:Hello, I am restoring a few radios from the 40's and I need to replace a few tubes. I have on hand the right replacements, but are newer and smaller than the original ones in the 40's. For instance A 6V6G with a 6V6GT or 6V6 and 6V6GTA. Are the smaller or even black metal tubes as good or better than the old larger ones. Although I do like the look of the loder tubes.
:Thanks
:David
I have had a set with all metal tubes. I rather appreciated it. Still, I like the big glass ones a lot. The big glass provides more cooling and strength than tubular glass. However, most radio tubes don't operate that hot. Guitar and organ amplifier tubes, horizontal output television tubes, etc., run hot enough to need the curved glass.
Thomas
Performance-wise, I really can't tell any difference between glass and metal.
Also, the elements in an old bulb style tube may be assembled in a perfectly straight manner. They are assembled on a base unit of glass, however, which is LATER attached to the glass bulb. Sometimes it is attached crookedly, which makes the electrode unit tilt to one side. With all of the glass ST style tubes I've owned, though, I've never seen one sag or change position internally, even after many many hours of use. I have one #280 tube in which the elements are almost at a 45 degree angle. All of the elements are perfectly spaced from eachother, though, and it is obvious that the tube was assembled this way. The base is simply crooked with respect to the bulb. In other tubes the wires are assembled crookedly in the base.
To Doug: Sometimes you can notice a difference between glass and metal tubes. If you remove the shields from the RF sections of a radio, you can mimmic changing from metal to glass tubes. The shields act like the metal on metal tubes. I notice more distortion and tendency towards oscillation with unshielded glass tubes. It all depends upon the circuit and the tube/component layout. In amplifiers which tend to hum, replacing glass tubes with metal tubes will tend to reduce hum. If you hold your hand near the 1st AF tube, especially one with a grid cap, you can mimmic this situation. Replace with a metal tube and the hum will be significantly reduced or it will disappear. I have this trouble a lot with my Crosley 1117. If you place your hand anywhere near the cabinet on the left side, it'll hum more loudly. If you put in a metal tube, however (6K5 instead of 6K5G), the hum will go away. I suppose I could also shield the grid lead with a long spring connected to the chassis. Hum will be more noticeable with poorly designed radios that utilize push-pull output than with radios which have single ended output (one output tube). Since push-pull emphasizes bass, the hum will be more prominent. With many well designed radios, you cannot tell the difference between glass and metal tubes, which is nice, because I prefer glass. Of course audio hum can also be caused by misplaced grid wires and/or poor shielding if misrouting the wires is necessary (placing them near an alternating current source, which is normally considered bad practice).
Also, the larger surface area of the glass in an ST style tube does aid in cooling. You are absolutely right, though, about heat radiating through the glass and not being captured by the glass. Here's an interesting thing not exactly related, but interesting just the same: If you paint a radiator any color you want so long as it's not metallic, it will radiate about the same amount of heat. If you paint it with metallic paint of any kind, however, the infrared light is blocked and absorbed by the metal. The radiator can put out as much as 15% less radiant heat. It will still heat the air passing by it the same way, of course. Another interesting thing is that if you want to reverse the damage you have done, all you have to do is paint over the metallic paint with any other nonmetallic paint. Strange.
Thomas
I went over to my parents' house to get a few tubes which were still over there. My key purpose was to retrieve a UX-371A. I really wanted it. Then, as I had the four tattered old boxes in my hands, with no end flaps, that tube slipped out and hit the floor (hard wood). It didn't break, but it might as well have. The innards were firmly pressed against the glass, and everything was out of alignment between the elements. The filament looked terrible. I gave it a firm, solid whap against my hand a few times, which amazingly straightened everything out perfectly. It is centered as well as anything could be centered. However, the filament is now broken. The tube is useless unless someday I can find a glass worker who can open up the bulb for me. Then I can mend the filament or something and he can reseal the bulb and evacuate it. I don't see this happening, but I save rare tubes like this with the hopes that someday I may have money and I may know such interesting people as glass workers (blowers, etc.).
I guess that's why Eveready came out with the four pillar tube. The four pillar tube isn't likely to take a nose dive like ordinary tubes of this vintage.
Thomas
MRO
Warren J. Dougherty, a serviceman of Kinkaid, Kansas, has called to our attention one of the factors most overlooked when replacing G-type tubes with the more newly-designed GT tubes. Invariably these GT tubes, with metal base, have their No. 1 pin grounded. Inasmuch as many manufacturers use all available socket pins for common tie-points, considerable damage may be caused when grounding the No. 1 pin. A particular case is the RCA Model BT-42 which was originally equipped with G-type tubes. When changing to GT tubes with metal base, the condenser, C16, 400-µfd. condenser and 1-megohm resister, R7, in the plate circuit of the 1H5 should be disconnected from the No. 1 pin of the 1N5 tube socket, as in this particular case, it was used as a common terminal connector. The metal base occasionally will short out the plate voltage from the 1H5 tube, resulting in a nonoperating receiver.
:When I got to any tube substitution manual I find that I can replace G types with GT, GTA. Can this cause problems.