filter cap max voltage?

[grindliner]

Do I need to be concerned about putting 455v into a cap rated for 450v?


they are LCR 32/32mf 450v.

[HTH]

hmm, you're asking for trouble running it so close to the maximum voltage. rule of thumb is to run at around 20% below the max voltage specs, i.e. 360v, but I'd personally be comfortable running up to 400v on a 450v cap.

run a pair of caps in series if the voltage specs are right on the edge, two 450v caps in series give you a working voltage of 900v and the voltage gets split equally between the two caps. of course, you need to have the same size caps (i.e. two 100uF caps) and balancing resistors across each cap (56k to 220k is fine).

[grindliner]

with my cans are they considered series or parallel?

ie 2- 32mf 250v in series, or 2-32mf 450v in parallel.

[novosibir]

Filter caps do usually have a safety margin of 5...10%, hence 472V on a 450V rated cap still should be safe.
But the longevity of filter caps is diminished, when you apply more than 70% of their rated voltage to it.

Larry

[flemingmras]

with my cans are they considered series or parallel?

ie 2- 32mf 250v in series, or 2-32mf 450v in parallel.

The two cans are first paralled with themselves (the positive terminals on both caps are connected together), then the two cans are placed in series with each other. This is a parallel/series configuration (or series/parallel depending on your preferred semantics). With your values, the circuit will see this as a 900 Volt cap with a total capacitance of 32uF.

Now if your rectifier is the bridge rectifier type with the center tap connected to the spot where the caps connect in series, you're actually only applying whatever the center tap voltage is to each cap independantly. This is because each capacitor is only seeing the center tap voltage, which is 1/2 the full winding voltage. The caps together supply double their charge voltage because they're in series just like batteries in series do, hence the "full wave voltage doubler concept.

Of course, the full winding puts out double the center tap voltage so it's not really a "doubler" per se, but having the center tap hooked up between the caps ensures that the caps see exactly 1/2 the full winding and one doesn't go overvoltage. It's just another way of doing the same thing that bleeder resistors do. There's "more than 1 way to skin a cat" so they say.

[grindliner]

Well, I may just scrap the caps and change my plan, as they came from an amp made in 1988. I have a new pair of 50/50mf 500v I got from Metro.


my original intention was to start out with a ss rectified JTM45 using the trannys from my Jackson JG2, then put it throught the paces of of a JTM50, then a 1986/1987 to see which I like best.

My PT is unknown, the amp was made in england, rumored to be Laney, but they told me they did not make it. My secondary windings are 2-red, 2-black, and 2-yellow. in the original amp, with one black lead grounded, I have 325v on the HT, 6.3v on the yellows, and the remaining black lead is 50v, and was used as the bias supply. it was putting about 408v on the grids.


Now that being said. I need to ground the unused terminal on the bridge rectifier to get my proper B+.


flemingmras, regarding the above about grounding the rectifier, in my post on that subject, you stated:

"Also if your PT doesn't have a center tap, you'll wanna make sure you have 2 watt 100K resistors across the first bank of filter caps for load balancing across the caps."

Do you mean bridging both positive leads on each can with the resistors?

[Structo]

Balancing resistors are used when you use two caps the same value that are wired in series.

Here is a layout of that type circuit.

The resistors go across the positive ends and the negative ends but note the red dotted line showing a series connection between the caps.

In this configuration the total capacitance would be 50uf @ 700v.

These resistors balance the voltage between the caps and also act as bleeder resistors when the amp is powered down since the top left - end of the cap goes to ground.

[flemingmras]

Well, I may just scrap the caps and change my plan, as they came from an amp made in 1988. I have a new pair of 50/50mf 500v I got from Metro.


my original intention was to start out with a ss rectified JTM45 using the trannys from my Jackson JG2, then put it throught the paces of of a JTM50, then a 1986/1987 to see which I like best.

My PT is unknown, the amp was made in england, rumored to be Laney, but they told me they did not make it. My secondary windings are 2-red, 2-black, and 2-yellow. in the original amp, with one black lead grounded, I have 325v on the HT, 6.3v on the yellows, and the remaining black lead is 50v, and was used as the bias supply. it was putting about 408v on the grids.


Now that being said. I need to ground the unused terminal on the bridge rectifier to get my proper B+.


flemingmras, regarding the above about grounding the rectifier, in my post on that subject, you stated:

"Also if your PT doesn't have a center tap, you'll wanna make sure you have 2 watt 100K resistors across the first bank of filter caps for load balancing across the caps."

Do you mean bridging both positive leads on each can with the resistors?

No...as was stated above, the positive terminals on each cap get connected to each other (i.e. each cap has two positive terminals...the two positive terminals on each cap get connected to each other) while the resistor goes between the joined together positive terminals and the negative terminal on each cap. Then you wire the negative terminal of one cap to the joined together positive terminal of the other.

It will look something like this -



As you can see, the positive terminals on each cap are connected together, placing both caps in the can in parallel. This makes each can a 100uF capacitor rated for 450 volts. Placing the cans in series with each other halves the 100uF down to 50uF, but doubles the voltage handling of both together to 900 volts.

The bleeder resistors form a voltage divider across the total B+. This splits the B+ voltage across each resistor. Due to the resistors being equal in value, each one will have the same voltage drop across it that is equal to 1/2 the total B+ voltage (the sum of all voltage drops in a series circuit will always equal the total supply voltage). This ensures that both caps see the same voltage and that one cap isn't getting more voltage than the other, possibly going into overvoltage.

The resistors in the above pic are a 56K 2 watt type. You can use any value above this if you want...just make sure they're both equal in value. If you plan to use the spec'ed 56K, make sure they are a 2 watt resistor.

Going off of your 325VAC HT secondary, multiplying this by 1.414 tells me that you should probably see about 460 volts DC at the supply with no tubes installed. Will probably drop by about 20-25 volts with the tubes in and biased. With no tubes in, each cap should see about 230 volts (1/2 of 460).

In regards to wiring the bridge rectifier...

The two red leads go to the AC input of the rectifier. DO NOT GROUND EITHER OF THE RED WIRES FROM THE TRANSFORMER!!! They'll get their ground through the negative terminal of the bridge rectifier, which DOES go to ground. The negative terminal of the rectifier will switch the red legs to ground with the AC voltage depending on which red wire happens to be negative at a given point in time. The positive gives you your DC B+ output and the rest of the supply is the same.

[grindliner]

Gotcha! thanks for the help guys.

And thanks for going into greater detail flemingmras

[flemingmras]

Gotcha! thanks for the help guys.

And thanks for going into greater detail flemingmras

One more thing...

Any groups of filter caps connected in series need to have these bleeder resistors on them just like they are shown above. Most Marshalls have both the power tube plate filter caps and the power tube screen filter caps connected in this fashion. Make sure any filter caps that are connected in series have these resistors on them.

[grindliner]

Ok I'm back to wiring, and confused.

Going off the metroamp 50 watt instructions, using my PT, the 2 Chassis mounted cans (F-1 and F-2)are not connected together. Do I still use a 100K 2W resistor from one of the Positive leads to the negative lead on F-2 (with both + leads connected to the standby sw), or do I still do it to F-1 as well?

[flemingmras]

Ok I'm back to wiring, and confused.

Going off the metroamp 50 watt instructions, using my PT, the 2 Chassis mounted cans (F-1 and F-2)are not connected together. Do I still use a 100K 2W resistor from one of the Positive leads to the negative lead on F-2 (with both + leads connected to the standby sw), or do I still do it to F-1 as well?

OK that means that they're not series connected and one serves as the plate filter while the other serves as the screen filter, while the choke connects between the two cans. In this case, you wouldn't use the bleeder resistors at all.

This is the schematic for your power supply as your amp sits now. You can get a couple of 100uF 450 volt caps and wire them between the negative terminal of each LCR cap and ground if you're still worried about the voltage handling.