage as they do so, indicated by the slope angle of the line in between the pulsed DC peaks.

The slope angle of this line is indicative of and determined by the discharge rate of the filter caps. The faster the discharge rate of the filter caps, the steeper the angle of this slope, and vice versa.

As long as there is an angle in that slope line, we're still not where we wanna be, but most of the filtering has been done and we are now much closer to a steady DC supply voltage. So what do we do? Add more capacitance!

By adding more capacitance to the circuit, the filter circuit will be able to store more electrons, which gives them more electrons to supply for a longer period of time without dropping voltage. Once this happens, the output of the power supply looks more like this.



The line representing voltage now stays constant between the peaks because of the higher capacity of the filter circuit...it takes longer to deplete the supply of electrons because there's simply more electrons available in the filter circuit to supply to the amplifier circuit.

We can add capacitance in 1 of two ways -

* Increase the value of the filter caps at the first filter stage or...

* Add more filter caps across the first filter stage

Most tube amplifier filter circuits use the latter approach. This allows them to use resistors between each stage that act as a multi-stage voltage divider to obtain different operating voltages for the preamp, phase inverter, and output stages. The resistors between each filter stage also provides isolation between the preamp, driver/phase inverter, and output stages so that each stage has its own "temporary battries" to pull current from and current is available on demand at each stage within the amp without one stage trying to steal current from another (i.e. "intermodulation distortion). Because there is a resistor that the current must flow through to get to the preceding amplifier stage, the current sees the stage hooked up to its capacitor source as the "easiest path" and instead flows through the stage that's connected to the filter cap itself instead of being "robbed" by the preceding stage.

Tonal Effects of Filtering

The amount of capacitance that a filter circuit has can have a great effect on tone as well as the dynamic response of the amplifier. Typically you won't notice it at low volume so much as you will at high volume, depending on how much capacitance the filter circuit has.

What happens is that when the amplifier is driven hard, the plate resistance of the tubes is at its lowest, allowing the amp circuit to draw the maximum current that it can from the power supply. If the current demands of the amplifier exceed the amount of current the filtering circuit can supply (there it is again, supply and demand), you will experience a voltage drop at the supply, which some refer to as voltage "sag". This will have a great effect on how loose or tight the amp sounds. More sag = looser sounding/feeling amp whereas less sag = tighter sounding/feeling amp.

Now this only comes into play if you have an amplifier that originally had a power supply that couldn't keep up with the amplifier's current draw demand and you bump the filtering up in the amplifier so that it can. If the filter circuit in your amplifier is already of high capacitance and can keep up with the current draw demand of the amplififer as it stands now, bumping the filtering up at this point won't make much of a difference, if any at all. You will eventually reach a point of diminishing returns.

Summary

The filter circuit is nothing more than a temporary power supply comprised of "temporary backup batteries" that supply the amplifier with power while the DC pulses from the rectifier are falling and rising. By adding more capacitance, our filter circuit can store more electrons and better supply the current demands of the powered circuit. The better the filter circuit can supply the current demands of the powered circuit, the less the filter circuit will discharge between the pulsed DC peaks and the straighter that slope will be. The trick is to get that slope between the peaks as straight as possible. The straighter that line between the peaks, the more steady the DC voltage at the power supply. It's all about supply and demand...the current supply of the filter circuit must be equal to or greater than the demand of the amplifier circuitry. The better it can do this, the more steady the supply voltage will be.
Attached Images
File Type: jpg PulsedDC.jpg (23.7 KB, 355 views)
File Type: jpg FilteredPulsedDC-1.jpg (13.4 KB, 356 views)
File Type: jpg FullFilteredPulsedDC.jpg (13.2 KB, 356 views)
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Last edited by Wilder Amplification; 10-17-2009 at 06:04 AM.
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Unread 10-17-2009, 01:54 PM   #2 (permalink)
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Re: Filter Caps Explained

Excellent info once again, Jon. Please keep it coming!
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Unread 10-17-2009, 03:09 PM   #3 (permalink)
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Re: Filter Caps Explained

Stuff like this is why I dig this forum.
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Unread 10-18-2009, 03:49 PM   #4 (permalink)
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Re: Filter Caps Explained

Excellent info. Thanks for writing it.

Question. How often should these things be replaced? My amp is 20 years old and the caps have never been replaced.
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Unread 10-18-2009, 03:55 PM   #5 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by wegman View Post
Excellent info. Thanks for writing it.

Question. How often should these things be replaced? My amp is 20 years old and the caps have never been replaced.
On a 20 year old amp I would definitely replace them. The dielectric in filter caps does dry out over time. I've seen some Fender amps from the mid 60s that actually have shit leaking out of them...just like batteries when they sit overtime.

Also, cap values drift as they age over time. If you like the way the amp sounds/feels as it stands right now, I have a capacitance meter that will allow me to measure the value of your current caps, then see if I can find filter caps that are close to the same value so that you don't lose the tone/feel of the amp with its current caps. If you replace them with caps that are the same value as the "marked" value, the amp will go back to how it performed when it was brand new and you may or may not like that.

If you decide to go this route, you would have to mark the caps as to where they were in the amp as you remove them so that I can mark where each replacement cap will go in the amp.

Thanks for the kind words! I enjoy writing tech articles for those who express a genuine interest to learn. I know how it is when you want to learn, so you dive into a book and within the first few sentences you've lost interest because of the way the material was worded...or you read someone's simplified tech stuff, and they didn't do it correctly or didn't do it in a way that correctly explains it, which is what motivates me to write my articles.
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Unread 10-18-2009, 04:02 PM   #6 (permalink)
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Re: Filter Caps Explained

[QUOTE=Thanks for the kind words! I enjoy writing tech articles for those who express a genuine interest to learn. I know how it is when you want to learn, so you dive into a book and within the first few sentences you've lost interest because of the way the material was worded...or you read someone's simplified tech stuff, and they didn't do it correctly or didn't do it in a way that correctly explains it, which is what motivates me to write my articles.[/QUOTE]

Oh my pleasure. It is really cool that there are those of you gracious enough to share your knowledge.

If you be so kind to answer one more question. What does a cap job cost roughly. I have a JCM 800 2210 (1989)

Thanks again.
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Unread 10-18-2009, 04:05 PM   #7 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by wegman View Post
Oh my pleasure. It is really cool that there are those of you gracious enough to share your knowledge.

If you be so kind to answer one more question. What does a cap job cost roughly. I have a JCM 800 2210 (1989)

Thanks again.
Does it have the filter cap cans that mount above the chassis (the can looking things around the transformers and tubes in case you don't know)? If so, how many?
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Unread 10-18-2009, 04:12 PM   #8 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by Wilder Amplification View Post
Does it have the filter cap cans that mount above the chassis (the can looking things around the transformers and tubes in case you don't know)? If so, how many?
I think it best if I provide a picture.


stuff 003.jpg
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Unread 10-18-2009, 04:30 PM   #9 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by wegman View Post
I think it best if I provide a picture.


Attachment 1409
OK those big blue cans sticking up that say LCR...those are your filter caps. Count how many of those your amp has. I believe there's one further over by the preamp tubes.

Also, since Marshall did shit differently in previous years, are there 4 behind the power valves or two?
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Unread 10-18-2009, 04:34 PM   #10 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by Wilder Amplification View Post
OK those big blue cans sticking up that say LCR...those are your filter caps. Count how many of those your amp has. I believe there's one further over by the preamp tubes.

Also, since Marshall did shit differently in previous years, are there 4 behind the power valves or two?
There are 3.
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Unread 10-18-2009, 04:38 PM   #11 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by wegman View Post
There are 3.
In that case you're lookin' at about $100. Those cans sell for about $12 a piece + $60 for labor. This is what I charge anyway.
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Unread 10-18-2009, 04:42 PM   #12 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by Wilder Amplification View Post
In that case you're lookin' at about $100. Those cans sell for about $12 a piece + $60 for labor.
Thank you for the information. I really dig reading your stuff. Please keep it coming.

John
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Unread 10-18-2009, 04:51 PM   #13 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by wegman View Post
Thank you for the information. I really dig reading your stuff. Please keep it coming.

John
You're welcome...and I plan to keep it coming. Stay tuned.
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Unread 10-20-2009, 08:03 PM   #14 (permalink)
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Re: Filter Caps Explained

great info john !!

i vote John for pres. !!!
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Unread 10-21-2009, 10:11 AM   #15 (permalink)
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Re: Filter Caps Explained

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great info john !!

i vote John for pres. !!!
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Unread 10-26-2009, 06:41 AM   #16 (permalink)
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Re: Filter Caps Explained

Hi thanx for great info.
I'm going to replace filtercaps in two of my marshalls (JTM 45 RI and 2061 lead&bass)
Where can I order some? (I live in Norway, Europe)

It seems like both amps use same type of filtercaps, 50µF/500VDC, or should I look for more info?
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Unread 10-26-2009, 07:20 AM   #17 (permalink)
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Re: Filter Caps Explained

Very good explanation and valuable information.

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Unread 10-26-2009, 10:22 AM   #18 (permalink)
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Re: Filter Caps Explained

Good reading.

To see those waves as per your diagram one would need an ocilloscope correct? Or is there a way to know our peaks are flat - the line is even using a multi-meter?
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Unread 10-26-2009, 10:41 AM   #19 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by Birk View Post
Hi thanx for great info.
I'm going to replace filtercaps in two of my marshalls (JTM 45 RI and 2061 lead&bass)
Where can I order some? (I live in Norway, Europe)

It seems like both amps use same type of filtercaps, 50µF/500VDC, or should I look for more info?

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Hvordan har du det?

You can try here for parts in Europe Tube Amp Doctor Verstärker Röhren Spezialist - Tubes and Amp Kits
They fairly reasonable on shipping and not bad in pricing.
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Unread 10-26-2009, 11:32 AM   #20 (permalink)
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Re: Filter Caps Explained

Thanx, MajorNut. I was a bit in a rush, so now I've ordered from Eurotube.
I'll check out TAD next time.
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Unread 10-26-2009, 12:42 PM   #21 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by Apostle Tone View Post
Good reading.

To see those waves as per your diagram one would need an ocilloscope correct? Or is there a way to know our peaks are flat - the line is even using a multi-meter?
Yes but you would have no way of hooking a scope up to a 500 volt supply. Way too high of a voltage.

With a multimeter on the rectifier output with no filter caps hooked up, you would still see the RMS value of the AC voltage. The reason you end up with the peak value of the AC when your filter caps are hooked up is because the filter caps charge up to the peak value of the incoming AC. The load on the power supply (i.e. the current flow demand of the circuit it's powering) will have an effect on this as the powered circuit draws current from the filter caps. The bigger the caps, the more current available to draw, hence the less the caps will discharge between the positive peaks.

People have already done the experimenting to figure out how much filtering is required to get the "ripple" out. It's usually about 95% gone by the 2nd stage of filtering, and most valve amps have a minimum of 4 filtering stages (Marshall has 5). If there were any ripple, you'd hear a 120 Hz hum coming from the speakers. There is a bit of ripple after the first stage, which powers the output valves, but since the output valves are working out of phase with each other, the ripple gets canceled out and thus does not get injected into the audio signal.

But the one thing you CAN experiment with is that the filtering affects how "loose" or "tight" it sounds/feels. The bigger the filter cap values, the tighter it will sound. This only applies when running the head flat out cranked though since that's when the amp is putting full load on the power supply. The bigger the filter caps, the less power supply loading the amp circuitry provides, and the tighter the sound/response, and vice versa.

Of course, if you have an amp that already has a high amount of filtering, stepping the filtering up won't have much of an effect. But you could always reduce the filter cap values to loosen up a tight amp, whereas you would use higher value filter caps to tighten up a loose amp.
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Last edited by Wilder Amplification; 10-26-2009 at 03:14 PM.
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Unread 10-30-2009, 02:14 AM   #22 (permalink)
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Re: Filter Caps Explained

Great reading that went right over my head after the buckets analogy.

I would suggest someone interested in a filter cap job wait for a re-tube or other repair so the labor charges can be piggybacked.
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Unread 10-31-2009, 06:19 PM   #23 (permalink)
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Re: Filter Caps Explained

I'm making a cap discharging tool, and I'm a little confused about the resistor to use. I've read that some guys use a 100k 5w, and others use a 15k 5w, and still others use a 10k 5w. In addition to that, some guys use a "cement" resistor.

Considering Ohm's law (I was an auto technician for 15+ years in a shop) the higher the resistance, the longer the caps will take to drain because higher resistance resistors limit current flow moreso than lower resistance.

So, what is the right one to use?

Also, does it matter if it is a "power resistor" or an "ohmite" resistor, or a "metal oxide" resistor? And I know they also have "cermet" ones which are a cross between ceramic and cement.

Will it make any difference?

If one of the amp "gurus" could shed light on this for me, I'd appreciate it.
(BTW, I'm using it on my MKII Super Lead 2203 JMP head, model year: 1980)


Thanks.
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Unread 11-01-2009, 11:52 AM   #24 (permalink)
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Re: Filter Caps Explained

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Originally Posted by kernal View Post
I'm making a cap discharging tool, and I'm a little confused about the resistor to use. I've read that some guys use a 100k 5w, and others use a 15k 5w, and still others use a 10k 5w. In addition to that, some guys use a "cement" resistor.

Considering Ohm's law (I was an auto technician for 15+ years in a shop) the higher the resistance, the longer the caps will take to drain because higher resistance resistors limit current flow moreso than lower resistance.

So, what is the right one to use?

Also, does it matter if it is a "power resistor" or an "ohmite" resistor, or a "metal oxide" resistor? And I know they also have "cermet" ones which are a cross between ceramic and cement.

Will it make any difference?

If one of the amp "gurus" could shed light on this for me, I'd appreciate it.
(BTW, I'm using it on my MKII Super Lead 2203 JMP head, model year: 1980)


Thanks.
The only special tool needed is a wire with alligator clips on each end. Clip 1 end to the chassis while clipping the other end to pin 1 of the first preamp valve. This turns all the dropping resistors in the amp's power supply and the plate resistor on the first preamp valve into a big "bleeder resistor" and discharges the caps within a couple of minutes. The resistance value of all the dropping resistors and the plate resistor is high enough to limit the discharge current to within the rating limits of the resistors.

Leave the jumper in place while you're working on the amp so that the caps don't try to "remember" their charge and build the charge back up. This is how the majority of us amp techs have done it for a long time and works flawlessly.
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Unread 11-01-2009, 05:09 PM   #25 (permalink)
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Re: Filter Caps Explained

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Originally Posted by Wilder Amplification View Post
The only special tool needed is a wire with alligator clips on each end. Clip 1 end to the chassis while clipping the other end to pin 1 of the first preamp valve. This turns all the dropping resistors in the amp's power supply and the plate resistor on the first preamp valve into a big "bleeder resistor" and discharges the caps within a couple of minutes. The resistance value of all the dropping resistors and the plate resistor is high enough to limit the discharge current to within the rating limits of the resistors.

Leave the jumper in place while you're working on the amp so that the caps don't try to "remember" their charge and build the charge back up. This is how the majority of us amp techs have done it for a long time and works flawlessly.
OK, thanks John. But for simplicity, I was thinking about just grounding one lead to the chassis (while holding the other end w/ insulated stork needle-nose pliers), and touch each of the + cap leads, and then verify with a DVOM the same way (chassis ground to cap +).

But, RE: the resistor in the discharge lead

Will this work for my amp, a MKII 2203, model year 1980? (see attached thumbnail)
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Unread 11-01-2009, 06:15 PM   #26 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by kernal View Post
OK, thanks John. But for simplicity, I was thinking about just grounding one lead to the chassis (while holding the other end w/ insulated stork needle-nose pliers), and touch each of the + cap leads, and then verify with a DVOM the same way (chassis ground to cap +).

But, RE: the resistor in the discharge lead

Will this work for my amp, a MKII 2203, model year 1980? (see attached thumbnail)
You're overthinking it. Grounding pin 1 of the first preamp tube to the chassis with the alligator clip jumper like I mentioned is as simple as it gets. Once you clip the jumper in place, you can verify that the caps are discharged by measuring voltage at the HT fuse socket. All of the filter caps are connected together through the dropping resistors in the amp so clipping the jumper between pin 1 of the first preamp valve will discharge all of the caps through the dropping resistors in the power supply and the jumper to ground from pin 1.

As I mentioned in my previous post, this is how us amp techs discharge filter caps so if it didn't work, why would we do it, let alone recommend it to others? It works on every valve amp and is the absolute simplest method.

Not to sound rude or anything...I say this solely for the sake of personal safety, but if you're having trouble visualizing why this method works, looking at a schematic of a 2203 (or any valve amp for that matter) should spell out for you why it works. If you can't figure out why it works by simply looking at a schematic, then you probably shouldn't be poking around inside of an amp containing voltages higher than your wall voltage.
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Last edited by Wilder Amplification; 11-01-2009 at 08:34 PM.
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Unread 11-01-2009, 09:16 PM   #27 (permalink)
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Re: Filter Caps Explained

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Originally Posted by Wilder Amplification View Post
You're overthinking it. Grounding pin 1 of the first preamp tube to the chassis with the alligator clip jumper like I mentioned is as simple as it gets. Once you clip the jumper in place, you can verify that the caps are discharged by measuring voltage at the HT fuse socket. All of the filter caps are connected together through the dropping resistors in the amp so clipping the jumper between pin 1 of the first preamp valve will discharge all of the caps through the dropping resistors in the power supply and the jumper to ground from pin 1.

As I mentioned in my previous post, this is how us amp techs discharge filter caps so if it didn't work, why would we do it, let alone recommend it to others? It works on every valve amp and is the absolute simplest method.

Not to sound rude or anything...I say this solely for the sake of personal safety, but if you're having trouble visualizing why this method works and you can't look at the amp schematic for a 2203 and see why it works, then you probably shouldn't be poking around inside of an amp containing voltages higher than your wall voltage.
I really want to learn. I have had this amp for almost 30 years, and since I do have some electrical/electronic skills I think I can handle it. I depended on somebody for a long time and the guy is no longer around, and haven't found anybody worth taking my amp to that is nearby to me (and won't try to rip me off).

Sorry, I misread your post, the part about all you need to do is "connect jumper with alligator clips...to pin 1 of V1." I didn't read your post closely enough. I get it now, after re-reading it. No resistor needed, because the existing resistors in wired in the circuit will do the job.

I wish I had a better schematic. The link was dead that you provided in another post for my amp. I got an error when I clicked on it. I didn't want to bug you for another link (as I'm probably already being a "PIA") Sorry.

So I went to "scematicheaven" and downloaded one. However, it is not very clear because some of the numbers are pretty fuzzy and I can't make them out (even when zooming in). All the other scematics for the 2203 that I have found are identical to the one I have...and not very clear. I know there is a "Marshall book" that has them in it, ebay perhaps?

Anyways, looking at the crappy schematic I have I can clearly see why draining pin 1 would discharge the 50 microfarad caps (4 of them) provided I have correctly ID'd pin 1 of V1.

Looking at the amp, pin of V1 is the one next to the key (the space with missing pin) that is connected to a plate (either section 1 or 2). Section 1 plate will be next to the key, and section 2 plate (pin 6) is not next to key.

Am I correct?

I can also see why checking for voltage at the HT fuse socket will indicate voltage on the + side of the caps.

Please forgive my "novice-ness," but what about the other four 100 MFD caps I see in the schematic? Two of them are wired into the V7 circuit, and the other two into the rectifier bridge?

Do I need to drain them as well? Or do they not hold a charge unless the amp is powered up?

See attachment for the schematic I'm referring to.

Thanks.
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Unread 11-01-2009, 09:36 PM   #28 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by kernal View Post
I really want to learn. I have had this amp for almost 30 years, and since I do have some electrical/electronic skills I think I can handle it. I depended on somebody for a long time and the guy is no longer around, and haven't found anybody worth taking my amp to that is nearby to me (and won't try to rip me off).

Sorry, I misread your post, the part about all you need to do is "connect jumper with alligator clips...to pin 1 of V1." I didn't read your post closely enough. I get it now, after re-reading it. No resistor needed, because the existing resistors in wired in the circuit will do the job.

I wish I had a better schematic. The link was dead that you provided in another post for my amp. I got an error when I clicked on it. I didn't want to bug you for another link (as I'm probably already being a "PIA") Sorry.

So I went to "scematicheaven" and downloaded one. However, it is not very clear because some of the numbers are pretty fuzzy and I can't make them out (even when zooming in). All the other scematics for the 2203 that I have found are identical to the one I have...and not very clear. I know there is a "Marshall book" that has them in it, ebay perhaps?

Anyways, looking at the crappy schematic I have I can clearly see why draining pin 1 would discharge the 50 microfarad caps (4 of them) provided I have correctly ID'd pin 1 of V1.

Looking at the amp, pin of V1 is the one next to the key (the space with missing pin) that is connected to a plate (either section 1 or 2). Section 1 plate will be next to the key, and section 2 plate (pin 6) is not next to key.

Am I correct?

I can also see why checking for voltage at the HT fuse socket will indicate voltage on the + side of the caps.

Please forgive my "novice-ness," but what about the other four 100 MFD caps I see in the schematic? Two of them are wired into the V7 circuit, and the other two into the rectifier bridge?

Do I need to drain them as well? Or do they not hold a charge unless the amp is powered up?

See attachment for the schematic I'm referring to.

Thanks.
The caps that are wired to the rectifier are part of the full wave bridge voltage doubler circuit, which is actually just a voltage balancer across the first stage filter caps so that each cap sees exactly 1/2 the power supply voltage.

The caps that you see that are "wired into the V7 circuit" as you put it are the screen grid filter caps.

The "pin 1 to chassis" jumper will discharge those caps as well. If you look a b
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Unread 11-01-2009, 09:40 PM   #29 (permalink)
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Re: Filter Caps Explained

Quote:
Originally Posted by kernal View Post
I really want to learn. I have had this amp for almost 30 years, and since I do have some electrical/electronic skills I think I can handle it. I depended on somebody for a long time and the guy is no longer around, and haven't found anybody worth taking my amp to that is nearby to me (and won't try to rip me off).

Sorry, I misread your post, the part about all you need to do is "connect jumper with alligator clips...to pin 1 of V1." I didn't read your post closely enough. I get it now, after re-reading it. No resistor needed, because the existing resistors in wired in the circuit will do the job.

I wish I had a better schematic. The link was dead that you provided in another post for my amp. I got an error when I clicked on it. I didn't want to bug you for another link (as I'm probably already being a "PIA") Sorry.

So I went to "scematicheaven" and downloaded one. However, it is not very clear because some of the numbers are pretty fuzzy and I can't make them out (even when zooming in). All the other scematics for the 2203 that I have found are identical to the one I have...and not very clear. I know there is a "Marshall book" that has them in it, ebay perhaps?

Anyways, looking at the crappy schematic I have I can clearly see why draining pin 1 would discharge the 50 microfarad caps (4 of them) provided I have correctly ID'd pin 1 of V1.

Looking at the amp, pin of V1 is the one next to the key (the space with missing pin) that is connected to a plate (either section 1 or 2). Section 1 plate will be next to the key, and section 2 plate (pin 6) is not next to key.

Am I correct?

I can also see why checking for voltage at the HT fuse socket will indicate voltage on the + side of the caps.

Please forgive my "novice-ness," but what about the other four 100 MFD caps I see in the schematic? Two of them are wired into the V7 circuit, and the other two into the rectifier bridge?

Do I need to drain them as well? Or do they not hold a charge unless the amp is powered up?

See attachment for the schematic I'm referring to.

Thanks.
Well since you took the time and made the effort to study the schematic to the best of your ability, I'll help you out a bit. Don't worry about your "noviceness". As long as you show a willingness to read and figure things out on your own I don't mind helping you out when ya get stumped(help me help you kinda thing).

The caps that are wired to the rectifier are part of the full wave bridge voltage doubler circuit, which is actually just a voltage balancer across the first stage filter caps so that each cap sees exactly 1/2 the power supply voltage.

The caps that you see that are "wired into the V7 circuit" as you put it are the screen grid filter caps.

The "pin 1 to chassis" jumper will discharge those caps as well. If you look closer at the schematic you'll see that everything from the doubler filters to the 50uF filter at V1 is all strung together with dropping resistors. From the rectifier to the grounding jumper, the discharge path goes -

Rectifier filter caps -->Choke-->Screen Filter Caps-->10K resistor-->10K resistor-->phase inverter filter cap-->10K resistor-->V2 filter cap-->10K resistor-->V1 filter cap-->V1's 100K plate resistor-->Ground

Also, you are correct on the pin orientation. Another help in identifying pin 1 is that it's the only pin next to the "missing pin" keyway that has a blue wire on it.
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Unread 11-01-2009, 09:57 PM   #30 (permalink)
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Re: Filter Caps Explained

Ahhhhh, got it! I don't look at schematics too often - hardly ever did in the auto bizz...only once in a while (like when I went to school for it 15+ years ago ) In the real world, schematics take too long in a flat rate auto repair shop. But I did use my DVOM, VATs and scopes extensively to find electrical probs.

But 99% of the shops I worked were the "replace parts until the problem goes away" type. If the service mgr even seen you bust out a repair manual or a schematic you'd be fired on the spot for "wasting time" and being "inefficient." Not the way I would have preferred to do things, but it was the way I was told to...or look for a new job.

Anyways, I'm out of work, and I would take it to somebody but I am pretty strapped for cash now, and like I said, I've had trouble finding a decent amp tech in my area (I live near Modesto CA). I took it to some clowns nearby and what a bunch of A-HOLES!

They charged $80 up front to look at it, kept it for like 3 months, and gave me nothing but excuses why they never seemed to have time to check it out. Then I demanded it back, and they refused to give it back to me (they kept it for 2 MORE weeks after I demanded), and I had to threaten to call the police on them (and report that they stole it), before they finally gave me my amp back. They refused to refund my $80.

I'd almost bet on it that they probably tried to sell it or something...and then were going to tell me it got stolen. I filed numerous BBB complaints about them and they have since gone out of business.

Anyways, thanks John.

I will give it go hopefully sometime this week (or next weekend). I am taking classes now in school (trying to learn a new occupation) and I have homework, so I better get crackin. I can't afford to spend anymore time on my amp for a while (it hasn't worked for close to 3 years anyways). I will get to it though as my other amp (a crappy Randall SS RG 100) is a MAJOR P.O.S.

But again, thank you.
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