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rang out some OPTs. have a question
rang out some OPTs. have a question
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Old 12th July 2018, 02:59 PM   #21
Kay Pirinha is offline Kay Pirinha  Germany
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No. This would rule out the inductances of both halves, leaving only the stray inductance between both.


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Old 12th July 2018, 03:11 PM   #22
mashaffer is offline mashaffer  United States
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The interesting thing is that even a small about of inductance gives near perfect matching in the mid to upper frequencies. The amount of inductance simply moves that matching lower in frequency so if the amplifier is intended for limited bass output (bi-ampimg for example) the inductance is not critical.

In this circuit even 4 or 5H gets you down to 100Hz before diverging by more than a couple of dB. Will have to check the effect of adding some more resistive tail.

rang out some OPTs. have a question-indtail-png

Ran a quick sim. Additional resistive tail limits the maximum deviation but not the frequency at which diversion comes.
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Last edited by mashaffer; 12th July 2018 at 03:23 PM.
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Old 13th July 2018, 01:16 AM   #23
PRR is offline PRR  United States
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> even 4 or 5H gets you down to 100Hz

5H at 100Hz is 3K reactance.

3K is much more than 1/Gm of a 12AU7.

OTOH, at say 10mA total tail current, a dumb/cheap 3K resistor drops 30V. So a say -24V rail (or elevated grids) and 3K resistor does the same as 5H does at 100Hz, except also all the way to zero Hz.
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Old 14th July 2018, 01:31 PM   #24
mashaffer is offline mashaffer  United States
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The EV is only rated at 5W v.s. the 10W rating on the other but given their reputation it might be a conservative rating. I will probably try both and see how they shake out. If they end up not being worthy of 50Hz output in the 5-10W range I can use them for the "Music Box" project that doesn't require lower than about 100-150Hz and drop $50 or $60 on a pair of Edcors.

I should also probably try to "measure" the primary inductance.
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Old Yesterday, 06:35 AM   #25
6A3sUMMER is offline 6A3sUMMER  United States
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If you are going to use an inductor as a current source for a LTP, consider the real impedance of the 2 coupled cathodes.

At 250V P-k and 10.5mA, the 12AU7 Gm is 2200uMhos (455 Ohms), and u=17.
(Yes, with a 300V supply, the plate voltage will be lower than 250, & other values will change too, but this is just to get a ball park set of values).

Post # 22 has plate loads of 22k Ohms.
22k/17 = 1295 Ohms
The impedance of one cathode is 1/Gm + Rp/u; 455 Ohms + 1295 Ohms = 1750 Ohms.
The LTP cathode pair is 1750/2 = 875 Ohms.

I once used a Hammond 20H 100mA choke as a current source for a pair of 6C45pi tubes, running about 10mA each. The laminations that were enough for 100mA at 120Hz worked reasonably well at 40Hz and 20mA. It had quite low distributed capacitance, so it also worked well beyond 20kHz.
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