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Old 14th March 2014, 04:00 AM   #21
AJT is offline AJT  Philippines
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sorry, i mean the screen grid resistor....
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Old 14th March 2014, 01:44 PM   #22
coffin is offline coffin  Taiwan
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I soldered two diodes and the arcing was gone as predict. Result!
But now the sound of this RIAA seems to lack the low frequency......I will test it next week some time.
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Old 14th March 2014, 03:59 PM   #23
kevinkr is offline kevinkr  United States
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Good start, now to figure out why the bass is not right.
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Old 23rd March 2014, 06:23 AM   #24
coffin is offline coffin  Taiwan
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I had simulate the circuit on TINA-TI, it seemed no major problem on bass. (well it did have some minor deviation from enhanced-RIAA curve within 0.5dB)

But when I tested on osc scope, the response rolled off at around 180Hz@-3dB / 50Hz@-6dB. On plate of the triode, there's no roll-off. On the grid of Pentode, no significant rool-off as though there's some 120Hz modulation due to the probe. And the both signals of OPT were the same - rolled off. I paralleled a 1000u/100V cap on cathode resistor of the pentode with no effect. And both channels rolled off.

So maybe I'll try to swap the pentode to first stage to see what will happen.

cheers

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Old 23rd March 2014, 05:04 PM   #25
kevinkr is offline kevinkr  United States
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That is one possible course of action, and there are others.. First I would see what the effect of reconnecting the pentode in triode mode is, I expect the high rp of the pentode is excessive for the available primary inductance of your transformer. Another possibility would be to shunt the primary with a 5 - 10K resistance - this will reduce the available gain, but with it the source impedance as well.
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Old 24th March 2014, 08:32 AM   #26
coffin is offline coffin  Taiwan
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Thanks Kevin.

I shunt a 10K resistor on primary and the -3dB drops to 40Hz, but in the same time the gain drops 9.5dB.

And then I change to triode connection, -3dB further to 13Hz and gain drops 10dB. I will listen to this later.

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