Gutting a 12sq7

[Zipslack]

Well, it turns out my only 12sq7, that I'm using in a project, is microphonic (tinny rattling vibration).  Since my hole and socket are octal size, I'm contemplating gutting it and wiring a 12av6 to it's base. Does anybody foresee any issues, or know of anybody that has done this?  I want to keep my socket for when I find another 12sq7.  I'm guessing there is a bottle under the metal shield - is it possible to remove without compromising the glass, just in case it's only the shield rattling?

If nobody has suggestions, I'll take pictures and post results later.

[Jack_Hester]

Later on this morning, when I go out to my shop, let me look through some of my 'non-standard' tubes and see if I have one.  What about a 6SQ7?  Can you use one of those, if I find one?  Just have to drop the heater voltage.  I'll PM you with what I find.

Jack

[Zipslack]

Thanks, but I can probably salvage one - I'm constantly coming across old radios and phonos. I just usually pass them up because I never had a need for those tubes.  I have 2 12av6 collecting dust and thought it might be an interesting experiment.

[HotBluePlates]

I'm guessing there is a bottle under the metal shield - is it possible to remove without compromising the glass, just in case it's only the shield rattling?

There is no bottle under the glass. The glass envelope tubes came out later due to advances in certain tube-making techniques that allowed good vacuum and seals in an all-glass package.

If you want to make an adapter, use a 7-pin socket and an octal plug (like the empty octal plugs Doug sells for a bias probe). Triple-check the relative pinouts for the two tubes, and attach leads to the 7-pin socket first. The stripped ends will pass through and out the pins in the octal plug before soldering the octal plug.

[kagliostro]

+ 1 for the adapter

K

[TubeGeek]

I have a GE 12SQ7 I don't need or plan to use.  It's yours if it helps you out.

[Zipslack]

Actually found an Olympic phono/radio at a local junk shop...if he decides to take my $10 offer I'll get it.  Thanks for the offer, though.

I think I'm just going to solder wire straight to the pins and into the proper connections on the sq7 base.  BTW, it turns out there is a bottle inside the can, but it appears to be glued tigether.

[kagliostro]

If you have trouble on inserting the 7 pin tube in the opened old 12sq7 or you don't have a tube to sacrifice

I think the HBP adapter proposal is fine (is not easy to solder directly to tube pins)

otherway, if you want to preserve the metal look, you can use a base like that indicated by HBP and a copper shield (you can find it at hydraulic shops) black painted

here an example (it is a SS rectifier to be used instead of vacuum rectifier that sells Weber)



K

[Jack_Hester]

I found two 12SQ7's and a number of 6SQ7's.  One of the 12's tested bad.  It looked bad, too.  The other was a NOS that tested weak, but usable.  A couple of the 6's tested way better.  Anyway, I have each, if you decide to go that route.

Jack

[HotBluePlates]

BTW, it turns out there is a bottle inside the can, but it appears to be glued tigether.

If you continue de-constructing that tube, saw through the side of the envelope.

I think you'll find that's not a bottle inside the metal envelope, but just a glass seal at the bottom. The glass doesn't continue up the sides.

I'll recheck some of the old books, but I believe that is what they show from photos of these tubes (which show the internal structure of tubes; the book predates most all-glass tubes).

[Zipslack]

Thanks for the info...I decided not to bother wasting more effort on a bad tube.  I saw the glass on the bottom and assumed it went all the way - learn something new every day.  It's also soft of strange that the glass pinch-off nipple goes out the bottom into the center locking pin.  I salvaged a PCB-mount socket from the junk radio that provided the 12av6, so I'll use that to connect to the SQ7 base to make my adapter - still in the $0 ballpark.  Wire the pins to the proper places, use a little hot glue to solidify the adapter assembly, and I'll have a handy little adapter for emergency use in the future.  I think hot glue will be okay since the tube is mounted upright and it will be between the PCB plastic socket and the SQ7 base.

I don't know that I'll need a shield, but if I do, the junker radio had a cheapo shield on one of the tubes (overlapping roll of thin metal with a wire soldered from it to chassis).

For everybody that offered, thanks, but save your postage.  I appreciate it, but I'm sure I'll come across some more during my garage sale/flea market scrounging - and people that know about my hobby will sometimes bring me free donor junk (like the radio and pthe honograph that supplied parts for my little amp project that led to this).  I just wanted to see if I could find something useful to do with these otherwise "unloved" tubes that I had collecting dust.  The flea market that had the Olympic radio/phono will probably work out - wait and week and he'll probably take my $10 offer just to get rid of it.  He also had a real nice cabinet phono/radio (with the drawers you pull down and the phono pivots out) that he's willing to take $40 for (non-working according to him) - it has a beefy power transformer, 2 6V6's (P-P?, field-coil speaker), 6SF5, some various metal-base 7** tubes (7A7 and others).  I think he'll probably take $50 for both if I give him a week to ponder it.  Plus my wife really wanted the console cabinet to use as a storage/bookshelf cabinet.

[Zipslack]

Finished product.  The 12av6 had a lot more noise, so I installed the cheap shield - fortunately, the shield pin lined up perfectly with the #1 shield pin of the 12sq7.  Hot glue to stabilize things with a little heat shrink for extra safety.  It works great, but isn't quite the same as the sq7 and still has more hum even with the shield. The hum is fine as long as I keep a hand on strings or bridge. Here's some pics

[Zipslack]

And in operation...

[kagliostro]

As is a very cheap mod

did you considered to have elevated heaters ?

K

[Zipslack]

Nah, but I may have to look into it - my cheap, simple amp is getting more complicated.  With the 12SQ7, I didn't have as much of an issue.  The substitute is okay for an emergency spare, though.  I think I have a 200 Ohm pot somewhere....

BTW, I posted a soundclip of the actual amp using the 12AV6 to the original thread http://www.el34world.com/Forum/index.php?topic=14602.0

[PRR]

> elevated heaters ?

The way he is heating it, that's not possible. (One side of the PT is common.)

Sure, he could get more complicated, but this is supposed to be simple.

Anyway, with a 4-inch speaker, there is NO 60Hz coming out of the speaker.

When he runs it into a eight-10" full-stack with honest 60Hz response, he may want to push the 12V lead to the preamp heater into a least-hum route.

And maybe not, because total gain is not large.

[birt]

if the hum is less loud when you ground the strings there is a ground problem. your new tube probably has more gain so you can hear the hum better. or maybe you introduced the problem with a bad solder connection.

[Zipslack]

Yeah, I'm thinking there's a ground issue somewhere.  It wasn't there with the 12SQ7 in place, even when connected to my Blue Jr. III's 12" speaker - just your basic low-level SE AC-heater hum, but nothing bothersome.  I'm wondering if my adapter leads needed to be routed differently - the heater wires had to go across the socket to mate with the 12av6 pins.  I need to check out my grounding scheme, too.  I had the shield going straight to chassis, but everything else is a modified-star -- power grounds to one point on chassis, signal grounds to another point on chassis, ground wire from the 3-prong to the chassis.

PRR, good call on the balance pot - I had forgotten that one side was grounded for the voltage doubler.

[PRR]

> one side was grounded for the voltage doubler.

And mainly because you use ONE winding to get both A (heaters) and B+.

With a separate heater winding, it floats, and can be tied to a modest positive voltage to repel a specific type of heater leakage.

You can also Balance it, so hum fields tend to cancel.

But this simple power scheme does not allow either option.

You could also build a MUCH (4X) larger DC supply. Now run your heaters on DC. They will be naturally "elevated" too. However it's 50mA for the plates and 150mA for the heaters. And poorly-filtered heater power running near sensitive pins is *worse* than pure AC (especially with small speakers that drop 60Hz but emphasize 180 360 etc Hz). And you get back to the issue of 62V heat and 100V B+. Unless you accept much less power (you might; have you tried 62V to the plates for a small-small amp?), you need complications on your supply to drop 130V to 62V heat, or get both 130V and 62V well-filtered from the same doubler.

It's easy to get wild ideas. I have a 3-tube Kent in the garage. I realized that 12SL7 12SL7 50L6 50L6 is a fine line-up for 4 Watts and tone-nets, and the heat can be same as B+. But at that point there's nothing left of the Kent except the cardboard cabinet, and four Octals with four pots is no 1-hour build. I need to panel the loft and rewire the fusebox, I shouldn't get started on a time-sink. (But then again, PartsExpress has a closeout speaker to beat the pee out of the Kent's sad squawker for only $7....)