Octal tubes and microphonics?

[Rhodesplyr]

While we're on the topic of octal tubes, I was wondering if most high-mu octal tubes tend to be microphonic in combo amps.

Or, to ask the question another way, are the larger plate structures in octal tubes inherently more liable to ring at audio frequencies?

I was recently working on a guitar amp that uses two 6SC7s, and, out of six I have on hand (four 6SC7 and two 6SC7-GT), I was only able to find two that would work without ringing badly at certain frequencies--and that was with the help of silicone tube dampers.

And are there any tricks--instead of or in addition to tube dampers--that can help minimize the tendency to microphonic resonance if you're stuck with a combo amp that uses octal preamp tubes and reengineering the whole thing is not an option? The problem seems worst to me in amps that put the tubes right behind the speaker.

I love separate heads and speaker enclosures.

[greekie]

Mount the sockets on rubber grommets. Tie some thick, flameretardent material around them, to get some absorbant mass around the glass bottle.

YM2C

[soundguruman]

It's not really the tube that's microphonic. But people do blame the tube for what is really the design of the circuit, causing a ringing noise.
Tube dampers, such as the foam damper on a Hot Rod Deluxe, work very well. The old school was to mount the socket on a rubber O ring or Grommet, But that does not work as well as a real tube damper..
Some people have started selling silicon O rings as "tube dampers," which they are not really.
These days, it has become standard to add snubber caps to the circuit, such as a 7 pf 1000V silver mica cap between plate and grid of V1 (negative feedback tweak).
But it is not really the tube, it is the design of the circuit, too much gain in a single stage, etc...a properly designed layout and construction won't have that problem, despite whatever tube is selected.
Now I like the 6J7, and have gone to enormous lengths to isolate it from acoustic waves reaching the envelope.

It is the sound, from the speaker, that travels through the air, and reaches the guitar pickup. The pickup channels that acoustic wave back to the input of the amplifier, forming a loop. And that's where your ringing is coming from, not the tube's fault at all really.

[Rhodesplyr]

It's not really the tube that's microphonic. But people do blame the tube for what is really the design of the circuit, causing a ringing noise.

It is the sound, from the speaker, that travels through the air, and reaches the guitar pickup. The pickup channels that acoustic wave back to the input of the amplifier, forming a loop. And that's where your ringing is coming from, not the tube's fault at all really.

Ah, so you're saying that it's positive feedback, reaching a peak at certain frequencies. Perhaps I can test that by sending a
signal through the amp from a synthesizer or other source unlikely to be significantly affected by sound from the speaker.

On the other hand, I can damp the ringing by putting my hand on the tube while playing a note that makes it ring. I agree that the silicone rings don't work as well as one might hope.

It's a terminological quibble, but a capacitor from plate to grid of a tube is not technically a snubber, as I understand the use of the term. What you're talking about is bandwidth limiting by applying negative feedback at high frequencies. I have heard of it being called a "Miller capacitor."

[soundguruman]

Yes it is for sure positive feedback, which is often misunderstood. Yes can call it a Miller cap.
Fender and Marshall have used the Miller method on many amps.
Mesa arranges the circuit tracks on the board to act as capacitors, to increase the interelectrode capacitance, same idea.

[Rhodesplyr]

Driving the amp with my synthesizer, I get the same rattle. Perhaps I used the wrong term when describing it as 'ringing.' I can damp the rattle with my hand on the tube's metal envelope, and I can feel the chassis vibrating as well. I can damp it pretty well with two silicone rings, but it's not perfect.

Thus, I think the mechanism is simply speaker to chassis/tube.

I think that either isolation mounting or a spring retainer would help.

[Wombaticus]

It's not really the tube that's microphonic. But people do blame the tube for what is really the design of the circuit, causing a ringing , etc.....a properly designed layout and construction won't have that problem, despite whatever tube is selected.

This is just silly. Of *course* tubes can be microphonic.. i.e. constructed in such a way that a mechanical resonance is readily established at certain frequencies. Sure the acousto-mechanical coupling amounts to a form of positive feedback, but such feedback certainly isn't a flaw that can be reasonably considered inherent to the circuit. And yes, some of the octal preamp tubes are much more prone to this well-attested phenomenon than are others. 6SC7s in particular are notorious for this. I have also had difficulties with 6SL7s - and this in circuits where substitution of a higher mu noval tube such a 12AX7 effectively eliminated the problem.

[J M Fahey]

It's not really the tube that's microphonic. But people do blame the tube for what is really the design of the circuit, causing a ringing noise.

How would you know? Specially not being there.
Even more, because Rhodesplayr (who is there) states

On the other hand, I can damp the ringing by putting my hand on the tube while playing a note that makes it ring.

and then again:

I can damp the rattle with my hand on the tube's metal envelope, and I can feel the chassis vibrating as well. I can damp it pretty well with two silicone rings,

Unless, of course, you call rhodesplayerīs hand a "snubber cap"

[Steve Conner]

"Miller cap cures microphonics" is another of these weird tube myths that won't go away. I've heard it often and never quite believed it.

The capacitor can certainly cure high-pitched squeals caused by electrical feedback through stray capacitances. It does this by rolling off the gain of the unwanted feedback loop. But that's a totally different thing to mechanical feedback. Maybe it can make a slight difference to the electric part of the loop, but my feeling is that it would be impossible to get enough rolloff to slug microphonics, without dulling the tone.

In hi-fi amps, global NFB seems quite effective at killing microphonics in the power and driver tubes, but that's a LOT of feedback, at least 10x more than anyone ever used in a guitar amp.

[boots]

Can somebody fill me in on the silicone dampers? Are they essentially a cushion under the socket, or are they something that actually stabilizes the tube itself?

I have become curious about octal preamp tubes because I have a small stash of them.. Sounds to me like it might not be worth the trouble and expense of building an amp around them, but I would certainly keep my eyes peeled for an old Champ or Deluxe with 6SC7's just to satisfy my curiousity about how they sound.

[Rhodesplyr]

Silicone dampers are silicone rubber O-rings that can tolerate high temperatures. Google or search eBay for sources. You roll them down the tube envelope to the position that best damps microphonic behavior. Sometimes one will work, but it can require two or more. And sometimes, they aren't enough, no matter how many you use.

The problem with some of the octals being microphonic is their behavior in combo amps where they're right behind the speaker. If you built a head with a separate speaker enclosure, you'd probably be safe.

[boots]

Thanks. I have been building exclusively combos because damn, they are cool. Nothing beats being able to haul a small combo amp around with you. But I definitely see the advantages of separate head/speakers, and I think my next projects will be a stand-alone head and a separate 1x12" cab. I'd love to build up a sealed cab with a Celestion Blue or a Weber Blue Dog, with the option of removing half of the back panel.

All it takes is a little more money, a lot more time, and a bit more of neglecting my family while I tinker in the shop. The next build will probably have to sit on the back burner for a spell while my family recovers from my last project....

[Rhodesplyr]

Nothing beats being able to haul a small combo amp around with you.

I say this all in good fun, but as a keyboard player, I can't find that much sympathy in my heart for guitar players in terms of complaints about the weight of their gear. :-)

[J M Fahey]

Fine ... so far, but guitars arenīt getting any smaller nor lighter, while Iīve seen featherweight keyboards with Himalaya weight sound stored inside.
Of course, if you still stick to a real "Rhodes" .... may God help you.

[Rhodesplyr]

This is a completely random idea that occurred to me, but does speaker phasing ever have any impact on microphonic tube misbehavior?

I'd guess that the answer is 'no,' but I thought I'd ask it anyway.