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ST-70 Custom Died - Where To Start?

Since taking Dave Gillespie's advice and installing screen stability resistors on most of my tube gear, and by me adding a power conditioner, I have since not had any tube arcing or failures. The power conditioner keeps the voltage rock steady at 120 V so the tube bias stays very steady after warm up. If I run any of my vintage tube gear without the power conditioner, I use my dual outlet bucking device that drops voltage down 6 V from the average wall voltage of 124 V. I figure output tubes are becoming a big investment anymore, so I'm very pleased with what I've done to help protect them.
 
Preliminary report from moewillis identifies one of the small tubes as the culprit. I recall someone asking point-blank in this thread if I tried replacing them, and I looked for that to give 'em some credit, but whoever asked either edited it out, deleted it, or I am overlooking it. I was pretty sure the problem would just wind up making me look stoopid, and I'm actually pinning my hopes on that. At any rate, he received the amp today and I had included the full set of old tubes as well as a full set of new - all twelve of each - in case they might help in diagnosis. Seems at least that wasn't a stoopid move. In my own defense, I didn't/don't feel up to figuring it out due to illness, meds, being a whiny pu$$y, and an idiot to boot. Well, crap, that ain't much of a defense, come to think of it.

So he checked the old tubes and found one of the little ones bad, very bad (which I did not replace when I purchased the amp a few months ago, despite not knowing their history - the big ones, yes I replaced, but the little ones, no). Further proof of my defense, you see. Now before everyone gets congratulatory, this is a preliminary finding. However, with the new tubes in place the EL34s biased up perfectly and moewillis is about to test it in his main system.

He says to ensure that there are no other issues, he should keep it a few months for daily monitoring. Does that seem about right?
 
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I agree some extended monitoring is necessary but a few months may not be necessary...just saying. A couple of weeks should be enough time to see if the issue is resolved or not.
 
tubeampking said:
I agree some extended monitoring is necessary but a few months may not be necessary...just saying. A couple of weeks should be enough time to see if the issue is resolved or not.
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...and that will be not. Morgan just reported that he got no sound but crackles despite the tubes biasing well. He's opened it back up, dug a little deeper and located a blown cap. While he's in there he will replace some others. Will get back to it tomorrow - says it shouldn't be a big deal to get it straightened out. Seems I did the right thing in sending it to him after all. Of course the "several months" I attributed to him was merely my own sense of humor - he mentioned a week or something, with one of those winky kind of emoticons.
 
musichal said:
I recall someone asking point-blank in this thread if I tried replacing them, and I looked for that to give 'em some credit, but whoever asked either edited it out, deleted it, or I am overlooking it.
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:D

That was actually me via pm... I couldn't help myself :D

Should get some more time tomorrow. A pic for fun. Notice anything odd here??

image.jpg
 
barefootbrew said:
Moe,

I'd love to see a pic of the inside of that baby...if you have a chance to post one.
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absolutely! if hal doesnt have any problems with it i can post some pics as i go. i dont typically post too many pics of other peoples stuff. it is a really nice unit. it would be nice to have a schematic (as its barely even an st-70 besides the a-470's) but i can make it.

the unit is working and biasing fine but with 0 output. i tested the output transformers and they seem ok. obviously the cap is a problem but im going to pull that board and the other corresponding board and test all of the components before i place any parts order. i was so excited when i found that bad tube and everything biased in the garage with no signal running through it. i was a little bummed when i hooked it up and got nothing out of it. i didnt have a ton of time to work on it last night but i should be able to spend an hour or two on it this evening.
 
i havent tested as many tubes as some of you guys but i had never seen one so bad. i ran a shorts test and it passed. i flipped over to emissions and it pegged on 0. when i flipped to grid leakage it pegged the gauge past bad immediately and that wasnt even warm. i pulled it, let it cool and tried a grid leakage test again. as soon as it started to warm it pegged past bad immediately. 0 emissions again. bad tube
 
moewillis said:
absolutely! if hal doesnt have any problems with it i can post some pics as i go. i dont typically post too many pics of other peoples stuff. it is a really nice unit. it would be nice to have a schematic (as its barely even an st-70 besides the a-470's) but i can make it.

the unit is working and biasing fine but with 0 output. i tested the output transformers and they seem ok. obviously the cap is a problem but im going to pull that board and the other corresponding board and test all of the components before i place any parts order. i was so excited when i found that bad tube and everything biased in the garage with no signal running through it. i was a little bummed when i hooked it up and got nothing out of it. i didnt have a ton of time to work on it last night but i should be able to spend an hour or two on it this evening.
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By all means, please post the pics. Need an assortment of wallet-size and glossy 8x10s.
 
Thats one of the small signal tubes? Sort of unusual for one of those to die so badly, they typically just slowly dwindle away after a whole lot of run time. Wonder if its got a vacuum leak. Kind of sounds like a tube with some air in it.
 
gadget73 said:
Thats one of the small signal tubes? Sort of unusual for one of those to die so badly, they typically just slowly dwindle away after a whole lot of run time. Wonder if its got a vacuum leak. Kind of sounds like a tube with some air in it.
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I foolishly failed to replace the small tubes when I bought the amp - their history unknown to me. I was thinking (debatable) that small tubes lasted about fifty years, or something. I immediately replaced the power tubes - each one had their guide pins broken off, which seemed odd, and I should have realized that the seller just sent me cast-offs. At least it seems so.

Am assuming, Morgan, that you will use all new tubes I sent once you get the amp going - for your listening checks.
 
gadget73 said:
Thats one of the small signal tubes? Sort of unusual for one of those to die so badly, they typically just slowly dwindle away after a whole lot of run time. Wonder if its got a vacuum leak. Kind of sounds like a tube with some air in it.
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yes, it was a 6ej7 (?) i think, im not in front of them at the moment

musichal said:
Am assuming, Morgan, that you will use all new tubes I sent once you get the amp going - for your listening checks.
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yes, i already marked the bad tube and took its buddy out as well (though it tested good). i wanted a matched pair. i used the old tubes on the other pair but those tested fine.
 
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what kind of " " electrolytics are those, what are the supposed values vs schematic values, and why the heck are (inexpensive or the C-word) Chinese/Taiwanese caps doing in a nice amp like this? At the risk of offending the original builder, why aren't known quantity major maker Japanese or American made low ESR caps installed assuming availability, size and voltage ratings are met? Are the caps run at / near their limits, or are they just old and failing? Any evidence of venting below on the circuit board.

Does one know the age of this amp and when it was built? And if the cap is failing as with the failure of the tube, other parts of the circuit will be stressed so looks like Moe has more troubleshooting to do, especially all the coupling caps.

Some people say the big bad cap debacle of the 1999-2007 era of consumer electronics was created when Japanese cap makers came up with a new water and additive formula for low ESR caps which gave much better specs in smaller packages with lower cost - win, win, win. Some industrial espionage led to an incomplete formula being used by rival Taiwanese cap makers supplying the rapidly expanding motherboard industries and then consumer electronics like TVs, media players, and other consumer electronics. Even Japanese electronics makers were enticed by the cost advantages and used Taiwanese caps to their eventual detriment. All products using these bad caps were experiencing failure symptoms within 6-24 months of initial turn on. So IMO its important to get absolutely newly made caps and avoid any, ANY Taiwanese or unknown brand name caps, or any NOS caps from that era, no matter how nice they look - and yes, Rubycon had a run of these as well because they had a supplier of theirs make the caps in Taiwan.
 
moewillis said:
absolutely! if hal doesnt have any problems with it i can post some pics as i go. i dont typically post too many pics of other peoples stuff. it is a really nice unit. it would be nice to have a schematic (as its barely even an st-70 besides the a-470's) but i can make it.
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Didn't musichal post a detailed description and schematic in his first post (seemingly long ago)?

Edit: I see now that there is a schematic but no part values, only identifiers. So you're probably going to have to have a go (and a magnifier with light source) to read the values off the installed components.

Might a PM to rustnrot get you a full schematic with parts values and parts list?
 
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