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That’s a Sylvania tube, the print is green, no, it’s blue

by Phil Taylor

If you’re the owner of an Effectrode PC-2A Compressor, Mercury fuzz, Fire Bottle boost or Glass-A buffer then you possess something rare and quite special as every one of these pedals contains a vintage subminiature ‘pencil’ tube. Effectrode have gone to great lengths to track down the finest quality legacy tubes from the golden era of tube manufacturing and furthermore we hand-select only the best of these to use in our effects pedals. This short article tells the tale of our efforts to gain some insight into factors affecting the quality of these little thermionic devices—believe it or not, one of our selection criteria was the colour of the print on the tube!

Philips ECG green-print and blue-print 12AT7 tubes

Philips ECG green-print and blue-print mil-spec JAN (Joint Army Navy) 12AT7WC tubes.

Am I getting old? You might well ask this question after taking a second look at the lettering printed on the side of new old stock Philips 12AT7 or 12AX7 tubes—sometimes it’s blue and sometimes it’s green. This variation in livery is not due to failing eyesight, but a deliberate change made by Philips. But why?

Well, Philips are a Dutch company, a giant in the electronics industry and during the mid to late 20th century they bought out several competitor tube manufacturing companies, including Mullard, Amperex, Tungsram, Miniwatt, Valvo and Sylvania. Sylvania had a (well deserved) reputation for making tubes of outstanding quality, rivalling even those made by the mighty RCA (the company that developed the 12AX7 tube). Philips purchased Sylvania in 1981 not solely for their ability to manufacture mil-spec tubes but because they had a view of gaining access to lucrative American government and military contracts. Their acquisition of Sylvania came rather late in the day—tube manufacturing operations ceased in their Sylvania plant just seven years later in 1988.

The green front door entrance to Sylvania Plant, Emporium

The green front door entrance to the Sylvania Radio Tube Plant, Emporium, Pa.

During the short time Philips ECG (Electronic Components Group) operated the Sylvania factory in Emporium, Pennsylvania they had little opportunity to make any significant adjustments to the manufacturing process and tubes produced by the plant were still based on Sylvania’s original designs and tooling. The only obvious cosmetic alteration was the change in name and the change in colour of the enamelled lettering on the tube glass from green to blue in 1986—blue was Philips’ brand colour, whereas green had been Sylvania’s brand colouring from their early days in the 1920s. Over the course of their operation Sylvania used green on their tube boxes, technical literature, merchandise, such as clock radios, enamel advertising signs and thermometers and even the doors and windows on their factory in Emporium were painted green too. The green livery and their oak leaf logo was almost certainly inspired by the verdent Pennsylvanian countryside—there are over 30 species of oak trees in this part of America (only 2 oak species in Britain).

On closer inspection there are other cosmetic differences in the blue-print tubes that were produced by the Emporium plant from 1986 onwards. For instance, tube plates are grey rather than black and this leads one to question whether other, more critical changes were made to the metallurgy and chemistry of the grid and cathode coatings and if these changes affected tube performance in any way. Effectrode were particularly interested to learn what we can about the tube manufacturing processes employed in the Sylvania/Philips Emporium plant as we utilise their subminiature pencil tubes in our compact effects pedal designs.

Black plate and grey plate compared

Black plate (green-print) and grey plate (blue-print) 6111WA tubes appear to have been manufactured with the same machine tooling.

But without proper acoustic/vibration test equipment, a disciplined test regime and a decent sample set, ideally hundreds of N.O.S. tubes from different batches from each year, it’s virtually impossible to measure any significant statistical variations in tube quality (utterly impossible if you’re just purchasing one or two of these tubes at time). However, Effectrode have been working with Philips ECG tubes for over ten years. During that time we’ve tested and hand-selected many thousands of subminiature 6021, 6111 and 6112 tubes for low electrical noise and microphony, and we’ve learned that some batches of tubes aren’t quite as good as others. Our tube rejection rates for these types of subjective failures run anywhere between 0% to 50% for a carton of 100 tubes.

For example, whilst testing a box 100 green-print 6111WA tubes (batch dated October 1985) we noted consistently low electrical noise and low microphony. These were what we consider “excellent” tubes and grade them subjectively as an “A”. Their performance was exceptionally consistent too and even the poorest performing tubes within this batch would be considered “good” or, according to our grading system, a “B”. Upon completion of testing the green-print tubes we began testing a batch of blue print tubes (batch dated February 1987). It soon became apparent that the blue-print tubes were generally more microphonic than the green-print tubes and also showed wider variablity in microphony. Many ended up being graded as a “C”, that is, having adverse microphony—these are considered as unusable tubes—and far fewer of this batch made the “A” grade.

Comparision of Microphony in blueprint and greenprint 6111WA pencil tubes

Philips ECG blue-print 6111WA tubes seem to show wider variability in microphony than green-print 6111WA tubes.

At this point one might reasonably conclude that Philips blue-print tubes are inferior to the green-print variety. Afterall, the Emporium plant was the last man standing—blue print tubes were manufactured from 1986 onwards—they were the end of the line for tubes made in the U.S.A. Perhaps the machines and tooling required servicing or were wearing out? Or maybe less care was being taken with manual assembly on these last production runs? This seems plausible, that is, until we discovered “poor” batches of green-print tubes and they even occur within the same year as the “good” batches! Doh! I’m sure the reader can appreciate our frustrations at not being able to locate those elusive cartons of near perfect tubes that pass our selection criteria with 0% rejects.

Sylvania 6021 pencil tube

A closer examination of this 6021 pencil tube manufactured in 1967 reveals that it’s identical to the last tubes that came off the Sylvania, Emporium, Pa production lines in the late 1980s, indicating the same fabrication process and materials were used over that period.

So we need to take a step back, reconsider and adjust our theory—did we just get lucky with our initial batch testing? Perhaps it was never possible to control the manufacturing process closely enough to ensure tight consistency of tube quality in the first place? Or perhaps this variablity only crept into the manufacturing process in the later years after Philips took over the factory? The only way to answer this is to test batches of sealed cartons of Sylvania tubes from even further back in time, say the 1970s, 1960s or 1950s. But we’ve never been lucky enough to find even just one sealed carton of tubes from the halcyon days of tube manufacture. It’s highly unlikely that they would remain unopened in their original carton for all those decades to the present day—at the time they were made there would have been a demand to put them into service.

A Sylvania 6021 pencil tube manufactured in 1956

Great Scott! A gold-print (a staggering technological achievement that was beyond even Mullard!) Sylvania 6021 pencil tube manufactured in 1956 for Leland Electric Co. Sorry, this is as far back as this time machine travels!

Individual tubes can still be got hold of though and we’ve tested a few from the late 1960s and 1950s. Some of them test “good” whilst others are “poor”. However, with such a small sample set size it’s impossible to know with any certainty whether early subminiature tubes were superior (or inferior) to the last ones rolling off the production lines in Emporium during the late 1980s. Perhaps we’ll never know. These tubes are a dwindling resource and soon will come a time when there are not enough of them available to perform reliable statistical testing and then, not long after this, there will not even enough to use in Effectrode pedal designs. This really is the end of a golden era in tone unless… Unless someone cares enough about these tubes to begin making them again—there are still people around who know about this stuff you know!

Hygrade Sylvania Plant

Old postcard showing the Hygrade Sylvania Radio Tube Plant, Emporium, Pennsylvania. This red brick factory was built in 1929 and still stands today, empty and derelict.


Further subjective testing (audio tests for hiss and microhony undertaken in October 2017) of green-print 6112 subminiature tube batches manufactured in the Emporium plant in 7/85 and 2/86 appears to support the hypothesis that there was a fall off in tube quality from 1986 onwards. To obtain a box of 100 “A” rated tubes from year 1985 133 tubes had to be tested, whereas to obtain a box of 100 “A” rated tubes from 1986, 173 tubes had to be tested. Or, to put it another way 33 tubes from the 1985 batch were rejected (75% pass rate) and 73 tubes from the 1986 batch were rejected (58% pass rate). Tubes from 1986 were generally noiser in terms of hiss and microhpony indicating that the cathode manufacture and packing of electrodes within the glass envelope was substandard. The wider variation in micophony and noise in 6111 blue-print tubes manufactured in 1987 described earlier in this article support the case for a fall off in quality of tube manufactured from 1986 onwards.

If you’d like ot find out more about the history of Sylvania it’s worth visiting Vintage Sylvania. Additionally this fascinating short piece written by Sylvania engineer W. A. Dickinson gives an excellent first-hand account of tube manufacturing at Sylvania. Oh, and just one more thing. There are yellow and red print Sylvania tubes too. Patuxent Communications website comprehensively lists all the different Sylvania tube boxes and print colours that Sylvania developed over their long history, along with many other N.O.S. tube manufacturers too. Not to be missed!