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6AQ7 VS 6SL7

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Posted on October 24, 2016 at 08:16:16
Triode_Kingdom
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Location: Central Texas
Joined: September 24, 2006
 There are times I need a single section equivalent of the 'SL7. The 6AQ7 looks like a good candidate, but I recall online posts where someone claimed that the operating conditions aren't the same. Looking at the data sheets, the two appear identical. Is there something I'm missing? Anyone here used the 6AQ7?





 

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If it looks like a duck, quacks like a duck, posted on October 25, 2016 at 07:32:14
Chip647
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Posts: 2652
Location: The South
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 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.

 

Looks like both half share same heater filament, posted on October 25, 2016 at 07:47:14
FenderLover

I have no idea if that effects the triode side --- even with the anodes not connected on the diode.

 

RE: If it looks like a duck, quacks like a duck, posted on October 25, 2016 at 07:58:07
Triode_Kingdom
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Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 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.










 

RE: Looks like both half share same heater filament, posted on October 25, 2016 at 08:13:17
Triode_Kingdom
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Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 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.


 

Verrückte ente!, posted on October 25, 2016 at 12:07:59
FenderLover

8^)

 

RE: If it looks like a duck, quacks like a duck, posted on October 25, 2016 at 12:54:00
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 In my RC-19 RCA tube manual, the RC chart #5 applies to 13 different types, including both of these. FWIW.

 

RE: 6AQ7 VS 6SL7, posted on October 25, 2016 at 14:38:20
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
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 I have been extremely happy with the 6K5GT in a SE 2A3-40 amp. It is hard for me to compare and contrast this tube as a driver for the 2A3-40 because my other implementation which uses the Mullard E180F has a different power supply and output transformers. What I can say is that I run it at Ep = 155V, Ip = 1.3mA with Eg = -1.3V and it is clean and transparent sounding; very much like the E180F. The 6K5 can not drive the 2A3-40 to full output since the output tube starts to draw grid current several volts before the grid hits 0V and this disrupts the function of the 6K5. The amp still manages to wrestle 3W out of the 2A3-40 with 18.8W plate dissipation.

I don't know if the cylindrical plate has any bearing on the sound of the 6K5, but these cylindrical plate tubes from the 1930s and 1940s do seem to offer good sound. I have been so happy with the 6K5 that I am converting the amp to 6F5 and using it to drive a EML 320B. I am still in the construction phase and haven't tested it yet, but I expect to use a source follower to buffer the 6F5 from the 320B. This will require a coupling cap and I hope the added components don't affect the clean presentation of the 6K5/2A3-40 amp.

 

RC-19, posted on October 25, 2016 at 15:36:50
Triode_Kingdom
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Location: Central Texas
Joined: September 24, 2006
 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!







 

Dont forget about the 7-pin miniatures, posted on October 25, 2016 at 18:32:59
Alpha Al
Industry Professional

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Location: N. Carolina
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Contributor
  Since: December 3, 2015
 6AT6, 6AV6, etc. are excellent tubes, and plentiful.

I've also has good luck using 6J5, especially the ones with cylindrical plates.

 

curiouser and curiouser ..., posted on October 25, 2016 at 19:33:07
Paul Joppa
Industry Professional

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Location: Seattle, WA
Joined: April 23, 2001
 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.

 

RE: curiouser and curiouser ..., posted on October 26, 2016 at 10:30:09
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 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.


 

RE: curiouser and curiouser ..., posted on October 26, 2016 at 20:06:25
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 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.

 

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