Replacing the 6au6

[2string1]

Thinking about replacing all the 6au6's in my Fisher 500B. What is the purpose for these tubes and what is recommended?

 

[RossW]

6AU6 is probably part of the tuner section. Unless they are shorted or test really bad I would recommend leaving them.

 

[2string1]

I'm having it restored and wondering if I should take out the originals and replace with NOS? Not sure what brand though... 6au6's are cheap. I'll probably leave in the 12ax7's telley's.

 

[Tom Bavis]

Tuner alignment will be affected - so replacing weak 6AU6s may not improve performance (and may hurt) without realignment.

 

[ab0ez]

6AU6's are most commonly used in the IF stages of receivers. They also usually last forever! I would take each 6AU6 out singly and test them. If they test good, leave well enough alone. Just remember, if you replace them all you will probably need a complete alignment done on the entire tuner section of your 500B.

 

[2string1]

As I said...I am having it restored...so yes it will be re-aligned. All the 6au6's are Fisher branded but one. That one is a Realistic Lifetime. Should I just replace that one or all 6au6's with NOS?

 

[ab0ez]

In that case, I recommend some NOS Sylvania JAN 6AU6WB tubes. They aren't expensive, readily available and perform well. If you search a bit you can also still find Telefunken and Amperex Bugle Boy 6AU6 tubes for relatively cheap....for now.

 

[2string1]

What does the WB stand for? Anybody else want to make a recommendation?

 

[electroking]

What does the WB stand for? Anybody else want to make a recommendation?

W (War!) stands for a military tube, and B is just some suffix for a revision
over W and WA.

Many tube types are mostly available with the W suffix as the last large scale
buyers were military organizations.

 

[gearhound]

I've rolled quite a few 6AU6 tubes in my 6AU6/12B4A flea-amp.
I like either RCA or Tung-Sol (USA) 6AU6 tubes.
Either brand in generally DIRT CHEAP as NOS tubes go.

Steve

 

[ab0ez]

From the Tung-Sol data sheet, it describes the 6AU6WB as "For industrial and commercial applications with a high degree of reliability". "It is characterized by long life and stable performance under severe conditions of shock, vibration, high temperature and high altitude. The tube may also be operated under stand-by conditions".

In general, these tubes were tested a bit more rigorously as they had to pass mil-specs.

Jim, AB0EZ

 

[2string1]

Do these tubes in the tuner section make a difference in sound? Will a $50 telefunkin make difference vs a $12 Military spec GE or RCA?

 

[ab0ez]

Indirectly, yes. You have to remember these tubes are used in the IF section where the audio signal has not yet been demodulated (or extracted) from the carrier frequency. However, the more linear, distortion free and lower noise floor your IF strip is, the better you can expect the resultant audio signal to be. And it just may turn out that the $12 surplus mil spec tubes will work better than a $50 Telefunken in your particular receiver/tuner/amp...although that's way too much to pay for ANY 6AU6!


On the flip side, if the radio station you're listening to is simply playing 128K MP3 files and/or has their audio compressors cranked up way too high, then you would be hard pressed to hear any difference.

Jim, AB0EZ

 

[ab0ez]

You can also try some 6136 tubes. They are often referred to as a "premium 6AU6".

 

[2string1]

Thank You for that explanation. What about the expensive Telefunkens...will they be more distortion free and have a lower noise floor. I would think the military USA tubes would be better for that.

 

[ab0ez]

Thank You for that explanation. What about the expensive Telefunkens...will they be more distortion free and have a lower noise floor. I would think the military USA tubes would be better for that.

That will depend on your individual piece of equipment, operating parameters, your ear, etc.

I would probably buy a sleeve of 5 JAN/JHS 6AU6WB's or 6AU6WC's and live happily ever after. I managed to buy a few NOS Telefunken 6AU6 tubes for less than $10, still in the boxes. However, in my old Dynaco FM3 tuner Amperex Bugle Boy 6AU6's work the best....for me...with my tuner...to my ears...

Good luck and how about some pictures of your 500???

Jim, AB0EZ

 

[2string1]

Thanks...I'll probably get a sleeve of GE JAN 6AU6WC. 25 bucks vs 200 is a big difference. Probably won't here the difference in the 500.

 

[electroking]

By the way, a tube working in the IF chain of an FM receiver does not need to
be particularly 'noise free'. The signal level is too high for a 'noise free' tube to
make any difference. Do not invest good money where it won't help!

 

[2string1]

Thanks...$25 is not a big investment for a sleeve. I almost lost my head and bought the telly's just for prestige, but with the help from everyone I found it and saved a lot of money.

 

[BinaryMike]

You have to remember these tubes are used in the IF section where the audio signal has not yet been demodulated (or extracted) from the carrier frequency. However, the more linear, distortion free and lower noise floor your IF strip is, the better you can expect the resultant audio signal to be.

In any well-designed receiver, noise is a function of front-end performance. Noise in the first RF amplifier stage normally dominates all others. And FM IF strips are anything but linear -- they're designed to amplify the weakest signal enough to achieve hard clipping in the last stage or two. Demodulated audio distortion is essentially a function of FM detector linearity, which is strongly dependent on alignment if it's one of the vintage types, e.g. discriminator, but IF alignment also has some influence.