Cathode stripping

[dhuebert]

Since EWB brought it up...
I used to believe in cathode stripping and built a standby switch into my first ST35. Then I started hanging out with Gar Gillies, who never put a standby switch on an amplifier to my knowlege. I have a 1974 Garnet Minibass that is still running the original tubes just fine thank you. According to Gar he has seen 35 YO amps come into his shop for "tune ups" that have their original tubes in them no problem. I saw a late 70s Session Man recently that had a sticker on the back from a music store in Los Angeles that had: you guessed it, yada, yada, yada.

Discussion anyone?

Don

[erichayes]

Hi All,

Don, I felt the shudder of that gauntlet hitting the ground all the way out here . . . no, wait. It was an earthquake.

I have a quartet of original Tung Sol 6550s that're older than God which I use as first-testers when prototyping and repairing. The getters are pink, but they still test good on my 752 and the curves look OK on the 570. Furthermore, they can put out 50 watts in a 50 watt amp, and 100 watts in a 100 watt amp.

I also have a quartet of "N"OS Tung Sols (less than 10 hours on them) that I bought when I saw the writing on the wall regarding NOS output tubes. I get them out only when I want to make a point with the JJs, with which I A-B them.

I also have a quartet of 6550EHs that came out of a Marshall which never had the standby switch used (the guy was told by the previous owner that it was bad for the tubes) and have, probably, 50 hours on them. I keep them around mainly to sell JJs, but they see occaisional service as first-testers as well. They have good looking getters, but there's noticeable cathode-flash on the envelopes. They test acceptable on the 752, but are pressed to get full wattage out in actual use.

The NOS 6550s kill the EHs and seriously wound the old 6550s in listening tests. The old 6550s do OK with rock music, where the dynamic range is fairly limited, but can't hold a candle to the new ones when it comes to orchestral music. The EHs couldn't even keep up with the old 6550s on rock--which was why the guy replaced them in the first place.

I've drawn some conclusions from these tests, which I've alluded to in previous threads:

First, cathode stripping is real.

Second, old cathodes were made with much better material than are new cathodes.

Third, tired tubes can sound perfectly good until they're compared with new tubes in a short time frame. Comparative memory is not one of the brain's strong suits.

I have too many case histories to list that reenforce the cathode stripping phenomenon. However, it was not regarded as a problem back when tubes were plentiful and discounting was the norm. Now, the general quality of tubes (yes, even JJs) is inferior to the old stuff, and we're paying premium prices to boot.

My advice is: even if you believe cathode stripping (and baking, to open another can of worms) is a fairy tale, treat your output tubes with respect and don't subject them to static extremes . . . the musical ones are enough of a challenge to them.

[dhuebert]

Ya know, I don't mean to be confrontational, I just want to be clear. Your answer is just what I was hoping for, someone with more experience than I've got to put my personal observations in context. Which I must say was nicely done here. Thank you.

Having said that it amazes me that modern materials and construction are not superior. Let me illustrate: Years ago while rebuilding a Volkswagen motor I asked my Guru if he had any old stock engine bearings, he explained that materials science had come so far since the 50s that there was no point using that old crap. Is this not the case for tubes as well? Gar has repeatedly told me that the new tubes are not nearly as good as the old stuff so I believe it. My personal experience with E-H 6v6 bears this up. As for sound quality and longevity I can't say, Sovtek are good for me

Don

[EWBrown]

The modern Russian and other "Eastern European" 6L6s (G, GC, GT, etc) and 6V6s are pretty tough and rugged, even the JJ 6V6s can be used in some 6L6 applications, and get away with doing this. Bob over at Eurotubes has done this with, good results, in some guitar amps. He has a few other interesting tricks, like "integrated quads" in whch a PPP amp gets one pair of 6L6GCs and one pair of EL34s, he claims you can get the best of both worlds this way, though I wouldn't necessarily want to try this in a "hi-fi" amp.

I don't have a lot of faith in most of the mid-quality Chinese made beam power tubes and power pentodes, but they are slowly getting better. The Chinese power triodes and 807s are said to be excellent.

I've had good results with Svetlana 6550Cs, and E-H KT88s.

/ed B in NH

[erichayes]

Hi All,

When WWII came along, the vacuum tube was no longer regarded as a parlor trick and the guys in the white coats at RCA and Western Electric went into overdrive. One of the breakthroughs was the development of cathode materials that could really put out. Strontium carbonate apparently was the big winner, with barium carbonate coming in second. In the manufacturing process (no one's telling how) these carbonates are changed to oxides that are loaded with free electrons when heated. The only problems were from contamination caused either in the "manufacturing process" or insufficient heater temperature (sound familiar?). In either case, the cathode became "poisoned" and its efficiency was greatly diminished.

The other things developed during the war were the technology to make compressors capable of drawing a "hard" vacuum, and glass composition formulas that would maintain that vacuum. These contributed greatly to a tube's longevity.

I suspect that the reasons current production tubes are inferior to most NOS tubes are a lack of attention being paid to the manufacturing process, and/or insufficient vacuum and/or lack of quality materials to make the oxide coating for the cathodes.