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'Power tubes near speakers could cause tuble rattle'
2009\05\11@215221 by Edson Brusque

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Hello,

    I have a question that maybe some of the tube guys on the list can
answer.

    Can a speaker's magnetic field interfere with tubes/valves and
damage it?

    Let me try to explain the situation. I've made some combo guitar
amps. They are just prototypes I'm making while studying tube circuits.
They uses a pair of 6L6 in the power section and gives close to 40W
maximum output power.

    Almost all the 6L6s (3 pairs) I've been using have developed tuble
rattle. I still have to make a simple single ended power amp to
individually test the tubes and discover wich ones are really
compromised but it seens all 6 have different levels of rattle. Tapping
the tubes with a finger while holding it near the ear, I can hear
something vibrating inside the tubes. The frequency/volume of the
vibrations changes from tube to tube and also depending on where I tap.

    This is really intrigging me. The power section works at about 450V
on the tubes plates. Plate current is about 45mA (each tube). Screen
grid voltage is about 420V and current is bellow 2mA. The power
dissipation seens to be well bellow the maximum allowed. I don't have
any fan blowing air to keep the tubes cool, but there's some airflow by
convection.

    Now, one the power tubes are very near one of the speakers (it's
two 10 inches). The physical layout, side-looking, is something like:

                    .
   +--+  +--+      /|
   |V1|  |V2|     / |
   |  |  |  | .--|  |
   '--'  '--' |  |  |
              '--|  |
                  \ |
                   \|
                    '

    I've been looking for a mail group about audio tubes/valves
circuits but haven't found anything interesting. Maybe there's a
"tubelist" somewhere?

    Best regards,

    Brusque



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Edson Brusque                    Stagetronics Eletro Eletrônicos Ltda
Research and Development                   Blumenau  -  SC  -  Brazil
http://www.ryan.com.br/netiqueta.htm             http://www.citronics.com.br
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Edson Brusque                    Stagetronics Eletro Eletrônicos Ltda
Research and Development                   Blumenau  -  SC  -  Brazil
http://www.ryan.com.br/netiqueta.htm             http://www.citronics.com.br
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2009\05\11@221103 by Richard Prosser

picon face
Brusque

Have you tried the "listen" test to a virgin tube? As I recall, most
tubes would have elements that would vibrate when shaken. Mostly the
various grids etc although the whole structure can also vibrate/rattle
a bit.

Is it causing a problem? Are you getting excessive microphony / feed back etc?

Since the grids etc. are sitting in a vacuum they will not be subject
to speaker vibration directly, can you mount the valves using a
damping material or clamp them to something solid?

I don''t thing any external magnetic field from a speaker would be
enough to cause a problem. (I have seen how a magnetic field from a
power transformer can effect a CRT tube however - took a while to
figure out why part of the trace was running "backwards").

RP

2009/5/12 Edson Brusque <spam_OUTbrusque.listasTakeThisOuTspamcitronics.com.br>:
{Quote hidden}

>

2009\05\11@223446 by John Gardner

picon face
a) Where the heck are you getting 6L6s?

b) Vibration coupled into the tubes is the most likely culprit,
   aka "microphonics", although other components can also
   be affected. Are you using tube pre-amps (12AX7?) They
   can also be affected by microphonics.

Magnetic coupling is a pretty low-order suspect in audio apps.

Just re-read your post. 450V plate voltage? Wow...

Sounds like fun :)

On 5/11/09, Richard Prosser <.....rhprosserKILLspamspam@spam@gmail.com> wrote:
{Quote hidden}

>> -

2009\05\12@120913 by Edson Brusque

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Hello everyone,

    thanks for all the replies. This will be a long email. :)

    I will not try to answer points already clarified by other posters
but I'll add some info when I find it necessary or cool.

Richard Prosser escreveu:
> Have you tried the "listen" test to a virgin tube? As I recall, most
> tubes would have elements that would vibrate when shaken. Mostly the
> various grids etc although the whole structure can also vibrate/rattle
> a bit.

    Yes. I've tested in the circuit and off the circuit (tube tapping
near the ear). The new tubes make some noises when tapped but it's very
lower than the damaged tubes noises.

> Is it causing a problem? Are you getting excessive microphony
> / feed back etc?

    Feedback/microphony is very low. But certain notes give rise to a
rattling sound. I'll record it and put the wave files on my server for
you to listen.

> Since the grids etc. are sitting in a vacuum they will not be subject
> to speaker vibration directly, can you mount the valves using a
> damping material or clamp them to something solid?

    The damaged tubes are not a big problem. They cost money but I can
just put them aside and buy new ones. The problem is that I'll probably
damage the new tubes if I don't find what's causing this.

John Gardner escreveu:
> a) Where the heck are you getting 6L6s?

    It's very easy to find it actually. There's a big market for guitar
amplifier manufacturers that uses a lot of them. One of the best places
on the web to find new tubes is http://www.tubedepot.com.

    Professional audio market are very big also. There's a lot of
(tube) compressors, equalizers and microphone preamps on the stores and
studios.

    "Audiophile" market are not so big but still present. Anyway, I
thing the "audiophile" people of today are wasting theirs money on home
teather equipments. Maybe I should manufacture oxygen-free video cables
and actually make some money.

> Just re-read your post. 450V plate voltage? Wow...
> Sounds like fun :)

    Well, not so fun when you're not carefull. It could bite hard. :P

    Some high power tube amplifiers go well above it. KT88 tubes are
rated for 800V IIRC.

Olin Lathrop escreveu:
> Or course they are, just not thru the air.  They are mounted to
>something, which is mounted to something else, which eventually can
>be vibrated by the speaker.

    Yes, you're right. My first suspiction was that with the tubes hot
and vibrating, something could get loose inside. Then, while looking at
that tube almost touching the speaker's magnet, I tought if the magnetic
field could have some guilt. Tried to find something about it on the web
and my books but found nothing.

> Why?  Is this because you want to learn about tubes, or because you
> prescribe to silly audiophool theories about sound purity, oxygen free
> cables, and whatnot.  If so, get a dead fish and start waving.  It's
> just as effective and a lot cheaper.

    Hahaha. Olin, I really love your sense of humor. If you go to
Masters2009 I'll be glad to buy you a beer.

    I can justify just by saying "well, I'm a guitar player" but that
doesn't tells the whole history. I was always intrigued about why a tube
amp sounds different from a transistor one. I've read a lot about it in
all those years but it seens that there's more nonsense than real
scientific information.

    So, I've tried to find it for myself and about two years ago I
started to study and build guitar amps. The side effect is that I've
learned a lot about other things. The tubes' inner workings are
relatively easy to follow and allowed me to have a better understanding
of semiconductors as well. I've also learned a lot about clearances,
parasitic capacitances and inductances, transformers, power supplies,
(psyco)acustics, EMI, ground loops etc.

    I surelly would recomend it. It pays. Sure, not everyone would
benefit from dealing with such 'old' technologies but I did (and do
everyday).

Mark Rages escreveu:
> No, he is talking about a guitar amplifier, not hi-fi.  Guitar
> amplifiers are intentionally overdriven, and tube overload
> characteristics are definitely different than normal solid-state
> circuits.  Some of the DSP simulators get very close though.

    Yes, that's true. DSP amp simulators evolved a lot on the last 15
years, to the point that a Boss GT-Pro (that I own) plugged to a good
solid-state power amplifier and a Celestion Vintage 30 speaker would
give you "that tone" without all the wasted power/heat.

    Still, there's some charm on 'real tube' amplifiers and guitarrists
everywere (even those that still have most of the brain cells undamaged)
are very fond of it.

    Best regards and thanks again.

    Brusque

--
---------------------------------------------------------------------
Edson Brusque                    Stagetronics Eletro Eletrônicos Ltda
Research and Development                   Blumenau  -  SC  -  Brazil
http://www.ryan.com.br/netiqueta.htm             http://www.citronics.com.br
---------------------------------------------------------------------

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