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In Reply to: RE: Interstage transformer to 600R recommendation needed posted by KevinC on December 06, 2013 at 12:44:24
Hi Kevin,
Thank you.
Actually, I wasn’t aware that the reflected impedance ratio is SQRT of the turns ration.
Thus, LL1892A Alt. Q with turns ration of 3.5:1 will reflect 600R on the secondary as 7.35K load to the tube, which isn’t bad at all.
The D3a will run at either 18mA or 22mA, with plate supply voltage is to be determined. (I will decide it, based on the final circuit bias point and plate current).
I believe that the static resistance of the primary in Alt. Q is 690R (two C windings in series). Is that correct?
What is the approximate inductance of LL1892A/18mA in Alt. Q connection and what the approximate inductance of LL1892A/22mA in Alt. Q connection?
While I’m at it, is there a way to calculate the LF attenuation, knowing the tube’s internal impedance and the primary inductance?
Alternately, how can I estimate the LF attenuation in each case?
The answer to the above question will also be part of the answer to my second question below, since I may use a preamp with 6V6 @ 22mA and not E55L @ 55mA. At the moment I’m considering both options.
Follow Ups:
If you use the C windings side of the LL1692Aas Q does, the DC resistance is 690 ohms.
The data sheet says 95H for 18mA and calculations give an approximate 75-80H primary inductance for a 22mA gap.
If you look at the data sheet for the LL1620/1623/1627/9202, at the bottom of the third page there is a calculation for "small signal" (response low frequency limit) bass roll-off. I'd give you the answer here, but the text nature of the forum makes it very difficult to communicate equations.
Kevin Carter
K&K Audio
www.kandkaudio.com
Hi Kevin,
Thank you.
My math is quite rusty. I tried to fiddle with the equation from Lundahl datasheet, though I’m not sure I’ve got it correctly.
I came up with the following formula, to find out Low Frequency 1dB roll off:
f =(( L*pi)/(R/2pi))*2pi
Is it correct?
Sorry, but I don't like interpreting ASCII equations, because I usually do it wrong compared to the intent of the writer. So I will write out my reduction of the equation as I use it.
The Response Low Frequency Limit as defined in the LL1620 et al, data sheet =
the arithmetic parallel combination of Rload and Rplate divided by the multiplicative product of pi (3.14...) and the primary inductance.
Kevin Carter
K&K Audio
www.kandkaudio.com
Hi Kevin,
Thank you very much.
I got it and now I can calculate all transformers I’m interested in.
The LL1692A/18mA, in the circuit I intend to use it in, has –1dB at 5Hz, while the LL1692A/22mA has –1dB at about 6.4Hz, which isn’t bad at all, sufficient for my needs.
The LL1620/60mA, in the circuit I intend to use it in, has –1dB at about 11Hz, which is sufficient for my needs.
Edits: 12/09/13
Now it's just a matter of deciding what to do first... :)
Kevin Carter
K&K Audio
www.kandkaudio.com
Hi Kevin,
Thank you.
I got my answers.
Only there is one thing I’m not sure about, concerning the equations in the mentioned datasheet.
I believe that in Rprimary it means the reflected load of the secondary onto the primary. Is that correct?
Yes.
Kevin Carter
K&K Audio
www.kandkaudio.com
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