6SL7 Tubes for Linestage Preamp

[kmolles]

I really like this tube.....I usually gravitate to the Sylvania's.....I also run Tung Sol 6SU7's, but expect to pay much more for them.

 

[stoutblock]

Best I have found is the RCA 5691 redbase.

 

[CZ4A]

In my experience the RCA red base 5691 is not microphonic when new. Microphonic tubes maybe an indication that the tube is old or well used. As tubes age the on/off cycles can relax the metal joints and make them more prone to microphony. This will happen to all tubes doesn't matter what brand it is and especially true for high gain tubes. Test results would not indicate this unless your tube tester has the earphone jack.
For 6sl7, which i use as a driver tube as well as in some of my preamps, the red base 5691 is one of the best but prices are high. Other good ones are tung sol, and sylvania. But, since the 6sl7 is a high gain tube the stability of the structure is important to keep the microphonics low. This is why it seems that the NOS red base 5691 (there maybe some 5691 that are fakes out there) is quietest overall.

I suspect the fact that most of the 6SL7s I own are used and have been heat-cycled a lot contributes to the microphony. It's not a huge deal to me since I only use them as preamp tubes in my headphone amps. The 1st-gen Tung-Sol 6SU7GTY is at the top of the heap, with the National Union (possibly T-S?) 6SL7GT black glass close behind. The Ken-Rad is pleasant but a little too smooth for my tastes. The GE 6SL7 has nice chime in the treble and is decent elsewhere. The Raytheon I have has only two micas, not three as I remembered, and the plates look like they were pulled off the 12AX7 assembly line!

As an aside, the worst tube for microphony I have is a NOS Raytheon 2C52, which is electrically an octal 12v-only 12AX7. Great sounding tube, but it rings like a bell if you so much as stare at it! Damping rings help a lot. It was made as a servo amp for military aircraft, so I'm assuming audio quality wasn't a consideration!

 

[primosounds]

The 2C52, i have wanted to try that tube. Is your a NOS tube? There are some available at a reasonable price.

 

[thorpej]

FWIW, JJ now makes new 6SL7s and 6SN7s. I keep meaning to get a few samples of each to play around with.

 

[byoungman1]

Best I have found is the RCA 5691 redbase.

+1

 

[kmolles]

I suspect the fact that most of the 6SL7s I own are used and have been heat-cycled a lot contributes to the microphony. It's not a huge deal to me since I only use them as preamp tubes in my headphone amps. The 1st-gen Tung-Sol 6SU7GTY is at the top of the heap, with the National Union (possibly T-S?) 6SL7GT black glass close behind. The Ken-Rad is pleasant but a little too smooth for my tastes. The GE 6SL7 has nice chime in the treble and is decent elsewhere. The Raytheon I have has only two micas, not three as I remembered, and the plates look like they were pulled off the 12AX7 assembly line!

As an aside, the worst tube for microphony I have is a NOS Raytheon 2C52, which is electrically an octal 12v-only 12AX7. Great sounding tube, but it rings like a bell if you so much as stare at it! Damping rings help a lot. It was made as a servo amp for military aircraft, so I'm assuming audio quality wasn't a consideration!

I agree 100% My Tung Sol 6SU7GTY (round black plates, black glass) have defeated any others I've tried in my gear (and I tried them all).

 

[juno]

I will try those if i can find them.

 

[CZ4A]

The 2C52, i have wanted to try that tube. Is your a NOS tube? There are some available at a reasonable price.

Yep, it is NOS as far as I can tell. It tested strong on my emissions tester and has little wear on the printing and pins.

 

[primosounds]

Yep, it is NOS as far as I can tell. It tested strong on my emissions tester and has little wear on the printing and pins.

Ok, thanks for getting back to me. If nos versions of that tube are noisy i will not pursue that one. I have lots of 6sl7 types.

 

[juno]

I just ordered 3 sets of tubes from Andy at Vintage Tube Services.

RCA 58-68 production years
RCA 5691
Sylvania 54-68 Chrome Domes

Should be a pretty well rounded group.
Once I figure out which I like best I will get more of the same for spares.

 

[juno]

In my experience the RCA red base 5691 is not microphonic when new. Microphonic tubes maybe an indication that the tube is old or well used. As tubes age the on/off cycles can relax the metal joints and make them more prone to microphony. This will happen to all tubes doesn't matter what brand it is and especially true for high gain tubes. Test results would not indicate this unless your tube tester has the earphone jack.
For 6sl7, which i use as a driver tube as well as in some of my preamps, the red base 5691 is one of the best but prices are high. Other good ones are tung sol, and sylvania. But, since the 6sl7 is a high gain tube the stability of the structure is important to keep the microphonics low. This is why it seems that the NOS red base 5691 (there maybe some 5691 that are fakes out there) is quietest overall.

I ordered a set of these, along with some Sylvania and RCA 6SL7s

 

[juno]

These just came last weekend. Had the day off so time today to try out the regular RCA. Sound great! I will leave them in a week and then rotate in the others a week at a time.