The PHILCO Phorum

I was working on a Scott 800-B. It uses 3 6SK7 tubes in the receiver. The radio had a mix of metal and GT style tubes (wrong for this radio).

I was rounding up a batch of metal RCAs  from my stash. I noticed different test values. I know that I have seen this before and probably just assumed an error or deference in the testers.

Well, today I looked it up in the Hickok testing chart only a 6SK7 is listed at 2000 (mu) - but on my 533 roll chart it is 1900. So I went to the RCA manual (1966). There are TWO sets of parameters, one for the metal tube and one for the GT (glass) tube.

The biggest difference is the plate voltage. A metal tube is speced at 100V. A gt tube is speced at 250V.

The transconductance is 2350 for the metal tube and 2000 for the gt.

There are other differences - mostly ALL of them. These are different tubes! I have observed the metal tubes as testing better than the gt - - well duh. So, it was really never good to sub a gt for the metal even if a shield is added. IT WILL WORK, but, since these are often in the front-end the transformer (trans-coupled) will probably need to be retuned - always.

With the difference in plate voltage, subbing a metal tube for a gt could be really bad. You know how some tube sockets will list both the gt OR metal version? I can't recall ever seeing one that says 6SK7/6SK7GT - have you?

So if you all know this, ignore, and, why did you not tell me?

Wow - very interesting. I would never have expected metal and glass to have different specifications. Why not make up a new tube number?

Looking further in the manual - not the Hickok tube chart - there are differences in:

6J5/6J5GT
6F5/6F5GT
6Q7/6Q7GT
6K8/6K8G/6K8GT
6SN7/6SN7GT and more in 6SN7GTA


But NOT in:

6K7/6K7GT
6R7/6R7G/6R7GT

Probably more examples. Need to watch closer for this.

One explanation for not noticing before is that I always use the factory spec tubes and most always try to use the factory brand or the brand that they used. AND that Hickok (for one) does not differentiate on most of these.

P.S. actually, I guess that I did notice, just never took the time to figure out why.

Once had a six tube Stromberg Bakelte set, had been serviced but did not play well, somebody had swapped a glass for a metal tube. When inserted a metal at the advice of a friend played much better.

So in a higher tube count set could have all kinds of issues if tubes got glasss/metal swaps.

Good luck.

Paul

Most people seem to think that the only difference is shielding.

Another aspect, though most of these are inexpensive, since the testers only indicate one flavor: What do you look for in a "NOS" tube - what does your tube vendor test for?

Good question.................

My friend is an Engineer at Raytheon and very sharp. He explained the difference to me and advised to check and see if a GT or not is actual spec. People find all kinds of tubes in wrong sockets so not surprising in these old things someone sticks in a glass for a metal, same numbers right?

Paul

I think too, that laypeople/beginners/audio guys want to see the fire inside (not literal, mostly) and you can't see through the metal - unless you are Superman. OH and P.S., don't touch the metal tubes while on - unless you are Superman!

Radios that have had metal/glass swaps require a bit of care during a triage. Not all metal tubes can "play" in a respective socket. A "Glass Only" socket may have pin #1 re-assigned to a B+ dropping resistor or some other "shocking" connection.

I got that as a painful lesson servicing a set at 12yo., gave myself a life-long rule, glass into glass and metal into metal, never mix the two... chas

RCA made a big push with metal and I have several of their sets that use metal tubes. They don;t seem to show in many of my Philcos, I can't think I have seen a metal tube in a Philco maybe later small sets. 

Paul

I never really knew there was much difference between metal and glass tubes except for shielding and hearing at least some were internally grounded. Never seemed to matter much to the old service guys around here either it seems as I have had several radios with glass tubes in place of metal tubes over the years. Usually I haven't had much issue with a glass tube swap, but have had some radios that definitely played better with the correct metal tubes in place. Usually I will try to swap metal tubes back in when the radio calls for it, but my supplies have been dwindling and it has never been easy to run across metal tubes locally. Even the now defunct TV shop in town was running low on metal tubes before they closed. Glass or GT types have always seemed more common to find around here, which might explain why the radios I find often have glass swapped for metal. Add to the fact that scrappers abound or did abound around here, it is possible that any stores of metal tubes were just tossed in the hopper.

Edit: I should say that my supply of earlier 30's, 6 volt metal tubes has dwindled. Still fairly easy to find later tubes for AA5 sets.

See this auction on Pay-Bay:

https://www.ebay.com/itm/Pair-GE-6J5-Met...%3A2334524

He states the 6J5tubes test at  2300 with a spec of 2600, which is the value for a gt. The RCA manual says that his tubes would test new at 3000. This would lower the value from 86% of spec to 76% of spec. He is probably testing to the single, Hickok test value.

This would take those tubes from "strong" to meh...

There is actually a third style of tube that you don't run across too much Stateside, and those are metal spray/spray shield tubes. they were quite common in European and Canadian radios of the 1930s through to the late 40s (at least in Europe). I think that they were just starting to appear during the final two years of Grigsby Grunow, and maybe supplied in the aftermarket for a few years after that as replacements. In Rogers built sets, with octals, they were purposely designed to accept either spray shield, or metal tubes, pin one is connected to the shield on both types, and even suggested that the owner could swap metal tubes into his Deforest-Crosley/Rogers/Majestic to see if it enhanced performance. This is quite handy as double ended metal tubes are easier to find, and you don't have add an extra  shield. One thing that may differ with regard to GT verses single ended metal style tubes is the inter-electrode capacitances, the few I have encountered have larger inner structures inside the GT style tubes. Of course I would expect something similar between double ended glass, spray shield, and metal tubes since the envelopes are longer on the glass ones.
Regards
Arran

Interesting... I'm guilty of swapping glass tubes in place of metal. I did find in radio that used a lot of metal tubes that it did work better with metal tubes in some positions. Since I didn't have a tube chart, I could only assume. In others, esp the AA5 sets, I didn't notice much difference. This is good info. Thanks for sharing! 

The metal tubes from 1936 were the first whole year of production. They were constructed differently as noted by the lump near the base/side of the tube and mostly having stamped tube numbers on the steel can. I am always looking for these as most large manufactures adopted them at first only to change back to glass the next year/years. A good 5Z4, in the can with holes in the side, is almost impossible to find. the others were:

6A8
6C5
6F5
6F6
6J7
6K7
5Z4

Besides the 5Z4, these early metal tubes may have been less reliable than the later ones. It is probably better to use later ones in post 1936 radios.

6D5 was proposed

Paul