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Can a failing 7199 sound like this?

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I was listening to my Sherwood S-5000 last night when a peculiar this started to happen. About every 7-8 seconds a soft zzzzz sound for one second on one channel with no music distortion.

Long story short, swapped the 7199s side to side and the zzzz followed one tube.

I've never really heard a tube fail in this manner??? Usually they just start humming or distorting. But not "pulsing".

My next post or question will be how do I rewire the sockets for a 6U8A tube. I know I can just get the adapter but why add more pin problems since I'm never going to get anymore RCA 7199s
 
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maybe internal leakage developing and you're hearing it start to arc between elements. Just a guess.

move the connection on pin 2 to pin 6
move connection on pin 6 to pin 7
move connection from pin 7 to pin 2
 
Second post:
is this the correct method of rewiring a 7199 socket to accommodate a 6u8a

1: Remove connections from pin #2. These will be moved to pin #6.
2: Remove connections from pin #6. These will be moved to pin #7.
3: Remove connections from pin #7. These will be moved to pin #2.
4: Mark Chassis to indicate the tube change.

Do I "need or should" make any other changes?
 
I own nothing that ever ran a 7199, so I can offer no opinions on replacing it with anything else. I do own a couple amps with a 6U8 that work fine. I've never tried any other tubes in that spot though.
 
The insulation has degraded and is leaking. Once that starts it only gets worse

The 7199 has a heater-to-cathode limit that is far below what it experiences in the ST-70. This is why they fail. It was why the tube failed in TVs, too, which was the intended use. I've elsewhere extensively written about this and the electrochemistry. Metal migration is the biggest issue.

Elevated heaters will prevent the problem for the replacement. This is a good idea for all tubes, as the high heater-to-cathode potential causes electrochemistry and degradation of the heater insulation. The leakage manifests as hum, low at first, increasing as the insulation disintegrates until an arc occurs and burns out the tube.

The 6GH8 can be used with an adapter socket and is considered to be a good substitute. The Sovtek "7199" briefly sold was not actually a 7199 but was instead a repinned 6GH8.
 
Agreeing with the comments above, the 7199 was designed by RCA as an audio tube. It had very close tolerances and will sometimes have an H-K (heater-cathode) short. It is not all of the time but not unheard of either. I have several which show a slight short.

HH Scott used the 7199 in early amplifiers and eventually switched to the 6U8 mentioned above. Rewiring a point to point amplifier is relatively easy. However, one needs to be careful when printed circuit boards are involved. It can easily become anything from a disaster to "Amateur Hour".

ESRC Vacuum Tubes has them listes NOS for $22.00 each (other than being a repeat customer, I have no association with them). For sure, Sylvania and RCA made them. GE may have been rebranded; maybe not. (ESRC is in Florida)

I realize that harpoons may fly my way but I am a very strong supporter of NOS tubes (purchase with caution!).
 
BTW, nothing is wrong with a 6BL8, 6GH8 or 6U8. The latter two were used in television receivers and are plentiful and inexpensive.
 
kvflyer said:
the 7199 was designed by RCA as an audio tube
Click to expand...

That was my understanding as well. I think the idea was to improve upon things like the 6U8 or 6GH8 by optimizing it a bit, or at least more tightly controlling the build so it was less variable than the other triode/pentode tubes.
 
gadget73 said:
That was my understanding as well. I think the idea was to improve upon things like the 6U8 or 6GH8 by optimizing it a bit, or at least more tightly controlling the build so it was less variable than the other triode/pentode tubes.
Click to expand...

Thatis exactly my understanding as well. RCA designed a few tubes especially for audio and they had a full page ad in several publications. I think that the 6973 was another one.

But back to the original question, if a problem follows a tube when it is moved to another socket, chances are that the tube is the problem.

EDIT: I did a search and the ad that I mentioned has a big "4" on it. The othertwo tubes are 7027A and 7025, from 1958/59. Sorry to wonder OT...
 
kvflyer said:
Agreeing with the comments above, the 7199 was designed by RCA as an audio tube. It had very close tolerances and will sometimes have an H-K (heater-cathode) short. It is not all of the time but not unheard of either. I have several which show a slight short.
Click to expand...

Leakage occurs in any tube with elevated heater-to-cathode potential. Current flow gradually erodes the insulation until the leakage becomes an arc. Leakage is problematic in a gain stage for obvious reasons.

Briefly, the heater wire is coated with an insulator having a certain breakdown voltage. That's simply the nature of insulation. In the case of alumina used for heaters, it is about 75 VDC / mm. As the the cathode-to-heater voltage approaches this breakdown voltage current increasingly flows between the cathode and the (lower potential) heater, degrading the heater's insulation (which flakes off for further degradation) and ruining the tube. But even before the insulation is severely damaged leakage occurs. It is that flow of leakage current which causes the hum. Once the insulation begins to degrade nothing can save the tube. This was reported to be of the causes of premature 7199 failure in the Dynaco ST-70.

Circa 1995 Matt Kamna reported the the 7199 leakage issue as being caused by heater-to-cathode limits, so this was a well-known issue.

The solution is moving the relative potential of the heater up so it is lower than the cathode-to-heater breakdown voltage point. This is published for the tubes. A number of amplifiers do not properly elevate the heater voltage. A resistor ensures you don't pull too much B+ current into the heater supply. A mA is all you need, since it is a potential not a current.

The published limits are about 90 V, but RCA and others published studies showing that any difference above a few volts difference begins to migrate metal. Either nickel from the cathode sleeve into the tungsten filament or tungsten and alumina into the nickel cathode sleeve. This is why the tube specifications list a different value for the positive potential difference vs the negative potential difference; the polarity determines the direction of migration. The higher the voltage the faster the migration.
 
OP is running a Sherwood S-5000. Not sure what its got going on for voltages, but it wouldn't surprise me if its near or beyond the tube's limitations.
 
gadget73 said:
OP is running a Sherwood S-5000. Not sure what its got going on for voltages, but it wouldn't surprise me if its near or beyond the tube's limitations.
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Well, I was using the ST-70 as a well-studied example of the problem.

Most amplifiers using the 7199 do not have elevated heaters and the insulation is not good for 90 V for very long. When the 7199 was used in televisions it failed because the HK potential difference was typically about 200 V. RCA at one point bumped the spec up but the insulation was no thicker and thus no more durable.

I grabbed a S-5000 schematic and saw no evidence of elevated heaters, and the tube is running at whatever full front end-voltage is present. That wasn't on the piece I saw so I can't tell you the voltage. But it would likely be > 90 V.
 
Retrovert said:
Well, I was using the ST-70 as a well-studied example of the problem.

Most amplifiers using the 7199 do not have elevated heaters and the insulation is not good for 90 V for very long. When the 7199 was used in televisions it failed because the HK potential difference was typically about 200 V. RCA at one point bumped the spec up but the insulation was no thicker and thus no more durable.

I grabbed a S-5000 schematic and saw no evidence of elevated heaters, and the tube is running at whatever full front end-voltage is present. That wasn't on the piece I saw so I can't tell you the voltage. But it would likely be > 90 V.
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TV sets usually used consumer grade tubes with the alphanumeric RETMA type designation, I have never seen four digit industrial/special tube types in a TV. I do believe the 7199 was an audio specific tube.

The 6GH8 has a bad reputation amongst TV repair men for developing heater to cathode leakage, and needing early replacement. This is why every tube collection from a repair man is usually full of some 6GH8. I had them out last several pairs of output tubes in a dynaco though, and believe them to be a worthy substitute for the hard to find 7199.

The only thing I would strongly suggest is if you plan to mess with this tube, buy RCA, GE, or Sylvania tubes only! Due to its popularity and high failure rate in colour TV, lots of poor quality 6GH8 imported and off brand tubes flooded the market, and they are more or less junk.
 
I decided to go with the Amperex 7643/E80CF

If I was going the route of 6GH8 I think I would have used a good quality 6BL8
 
maxhifi said:
The 6GH8 has a bad reputation amongst TV repair men for developing heater to cathode leakage, and needing early replacement. This is why every tube collection from a repair man is usually full of some 6GH8. I had them out last several pairs of output tubes in a dynaco though, and believe them to be a worthy substitute for the hard to find 7199.
Click to expand...

I've explained this: most applications exceed the heater-to-cathode ratings. This gradually destroys the tube. Abusing the heater insulation is 100% guaranteed to cause the tube to premature fail. Some tubes fail sooner, others fail later. Elevating the heaters (using a voltage divider with two resistors) trivially solves this problem.
 
Retrovert said:
I've explained this: most applications exceed the heater-to-cathode ratings. This gradually destroys the tube. Abusing the heater insulation is 100% guaranteed to cause the tube to premature fail. Some tubes fail sooner, others fail later. Elevating the heaters (using a voltage divider with two resistors) trivially solves this problem.
Click to expand...

In colour TVs like the RCA CTC38, the cathodes were pretty close to ground, but yet the tubes still failed with heater/cathode leakage. Some people claim RCA just ran the tubes too hard, or that 6GH8s are just a bad design. In that sort of application, the GE and Sylvania tubes last longer than an RCA.

Elevating heaters as you suggest above is almost never a bad idea, in cathode bias amps it's easy just to use the cathode voltage from the output tubes to bias the heaters positive a bit. It also helps reduce hum.
 
Retrovert said:
I've explained this: most applications exceed the heater-to-cathode ratings. This gradually destroys the tube. Abusing the heater insulation is 100% guaranteed to cause the tube to premature fail. Some tubes fail sooner, others fail later. Elevating the heaters (using a voltage divider with two resistors) trivially solves this problem.
Click to expand...
Does it matter which side of the heater you elevate?
 
triode17 said:
Does it matter which side of the heater you elevate?
Click to expand...

Ideally the center tap, of a filament winding. But if there isn't one, no, it doesn't matter. You have to make sure that the filament transformer or winding isn't grounded at all before you do this, though.
 
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