Need Advice Regarding 6SL7 Preamp Tubes

[lousy13]

I need some advice about 6SL7 preamp tubes. I have a Vintage 47 Spectator that takes a 6SL7 and I've tried two new Tung-Sol 6SL7GT tubes and both are microphonic at least that is my assumption. When I play an open note on the low E string I get that fizzy sound and it becomes more pronounced when the volume is increased.

My questions are:

1) Is this sound consistent with a microphonic tube?

2) Is there a better brand of tube I should buy that is more consistent than these new Tung-Sol tubes?

3) Is it possible that there is some vibration in the chassis or cabinet?

 

[schmee]

I've never thought of microphonic as "fizzy". More of a squeal or high harmonic note that sustains.
Is the speaker ancient? Have you tried plugging into another speaker? I test old speakers on the low E string and a bad voice coil will often be bad in the E to B range on that string, louder is badder usually too.
The 6SL7's were noted for being microphonic IIRC.

 

[VintageSG]

1) Yes. Can be. Can be other things too. Try a speaker cab/different speaker. Is the power supply stable?

2) NOS, or explore NOS Soviet alternatives/clones ( Reflektor 6H9s/6n9 ), either of which is likely less expensive than new production. Won't hurt to have spare(s)

3) Possible.

 

[slider313]

The new Tung Sol 6SL7's I've tried have been quiet. You may want to try moving the lead dress, one wire at a time, to see if that helps.

I've had power tubes produce sympathetic overtones on certain notes also, so don't rule them out.

 

[lousy13]

Thanks schmee, Vintage SG and slider313 for the input's. This gives me a couple of things to troubleshoot. I really like this amp for 50's/60's Chicago and Jump blues stuff just need to eliminate the noise.

 

[lousy13]

Just a note. The amp is only a couple years old so I don't think the speaker is a problem but I might load a different one for no other reason than to eliminate that as a problem.

 

[Paul G.]

Microphonic tubes tend to sound "ringy" or can "howl", but not usually fizzy. While the amp is on, just tap the tube lightly, you'll know right away if the tube is microphonic.

 

[lousy13]

Microphonic tubes tend to sound "ringy" or can "howl", but not usually fizzy. While the amp is on, just tap the tube lightly, you'll know right away if the tube is microphonic.

I think I confused this by using the word fizzy. "Ringy" is a better way to describe it.

 

[Nickfl]

Microphonic tubes tend to sound "ringy" or can "howl", but not usually fizzy. While the amp is on, just tap the tube lightly, you'll know right away if the tube is microphonic.

This. Tap the tube with a chopstick while the amp is on, if this produces a glassy tapping sound through the speaker the tube is microphonic.

Microphonic tubes aren't necessarily a problem, but they can make the amp unstable or cause a screechy ringing overtone, in which case they need to be replaced or moved to a different position if you have somewhere else you can try to use them.

Depending on how bad it is, you may be able to eliminate the problem using silicon o rings placed around the glass envelope to dampen the tube. I've not had much luck with this, but its worth getting a few $0.20 o rings to give it a try. Grommets placed between the tube socket and chassis to dampen vibration are another idea I've seen (Fender used them on some amps apparently).

Are these the tubes that came with the amp or did you replace the originals? Octal preamp tubes are unfortunately prone to this issue, especially 6SL7 and 6SC7 I believe.

 

[lousy13]

One tube is the original and the other is one I purchased. They are both new Tung-Sol tubes and they both sound the same when I switch them out.

 

[slider313]

If both tubes do this, then I would start with moving the lead dress attached to that particular socket. You will have to do this with the amp on, using a plastic probe or wooden "chopstick", one wire at a time. Sometimes wires are routed too close to each other or sometimes just lifting one off the chassis does the trick.

 

[lousy13]

Thanks slider. I will give that a try. By the way Duane Allman is the bomb.

 

[lousy13]

Just to finish this thread. I bought a Sovtek tube from TubeDepot and specified low microphonics and put it in today and no noise.

 

[radiocaster]

Just to finish this thread. I bought a Sovtek tube from TubeDepot and specified low microphonics and put it in today and no noise.

Interesting. Is it just a relabeled and tested NOS? All Sovtek octal preamp tubes I've seen in pics are.

If it is, it should have Cyrillic writing on one side and be marked 6H9C. Also the month and year of manufacture.

 

[lousy13]

This is it. This is what TubeDepot has to say about it on their website. "The Sovtek 6SL7 is a suitable replacement for any 6SL7 tube. Sovtek 6SL7 are NOS Soviet era tubes made in the mid 70's in the Novosibirsk factory. Some tubes may have dust on them, but are new and unused. Good reliability at a great price.

 

[teleman1]

This. Tap the tube with a chopstick while the amp is on, if this produces a glassy tapping sound through the speaker the tube is microphonic.

Microphonic tubes aren't necessarily a problem, but they can make the amp unstable or cause a screechy ringing overtone, in which case they need to be replaced or moved to a different position if you have somewhere else you can try to use them.

Depending on how bad it is, you may be able to eliminate the problem using silicon o rings placed around the glass envelope to dampen the tube. I've not had much luck with this, but its worth getting a few $0.20 o rings to give it a try. Grommets placed between the tube socket and chassis to dampen vibration are another idea I've seen (Fender used them on some amps apparently).

Are these the tubes that came with the amp or did you replace the originals? Octal preamp tubes are unfortunately prone to this issue, especially 6SL7 and 6SC7 I believe.

They are also prone to sounding incredible.

 

[teleman1]

This is it. This is what TubeDepot has to say about it on their website. "The Sovtek 6SL7 is a suitable replacement for any 6SL7 tube. Sovtek 6SL7 are NOS Soviet era tubes made in the mid 70's in the Novosibirsk factory. Some tubes may have dust on them, but are new and unused. Good reliability at a great price.

pm'd

 

[teleman1]

Have someone imitate the noise you hear while they play your guitar. tap on the power tube and the 6sl7. See if it instigates the noise.

 

[BobbyZ]

Octal preamp tubes can get so microphonic that the "tap test" becomes useless. My Reverberockes run two 6SN7s and two 6SL7s, when one gets bad anywhere you tap the amp you hear it. End up swapping tubes, hopefully with a non microphonic tube, until you find the bad one. A real pain sometimes but worth it.
I need to order some 6SL7s, could only find a couple the other day. I've got a bunch of 6SN7s, apparently that was amore common tube back in the day.

 

[lousy13]

I did swap the preamp tube with another and I no longer have the problem. Definitely a microphonic tube. Luckily the amp is small and only runs one preamp tube so it was not a costly diagnoses or fix. I love the Octal sound for certain older blues sounds.