Discussion in 'Tube Audio' started by nyindallas, Aug 16, 2008.
Curious what the symptoms are when power, rectifier and pre-amp tubes are failing? Thanks
Rectifier: Usually work great until failure, then causes fuses to pop when bad. Old school rectifiers rarely wear out. Sometimes you will see a voltage drop with a weak tube, but you need to be able to measure that. I will buy an old USED Mullard or RCA before buying a NEW JJ/Sovtek/Ruby etc.
Power: Lose clarity, power, lack of "punch" when volume turned up and overall more weak sounding. When they fail or short out, they also cause fuses to pop.
Pre-amp: Hiss, popcorn-type sounds, weak signal. When pre-amp tubes go bad, they you have to pull them to test them. I had a 7199 PI tube in a Dynaco Stereo 70 that was bad, but the only thing it did was cause a "crackling" sound when the volume was turned down. It was a Sovtek with relatively low hours (what else is new), replaced it with an NOS Tung-Sol. Problem solved.
It depends on how the tube fails. They can get noisy, microphonic, the heater can blow, they can short, develop hum, or just wear out. The symptoms will depend on the circuit. A tube with a particular defect might work fine in one circuit, but not at all in another.
Rectifiers: Yeah, the B+ plate voltage may drop or the amp might be hard to bias (drifts around, won't settle). Don't know about old/NOS vs. new reliability. I've got a cheap Chinese AR54 in my Dynaco ST-70 and I can drive both channels to full power into an 8-Ohm load with no problems or overheating. My experience has been that degraded rectifiers don't always have audible results. Hum is more from dried out filter caps and/or leaky Selenium rectifiers in the power supply rather than the tubes themselves.
Pre-amp: One or both channels might have a rumble (if the tube has gone microphonic) or static hiss/crackle (if the grid has eroded). Harder to spot a bad tube, will agree there, but I disagree with the comment about the Sovteks. They're not that bad a brand (the Chinese tubes are much worse in quality/consistency).
Power: Again, mushy or weak sound. Typically will either blow fuses (shorted) or no output (open) when they fail. May fail abruptly or shriek during their death throes.
General: Tubes can become leaky over time and glow blueish. While this diminishes the tube's performance, they can still sound quite good. If the heater goes, then it's game over as there's no more electron flow from the cathode. If a tube goes microphonic, it may still sound OK so long as it's not played with or touched. Power tubes can also "red plate" which can be visually seen and indicates possible thermal runaway. Keep a close eye on those. My '60 or '61 Dynaco still has her original RCA 6CA7s and 7199s, two of which are blue and one is red plating, but it can still put out full power for both channels and sounds great.
Forgot to add...
If you want to see red plating/thermal runaway, just take the ol' Variac up to 130-140VAC and watch the power tubes really glow! Just don't do this with some NOS Mullards. Chinese tubes are good for this abuse.
wear gloves when you handle tubers the oil in yer skin can permeate the glass which is porous.
Yep just like when you replace a halogen bulb on yer car wear GLOVES AND KEEP YER FILTHY HANDS OFF THE GLASS HEHE :thmbsp:
I had two Chineese 300B's get hissy after 4 maybe 5 years, otherwise still sounded good. I keep them on hand as spares. I could have kept them in longer. Tubes in a MPX circuit (my MR67, MX110, Scott 340B) will cause loss of stereo. I've had a 6SL7's heater gradually fade out but never quit resulting in low output.
Amazing how tubes can go on functioning for decades when many original manufacturers listed a few thousand hours as the "expected" lifespan. Can't help but wonder how many perfectly good tubes have been disposed of over the years just because they reached 10,000 hours...
EDIT (afterthought) I don't even know how old the tubes (all different mfrs) in my recently acquired Hammarlund 145 X are, but it still works great.
No disrespect intended, but I humbly dis-agree! I have never worn gloves while replacing or removing tubes. Nothing happens to them if you don't wear any protection. I have been handling tubes sinse I was 6 years old (I'm 45 now), and am still using tubes I had when I was 6, and they are still going strong. What's supposed to happen to them if you don't use gloves? Not a thing that I can think of. The glass doesn't crack, does not effect tube performance, and besides, halogen bulbs burn way more hotter than any tube ever will! It's a total myth to have to handle tubes with some kind of protection.
Think of this: Has your TV screen or computer CRT ever gone bad because there are fingerprints all over them? NOT!
glass is considerably thicker on a crt screen. The guy I get all my nos tubes from, when he handles them he always wears neoprene gloves.
This takes me back way back to when I was little and I put my gruby finger prints on my uncles dynaco 70 with out him knowing, yep my prints are still there to this day.
If the oil from my hands did this to chrome what do you think it will do to a tube.
Just like fingerprints on a Buick mean little compared to fingerprints on Cindy Crawford...
This guy has some good advice.
Hrmm....makes me want to become a cop so I can dust for prints....
Leaving fingerprints on tubes is a bad practice if one wants to maintain strict adherance to protocol in the handling and use of vacuum tubes. While small signal tubes may bear finger oils without issue the tubes that run hotter in application will make cleanliness of the glass a more serious concern. An old 6L6G may do OK with grime on its glass, but a more modern 7591 or EL-34 may take exception, and suddenly have the envelope fail.
Owing to the high vacuum contained in a typical tube I have my doubts as to the porosity of the glass used in the containment of same. And the amorphous nature of glass leads to many an argument as to its state as solid, or perhaps a very high viscosity liquid.
The attention here needs to be aimed at the temperature signature over the area of the enclosing glass. If there is a foreign substance of a different specific heat than the surrounding air, and the glass beneath such contamination heats at a slower rate than the surrounding glass, there is an anomaly in that area as a result. If the area of clean glass temperature to the temperature of the area of anomaly has a sharp enough transition there will be an immediate and concentrated area of stress that may cause the glass to crack, thus causing immediate failure.
In the real world tubes get dirty, and--while usually they will be forgiving, and function for many hours--sometimes they fail for the contamination on them. If you can take steps to clean them, or can take steps to avoid getting tubes dirty in the first place, why take a chance....?
As to symptoms indicating tube failure the ones mentioned in above postings all have merit in my belief. But there are many more. I've seen tubes fail in the gol-durndest ways.
I had a compactron in a portable TV develop a G1 to K short after the tube had been operating for 1 minute and a few seconds. This resulted in complete sync failure. The customer loved the set, and really wanted it fixed, but the compactron proved to be made completely of unobtainium. After some meditation I went and grabbed a car battery and horsed it onto the service bench. I set up jumper leads to the set, and switched the TV on, and then made the final connection when the grid shorted to the cathode. The set operated with that tube for many years thereafter.
Many times a tube is condemned when it is an associated component that is causing the malfunction. I've seen tubes that perform unfavorably in one application that perform well in a different one. Microphonic tubes, while not favorable in phono preamp application will often work fine as cathode-follower output..... :thmbsp:
I'm with the school of thought that a tube works best when spotlessly clean. Back in the old days when all my family's stuff was hollow-state, I used to use a soft cloth with some rubbing alky and regularly polished the tubes in the TV, CM console stereo, GE tabletop radio, etc which we used around the house. The moment I got my Hammarlund from EPrey, That's the first thing I did; polish the tubes. Seems to me that a clean tube (like Old1625 says) is more important if it tends to run particularly hot, but low power signal tubes would be more forgiving.
There is a world of difference between metals that rust and glass... That is an apples to oranges comparison and holds no value to the discussion.
Not a thing! Like I said, I have never had a tube fail in any way by handling it. NEVER!
I Agree Cademan. No truth that tubes will shorten their life by handling with bare hands.
I hope the last two that posted will not have to eat their words. Again: it is all a matter of application, and the maximum bulb temperatures attained in such application. I, too, have seen the plastic sheath of a power cord intimate with the sides of output tubes in guitar amps owned by sloppy musicians, and have seen nothing happen other than I have to replace the cord, and scrape the residues of the old cord that have melted onto the glass, and have the tubes go on to give many hours of service.
But I've also seen many failures as well in these circumstances where it is obvious that contamination of the glass surface was the culprit--hands-down.
I've been in the service industry since the early '70s, and know that it is all a lottery as far as contaminated glass is concerned, and what can happen, and have learned that it is best that such be kept clean.
What more can one ask?
OK, OK, you've had your experience, but what about my experience? I am also a tech from the late 70's, and still I have never seen the things you have described here. So.......am I un-experienced? Not really but........I still have not seen catastrophic failure that you have described here. Maybe all of the amplifiers and receivers that have been through my fingers have never been abused or have had the tubes touched by human fingers. This is just my experience over a 30 year period. I don't dis-agree with you, but I have never seen it.
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