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Old 25th April 2003, 09:37 PM   #1
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Default Tube failure

I had a strange thing happen to my new OTL the other day and was wondering if anyone could shed some light. The design uses the EL509 (I'm using the Ei EL519) with the screen grid tied to the plate with a 100 ohm resistor. Anyhow, during a very loud party, with only about 100 hours of run time I think a tube failed. The 100 ohm resistor got red hot and the -170V rail fuse blew. I can see only one possible failure but am curious to know if is common or even possible. Could the screen grid have shorted to the cathode? I could see the resistor getting hot if the plate connection opened, but it wouldn't have blown the fuse. Prior to this, there was some crackling coming from the speaker. When I replaced the fuse, the crackling returned, and when I replaced the tube, all seems ok. Any thoughts?
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Old 25th April 2003, 11:59 PM   #2
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Default TUBES CAN FAIL TOO.

Hi,

Quote:
The 100 ohm resistor got red hot and the -170V rail fuse blew.
This can happen with any tube amp...a slight mismatch and the weakest is taking all the heat.

You better measure the 100R resitor that took the heat too and replace it in case its value has drifted.

I don't think the tube you replaced is faulty but it may very well be unfit for that position in the amp.

If you want to find out than you'll need a tube tester or in case of the short you suggest - which I don't think is the case- a DVM can tell you that too.

On a general note, OTLs take time to settle in properly and I wouldn't use them for parties unless they had a couple of hundreds of hours on the counter.

Cheers,
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Old 26th April 2003, 12:59 PM   #3
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Thanks Fred,

It was a wire wound resistor, and it's still 100ohms funny enough. I'm going to replace it anyway. The other reason why I thought the tube was at fault was the 'crackle' that went away when it was replaced. While I'm here, if you don't mind my asking, wouldn't the plate be taking all the current since the screen grid has an extra 100 ohms on it? As for letting it settle in for a good long time, well, it's kinda late for that. It was a good party though I do have access to a couple of tube testers, but neither will do magnoval based sweep tubes.
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Old 26th April 2003, 10:46 PM   #4
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Default OTL.

Hi,

Quote:
It was a wire wound resistor,
I wouldn't use a WW in that postion unless it's a non-inductive one for starters.

Quote:
While I'm here, if you don't mind my asking, wouldn't the plate be taking all the current since the screen grid has an extra 100 ohms on it?
Well, the 100R isn't going to make the difference...

Quote:
It was a good party though
Over and out.

Cheers,
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Old 27th April 2003, 04:51 PM   #5
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gotcha, thanks much.

Chris
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Old 17th May 2003, 12:23 PM   #6
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Default Bruce says....

me;

"A member has stated that he feels it somewhat bad practice to spec wire wound resistors for in the output section of the T8 without going non-inductive. I said that you would only do so if you felt it made a difference, but that I would ask you.

What say you?

BR;
"The inductance of a 2 ohm resistor is so very low that it is totally insignificant in this circuit. The output stage shows absolutely no
effects of inductive loading. Ask the "critic" what the inductance of these resistors are."

I'll spare you the rest of the details, but I suggested that as I didn't specify, and he defended the 2 ohm cathode resistors (I forgot they were there, so contrary to what I had said earlier, the plate is not unimpeded in this manner at all), that he felt the 100 ohm plate to screen resistors were less important. His answer, those resistors carry no audio, only DC.

I like Bruces approach to design as I said before, he's very pragmatic, no expensive or complicated stuff that doesn't make good engineering sense. That's why I like it.

Have you read his book 'Audio Reality'? He explains his OTL patent and his design choices, that's why I built it. And the preamp too.

I'm not sure I need to torture my amp with such a test as you describe, but I might. I know somewhere he says it's stable while driving a square wave into a capacitor, I'll try and dig up the details.

Chris
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Old 17th May 2003, 10:02 PM   #7
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Default Frank says...

Hi,

Still ticked off over nothing, are we?

Let's look at some facts here:

Ask yourself what would be the better ( not necessarily the cheapest) component in that position?

A non-inductive resistor, right?After all in audio we try to avoid non-linear components, not?

Ask my advise and I invariably will give you the best component I know of unless you specifically ask me what the most economical component would be.

It is not good practice IMHO to add inductive components that are invariably non-linear...same goes for adding stray capacitance and what have you.

As you state it, BR replies besides the point at hand but nonetheless I still disagree with him if he claims that that resistor is not in the signal path...let that be a disagreement between him and myself.

Quote:
(I forgot they were there, so contrary to what I had said earlier, the plate is not unimpeded in this manner at all), that he felt the 100 ohm plate to screen resistors were less important. His answer, those resistors carry no audio, only DC.
Those 100R resistors tie plate and suppressor grid together,making it part of the plate, hence the "triode mode", if you or anyone else think the plate carries no signal (AC) than I let me know what does.

Quote:
I like Bruces approach to design as I said before, he's very pragmatic, no expensive or complicated stuff that doesn't make good engineering sense. That's why I like it.
Rest assured, I like those designs too...
But I can always find ways on how to improve them...
The design published in GA back in 1990 was in some ways more appealing to me and in some respects better and more developped.

Quote:
Have you read his book 'Audio Reality'? He explains his OTL patent and his design choices, that's why I built it. And the preamp too.
I hold some copies of the articles regarding this design and some others.
It may sound arrogant from my part but I don't buy a book that's not going to teach me anything.
There are others out there that are far more interesting to me at least.

Quote:
I'm not sure I need to torture my amp with such a test as you describe, but I might. I know somewhere he says it's stable while driving a square wave into a capacitor, I'll try and dig up the details.
His GA article OTL passed it (admittedly that was my 80W version of it) , so does my own OTL.
And I tested it with real music, not just some sine waves.

If you like to look at the GA article regarding that OTL and the one offered in the book you will see where corners have been cut.
BTW, I am still wondering what exactly was patented...but that's another issue.

Now, if you think you can teach me something than please go ahead, somehow however I feel I have a 25 year worth headstart.

Cheers,
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Old 17th May 2003, 10:10 PM   #8
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Quote:
After all in audio we try to avoid non-linear components, not?
Well, there go tubes and semiconductors. Back to stone knives and bearskins.
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Old 17th May 2003, 10:16 PM   #9
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Default SMARTIES AND M&MS

Hi,

Quote:
Well, there go tubes and semiconductors. Back to stone knives and bearskins.
We're talking about PASSIVE components.

Cheers,
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Old 18th May 2003, 06:54 PM   #10
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Default My my, where to start.

Ticked off? Not at all. As I said earlier, before I'd go off replacing 48 resistors, which had been spec'd by the designer, I would have done the research and come to my own conlcusion.

Quote:
"Ask yourself what would be the better ( not necessarily the cheapest) component in that position? A non-inductive resistor, right?"
Only if it makes a real difference. Either in a DBLT or on the measurements. If a component doesn't make a difference, and it costs more, then in this case it is not "better" at all. We're not building radars here. So in this case, I've asked twice, and now B.R. has asked as well, what is the inductance of those resistors, and further, how would it affect the circuit?

Quote:
"As you state it, BR replies besides the point at hand but nonetheless I still disagree with him if he claims that that resistor is not in the signal path...let that be a disagreement between him and myself."
Well I'm sure Bruce could explain his postition to me, can you explain yours?

Quote:
"Those 100R resistors tie plate and suppressor grid together,making it part of the plate, hence the "triode mode", if you or anyone else think the plate carries no signal (AC) than I let me know what does."
No one said the plate doesn't carry audio, what he said was the 100ohm resistor carried DC only. I had made the same assumption that you have,(that the resistor makes the screen part of the plate), but based on Bruces reply, there's something I missed, I'm going to try figure out what that was.

Quote:
But I can always find ways on how to improve them...
Well don't keep us in suspense I'm sure that if you have some useful improvements, Bruce would most certainly appreciate your input, I know I would. I'm serious here, what do you propose? I am assuming topology changes, as component changes are within pretty much anyone's ability assuming they can read a spec sheet, and unless they make a difference at the speaker terminals, or add to reliablility or such they are not improvements at all.

As for the GA vs AR version of the OTL, I don't see any cut corners at all, perhaps you can point them out. Since you photocopied parts of the book, there was obviously something you could learn, therefore your statment not only sounds arrogant, it is, in fact, arrogant. Furthermore, if you can't see what the patent was about, (hint: 3 zener diodes) then either you didn't understand what you read, or, you didn't read it. Me, I've read it 7 or 8 times now, and I think I'm finally beginning to get it. I, however am apparently 25 years behind, so I have an excuse.

Quote:
"And I tested it with real music, not just some sine waves."
Square waves actually, and I did dig it up, it was a 2uf cap in parallel with an 8ohm resistor. Tougher than music I'd say.

I never said I had something to teach you. I do, however think you've got lot's to learn.

Chris
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