Tinkerra "GOAT SHIELD " is the metal shield around the valve. The one that hugs the outside of the valve is branded "Goat". They minimise interference between the valves.

A goat shield is a valve shield that fits close to the shape of the globe like a pair of socks would fit on your feet. They come in two halves and stay together with a wire clip.

https://www.bing.com/images/search?view=detailV2&ccid=bAhPUFxF&id=FDDFB43206D58E59421C67...

Yes, what they said. It's mandatory to have one if your radio has a 6U7G IF amp. Without it the IF stage would self-oscillate. A goat shield is sometimes fitted to the the first AF amp too (usually 6B6G (double-diode triode), or 6B8G/6G8G (double-diode pentode)). I would expect to see these shields on the pre-octal versions of these valves. It's not common to see them used anywhere else.

Hi all,
Okay, most of my radios have these shields as you describe ... didn't know that they were called 'goat'' shields ... thanks.

Normally the real ones have "Goat" impressed into them.

What is missing is whether they are complete? All should have a pressed sheet metal tail which is keyed for the octal spigot and pin 1. Pin 1 on most of the Octals (NOT ALL) is a grounding pin. That also applies to Philips Metalised and Metal tubes. The shield is useless if not grounded. Removing Philips metalised tubes by the envelope can break the bond between pin & metallisation.

Octal pin one is the cause of many nasty incidents, like a live 6L6 in a KT66 socket, when some Goose uses it for a tag of convenience and a metal tube is subbed for glass.

The envelope they fit is "S"

Marc

Manufactured by Fred H Goat, Brooklyn NY.

https://i.pinimg.com/564x/ba/0e/05/ba0e05fc3c4242901df7756bc9dbdb89.jpg

I have discovered that an intermittent cause of instability in my STC 830 radio is poor contact between the aluminium shields and their bases which are pressed from some sort of chromate or phosphate treated steel. The aluminium can be cleaned up on the inside of the can with 800 grit wet and dry, but the bases if cleaned up this way would probably rust. Luckily cleaning the aluminium only so far seems to be a cure, but I would rather have a more positive connection which I am yet to come up with.

STC830 thats a easy fix. Solder (yes aluminium can be soldered) a piece of pickup wire to the can and then to the chassis. I have done this many times.

Carl, have soldered stainless steel using phosphoric acid rust converter as a flux - might work on aluminium too.

Your solution would require a lead about 150mm or more long to allow the can to be lifted off and got out of the way to remove the valve, and for looks would rather put the lead inside the can. I don't suppose that would cause a problem as long as it doesn't contact the valve - might need silicone or glass fibre insulated wire.

Yes basically what I do is sand the aluminum clean and just use Flux and my scope iron.

Hi Rob, re post #44 and the SC4A19 "air hostess".

That was a circuit arrangement that interested me at one stage.
The key to the performance is the combination of the 6AR7 and the KT66.
The KT66 is very important as it has a gain of about 3 times a 42 or 6V6.
That allowed the designer to almost get "5 valve" audio gain, as good as a 6SQ7 + 6V6.
Radiotron was very proud of the 6AR7 and they said was meant to be reflexed which it is not in this circuit.

This circuit looks like a penny savers special, with everything that was not essential removed!
I am amazed the used a choke in the HT, later circuits got a resistor, much cheaper and who cares about hum.

I never got around to trying one IFT to see how the bandpass and gain wound up.
With a 6V6 in place of the KT61 the audio gain would be miles down and sound lame.

I think it would perform well with a short wire antenna in the city and suburbs with a KT61 style output and a bit of investigation of the tuning coils. The sensitivity was quoted as 20uV.
Got to agree with the volume control, I get twitchy about pushing valves up and down their slope to control AF gain.
I have changed a couple of sets from cathode to normal grid control for volume without problems.

The set would be an interesting one to play with to see where the designers were going, they were defintely watching their pennyies on that one.

Fred.

I also find this circuit interesting Fred. It's quite alien compared to the 4A18 and 4A20 models. I did play with one several years ago and from memory, it was quite a performer, I can't recall what the selectivity was like though but can only assume that it was compromised . I would love to have seen the design notes on this circuit. There was obviously some sort of clear objective even if it's not apparent these days. The volume control seems to be a natural extension of the AGC circuit. The designers must have had their reasons for its placement here as apposed to the grid of the KT61? I might have to read up more on the subject in RTDH for any clues?

I do have a Stromberg Carlson Air Hostess awaiting repair, but unfortunately it's the 4A18 model. I was so hoping it was the 4A19 model. Never-mind.

Interesting. So it is capable of working. I'm pretty sure I don't have a KT61, but I didn't do a completely exhaustive check. If there was such a thing as a triode-pentode octal output valve I'd be tempted to wire it in. Or perhaps look at the theoretical possibilities of reflexing the 6AR7GT.

Perhaps I could revisit it at a later stage. Or not. I don't know.


EDIT: I took another look at the blurry diagram at radiomuseum.org, and found the photos they have don't correspond to the schematic. Their radio has 2 IF transformers, where mine has just one, as per the diagram. Perhaps Stromberg-Carlson realised there was a problem and made a better version.

Yes Robbbert, I did see that photo at radiomuseum.org also and didn't know what to make of it. The earlier model 4A18 is reflexed using a 6G8 and a 6V6 output but does have 2x IF transformers. Was this the better model? It seems counter intuitive if is was.

Goat shields fitted anything with an "S" envelope. There is such a thing as a conductive glue, which is rather handy for repairing Philips metallisation.