Tube Amps / Music Electronics
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|11/14/2000 8:58 PM|
||Re: Randall's findings on the outer foil terminal|
If you use a shielded cable to a couple alligator clips, connect the cap. across the clips, hold the cap. & listen to the noise level. Now reverse the cap and listen again. Find the orientation with the least hum. The outside foil will be the lead connected to the shield. You can also do this test with an oscilloscope. It's even easier when you can see the noise on the scope. I was thinking of building a box with treminals to connect the cap to and a switch to reverse the connections. That would make for easy lead identification.
Thanks to Randall Aiken for the idea. This is a very simple but effective test procedure.
|11/15/2000 3:31 AM|
After reading Randallís posts last week I tested about 20 caps (Orange Drop 715ís and Mallory 150ís) using a tube oscilloscope. I saw definite differences on the scope by holding the caps, as Randall described, then switching positive and negative leads. After testing the first few I set up a DPDT toggle switch to quickly switch the poles (faster and easier). I got through all caps in just a few minutes, then repeated the procedure with many of the caps to make sure it was repeatable, which it was, consistently. I marked the outer foil lead. Havenít used them yet but will be soon be placing them in a amp Iím building. Ė Dan
|11/15/2000 5:10 AM|
||A simple ss circuit to determine orientatiion?|
I tried clipping both leads to the caps and it was so quiet I could hardly hear any hum at all either way (I even turned on a fluorescent light just to add some RF hash to the environment!). I was thinking of maybe wiring in a 100k resistor in series with the hot lead just so the signal would not be completely muted. BTW I was plugging the test cable into a guitar amp... do you think that just a small ss amp would work better? (Less likely to get noise from the trannies and tubes...)
The only obvious difference I could hear (and see on my DMM) was when I'd tap the end with my index finger... that would add a lot of noise with one orientation and not that much noise with the other. But I wasn't able to figure out what that meant! (BTW you can hear a "knocking" sound on the 418P's... does that mean that they are microphonic? This was with just the hot lead connected.)
In any case by following the visual clues as to the orientation (which were sort of confirmed by ear) my Blues Express amp has a lot less background noise. I'm a believer!
I do think that a very simple ss circuit (one or two transistors) could be designed that would make it even easier to check these caps... you'd have to be as deaf as Da---, well, let's just say a deaf man, to not be able to hear the difference. And like you said include a double pole toggle switch.
BTW the issue of the outer foil was brought up here about a year ago, but I don't think that anything was done with the idea. Thanks again to Randall for picking up the baton and running with it!
Anybody out there who can brew up such a circuit for us?
P.S. Did you notice any pattern as to which lead was which? The person last year had talked to SBE, and they told him that it was strictly random. But then again that might have been a cop-out... They certainly do not want to guarantee that one particular lead will always be connected to the outer foil, but if you were to check their manufacturing processes I bet that it would be consistent for each batch... Unless they just dump a bunch of the epoxy dipped caps in a bin before stamping the numbers on them, it would take more time and effort to keep them random.
|11/15/2000 5:35 AM|
I hate to enter the thread, but...(Randall starts laughing now)...I spent a few hours doing this test last week and to put it simply, it works.
I went through a few hundred caps marking them all wioth a Sharpie after that.
I just finished a little tweed type amp and as a test I installed all the coupling caps backwards, (once I knew which is the shielded side), some of you who've stopped by know I have ton of 8' double tube flourescent lamps in the ceiling of my shop and one big one hanging 3 or 4 feet above my work bench.... with all the caps backwards I could hear them all, including the guy next door running his electric tools and moving my hand closer to the circuits made it a little worse, however, with them reversed, I was back to my normal quiet amp again with little or no effect by moving closer to the circuit!
Not a HUGE "Oh my God" difference but it was still enough testing for me that as I told Randall, I'll never build or repair an amp again without making sure I get the shielded side of the cap on the right side of the circuit! This is a "must do" now.
One caveat... the cheaper the cap, the harder it is to see solid results with this test.
When using my O'scope and my expensive polystyrene coupling caps, they shows about a 4-5:1 ratio from shield to not shielded, while the difference with cheap crappy caps are just barely noticable.
Oh.. I found NO cap company that followed a priinting pattern to the shielded side except very old caps from the good old days when manufacturing companies gave a damn about things like this.
|11/15/2000 6:14 AM|
So it is not just the Orange Drop style caps that all of this would apply to? For the cheap ones that it did not seem to make a difference, were they equally noisy in both orientations, or equally quiet?
Do you have any suggestions for a simple ss circuit that would make it as plain as the nose on your face to determine the correct orientation? I spent about 15 minutes with my test cord plugged into a guitar amp, and managed to come to definite conclusions on 5 caps... let's see, 3 minutes per cap times 10 caps times 100 amps per year... I guess you'd need to charge $5k per amp to make a profit!
Not a HUGE "Oh my God" difference...
Well, if you have been tearing your hair out (you do have some left- right?) trying to reduce the background noise on an amp for many hours it is like Moses coming down from the mountain with the 15, ur, 10 commandments...
I think that this discovery alone explains that mystery of all mysteries, namely, why some of our homebrew amps are noisier than others even though we use the same construction techniques.
P.S. Maybe you can tell us how you use your scope for this... are you amplifying the signal, or clipping the caps into an existing circuit or what?
|11/15/2000 8:33 AM|
I seem to have missed the part about which is the proper orientation in-circuit (once I figure out how to test them ). If I imagine replacing all the caps in the amp with polarized, would the shield end go where the negative of the polarized cap would go?
|11/15/2000 9:13 AM|
I seem to have missed the part about which is the proper orientation in-circuit
I hope I saved the earlier part of this thread before it scrolled off... or maybe Randall will write it up on his site.
I believe that he said that the lead connected to the outer foil would go to the "point with the lower impedance"...
Well, them big fancy nickel words throw me for a loop so Cousin Jed an' me jus' figger that it is a piece of shielded cable, and I would connect the "shield" to the point earlier in the audio chain. In the case of a coupling cap, the outer foil lead would be connected to the plate of the preceeding stage. In the case of a bass or mid cap in a tone stack, that lead would be connected to the slope resistor. Now if the cap was shunting some of the signal to ground, well I'd connect the outer foil lead to ground. Or similarly if the cap was bypassing a plate resistor I would connect the outer foil lead to the B+ side.
I'll have to look through my files to see if I saved the beginning of this thread (Is it just me or does it seem like these threads are scrolling off faster than they used to? At one point I believe that there could be up to 500 individual posts in each forum before the "toilet would be flushed", so to speak... )
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