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Old 19th July 2008, 11:40 PM   #1
Zap is offline Zap  United States
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Default Tube rolling causes failure...

I just couldn't resist the urge to try some vintage tubes that I got at a good price today, so I threw them into my Simple SE and fired it up.

They are 6550's and I had KT88's running just fine for the last few months now without any trouble. I was listening for a good 10 minutes when I noticed some audible distortion all of a sudden - I walked over, saw a red plate and quickly shut off the amp. A funny smell was present as well. Not good!

I quickly shut down the amp, but I then heard a thud as one of the capacitors apparently had unsoldered itself from the PCB and hit the bottom of the chassis.

The part in question is a 1500uF 50V cap, and the top is bulging out. I believe it goes to ground or to the OPT depending on the cathode feedback switch (so a cathode bias cap). The other channel appears fine and I don't see any other parts that look bad, even resistors from the top of the PCB. I am suspecting the tube may be somehow defective and went into runaway, or did the cap just decide to give up the ghost somehow?

The amp has seen many hours of use and never had an issue before so I suspect the wiring should be fine. It has even had indirect torture testing as I have fallen asleep once or twice and it was running idle for some time (I know that's really not a good thing to do). The tubes were tested on a good quality tester (Hickok 539c) for me, and should be free of any major defects. I've bought a number of good things from this source before. I'm sure he'll refund me if the tube is indeed bad, but I'm puzzled.

Any suggestions are most appreciated - I hope I can get it running again because I can't stand the only other (non-tube) amps I have anymore (not to mention people in our apartment building can be loud at times so the music is nice)

Thanks everyone!
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Old 20th July 2008, 12:51 AM   #2
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Your newly acquired tubes probably just pulled more current than your originals. Higher current resulted in a greater voltage drop across the cathode resistor. It must have exceeded the 50 volt rating of the original part.

I'd replace both cathode bypass caps with a 63 volt rated part. Try this one from Mouser: 647-UPW1J152MHD.
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Old 20th July 2008, 02:12 AM   #3
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I have blown the cathode cap in a Simple SE twice. Both times it was caused by a seriously gassy output tube. I posted pictures of 6V6's glowing bright purple in another thread. That caused one cap to spew its guts. I blasted another cap with a bad WE 367A.

For the cathode cap to get hot enough to unsolder itself it had to be subjected to serious overvoltage, probably 75 volts or so. This would mean a tube current of well over 100 mA. I have seen a gassy tube work OK for a while, but the current will creep up until a runaway occurs. This sounds like what happened in your case. A gassy tube will NOT usually show up on a tube tester unless it is really bad. All of the 6V6 tubes that were glowing brightly in this thread tested good, and worked good for 1 to 10 minutes, then blew up.

Photos - the death of a Simple SE and 6V6's

To fix the amp, first check the 560 ohm cathode resistor. It could have been damaged. I managed to blow one when the 367A lit up. If the resistor is OK just replace the cap and try the amp with known good tubes. If the resistor is open, change it too. It is unlikely that anything else got damaged. If the cap oozed any of its goo in the amp, clean it up. I use rubbing alcohol, but it contains some water, so make sure everything is dry before power up, and keep it out of the transformers. The goo can cause corrosion if left on metal parts.

The 63 volt cap is a good idea, and I have them in one of my amps. The voltage should never get near 50 volts in nomal circumstances.

After you have the amp working, if you decide to try those tubes again watch them carefully for any signs of red plate. A gassy tube will usually go into runaway within 10 minutes or so, but I have had it happen after an hour. If any red plate shows up, shut the amp off. Swap the tubes and try again. If the same tube glows in the other channel, the tube is toast.

As you found out continued operation with a gassy tube can blow parts. I have a Simple SE (the Industrial Amp) that gets left on for hours at a time. It is about 1.5 years old, still has the same tubes and has never blown any parts. I also have a naked circuit board that gets used for experiments, and general tube torturing. It has been blown up too many times to count including both blown caps.
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Old 20th July 2008, 03:53 AM   #4
Zap is offline Zap  United States
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Thanks for all of the great information! I will be getting the caps from Mouser and will replace both of the existing units. The cathode bias resistors can apparently take some punishment as they both read about the same between the good and bad channel (within a few ohms of 560).

The question I have now pertains to the tube that is probably bad. I did not see any gassy glow in the envelope, even inside of the plate (through the holes). The plate was glowing pretty evenly bright red on the tube in question though. The triple getters all still look pretty decent as well on both tubes. I thought that often times they would be mostly gone or discolored if the tube was significantly gassy like the 6V6's in your other thread.

Thankfully, I don't think the cap sprayed - I don't see a big mess like I've seen in other gear where explosions occurred. I wonder why the fuse didn't blow though.

Wanting to know for sure, I'll probably fire up the amplifier and pay close attention in a dark room for any activity after I replace the cap. If the tube is gassy, so be it I guess. It's just a shame because there were only two of the RCA (Tung-Sol) 6550's and the price was a steal even with a bad one. I definitely noticed a difference in the sound - makes me wonder how the Russian remake sounds. Does anything in particular cause the tubes to do this?
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Old 20th July 2008, 04:04 AM   #5
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Think positive. Maybe there's nothing wrong with the tube. It could've just been the cap decided to short out, cooking itself and the tube in the process. You'll probably find out for certain after you get the replacement parts in there.

For what it costs to ship stuff, you might want to order a couple spare caps when you place your order...
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Old 21st July 2008, 10:23 AM   #6
chrish is offline chrish  Australia
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While you are digging around inside, you might want to consider modifying the cathode resistor setup. I put in a higher value resistor with a three position switch that can either be open or parallel two other value resistors. This effectively gives switcheable optimum bias for 6L6, EL34 and KT88. Allows more tube rolling without blowing things up

See this thread Cathode Bias for SimpleSE

Cheers,

Chris
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Old 21st July 2008, 10:30 PM   #7
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by chrish
While you are digging around inside, you might want to consider modifying the cathode resistor setup. I put in a higher value resistor with a three position switch that can either be open or parallel two other value resistors. This effectively gives switcheable optimum bias for 6L6, EL34 and KT88. Allows more tube rolling without blowing things up

See this thread Cathode Bias for SimpleSE

Cheers,

Chris

I'd recommend this as well. FWIW I had a lot of original Tungsol 6550 at one time and found them to bias quite differently than more modern production. Seemed that for a given bias voltage (fixed biased amplifier) they ran way hotter than any of the others I had on hand. I actually had to change the bias supplies to accommodate them. Just possibly this is the case here, but it is also quite possible it is just bad.
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Old 22nd July 2008, 01:37 AM   #8
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Quote:
I thought that often times they would be mostly gone or discolored if the tube was significantly gassy like the 6V6's in your other thread.
I have an old GE 7591 that glows red after a few minutes of use. The getter looks fine, but sure enough no matter which socket I put it into the tube will glow red after a few minutes. It sounds great before it does, though.

Looking closely at the tube I see shiny spots on the glass where the holes in the plate are located. Like it spewed metal particles out from inside the plate structure.
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Old 22nd July 2008, 10:41 PM   #9
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Possibly stating what will be obvious to some, but how about connecting a multimeter or two up?

With "alien" tubes I have learnt the hard way to carefully test first. I have a mock-up test rig using some 350V with meters and adjustable cathode resistor to inspect tubes before putting them into an expensive amplifier. Otherwise one could put an 10 meg input resistance dvm on the control grid and another one on the cathode - in case of trouble one can usually switch off quickly enough to avoid damage. (I have one proper and two more cheap dvm's around, and have often been grateful for that expenditure.)
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Old 1st August 2008, 04:00 AM   #10
Zap is offline Zap  United States
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Okay, well it is time to revisit this issue as I now have 63V replacement caps! I put the new capacitors into the amp, put it back together and fired it up with the original EH KT88's. As expected it has been playing without incident for a few hours. I can't believe that it was only just over a week ago - it seems like forever and I really appreciate how good it sounds compared to the crappy solid state amps I have been listening to.

At any rate, what should/could I easily monitor to give me a good idea of what is going on when I decide to try the RCA 6550's? I want to watch for runaway and catch it before I blow another cap or red plate a tube. Interestingly enough, the bad cap does not register as a short on my meter.... but the bulging top definitely says "BAD".

Any suggestions on changing the bias to accommodate these tubes if they run differently from new production? In addition to Kevin's comment, a post I saw suggested that the cap may have been run at or over its 50V rating if the RCA (TS made) 6550's do indeed run hot.

One suggestion for everyone talking about the adjustable bias switch - a multi-position key switch. That way you could set bias and remove the key preventing a bad accident.
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