Gas gate?

[HuggyBear]

I have an Airline 14BR-735A It has one tube on the schematic (a 6T7G) labeled "Gas Gate" What does this do? It is a duodiode/triode, similar to an *SQ7 or a 6Q7/12Q7 (same 7V basing) - and it also has a 12SQ7 as a detector / first audio.

This Gas Gate is using the diodes and triode, the triode grid is grounded, plate and cathode are connected together and connected to the cathode of the 12SQ7. The diode plates are active and connected to other places, almost like this whole stage affects the bias on the 12SQ7.

What is this supposed to do?
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Don't regret growing older. It is a privilege denied to many.

-Unknown

[Tom Albrecht]

I can't figure this one out. What is the grid of the gas gate doing connected to the chassis ground (not circuit common)? Otherwise it looks like the triode section of the 6T7 is disabled (both plate and cathode at circuit common).

Are the diode sections of the 6T7 functioning as some kind of audio limiter? The only thing I can see it doing is limiting positive-going audio peaks. Maybe that serves as a "static supressor" ?
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Tom K6VL

[TonyC]

The diodes are forming separate AVC voltages for the IF and RF stages respectively. Not sure why the grid is tied to chassis.
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Tony Casorso

[Tom Albrecht]

Hi Tony,

That helps a bit with understanding it, but I'm still scratching my head a bit. What is the advantage of having separate AVC lines? Perhaps the gain curves of the different stages need different voltages?

And where did the terminology "Gas Gate" come from? When I first saw that, I thought maybe it was a thyratron or something like that, and it might be a crazy squelch circuit. Studying the circuit and looking at the tube specs made clear that was not the case.
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Tom K6VL

[TonyC]

Different gain curves for the tubes or a desire to adjust the gain of the IF and RF stages by different amounts. It also eliminates a path for stray coupling between the stages. The "gas gate" thing is new to me, but so is lots of this stuff . I was browsing and found a reference to gas gate in the google groups. They mentioned a Crosley 855. I looked at the schematic and it too uses AVC diodes separate from the detector diode but I didn't see the term "gas gate" on the schematic.

Edit: Maybe because the mixer is biased with cathode bias and the IF amp has a grounded cathode (I think, schematic is noisy), the grids needed to sit a different voltages. So, the need/desire for different bias schemes may have led to separate AVC.
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Tony Casorso

[HuggyBear]

[HuggyBear]

[Ed Engelken]

The "gas gate" is a diode connected from the AVC line to ground to prevent the AVC from going positive if one of the AVC controlled tubes gets gassy. That is according to Ghirardi in "Modern Radio Servicing." The idea is that if a tube goes gassy and it's grid goes positive it will drive the other AVC controlled grids positive and increase the current drain to critical levels and damage other good tubes and perhaps the power transformer. The gas gate prevents the grids from going positive and, in theory, prevent damage.

My Crosley Model 855 has a gas gate using an extra 6H6. Looks like an interestsing way to boost the tube count!!

EDIT: The gas gate is explained in "Radio and Television Receiver Circuitry and Operation" by Ghirardi and Johnson, pages 184-185.
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http://www.gvtc.com/~edengel/TARC.htm

[HuggyBear]

[TonyC]

I believe that the grounded grid forms the "gas gate" diode as the other two diodes appear to be connected to generate AVC.
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Tony Casorso

[Tom Albrecht]

I don't think they can generate AVC as they are connected. They conduct if the AVC line goes positive. That's where I got the funny idea that it might be some kind of "clamp" for static spikes.

I have heard of the AVC going positive, but have never seen it.
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Tom K6VL

[TonyC]

You're absolutely right Tom. I had my diodes backwards.
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Tony Casorso

[Tim Tress]

There was an article on the gas gate in the AWA Journal, not too long ago.

Tim

[Guest]

[jkaetzjr]

Comment on AVC going positive. This is by no means an uncommon phenomenon. Have seen it happen in older sets using "G" style tubes, "GT", but probably more common in miniature tubes that run hotter than the older types. Output tubes also can have grids that gradually go positive due to gas or control grid contamination. Happens a lot with 50 and 35L6s and 50B5s and 50B5s. Displays the same symptoms as a leaky coupling cap. Most AVC circuits use rather high value resistors that in effect are "grid return" resistors such that it doesn't take a lot of gas current to cause a positive AVC line. Frequently doesn't get noticed until you tune in a barn burner down the road at which point the RF/IF amps get overloaded enough to clip the modulation. TV sets also have the problem. Many years ago Philco made sets using 6CB6s in the IF with a very high grid return path. Once the tubes began to go gassy the pix would wash out completely due to overload in 30 minutes or so. There were many Philcos in which all 4 of the IF amps had to be replaced to make it work properly. Customer often tended to be skeptical when you began wholesale replacement of tubes in a set that was "working fine" until it quit suddenly!

[Curt Reed]

A lot of the older, more detailed tube manuals list the maximum grid resistance value for a lot of tubes. I have run across sets where the manufacturer did not pay any attention to this at all. Of course, tubes were cheap and available at the time, but it still makes a person wonder at times.
Curt
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Curt, N7AH
(Connoisseur of the cold 807) CW forever!

[Terry Judkins]

Philco included a 'gas gate' in many of their radios. Here is a snippit from the 39-30 schematic. Note the two diodes in the 75 clamp the AVC line to zero volts maximum (can't go positive). The plate of the 37 is used to develop the AVC voltage and the grid of the 37 is a detector.

[jkaetzjr]

Guess the gas gate is worthwhile to keep tube currents from getting out of hand especially in sets that don't use cathode resistors in the RF/IF stages but overload distortion overload can still be present on strong stations. Something is better than nothing, particularly if it doesn't cost anything extra!

[Norm Leal]

Notice this schematic has a 750K grid resistor for the 41 tube. Max grid resistor should be 500K. This is a case like Curt mentioned.

Using a higher value grid resistor gives a little higher gain and requires a smaller coupling cap for a specific frequency response. Too high a value resistor and grid can go positive after some use.

Neat idea clamping the AVC line so a gassy tube can't make it go positive. Even though the radio may operate still a good idea to replace a gassy tube.
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Norm