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Vintage threads from the first ten years

Re: Blackface Bassman filter caps

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6/1/2000 10:27 AM

Re: Blackface Bassman filter caps

Rick;

Here's the main points of why I don't like them.
First their construction quality is not very good. These were designed for commercial applications, whereas the Sprague and CDE caps were designed for industrial applications where they would be subjected to high heat and high vibration environments.

Second, and this is one that really bothers me, is that the caps are overrated. the 500v caps are actually rated for 500 volts surge and 450 volt continuous as opposed to 550/500 volt respective ratings of the Sprague and CDE caps. This means that in most amps they're being stressed to their design limits.

Third, the one's I've tested have ESR's all over the map. That brings the quality control of manufacture into question.

Bottom line is that I think most mfg's use them for the following reasons. They're cheap, small, and will most likely outlast the warantee period of the amp.

Carl Z

6/3/2000 5:47 AM

Carl:

So are Illinois Caps the ones with "IC" printed on them? Just wondering...

Steve Ahola

P.S. As for the test Ray was discussing, what sort of noises are caused by bad Ck caps? Lots of these older amps have intermittent noises, but I guess his test would be for a more constant noise...

BTW would the bad Ck caps show up when you measure the capacitance with a DMM? What if you measured DC resistance in a range like 20M to 2000M to determine if there was any leakage? (I was just checking my DMM and the reading for a 22uF cap doesn't change at all when I add a 1k8 resistor in parallel with it. This might be another non-intrusive way of checking older amps...) Thanks!

6/4/2000 12:50 AM
Carl Z	Steve; Yup, those are the ones. Kinda small and grey. I think that cap test might be a Ray exclusive. I've never tried it myself, and apparently neither has bruce. Maybe ray can give a little clearer explanation of the hows and why's of what he's doing. Carl Z

5/27/2000 11:21 AM
Ray Ivers	Rick, In addition to Ted's and Bruce's suggestions, you might also check out all the cathode bypass capacitors - it's OK to jumper across them briefly and see if the noise vanishes. It's a quick way to eliminate one possibility, and these caps are a known problem area in older Fenders. Ray Ivers

5/28/2000 3:50 PM
Bruce	For clarity.. Ray means jump across the cathode bypass caps with another similar valued cap, not piece of wire! Bruce

5/28/2000 11:29 PM

Bruce,

No, I meant with a wire (PREAMP TUBE CATHODE BYPASS CAPS ONLY!!! - sorry, I should have said this, but since the thread was on a blackface Bassman... I still should have said it). I've done it many times with no problems. It only takes a split-second to determine that the bypass cap is definitely bad. I've also tried jumpering with a known-good cap, and ended up clipping it in, and out, and in, and out... trying to make sure if there was any difference, and if so, was it caused by the new capacitor's charge temporarily 'fixing' the bad cap's problem, etc. - you'd probably be better off unsoldering the cap, I think. I certainly wouldn't want to leave the jumper on there for ten seconds, though.

Ray

5/29/2000 7:21 PM

I guess I need a better understanding of what you're doing!!

If you jump across the bypass cap, you've effectivly grounded the cathode... ZERO bias.
Is that what you are trying to do in this test Ray?

It's been my experience, in a testing situation, many people can't hear the difference between a 25uF cap and a 5uF cap when used as a cathode bypass cap in a preamp section.
Most of the old caps I test are way over their stated value anyhow so paralleling one only works if the cap has gone VERY low in value or has come unsoldered.
Shorting the cap out with a wire would bring the cathode to ground and adversely change the way the stage works!!
If the cap is already shorted, there will be no difference in the operation of that stage with the wire across the cap... actually I'd be surprised if the tube was still alive and working close to spec after any real operating time with a shorted bypass cap.

Bruce

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