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 Post subject: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 8:31 pm 
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Location: Northwest Minnesota 56501
I've got a early 50's Crosley and the schematic calls for 2 6SQ7- GT tubes The radio has two glass 6SQ7- GT installed...my supplier only has metal tubes...this may sound dumb, but I thought 'GT' meant glass tube...can I use the metal tubes without changing the performance ( audio wise ) of the radio? Thanks in advance for any information.
Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 8:43 pm 
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Yes.. no problem!
HOWEVER... check pin #1 on the socket carefully, because the metal tubes use this pin as ground and is common to the metal jacket.
The glass tubes did not use pin #1 .. therefore many manufactures used the pin as a tie-point for other elements of the surrounding circuit... so be careful. If pin #1 goes to ground.. ok.

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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 8:48 pm 
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Thankyou for the information...I guess I should have posted this before I recapped the power supply/audio amp on this Crosley...They are two separate chassis, and one of the tubes in question is on that chassis and the other is on the main chassis. Then the metal tube has to have or cannot have that pin grounded ? I'm a tad confused.
-Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 8:50 pm 
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Location: Northwest Minnesota 56501
The #1 pin on the glass tube in the main chassis is grounded.
-Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 8:54 pm 
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Location: Northwest Minnesota 56501
One more thing I just noticed is that the set has 6SQ7 -GT glass tubes but the schematic calls for 6SQ7 ! So I should be aokay with metal. Right ?
-Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 9:00 pm 
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The metal octals were the first tubes with that base, which is why they have no suffix.

Later glass tubes with the same innards have either a G or GT suffix (or variants like GTA, GTB, etc).

For example, the 6R7 was the original tube.
The 6R7G with the "shouldered" glass bulb was added later, and
the 6R7GT (glass tubular, with straight sides) was added even later.

Not all bulb shapes were used with all tubes. I show a 6SQ7 and a 6SQ7GT, but no 6SQ7G.

The W suffix indicates a mil-spec tube. These are generally more rugged than the commercial version.

You'll find other suffixes, like the 6SB7Y. The Y denotes a micanol base which improves stability during warmup.

- Leigh

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Last edited by Leigh on Apr Sun 07, 2013 11:29 pm, edited 2 times in total.

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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 9:01 pm 
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Yes.

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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 9:56 pm 
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Many thanks to everyone for their input !
-Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Sun 07, 2013 11:53 pm 
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Bill Bert wrote:
I thought 'GT' meant glass tube.
Bill


GT = Glass Tubular, denoting the shape.


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Mon 08, 2013 2:42 am 
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One does have to be careful in using metal tubes in a set not designed for them, specifically Philco and Zenith. If pin 1 is not grounded, the metal shell will "float", which can cause oscillation.

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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Mon 08, 2013 5:05 am 
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Tim Tress wrote:
One does have to be careful in using metal tubes in a set not designed for them, specifically Philco and Zenith. If pin 1 is not grounded, the metal shell will "float", which can cause oscillation.
Even worse...

On some radios that were not designed for metal tubes, pin #1 may be used as a tie point for arbitrary circuitry.

If that circuitry happens to carry high voltage, that voltage will appear on the metal tube shell.

- Leigh

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Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Tue 09, 2013 12:30 am 
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HI ALL:
Good discussion about metal vs. glass tubes. Here is my situation. Arvin Model 552AN. Rider page 16 (Arvin)-1. Schematic lists 12SA7GT/G, 12SK7GT/G, 12SQ7GT/G tubes and shows all pin 1 connected to chassis ground. The parts list (Rider 16-3) lists 12SA7 and 12SK7, assumed to be the metal variety, but a 12SQ7GT (glass). The set arrived with 12SA7 and 12SK7 (metal) tubes. Both test bad. What should I use (metal/glass) as replacements, or does it matter in this set.
TIA,
BOB


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Tue 09, 2013 3:26 am 
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BOB BONCHAK wrote:
The parts list (Rider 16-3) lists 12SA7 and 12SK7, assumed to be the metal variety, but a 12SQ7GT (glass). The set arrived with 12SA7 and 12SK7 (metal) tubes. Both test bad. What should I use (metal/glass) as replacements, or does it matter in this set.
Any set that was originally designed to use metal tubes, as this one was, should get metal tubes.

The metal shell provides shielding that is not provided otherwise in the set.

Use of glass tubes may result in oscillation, instability, false responses, or similar problems.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Tue 09, 2013 1:52 pm 
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A lot of the guys don't like the metal tubes because they can't see the filament glowing. As though it weren't a real radio unless it glows! A little silly in my view, but I'll admit it does provide a quick visual check that the tube hasn't burned out. Personally, I prefer metal tubes in the RF and IF sections because of the built-in shielding.

Power tubes, like the 6L6, or rectifiers are another story. They get so dang hot! Blister city! :(

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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Tue 09, 2013 3:19 pm 
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The metal tubes are cheaper to buy.


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 Post subject: Re: Vacuum tube numbers.
PostPosted: Apr Tue 09, 2013 7:28 pm 
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Location: Northwest Minnesota 56501
Many people, mostly teenagers like my two Grandsons, do not even know what a vacuum tube is let alone see one light up and bring in radio stations....therefore I think the glass ones are much more appealing. And as far as not being a real radio if you can't see it light up, why not slip in a solid state unit and rig up the tunning so the original dial pointer works and there you have it....no hum, reliability, and it will only use about 25 watts ! Not for me.....I love to see those tubes glowing. Here's a thought...what do we do when they decide to go all digital with radio broadcasts ? Hopefully I won't live long enough to see that happen.
-Bill


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 Post subject: Re: Vacuum tube numbers.
PostPosted: May Thu 02, 2013 1:13 am 
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Now I'm really confused. Obtained the Sams service data for the Arvin 552AN. The parts list and schematic both list all GT tubes, but the Sams photo clearly shows metal tubes , 12SA7 and 12SK7. Again, as in Rider, the Sams schematic shows all tube pin one going to chassis ground. Is this a case where GT and metal versions are truely interchangeable? TIA.
BOB


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 Post subject: Re: Vacuum tube numbers.
PostPosted: May Thu 02, 2013 4:31 am 
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Hi Bob,

It's very difficult to reverse-engineer shielding issues.

Metal tubes are "safer" from a shielding standpoint. To my mind they're the better choice.

- Leigh

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Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Vacuum tube numbers.
PostPosted: May Thu 02, 2013 1:09 pm 
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It should be noted that metal tubes were the original octals, and were produced by RCA.

Most of the "G" versions were designed by RCA's competitors, and may have compatibility issues like pin 1 connection, physical size, and shielding. RCA did not support "G" octals to any great extent.

"GT" tubes were introduced a few years later as more or less universal replacements. Most types contain internal shielding, so they can usually be used in place of metal octals. But there's no connection from pin 1 to the metal base shell (on some versions) so no shock hazard if used to replace a "G" tube.

GT tubes turned out to be so much cheaper to make than metal octals that RCA eventually adopted the design themselves.

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