Bias and cathode caps in mid '70's Fenders

[cartoonweirdo]

I am in the middle of fixing several mid '70's fender amps. Due to the fact that the filter caps were original I changed them out. I was going to do the same to the bias filter and cathode coupleing caps as well, but they are a style I am unfamiliar with. They are white and plastic looking and smaller than the old cardboard wraped electrolytics I am used to seeing in fenders. Does anyone know if these plastic ones are electrolytic?
Thanks,
Carl

[Cygnus X1]

I use polyprops, or polyester caps for those.
Many times there is a line at one end, but they are not polarity sensitive.
(Those ones aren't electrolytics).

If in doubt, you can test them with your meter.

[cartoonweirdo]

I was curious if the old ones were electrolytic or not. How does one test for that with a meter? I'm sure that the polyester caps sound worlds better.
Carl

[Cygnus X1]

Many of the Fender schematics are located at the Fender Amp Field Guide website (google it). If it doesn't have a polarity signal on the schem, it is usually not an electrolytic.

When pulling old caps, a good electrolytic will show a brief charge that can be seen by the needle movement in ohms on the needle action on an analog meter. It can also be seen on a digital, but easier to discern on a needle.
It will charge (jump), then discharge (drop). Primitive, but it works.

Or, for checking unreadable values, get a meter that gives capacitance readings. My inexpensive AW Sperry works just fine.

[cartoonweirdo]

Thanks Cygnus,

I have the paper schematics for almost all of the fenders until the '80's so this should be easy.

Please educate me from an electronic standpoint... Why is it that a high value capacitor of a certain composition (electrolytic or poly-something) will charge and discharge visibly (on your meter), but the other composition will not behave in a similar fashion?

Shouldn't any large value cap hold a charge when charged up and fall off slowly when measured during subsequent 0 volt conditions?

Perhaps I have the wrong idea?

C onfused....
Carl

[Cygnus X1]

It's been a very long time, but yes, the electrolytics ( I think) charge/discharge at different rates according to polarity. Other caps don't.

I don't do this anymore, btw...I just go by the schematics.
Someone else might be able to extract my foot out of my mouth!

Good luck!

[cartoonweirdo]

Cygnus I'm sure you are right, I just wanted to know what the actual mechanism at work was. I am self taught, so there are gaps in my knowledge when it comes to the more in-depth workings of the thing.
Thanks for a new bit of test information,
Carl

[Cygnus X1]

I wouldn't replace anything other than the electrolytics, anyways...unless there's obvious damage to the original tone caps. Or hissing/crackling resistors.

The poly's and ceramic are usually not a problem, in my limited experience.

[mesherm]

When you measure resistance with an analog or digital VOM, the meter applies a small current and voltage to the device under test to determine its resistance. Analog meters typically apply a higher voltage due to the low impedance of the meter movement.
When this charge is applied to a capacitor the capacitor starts to charge up to the level of the voltage applied by the meter. This initial current flow is high and is interpreted by the meter as a low resistance. As the capacitor charges, the current drops and registers as an increasing resistance. Electrolytic capacitors are generally much higher than film caps so it takes longer for them to charge making the effect much more noticable. If you had a film cap and electrolytic cap of the same value your meter would react the same on both.
The meter should eventually read infinity (or at least in the megohm range) on any capacitor when its fully charged or the caps integrity is suspect.
Not all meters react the same since they might use different voltages and currents so its best to have a know good cap of a similar value to use as a comparison until you get familiar with the meter response.

[Gingertube]

Look at the values printed on the side.

I think I know the capacitors you mean. I did a recap job on a Bassman 100W on the weekend. In the Bassman there were 3 of these. 2 were 5uF/50V and 1 was 25uF/25V. I both cases the value and voltage rating were clearly marked and one end had some + symbols around the perifery.
All had the Mallory brand name.
All were cathode bypass capacitors.

They are definitely electrolytics and I definitely changed them.

Its probably not as important to do these caps as it is to do the High Voltage AND Bias Supply Caps but the amp I was doing had run 27 years without trouble and I want it to run another 27 years - so I changed them.

ASIDE: The B+ on theis amp is according to the schematic +445V when operating. With all new caps I measured 454V. This made me glad that I replaced the old 20uF/525V with series connected 47uF/400V caps with 330K 1 Watt voltage share resistors across each cap.

Cheers,
Ian

[cartoonweirdo]

Hi Ian,
Thanks for the tip. One of mine was also a Bassman 100, and I did end up replacing the caps in question with new ones. It sounds much better.
Carl