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Post by EmRR on Aug 12, 2015 7:55:38 GMT -6
You could have a bad transformer, yes. Edcor doesn't really have any quality control. You have to test their transformers before installing. In my experience you'll get at least one bad one out of every 10 you buy. That's why they are only $12.
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Post by deepdark on Aug 12, 2015 7:58:25 GMT -6
holy crap, really? last time I ever got them. I'll try the current balance pot tricks and then, swap out the input transformer, and then the output if nothing change. Now that explain the ridiculous price (though the shipping is real robery)...
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Post by deepdark on Aug 12, 2015 8:23:21 GMT -6
I'm being thinking about the fact I'm loading the output at 10K, this is probably not a good thing. If I force my secondary to be 600ohm, ( strapping a 660ohm resistor across the secondary) could it help of it will just fucked it up even more? I don'T think the load will be the exact square root at the primary of the secondary, isn't it?
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Post by deepdark on Aug 12, 2015 8:30:36 GMT -6
oh and I forgot to mention, with the lows lost, came a bump at around 1Khz. Frequency start to fall at about 500hz. This is what I remenber from my test and forgot to list it here.
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Post by EmRR on Aug 12, 2015 8:48:42 GMT -6
Put any output load from 600 to 1K on there and measure again with pink noise or similar. With and without 6sq7. Then put test signal into input pot with input transformer disconnected and 6sq7 removed.
Any other transformer will be at least twice as much. Buy knowing this, and test first. They'll send replacements if you confirm bad immediately.
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Post by deepdark on Aug 12, 2015 9:16:36 GMT -6
allright. I'll plug it into an mic input, which is 3k and that should be a little better for testing. removing the 6sq7 will basically force the signal to flow from input amplifier to the output, no current control, etc. isn't it? If things remain the same with or without, then the trouble of low end loss will not be into the side chain circuit but localised between the input and output stage. And by removing the input transformer, and passing signal straight to the dual gang pot, we will see if thing change according to the frequency response. If not, then trouble is after the input transformer, but before, the output one. If I try another output transformer and things do not change, then the trouble sit between the 6SK7 and 6SN7. Am I allright?
Next time, be sure I'll not buy another one. I'll stick to Cinemag  |
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Post by EmRR on Aug 12, 2015 9:55:48 GMT -6
right
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Post by deepdark on Aug 12, 2015 18:41:00 GMT -6
Houra, I run the unit without the input transformer and it was teh culprit. Frequency is flat as hell. a 3-5 db drop at 50hz and about 7db at 17khz. So now, I need to convince edcor to send me another one XD. I just tested out with another edcor transformer and it is allright. So we find the culprit.
Next step, finding the culprit of the motorboating XD
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Post by EmRR on Aug 12, 2015 20:10:42 GMT -6
awesome
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Post by deepdark on Aug 14, 2015 8:42:03 GMT -6
Looking now at the 6SQ7 section which is motorboating. Looking first at the voltages, my plate is spot on 140V, so my current shoudl be ok there. I adjust pin 3 at 2.5v via the current control. The only place is the diodes section pin 4/5. I measured -0.8v instead of -1.42v. This is where I think I could have a trouble. Some thought about that part. Maybe C6 is where I got trouble. Maybe I should install it closer to the tube instead of mounting it on the turret board. Maybe distance is important here. If so, the trouble would sit before the 6sk7, so before compression stage, isn't it? Or maybe better 6sk7 tube match would help. basically, the diode are wired to send a half wave signal, so only one portion of the wave is then send to C1/R1, isn't it?
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Post by deepdark on Aug 14, 2015 9:08:56 GMT -6
and concerning my psu, with the 4 large 47uf capacitor, I don'T think it is the trouble we got here (in term of ripples shot into the circuit)!? Maybe too much filtering, though. I could replace the last 47u with something like a 22u.
The way I see it, is that 6SQ7 is controled by the output of V2. A portion of the signal is send from V2 pin 8 (output) to V4 via a coupling cap (C5) and R10 (threshold), which set the how hard we will limit. The amplified signal then exit at diodes plate 4/5, coupled with C6 and go through R11, and then to C1/R1 and attenuator pot. So where appear the motorboating? At the limiting stage, or after the amplification stage of V4, so between output of V4 (pin 4/5) and input stage? If occuring at the limiter stage, so having a good 6sk7 matched pair would help or since we only take output of V2, imbalance shouldn't be an issue?
I just try to figure out where did the motorboating start 
Another thing I'm suspecting is a not high enought power resistor. Exept the big 10 watts resistor and my psu ones that are 1w, all my other résistors are 1/2W. For exemple, R7 drop about 51V/27k = 1.89ma. 51v at 1.89ma should gave me a power rated low enought to use a 1/2w resistor. and R8 (current ctrl) is 2 watts. So probably not the culprit here.
one possibility is the shield. Maybe if i strap it straight to the tube socket mounting bolt it will shorten the run and help. Or maybe one of my lytic cap is leaky, or c6. |
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Post by deepdark on Aug 16, 2015 4:29:12 GMT -6
Other than that, do you have anything else in mind of trouble lie somewhere into what i already think?
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