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Originally Posted by jamato8 /img/forum/go_quote.gif
Your concern is valid but in practice there aren't many designers who have followed this very closely, so Woo can't be faulted in a way. Cary Audio had 1000uf behind 2 GZ34's in their SLA50 and a good GZ34 would and does last for years in the circuit.
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This is why I maintain that the vast majority of "designers" are hacks, they can't even read and follow a manufacturer datasheet.
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Now directly heated tubes that come up to rectification fast do fail but not the indirectly heated GZ34's. |
They all fail. Think about what happens in a momentary power interuption lasting maybe one to five seconds. The capacitors discharge yet the rectifier tube(s) are still hot, when the power comes back on it arcs over and fries.
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Turn the Woo 6 on and meter the DC as it comes up and has to fill the caps. It takes a good 15 seconds or maybe a little less. Now that is a slow ramp up of the voltage, which is great for the tubes (6DE7) getting the DC as their heaters are also coming on and the tube is not being hit with a lot of current all at once. Also the cap following the GZ34 can only be filled as the tube comes up to spec and it doesn't matter if it has 10 volts or 50 as it ramps up the cap is still being charged so since the 34 comes up slow it really isn't being stressed all that much. Also once everything is up and running there is no stress and it doesn't matter about the size of the cap as it is now charged (within reason). |
Wrong. Rectifier tubes do NOT deliver a steady state DC current when they're on, they deliver a spike of DC for the portion of the AC cycle where the voltage coming out of the rectifier exceeds the stored voltage in the capacitor. With a C-input filter there's a sharp current spike lasting anywhere from a few milliseconds to a few dozen milliseconds depending on the DCR of the transformer, the internal resistance of the rectifier, and the size and ESR of the input capacitor. With a full-wave rectifier, this happens 120 times every second. Which brings us to the next rating, Maximum Peak Current, in this case 750mA. This is the maximum current which can be allowed to flow on each AC cycle, given the size of the capacitors in the Woo 6 this rating is most likely being exceeded, and with low ESR Blackgates it's almost certainly exceeded.
To simplify a bit, the larger the input capacitor is, the shorter the current spike will be and the larger the current flow. Lowering the ESR of the capacitor has the same effect. This is how the manufacturer derived the maximum recommended capacitor size, if the cap is larger 60uF, the peak current flow will likely exceed the 750mA rated maximum given typical transformer and capacitor characteristics.
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Is 60uf or less ideal, sure but remember the manufacture was also trying to be conservative. For something like the 274B you would need to change the circuit to a 8 or less uf cap as it is directly heated and is known to be a more easily stressed rectifier. So my experience over the years has shown that the GZ34 will be and is fine in this circuit but that the directly heated tubes will be a little more effected. |
Design center ratings are conservative but that's no excuse to use a 330uF cap when the recommended maximum is 60uF, it's more than 5 times larger. Design center means you can push the specs around 10-20% at most with reasonable safely, not 500%.