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In Reply to: RE: 6AQ7 VS 6SL7 posted by Triode_Kingdom on October 24, 2016 at 08:16:16
it usually is a duck. The only real difference in the rating is a 50 volt less max plate voltage. Everything else the same as you found. I would probably ground the diodes.
Put it this way, design a circuit for both, what changes would you make? none.
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I looked again after reading your post, hadn't noticed the difference in max anode voltage previously. There must be a typo in the data, because even though 250V is the stated maximum, the chart of operating conditions shows it being used at 300V.
What's even more interesting is that in spite of basic characteristics for the two tubes being identical, the two charts aren't. For example, the 300V column with Rp of 240K shows different cathode resistors for the two tubes. The 6SL7 is measured with 2,100/2,500 Ohms (depends on the next grid load Rs), but the 6AQ7 is measured with 3,600/3,900 Ohms. Also, the 6SL7 exhibits somwewhat more gain than the 6AQ7 in this area of the chart, despite next stage grid resistors being the same. Oh well, those are minor differences, not enough to justify a claim that the two aren't the same.
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Buy Chinese. Bury freedom.
In my RC-19 RCA tube manual, the RC chart #5 applies to 13 different types, including both of these. FWIW.
Thanks Paul, I looked it up online following your post. The 6AQ7 is omitted from the charts in my RC-21, but RC-19 groups it with the 6/12SL7 series. Looks like I can use it with the same operating conditions. Just bought some more, I've got a good stash of these now!
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Buy Chinese. Bury freedom.
My RC-26 does not include the 6AQ7 in the main pages, and it has disappeared from the RC amp table - though it is still listed in the "replacement use only" section at the back.
The RC-19 might be the last manual with full specs. In addition to deleting it from the charts, my RC-21 only has a block of text under this type, indicating it to be an obsolete part. The original application must have been a radio where the diodes were used for AM detection or AGC. Maybe it was a WWII receiver produced in sufficient quantities to warrant the tube. Later models probably used separate diodes and/or were redesigned for miniatures. It's odd that none of these were produced with metal shells. Anyway, 'SL7s and 'SN7s were likely used in more applications than this tube, so they were manufactured and documented longer.
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Buy Chinese. Bury freedom.
The independent cathodes of the diodes indicate it was most likely intended for FM demodulation. Triodes with extra plates, but a cathode shared by all elements, were widely used for AM, but would not work for FM as normally implemented. Could have been for TV which has FM audio.
I have no idea if that effects the triode side --- even with the anodes not connected on the diode.
I don't think the potential for interaction is any different than when two triodes are powered from the same filament. At any rate, it's easy enough to ground adjacent pins 1, 2, and 3. That should take care of any potential issues. I'm sure I've seen commercial gear that used a tube like this with an unused diode section, but don't think it was the 6AQ7.
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Buy Chinese. Bury freedom.
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