Old tubes - please advice needed (6SC7, 6J7)

[slidincharlie (Carlo P)]

I have an old Oahu (Valco made) tube amp. In the preamp section it has a 6SC7 and a 6J7, both metal RCA (probably original).
I don't know the shape of these tubes. In particular I wonder why the 6J7 looks so odd:


What is that cap on top of the tube?
Is such a tube still in production, or should I look for a NOS or UOS tube?
Shall I keep the cap and braid in any case? Or should I get rid of them if I find a glass tube instead of metal)?

[dai h.]

I think you should probably be able to find NOS. The thing on the top is a connection (maybe the grid??). Some tubes have top connections. Maybe the tube looks weird because the body is made of metal instead of glass. I don't know much about the differences between the metal and glass versions. I suppose the metal ones provide better shielding.

[soundmasterg]

The metal ones are often microphonic, and some have connections to the metal shell too. Its best to look them up in the RCA manual and see what the specs are.

[Enzo]

Metal tubes were common in that era, but were replaced by glass versions. No, these are not in production.

Those are grid connections on top. SOme power tubes have the plate connected on top, but the connector would be very well insulated and not under a metal shield cap like yours. That can is the shield, note the shielded cable shield is soldered to it.

If a glass version exists it still would need the grid cap, and the metal thing shields that connection.

[slidincharlie (Carlo P)]

Thank you guys.
I know that old tubes had a metal envelope sometimes.
I have searched for a 6J7 or substitute (according to the TDSL database at Duncan Amps website) by KCA Nos Tubes and Watford Valves, to no avail.
I wonder if a glass 6J7 tube (in case I find one...) still has the grid connection on top. In other words, shouldn't I change anything with a new tube?
I attach here a schematic of the preamp section, in case it helps.
And this is the tube schematic:

[Tom Phillips]

Carlo,
The 6J7 is readily available even though it is out of production. Look for companies that supply tubes to radio restortion type people. For example, one source in the US is Antique Electronics supply at http://tubesandmore.com/
They have both the metal and glass type listed. They are not too expensive but it can be difficult to find one with low microphonics. Hopefully, you can find a supplier on your side of the world.
Regards,
Tom

[JC@]

Carlo try an EF36. It is equivalent, and you may be able to choose from a lot of brands. You may even find a mullard
It's glass (usually finished with conductive red paint), and a bit taller in size.

[slidincharlie (Carlo P)]

JC,
what about the top connection? Shall I put that metal cap on top of the EL36 too?
This is the EL36 schematic:


(BTW, the TDSL database does not give an EL36 among possible substitutes for the 6J7)?!?

[Tom Phillips]

Carlo,
I forgot to address your other questions.

Originally Posted by slidincharlie (Carlo P)

I wonder if a glass 6J7 tube (in case I find one...) still has the grid connection on top. In other words, shouldn't I change anything with a new tube?

Yes. It will still have the cap and you will not need to change the socket wiring.

Originally Posted by slidincharlie (Carlo P)

In other words, shouldn't I change anything with a new tube? I attach here a schematic of the preamp section, in case it helps.
And this is the tube schematic:

No need to change the top cap wiring after you get a replacement 6J7 or EF36. However, here is something to consider if you don't get enough gain. You may want to re-configure the input circuit. The schematic shows that the input signal will be attenuated quite a bit when only one of the three input jacks is used. The equivalent circuit is a voltage divider with 47k on the top and 23.5k on the bottom so you will loose 66% of your signal.

Regards,
Tom

[Alf]

Carlo, if you are going to use an El 36 use a porcelain anode cap that's safe.
I just bought 8 of them last week.

regards, Alf

[Tom Phillips]

Check again. You want an EF36 not an EL36. Otherwise, I'd stick with the 6J7 to be sure.
This page http://tdsl.duncanamps.com/show.php?des=6J7 has information that says the EF36 has "Different ratings or performance" than the 6J7.
In either case, the top cap connects to the grid and your amp has an additional shielding cap. The shielding cap is grounded as Enzo pointed out. I would not change it. The porcelain type cap is often used when connecting to the plate which is at high voltage and usually not shielded.

[JC@]

Originally Posted by slidincharlie (Carlo P)

JC,
what about the top connection? Shall I put that metal cap on top of the EL36 too?
This is the EL36 schematic:


(BTW, the TDSL database does not give an EL36 among possible substitutes for the 6J7)?!?

DON'T!!! PLEASE DON'T DO IT!!!!
EL36 is a completely different tube.
I meant to say EF36, sorry.
That is 6J7 = EF36
again: you would be looking for an EF36

[Tom Phillips]

Originally Posted by slidincharlie (Carlo P)

JC,
This is the EL36 schematic:

The diagram above is NOT the same as a 6J7 or an EF36. The above shows the plate connected to the top cap. I think that's why JC@ is concerned.
See my previous post.
Tom

[Rob Mercure]

Hey Carlo,

For several 1930s designs the RMA/EIA tube designation system was "hijacked" to produce single ended designs from previous double ended (top cap) designs. The 6SJ7 is indentical to the 6J7 but single ended (6SK7 the same but the SL7 and SN7 dual triodes don't follow this rule.) If you send me a box with the postage attached I'll send you a few 6SJ7s to play with - this would be the easiest socket conversion - I probably have some 6J7s also. Use the PM feature to contact me.

Rob

[slidincharlie (Carlo P)]

Sorry guys!!!
I haven't received any email notification of further replies after my post #8 above, so I thought that this thread was at a dead end.
Thanks for your replies.
I'll see if I find a 6J7 or an EF36 (sorry for the mistake above!)

[Tom Phillips]

Carlo,

I wondered what happened. I now realize that I missed some of the email notifications for this thread too.

I was looking at the photo of the installed tube that you posted and I just noticed that the shield braid seems to be disconnected from the top cap. Is that so?

Tom

[slidincharlie (Carlo P)]

No Tom,
it just depends on the photo angle. The braid is soldered onto the top cap.