Power Supply Mods

[WA4JK]

I have to replace my Filtering Caps and bleeder resister in my amp. So I'm thinking to myself, why not increase the capasitance and increase the resistance of the bleeders. All I see I would be doing is building in higher margin so less stress. The orginal circuit used 210uf @450vdc terminal caps and 50k@7w. I was thinking about 250-470uf, depending on what I can find. Only issue I have is I can not find anything above 450vdc on-line. I was thinking about 100K@10W for the bleeders. Does anyone have some input on changing from the designed valuse to higher values.
Does anyone know where other than Mouser I could find those value Caps and resisters at a reasonible price?
Anyone have a supply of those values surplus to their needs for sale?

[WD8BIL]

Is this a homebrew or commercial amp?
How many 450V caps are in there now?

[WA4JK]

It's commercial a Al-811. It has Four 210uf at 450. They seem to be a problem as well as the bleeders. Thanks Bill

[WD8BIL]

OK, so you have 1800VDC of capacitor at just over 50uf.

50uf is a decent amount but if you wish to increase it that's ok.
470uf@450VDC will give you the same voltage ratings with 117uf effective capacitance.

Going to a 500VDC cap will give you 2000VDC.

Mouser, Antique Elec. Supply and other suppliers have these caps.

[KM1H]

I dont understand what you mean they are a problem? Good quality caps are used and should give at least 20-25 years of service. The 1800V of available ratings is plenty as is the 52.5uF of output C.  The problem with that amp is the marginal transformer resulting in a rather poor 12-15% HV regulation. Doubling the C will tend to improve regulation BUT at the expense of more transformer heating. The extra energy storage (joules) has to be considered in case of a tube arc or other HV short event. There is no B+ safety circuit in that amp and a lot of components can become toast before the fuse opens.

Ameritron is shipping 270uF as replacements since that is what is used in all their amps these days and they are compatible.

Carl

[WA4JK]

Thanks Carl for the info. Your right on the marginal trany. I don"t need any more heat in that area. I have a  problem with the filter caps. So I have to replace them. That may not be the only issue, but I do not have anything execpt a meter and it is an amp meter. It's hard when you live in a RV. I need to get a good meter and dig into this thing. I'm shotgunning right now.

[Gito]

Hi

used 5 to 6  250 uf/330uf  caps in  series make the voltage ability to
2250 VDC/50uf(five 250uf caps /450v) or
 2700 VDC /55uf (six 330UF caps/450v).


When I read the manual the plate voltage of the al 811 is around 1500 too 1700 VDC,if You used four 450 VDC than the maximal voltage it can handle is 1800 VDC.

If Your line voltage is higher than normal, the voltage rating of the Caps can be exceeded  ,so the  caps can be damaged.

The 50 kohm resistor(parallel to each Cap) is an equalizing resistor /voltage divider to make each Caps has the same DC voltage across it

Since this series resistor has a 200Kom value (4 X 50 Kohm).than You can use  a 200Kohm : 5 = 40 Kohm or 39Kohm (using 5 Caps in serie) 0r 200Kohm : 6 = around 33Kohm (using 6 caps in series).for each Caps

So the total " bleeder " resistor stay the same -----200Kohm.also the value of the caps is still the"same" but with a higher plate voltage ability

needs a higher value of caps? use a bigger C -----450uf ? than You got 90 uf with 5 Caps,or  75 uf with 6 caps in series.

Gito

[WA4JK]

The board only allows for four caps. I think I need higher voltage caps but the same or close uF. Got to do some more testing, but I will keep the resistance the same but higher wattage. Thanks Guys.

[AB2EZ]

One additional consideration:

If you significantly increase the capacitance (individual capacitor value / number of capacitors in series), the tranformer will have to deliver significantly more total charge to the capacitor when you turn on the power supply.

If you use a step-start circuit, you may need to change the step-start time delay to accommodate the longer capacitor charging time.

If you don't use a step-start circuit, there will be a longer duration of relatively high current flowing in the transformer when the power suply turns on. This will produce additional heating in the transformer windings during the turn on period. It might even lead to fuse-blowing problems.

[Gito]

Hi

What Stu wrote is true ,using a larger uf ,makes the inrush current Higher
.
Why don't You modify the Capacitor bank,to day the size of these capasitors become much smaller than the the older one, when the place/room permit You can used like super glue to glue to each other,like I do.

This my bank  of  8 capasitor(homebrew) ,using PVC tube,glue them together with a step -start-time.

Gito

[KM1H]

Ameritron uses caps tested to 475V the last I checked and there is still sufficient headroom even with 450V caps up to a 123-125VAC line. If the line is excessive so is the filament voltage and its time to use an external bucking xfmr to get the input down in the 115-120V range, the lower the better as long as the filaments are in spec. The transformer will thank you also.

Carl