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Post by deepdark on Sept 8, 2015 20:26:14 GMT -6
that's exactly what I was thinking. Maybe my resistor isn't the right for the task. Here is what I've just put in there www.vishay.com/docs/28730/acseries.pdfIdeal for pulse operation, ceramic with lacquer coating, etc. etc. etc. Maybe I'm just at the edge of the limit voltage? I got there table and look at it. Limiting voltage is set by the square root of P*R, so square root of 5W * 8200r = 202V (probably the voltages across the resistor), which set it near the edge of the voltage I'm passing through. And if it's really what happened here, then I'll have learned something (actually, plenty of things haha). Anyway I'll just put a 10w in there and live with it. Since all my voltages are right and current are what they are supposed to be, I guess I should be fine, right? |
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Post by deepdark on Sept 11, 2015 6:07:09 GMT -6
Just changed the 5w wirewound for a Ohmite 10W and it seems to have cured the trouble. I let it run for about 5 minutes for now and no smoke, nor any fume or burning smell. It remains hot, but it's allright. So, 3.3W through a 5w isn't really optimal XD
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Post by jeromemason on Sept 11, 2015 15:39:09 GMT -6
A 10W resistor is getting hot where 3 watts has been calculated? I've never known a resistor more than twice the wattage to run even warm. Are you sure you've checked everywhere for shorts and possible mistakes? I'm not familiar with that board, but if there are via's maybe check around them to see if solder could had splashed up and bridged a connection.... I just really don't see why you are getting a toasty resistor that is twice the rated power. Its' extremely odd, the current going to the plate at what 18.5ma?? The most power I can come up with is 2.5W through the 8.2k. Assuming that your 10W resistor is getting hot, and that your plate voltage is indeed 138v it puts he current draw somewhere near 72ma. That's calculating that you're going into breakdown by overheating.
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Post by deepdark on Sept 11, 2015 19:42:32 GMT -6
Im not using a board,mit's a turret mounted one. Yup, i took voltages drop and current everywhere. Every single voltage and current is spot on. Because of my higher than soec b+, there is about 3,3w passing through the resistor, which is more than half the 5w spec. I should have take a 7w but i decided to take a 10w. When i say it is hot, it is not smoking or smelling. It's just running a little more than warm. In my point of view, 3w of dissipation must remain somwhat hot. Even in a resistor being higher than twice the current rating. 3w remains 3w, but the resistor just run into a safer side. It's my understanding of what is going on. As i said, i measured everything, and it's spot on.
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Post by deepdark on Sept 12, 2015 4:32:50 GMT -6
I just let it run for about 30-45 minutes and no smoke, no odor, by the way.
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Post by deepdark on Sept 12, 2015 9:25:51 GMT -6
Oh and i just tested it with a dynamic mic and it sounded quiet good. Nothing weird, gain selector works. No hiss. No burning resistor, etc. Run has expected.
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Post by EmRR on Sept 12, 2015 9:33:50 GMT -6
I agree it's very odd that the resistor is running hot given the rating. And, even a 5W is enormous for a preamp output stage, especially one rated for +14dBm max output with peak distortion. Theres a lot about that output stage NOT to like, really low headroom, really high power consumption, big heat output, huge drain on PSU. But, it's there in the manual. Weird f'in preamp. Plenty of things do higher output with much less current draw.
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Post by deepdark on Sept 12, 2015 9:42:38 GMT -6
I didn't know that particular output was rated to run that hot. I suspected first maybe some high freq oscillation, but in testing, i didn't noticed anything. And because the pre works, as expected, i don't really see what's weird here.maybe the fact my b+ is 20v higher is responsible??.
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Post by deepdark on Sept 12, 2015 9:53:28 GMT -6
And if that output really heat, all that heat must be dissipating somewhere, so at the plate resistor. Right? I don't know why it runs hot. I took every single voltages, vdrop, current. Everything is within spec. Maybe it has a reason why the original psu was regulated. Maybe this particular circuit is sensible to variation in current.
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Post by EmRR on Sept 12, 2015 10:07:30 GMT -6
It just runs hot. I bet they eat tubes fast. There's may be a clue in EMI overall system build that's not discernible. They drove 200 ohms, and this is a higher ratio output transformer but a very low value plate resistor. Nothing is adding up to me that would demand that list of constraints, when so many other preamps manage the same without such current draw. Compare to Altec 458, output of +24 with less current by going push-pull. Collins/Gates/RCA single ended, many do +18 pretty cleanly at well under 10mA total.
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Post by deepdark on Sept 12, 2015 10:14:50 GMT -6
You're right. I don't think the resistor being hot a big concern. With 7vac heaters and higher b+, maybe here i'm pushing the tube farther than emi. What do you think?
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Post by EmRR on Sept 12, 2015 13:27:38 GMT -6
Heaters and B+ are totally different issues that don't affect one another.
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Post by deepdark on Sept 12, 2015 14:05:04 GMT -6
Anyway, it works lol.
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Post by deepdark on Sept 12, 2015 14:52:46 GMT -6
What. Was speaking with higher b+ and heaters is more about the heat of the tubes, and the overall power dissipating through 8k2. Anyway, it's a pretty agressive pre, no real control over the input, it's pretty raw but it's cool.
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Post by deepdark on Sept 14, 2015 8:44:25 GMT -6
Some food for thought. If we look at it from the psu side, we know we have 308V at about 21ma, which is about 6.5W. We know we feed the input and output stages anode. Input, with such a small current consomption, would take no more than about 0.2w, right? Let's say 1w at the input, doesn't it let at least 5W at the output? And the only way for B+ to feed the output is from R11, isn't it? From that point of view, first we can say that the output is ridiculesly high, but also, we can ask ourselves where does all that power go?
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Post by EmRR on Sept 14, 2015 10:09:19 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA.
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Post by jazznoise on Sept 14, 2015 10:41:51 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA. This, and also the rated maximum draw and the actual draw of the circuit have to be very different. If a unit draws 30mA while idle it may well draw 50mA for the first millisecond as the caps are filled. Usually the PSU has to handle the current draw of the circuit + 50% though with circuits with a lot of big caps going closer to doubling is safer. |
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Post by deepdark on Sept 14, 2015 12:12:52 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA. You're right. PSU only provide what's needed. I look at it the wrong side. but still, the power remains the same, which lead me to think, as you said, the output is so damn gluttonous for such a task. If really output is about 5W, damn it's quiet big. E88CC must have to be replace often than normal, isn't it? |
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Post by deepdark on Sept 14, 2015 12:17:45 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA. This, and also the rated maximum draw and the actual draw of the circuit have to be very different. If a unit draws 30mA while idle it may well draw 50mA for the first millisecond as the caps are filled. Usually the PSU has to handle the current draw of the circuit + 50% though with circuits with a lot of big caps going closer to doubling is safer. Yeah you're right. I always double values to get the right power transformer, to be sure I'm not to close to the limit. My first filter cap is about 1uf/ma and generously round up so I get 47uf in this case, where I could have put something like 22uf, even 33uf. I had calculated about 2W through the psu, I got 5 sections so less than 1/2W / resistor. I put 1W résistors everywhere in the psu. In reality, here we have about 21ma and my power transformer is rated 69ma at the secondary, so I could tripling the initial valu and not exeeding the maximum rated value of the PT. |
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Post by EmRR on Sept 14, 2015 13:04:05 GMT -6
Actually, data sheet makes it look like the tube is run cooler than it could be. Probably last long enough.
It's a hot output stage because the 7:1 iron is run 10k:200 if my math is right. The more current you put into that the better its response is, and a higher ratio to lighten the load on the tube would yield even lower output levels, even though tube gain would also go up marginally.
We don't typically need to drive 200 ohms anymore, an American 600 ohm output would be a 4:1 and deliver higher max levels. A 15k:600 would still be a lower 5:1 ratio with greater output level and handle a cooler output stage.
So I blame EMI's need for a 200 ohm output stage.
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Post by deepdark on Sept 14, 2015 13:07:58 GMT -6
I used a Edcor 10K/600ohm, and typically, it will be load by about 10K if used straight into a ad/da.
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Post by deepdark on Sept 14, 2015 13:14:26 GMT -6
I agree it's very odd that the resistor is running hot given the rating. And, even a 5W is enormous for a preamp output stage, especially one rated for +14dBm max output with peak distortion. Theres a lot about that output stage NOT to like, really low headroom, really high power consumption, big heat output, huge drain on PSU. But, it's there in the manual. Weird f'in preamp. Plenty of things do higher output with much less current draw. is it the same reason for the big heat output and output stage running hot? |
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Post by EmRR on Sept 14, 2015 13:24:29 GMT -6
I used a Edcor 10K/600ohm, and typically, it will be load by about 10K if used straight into a ad/da. So you have more gain already, and all that output power is wasted on the original idea that it's driving 200 ohms. If you took the output transformer off it and drove it cap coupled only, the output stage would be seeing what was originally envisioned for load, and you'd have a lot more output level. |
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Post by deepdark on Sept 14, 2015 13:49:12 GMT -6
so all that heat, power consumption is related with the way output transformer and overall reverse feedback is handled at the output, and even with a lower ratio, i guess it will still runs hot because of the huge current flow, right?
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Post by EmRR on Sept 14, 2015 14:16:26 GMT -6
It's because of the output they chose, but it will run the same regardless until you redesign the tube stage.
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