Current Production KT-77, 6CA7 Type Reliability

JonL

AK Subscriber
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Just thought I'd share my recent reliability problems with a couple of tubes. Amp is a Dynaco Stereo 70 with a Dynamutt Octal driver board modified for lower gain with 6SN7 in place of 6SL7 driver tube. The amp was being used with its original Mullard rectifier and EL34s. A few years ago it went "CRACK" with a nice arc in the rectifier. I don't recall exactly how, but I traced the problem to one of the output tube sockets, and took the amp out of service to repair. I bought a quad of JJ KT77s to install, but before I could attend to the repair, my psychotic ex girlfriend shoved the amp off the kitchen table, breaking a couple of the 6SN7s and the brand new (NOS) GE GZ34 rectifier. Grrr. In my disgust, I abandoned the project for a couple of years, getting back to it about six months ago. I went through the amp, replaced the rectifier and output sockets, replaced the can cap, did not add any additional capacitance, put in old 6SN7s I had on hand, a brand new (Russian) Tung-Sol GZ34 and the quad of JJ KT77s. I biased it to the Dynaco spec (1.56 V with the original Dynaco cathode resistors), and kept an eye on the bias frequently. The amp sounded good, if perhaps a bit light in the bass. Now to the problems...

First problem was with a JJ KT77. I don't recall exactly what happened, but when checking the bias I determined that one tube (coincidentally or not, in the socket I originally diagnosed as the problem) was drawing excessive current from the bias circuit so I couldn't properly bias any of the tubes. The tube wasn't red-plating, but with that tube installed the bias voltage was being pulled way down. The tube had only a few weeks of use. I bought a replacement, and everything was fine until a few days later when "poof..." and the replacement lost vacuum due to a crack around the base. Defect? Damage in transport? Who knows. Got a warranty replacement. All good for a couple of months when... the replacement started to red-plate. (Still in that same position - but a new socket - where the original problem became evident.) I moved the tube to other positions and the red-plating stayed with the tube. OK, now I'm done with JJ KT77s. I purchased a quad of EH 6CA7s and installed them. Biased them, and they sounded fine. For a month. Until... "ZAP." One of them (in a different position!) started glowing like a barbecue. Tried it in different positions, and the red-plating followed. I've returned it for a warranty replacement, which I haven't yet received. In the meantime I've installed a pair of the original Mullard EL34s in that channel. They test pretty weak on my Hickok 6000, but they are sounding fine.

I've owned several tube hi-fi amps and about half a dozen or more tube guitar amps since 1973 when I built my first Dynaco SCA-35. I've only ever used tubes made "back in the day" until now and not once had a problem with a runaway power tube. Out of the 10 current production power tubes I've bought, 4 have gone bad in less than 2 months of use. Is this typical???
 
Sorry to hear this. I've had great luck with both JJ and Gold Lion KT77s. Not a failure. Spoke with Jim McShane a while back and he said that his customers have had great reliability from both the EH 6CA7 and GL KT77. He recommends them without hesitation.
 
I would check to make sure the grid resistor used in your rebuild isn't too high of value for the tubes. Sometimes, new tubes can't tolerate the same high value of grid resistors as old tubes, and this can lead to runaway bias. Also, it's possible the tube sockets or some other wiring got damaged in the drop to the floor - a momentary loss of grid voltage would be enough to ruin a tube. At this point I'd be looking at the amp, not the tubes.
 
The JJ77's of a few years back were notorious for failure. It's all over the web. Supposedly the reliability issues have been fixed in the newer tubes. As max said, I suspect the amp.
 
I really like the Siemens or RFT EL34 . They sound almost as good as a Mullard, at half the price , although they seem to cost about the same nowadays. And they are as reliable as anything else.
 
I've been using the same set of JJ KT-77's for a couple years now in my ST-70 without an issue. I know the earlier production 77's had a bad rap, but the new ones seem to hold up ok. Where did you get the tubes? I would be looking everything over in the amp also. Being that an EH 6CA7 blew too.
 
The repair work on the amp (including new output and rectifier sockets) was done after the fall to the floor but it certainly bears another look. I won't rule out the amp, but the failure modes haven't been entirely consistent... first failure was of a type I've not heard of... one tube loading down the BIAS voltage, implying (to me) a partially shorted grid (?). It was just the one tube. The cracked glass is of course unrelated to the amp. The two runaway tubes happened in different sockets, and the tubes stayed in runaway when moved to other positions. 3 of the 5 JJs I had in the amp worked fine for months, and are still OK although I took them out to use the EH 6CA7s. The JJs are about 4 years old, though never used until recently. I don't know if that puts them of a vintage where they were problematic.

The tubes all came from The Tube Depot, who have honored all warranty claims without an issue.
 
I have a set of EH 6CA7 in a Pilot SA-260. Its broadly similar to the ST-70. Same sort of output stage, voltages, etc. They've been fine, but the very first time I wound up the loud knob one of them arced internally. I added 100 ohm resistors between the screen grid and the screen taps on the output transformer and they have been perfectly fine since that time. Maybe worth adding the resistors for a bit of insurance.

+1 on checking the sockets to make sure the pins are snug. Also worth going over the wiring in the bias supply and maybe cleaning the bias pots just in case anything is flaky there.

also, good call on the EX.
 
In addition to adding Screen Stability resistors, many of the Russian power tubes have notably slimmer pins. I know you installed new sockets -- but their design is based on the standard pin size of the original vintage products. Therefore, it would be a good idea to tighten the receptor terminals in all of the octal sockets as well, so as to ensure there is good connection integrity between the tube pins and the socket receptors. At a minimum, it will at least eliminate the possibility of intermittent socket connections from the list.

Dave
 
Ongoing saga...
I've had the EH 6CA7s in the amp since the summer, and it's been fine until a few days ago. I turned on the amp, and left the room for a few minutes while it warmed up with no music playing. I heard an unusual hum and came back to find one tube moderately red plating. I turned off the amp and started some troubleshooting, measuring the idle current and moving tubes around, etc. I found that the non-red plating tube of that channel's pair was noticeably cooler than all the others in the amp. The good channel required about -37V (as I recall) to bias properly, but the bad pair wouldn't get to the target current even with the bias voltage decreased as much as the pot would allow. (I don't have independent bias pots for each tube.) My conclusion was that the non-redplating tube was drawing very little current, increasing the plate voltage to both tubes and making the stronger tube start to glow. Closer examination showed something very strange... the cathode on the weak tube actually protrudes above the plate structure about twice as far as for the other tubes, and the filament is much brighter at the bottom of the tube because it's no longer as completely contained within the cathode. It's hard for me to imagine how the cathode migrated upwards in the amp, but it's also hard for me to imagine that the tube went through matching and worked well for 8 months in this condition. Picture attached of the bad tube next to a normal one.
IMG_3496.jpg
 
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