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ST70 EL34 plate glowing like Rudolph's nose

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Posted on September 17, 2009 at 14:22:06
wjcorbin
Audiophile

Posts: 38
Location: Pennsylvania
Joined: July 22, 2001
 Hi folks,
Boy, two days and two issues/questions for the forum. I'm on a streak here. I was the guy who posted yesterday about whether to leave the ST70 cage on or off. Many good replies to that, so thank you to all who replied.

Well, today I'm listening to the Beatles Rubber Soul remastered CD (ST70 with cage on) through my recently refoamed/refurbished new large advent speakers thinking life can't get much better than this when I notice the volume on one channel is decreasing in the middle of a song and I'm also hearing some fuzzy distortion in sound. I look at the right side front EL34 through the cage and it's plate is glowing red, oops, this can't be good so I immediately power down ST70, then the PAS 3X.

Now after letting amp cool for a half hour I screw up my courage to start investigating. I take the cage off the ST70, get my DMM, and turn on just the ST70. For about 15-20 minutes I observe plates while also checking the bias on both sides of amp. After initial warmup both sides settle into 1.29 to 1.31 volts and then hang steady at these values. Plates stay the same cool gun metal grey I have grown to know and love. Next I briefly power down the amp, then power up the PAS 3X and then ST70 and again leave both on for 20 minutes with the same steady bias readings and cool EL34 plates. The only thing I have not done is actually run the CD player or tuner.

As background, I completely refurbished my ST70 about 4 years ago with Joe's premium driver board and above chassis quad cap replacement board. I also replaced all the EL34 tube sockets with gold plated contacts/ceramic tube sockets. My EL34's are Svetlana flying C's I got about 2 years ago. I use the amp maybe 6 hours a week on average, not real heavy use. I have only been using the refurbished Advents about a month now. Does anyone have any idea why the plate on my one EL34 started to go nuclear and then on testing stayed normal? Could the advents be putting such a load on the output transformers that would cause increased current through the EL34 plate? Could my having started using the ST70 with the cage had something to do with this? Excuse my ignorance, but right now I'm scared to turn the whole system on again.

Bill C.

RE: ST70 EL34 plate glowing like Rudolph's nose, posted on September 17, 2009 at 17:50:31
Joe Curcio
Manufacturer

Posts: 1094
Location: Northern CA
Joined: October 10, 2001
Contributor
  Since: July 1, 2002
 Bad output tube socket

I've posted a paper on this subject - might be a good read to better understand what is happening

RE: ST70 EL34 plate glowing like Rudolph's nose, posted on September 18, 2009 at 05:52:29
wjcorbin
Audiophile

Posts: 38
Location: Pennsylvania
Joined: July 22, 2001
 Hi Joe,
I did go through and read your troubleshooting guide, which is the white paper I think you are referring to(?)

I think I understand what you say about the tube socket. I will say that the EL34 sockets are high quality ceramic sockets I installed new 4 years ago. If I understand the socket theory, if the connection on pin 5 is bad, the bias will go to zero causing current increase on the plate. This morning I ran amp with tuner input on for >45 minutes and without the cage on. After initial stabilization, the bias on right sided showed 1.26 V and left at 1.28. (Which brings up another question. Should bias be measured with input source running?) After 30 minutes the right bias was 1.21 and left was 1.25. Both had gone down but both stayed at this level for the next 15-20 minutes. This is a little low since I like to keep it at 1.3V which I believe I had seen recommended here. But at no point did the plate runaway. Since the only time I have seen the plate runaway was when I had the cage installed, I'm wondering if this is a thermal issue. Maybe the added heat from cage on is causing say a bad solder connection (pin 5?)to open?

Bill C.

RE: ST70 EL34 plate glowing like Rudolph's nose, posted on September 19, 2009 at 05:10:55
xaudiomanx
Audiophile

Posts: 978
Joined: August 16, 2004
 You should never bias with an input signal going through the amp.

Paul

RE: ST70 EL34 plate glowing like Rudolph's nose, posted on September 18, 2009 at 07:24:23
Joe Curcio
Manufacturer

Posts: 1094
Location: Northern CA
Joined: October 10, 2001
Contributor
  Since: July 1, 2002
 That is correct - once pin 5 disconnects, the tube current will rise to near saturation.

Another point, I would forget about the 3rd digit on your DVM - there are too many _normal_ variables that will impact the current at that level. The numbers you cite are fine.

I would not consider heat to be an issue causing the bias current to change (cage vs no cage) unless is it severe (cage with holes covered for example). If your sockets are new high quality ceramics, the only other cause (other than an intermittent solder connection to pin 5) would be a leaky cap - but this would not correct itself.
RE: ST70 EL34 plate glowing like Rudolph's nose, posted on September 18, 2009 at 16:56:43
wjcorbin
Audiophile

Posts: 38
Location: Pennsylvania
Joined: July 22, 2001
 Thanks Joe for the thoughts.

Bill C.

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