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Old 8th May 2012, 12:38 PM   #21
euro21 is offline euro21  Hungary
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Quote:
Originally Posted by pieter t View Post
and measure the inductance with a good quality meter at 100 Hz, 1 kHz and 10 kHz.
...at 25mA DC current.
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Old 8th May 2012, 01:26 PM   #22
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Quote:
Originally Posted by euro21 View Post
...at 25mA DC current.
Yes, that would be best, but equipment measuring inductance with current flowing is not cheap
However "normal" measurements give a good indication, and when possible measure your choke to see if it has enough inductance (I think it is the limiting factor in your circuit).
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Old 8th May 2012, 02:36 PM   #23
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Quote:
Originally Posted by pieter t View Post
However "normal" measurements give a good indication, and when possible measure your choke to see if it has enough inductance (I think it is the limiting factor in your circuit).
I agree with you. The choke's inductance is the only thing I see as the bass limiting factor. There might be a "hidden circuit" inside that affects the bass but I just don't see it.

It doesn't hurt to measure the choke first before gutting the circuit. Just my two pennies.
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Old 8th May 2012, 03:18 PM   #24
DF96 is offline DF96  England
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The choke cannot be the problem unless this particular 12B4 has unusually high anode impedance.
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Old 8th May 2012, 03:28 PM   #25
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Let's just wait till the OP to measure everything is in spec.
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Old 8th May 2012, 03:39 PM   #26
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Quote:
Originally Posted by DF96 View Post
The choke cannot be the problem unless this particular 12B4 has unusually high anode impedance.
Do you see a more likely cause?
Even when the output capacitor is 0.33 instead of 3.3 uF, and the 33k input impedance of the Rotel amp is a 30k load together with the 470k load resistor, it is still a 16 Hz high pass which should not sound extremely bass shy.
There is not much in this circuit that can go wrong.
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Old 8th May 2012, 03:50 PM   #27
DF96 is offline DF96  England
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The change in load is so tiny compared to the 12B4 anode impedance that it can't possibly make any difference to the choke, assuming that the circuit is correct. Even an open circuit cathode decoupler would not raise the anode impedance by very much, as the 12B4 mu is so low. Assume a bad choke and a bad cathode decoupler: you still only get maybe 10% shift in LF cutoff when you change loads (10% is rough estimate - should be OK to within a factor of 2). Either the output cap is wrong, or the OP is not doing what he thinks he is doing or not hearing what he thinks he is hearing.
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Old 8th May 2012, 07:15 PM   #28
euro21 is offline euro21  Hungary
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Quote:
Originally Posted by pieter t View Post
equipment measuring inductance with current flowing is not cheap
My measuring set:
-simple power supply: 90VAC, diode, 220uF-100R-220uF
-simple CCS: DN2540 (heat sink), trimmer pot
-signal generator (1V, 10Hz-100kHz)
-RMS voltmeter
Attached Images
File Type: jpg My inductance measuring set.jpg (64.2 KB, 190 views)
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Old 20th August 2012, 07:12 PM   #29
6L6 is offline 6L6  United States
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Quote:
Originally Posted by Eli Duttman View Post
Replace the choke with a good CCS. Then, even the nasty IHF 10 KOhm "standard" I/P impedance will not be a problem.
I really want to try the 12B4.

Any information or schematic as to a suitable CCS? I'm in the dark here...

Also, what's the B+ in your schematic?

And are the values of chokes in the PSU critical? I have some that are in the ballpark, but not exact.
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Old 20th August 2012, 07:30 PM   #30
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Here are couple of simple CCS circuits:

Click the image to open in full size.

Click the image to open in full size.
Using two FET or transistor CCS with 12B4 is waisting of resources.
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