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Replacing caps in a tube amp
- Thread starter dt1561
- Start date
Doesn't usually make a difference. Newer parts tend to be smaller because the techniques to make aluminum foil thinner have been refined in the last 20 or 30 years. US-made filters of that type tended to be larger, since aluminum is cheap in the USA, compared to, say, Japan. The japanese capacitor companies had a cost incentive to use less aluminum, the US manufacturers didn't. You'll find that the larger ones were probably made in the USA, and that the smaller ones are not.
Just be sure to replace the bleeder resistors. Each capacitor should have its own an identical-value bleeder across it. This assumes that two or more of them are placed in series in this amplifier. When this is done, some manufacturers will use only a single bleeder resistor, placing the capacitors alone in series.
That's a risky thing to do. A separate bleeder placed in parallel with each series-connected filter cap serves to dvide the DC voltage equally across each series-connected cap.
If you leave them off, you are trusting the internal leakage resistance of each capacitor to be equal.
Equalizing bleeder resistors are just cheap insurance, compared to the cost of the filter in parallel with each one.
Thousands of sweep-tube amplfiers made by D&A would have one or more series strings of three of those parts, with no equalizing resistors on them at all. When D&A bought filter caps, it was in massive quantities, all from one manufacturing batch at a time. Parts from the same day's production run tend to match each other closely enough to get away with this. Just because they got away with leaving these parts out doesn't guarantee that you can get away with it.
Your local parts source can't promise that two (or three) caps they sell you will match in this department. The bleeders' exact resistance value is not so important, so long as the voltage on that cap doesn't cause the resistor to throw excessive heat. They just need to be equal, so the DC voltage is divided equally.
We use 240k 2 Watt resistors for that purpose. Nothing magic about that value. If you used a lower resistance, like 100k, a 5-Watt rating would be wiser. Or, if you you use a 470k, a 1-Watt rating is okay. Anything in that range will work, so long as each one in a series string is the same.
73
Just be sure to replace the bleeder resistors. Each capacitor should have its own an identical-value bleeder across it. This assumes that two or more of them are placed in series in this amplifier. When this is done, some manufacturers will use only a single bleeder resistor, placing the capacitors alone in series.
That's a risky thing to do. A separate bleeder placed in parallel with each series-connected filter cap serves to dvide the DC voltage equally across each series-connected cap.
If you leave them off, you are trusting the internal leakage resistance of each capacitor to be equal.
Equalizing bleeder resistors are just cheap insurance, compared to the cost of the filter in parallel with each one.
Thousands of sweep-tube amplfiers made by D&A would have one or more series strings of three of those parts, with no equalizing resistors on them at all. When D&A bought filter caps, it was in massive quantities, all from one manufacturing batch at a time. Parts from the same day's production run tend to match each other closely enough to get away with this. Just because they got away with leaving these parts out doesn't guarantee that you can get away with it.
Your local parts source can't promise that two (or three) caps they sell you will match in this department. The bleeders' exact resistance value is not so important, so long as the voltage on that cap doesn't cause the resistor to throw excessive heat. They just need to be equal, so the DC voltage is divided equally.
We use 240k 2 Watt resistors for that purpose. Nothing magic about that value. If you used a lower resistance, like 100k, a 5-Watt rating would be wiser. Or, if you you use a 470k, a 1-Watt rating is okay. Anything in that range will work, so long as each one in a series string is the same.
73
Toll_Free
Well-Known Member
nomadradio said:
The lower the value of resistance, the better the instantaneous regulation of the power supply.
Something to keep in mind when "redesigning" the power supplies of older tube equipment.
--Toll_Free
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