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Old 5th February 2012, 02:59 AM   #1
regal is offline regal  United States
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Default opt primary inductance and secondary load

Scenario. You find a 5k:8 parafeed transformer, which is rated with enough inductance at 20 hz for -3dB with an 100V signal across the primary for a 1.5kohm Ra tube with an 8 ohm speaker . Now you load this transformer with a 16 ohm driver, so the transformer presents a 10k ohm load. Good for linearity and for sake of argument the 100Vrms across the primary is good enough for the 16 ohm driver. So power went from 2W to 1W and you are OK with this.

How could this affect bass response ? I was in a debate that if I didn't parallel two 16 ohm speakers and present an 8 ohm load on the secondary that the -3dB@20hz point would shift to more than -3dB down @20hz?

I understand that increasing the load on the secondary is not going to magically increase the number of turns on the primary and give more inductance. But shouldn't the inductance stay the same independant of secondary load with the same tube Ra (assume I adjusted bias to maintain Ra) and hence the frequency response remain that same, just I lose 1/2 power by running a 16 ohm speaker on an 8 ohm tap not bass cut-off?
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Old 5th February 2012, 08:56 AM   #2
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Originally Posted by regal View Post
I understand that increasing the load on the secondary is not going to magically increase the number of turns on the primary and give more inductance. But shouldn't the inductance stay the same independant of secondary load with the same tube Ra (assume I adjusted bias to maintain Ra) and hence the frequency response remain that same, just I lose 1/2 power by running a 16 ohm speaker on an 8 ohm tap not bass cut-off?
Primary inductance stays the same. Connecting a lighter than rated load across it may compromise low and high frequency performance, but whether it's enough to make a big difference is another matter entirely.
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Old 5th February 2012, 11:47 AM   #3
regal is offline regal  United States
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Originally Posted by Miles Prower View Post
Primary inductance stays the same. Connecting a lighter than rated load across it may compromise low and high frequency performance, but whether it's enough to make a big difference is another matter entirely.
How would a lighter load compromise low frequency perormance? I mean with preamp OPT's (15k:600) it is common practice to drive an amp with 10k to 100k input impedance.
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Old 5th February 2012, 03:13 PM   #4
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The -3dB point occurs when the inductive reactance (Xl) = the parallel combo of the tube Ra and the reflected load. by increasing the value of the load the value of the parallel combo goes up pushing the -3dB point up. Quick calculations based on 10hy's of inductance put you at -3dB at 18hz for the5K load and close to 21hz for the 10K.

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Old 5th February 2012, 03:19 PM   #5
DF96 is offline DF96  England
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+1 on the explanation, but I haven't checked the arithmetic.
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