Microphonic 6SJ7

[Leevi]

I got an amp based on Gibson GA-50 wired and noticed that the preamp pentode
6SJ7 is microphonic. The problem exists only if the chassis is placed in the cabinet (combo).

The tube is a metal shielded NOS tube manufactured by GE probably in 50's/60's.

I found some tips how to eliminate the problem like separate the tube socket from chassis with a rubber ring,
use glass tube instead, use O-ring on tube etc.

Do those really work? Any other tips?

/Leevi

[Jack_Hester]

I just checked my 6SJ7's.  I have old used, and what appear to be NOSl metal case tubes.  And one glass JAN, that appears to be NOS.  You're welcome to try that one if you like.  PM me if you can't resolve otherwise. 

I've found quite a few microphonic metal case 6SJ7's in old amps.  Must be a characteristic of them. 

Jack

[DummyLoad]

I got an amp based on Gibson GA-50 wired and noticed that the preamp pentode
6SJ7 is microphonic. The problem exists only if the chassis is placed in the cabinet (combo).

The tube is a metal shielded NOS tube manufactured by GE probably in 50's/60's.

I found some tips how to eliminate the problem like separate the tube socket from chassis with a rubber ring,
use glass tube instead, use O-ring on tube etc.

Do those really work? Any other tips?

/Leevi

those tubes are *extremely* microphonic. the most stable i have found so far are the tung-sol 6SJ7GT which were also marked "MR" on the box. manufactured during WW2 the MR marking was to indicate that the tubes were to be used for maintenance and/or repair, since nearly all commercial and consumer product manufacturing was suspended for the war effort. unfortunately the tung-sol 6SJ7GT are rare. i have had some success with the sylvania barrel plate 6SJ7GT: those were packed in the green/black box.

to help microphonics with the MT8 type tubes, use a rubber mounted sockets and cover with cork or some other rubber damping ring. it's not a cure but it does help, especially in a combo. 


--pete

[smackoj]

I built an early Valco tweed Supreme amp which uses a 6SQ7 for the preamp. I had lots of trouble finding one that didn't make noise. I switched the tube to a 6SL7 (requires re-wire of socket) so I could have a dual triode and buy them for half the price of a 6SQ7.  I didn't try the red rubber 'O' rings but I know David Barnes at Vintage 47 Amps puts them on his version of the Valco Supreme that he builds and sells commercially.

 

[kagliostro]

As far as I can know there are two types of metal tubes

one has a glass tube inside, the other not





the use of a rubber mounted socket I think is the best choice

if the use of an O ring on a glass tube can be of some help

the use of O ring in a tube that has a metal shield, covering a glass I don't think can give much benefits

because can be an aid to smooth vibration on the metal but not on the under glass

So the rubber mounted socket seems a must to me

K

[Leevi]

Thanks for the responses!
I installed a rubber O-ring between the tube socket and chassis and used even rubber washers.
The microphonic problem disappeared, I didn't even need to use the damping ring on the tube.
The only worry is the temperature since the ring I installed is specified up to 125C (257F).
The 6JS7 is not running as hot as power tubes and the ceramic socket will isolate the heat, hopefully.


/Leevi

[kagliostro]

Go to a plumber shop and ask for red O ring, may be you can find it at a lower price than in an amp spare parts boutique

Ciao

Franco

[Leevi]

I bought the whole set for 15,99 Euro


http://images.biltema.com/PAXToImageService.svc/article/xlarge/86703


/Leevi

[kagliostro]

I got the same set for 10 € at a radio amateur fair (a pair of years ago)

but if you want the warranty about temperature resistance, you must get the red ones

Franco

[Leevi]

I think I'll first perform some temperature measurements there and do
as you proposed if needed.


/Leevi

[Leevi]

The temperature is not a problem. The measured value
after one hour playing was 35C (95F).

/Leevi

[kagliostro]

35° C is nothing, this summer things will be different, but think you can stay with the black ones


Franco

[HotBluePlates]

... the ring I installed is specified up to 125C (257F). ...

You can't boil water on the chassis, so you should be good.

The temperature is not a problem. The measured value after one hour playing was 35C (95F).

When I left Iraq, the temperature was almost 53C (127F). So your chassis is nice and cool.

[PRR]

Auto-parts stores stock O-rings rated for engine heat, which will be fine for all preamp uses.

They generally (here) have an open-set where you can buy just one/two.

Power tubes will run MUCH hotter high on the glass, but power tubes usually are not microphonic unless there is something loose inside (a bad thing-- if it gets in the wrong place the tube becomes a dead-short).

[PRR]

O-ring dimensions, USA-inch sizes:

http://catalog.precisionassoc.com/asset/PAI%20Standard%20O-Ring%20Dimensions%20&%20Illustrations.pdf  (1.6MB PDF)

The rubber-thickness can be 1/16", 3/32", 1/8", 5/32", 3/16", 1/4"

For tube damping you probably want a lot of rubber, 3/16 or 1/4.

6SJ7GT is 1-3/16" max glass diameter.
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6SJ7.pdf

1/4" rubber does not go that small ID, so try 3/16" rubber.

"Mold ID" 1-321 is nominal 1-3/16" ID, actual 1.162" ID, or 0.0255" tight. This may be a loose fit. Try it on the tube, or also get a 1-320 1-1/8" nominal (0.0875" tight).

[HotBluePlates]

I installed a rubber O-ring between the tube socket and chassis ... The microphonic problem disappeared ...

At least some (early) tweed Fender amps used black rubber O-rings between the tube sockets and the chassis. I had a '54 Princeton and a '55 Tremolux which both had these.

See the pic below, where the O-rings are difficult but not impossible to see.