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Post by Deleted on Mar 4, 2009 9:00:17 GMT -7
I'm getting ready to order a couple of Dario Miniwatt Ef86's from KCA for my Dr Z Stangray. It seems a general consensus here that this tube is great in the Stangray. Although I have some really good NOS EF86's for my Stangray (Mullard GE-Phillips) I simply have to try the Dario's. Due to the reviews on this site my curiosity has been piqued. When I go to KCA to order I'm given the standard option "Better than average" or I can alternatively choose "Best" for an added $10. What exactly does "Best" mean and is it worth the extra $10?
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Post by ajnewkirk on Mar 4, 2009 16:46:55 GMT -7
The "Better than Average" is describing the microphonics of the tube.
When ordering an ef-86 I always choose the "Best" option. To me it is well worth $10 to get one with lower than average microphonics.
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Post by smolder on Mar 4, 2009 18:32:18 GMT -7
I would agree. At the price of NOS tubes, get what you want and have no regrets. You'll pay half that again in shipping if you don't like it.
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Post by evandael on Mar 5, 2009 19:32:40 GMT -7
Regarding microphonics.. my amp has an EF86. Occasionally when I am playing I swear I hear the telephone ringing, but when I listen for it, there is silence.
Is it possible that the tube is going microphonic, or am I just hearing things? It could be other things rattling in the house.
Thanks,
-eric
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Post by myles on Mar 6, 2009 11:02:22 GMT -7
EF86 tubes vary all over the place when it comes to actual true gain. The spec for the tube is a gain of 38.
Here are the design specs for the tube straight out of the book:
Plate Voltage ................................. 250 V Grid No. 2 Voltage ............................ 140 V Grid No. 1 Voltage ............................ -2 V Amplification Factor .......................... 38 Plate Resistance (approx) ..................... 2.5M Ù Transconductance .............................. 1800 µ Plate Current ................................. 3.0 mA Grid No. 2 Current ............................ 0.6 mA
Depending how these are implemented in a circuit the gain can be much higher in some amps, above 400. Amps such as a Matchless DC-30 run these tubes at a very high gain level as an example.
The good news and bad news ....
Dr. Z amps typically do not ask these tubes to be in a very high gain application. EF86s that would be unusable in many EF86 based amps can work very nicely in Z amps. That is the good news.
The bad news is that only about 20% of all EF86 tubes I test (and all are NOS these days unless there is some new offering to examine) have a gain below 40 in the standard test setup. Once gain hits about 50 things are always more prone to microphonics. I find many at figures above this, over 50%. Some are over 100 and those are pretty useless in any amp and will howl or make other noises.
Since measuring true gain requires equipment that is costly and not all that easy to procure most of the tube vendors out there use a simple method of putting the tube in an amp or some sort of device which will allow the tube to operate so it can be tapped to hear the noise and also hear the tube's self generated noise. If the tube is a more quiet tube it will be a better tube. If you look at tubes that are considered the best of the best and measure their true specs you will always find (if the tube is not a rattle trap inside already) that the true gain is at spec or below spec.
There are very few EF86s out there with lower than design spec gain. I have seen numbers as low as 28 and these tubes are actually great. Unlike new production triodes that are generally low in gain the pentodes are generally too high in gain, new tubes or NOS ones.
In a 12AX7 amp in many cases higher gain is accomplished by feeding one amplification stage into another ... cascading gain stages. In the case of most EF86 designs the single gain stage of the single pentode is designed so the target gain is done in a single stage and if a tube is too gainy noise and instability is the end result.
In the past the Mullard EF86 was developed for use in phono preamps where very high gain was required but these preamps were solid little boxes in many cases that were on tables in living rooms or shelves, nowhere near a speaker and not in physical contact with other components.
In any case, I agree with the post from ajnewkirk in his selection of these tubes.
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